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libpng(3)                                                            libpng(3)




NAME

       libpng - Portable Network Graphics (PNG) Reference Library 1.6.17


SYNOPSIS

        #include <png.h>

       png_uint_32 png_access_version_number (void);

       void png_benign_error (png_structp png_ptr, png_const_charp error);

       void png_build_grayscale_palette (int bit_depth, png_colorp palette);

       png_voidp png_calloc (png_structp png_ptr, png_alloc_size_t size);

       void   png_chunk_benign_error   (png_structp  png_ptr,  png_const_charp
       error);

       void png_chunk_error (png_structp png_ptr, png_const_charp error);

       void png_chunk_warning (png_structp png_ptr, png_const_charp message);

       void png_convert_from_struct_tm  (png_timep  ptime,  struct  tm  FAR  *
       ttime);

       void png_convert_from_time_t (png_timep ptime, time_t ttime);

       png_charp   png_convert_to_rfc1123   (png_structp   png_ptr,  png_timep
       ptime);

       png_infop png_create_info_struct (png_structp png_ptr);

       png_structp   png_create_read_struct   (png_const_charp   user_png_ver,
       png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

       png_structp   png_create_read_struct_2  (png_const_charp  user_png_ver,
       png_voidp error_ptr,  png_error_ptr  error_fn,  png_error_ptr  warn_fn,
       png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

       png_structp   png_create_write_struct   (png_const_charp  user_png_ver,
       png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

       png_structp  png_create_write_struct_2  (png_const_charp  user_png_ver,
       png_voidp  error_ptr,  png_error_ptr  error_fn,  png_error_ptr warn_fn,
       png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

       void  png_data_freer  (png_structp  png_ptr,  png_infop  info_ptr,  int
       freer, png_uint_32 mask));

       void    png_destroy_info_struct    (png_structp   png_ptr,   png_infopp
       info_ptr_ptr);

       void  png_destroy_read_struct  (png_structpp  png_ptr_ptr,   png_infopp
       info_ptr_ptr, png_infopp end_info_ptr_ptr);

       void  png_destroy_write_struct  (png_structpp  png_ptr_ptr,  png_infopp
       info_ptr_ptr);

       void png_err (png_structp png_ptr);

       void png_error (png_structp png_ptr, png_const_charp error);

       void png_free (png_structp png_ptr, png_voidp ptr);

       void png_free_chunk_list (png_structp png_ptr);

       void png_free_default (png_structp png_ptr, png_voidp ptr);

       void png_free_data (png_structp png_ptr, png_infop info_ptr, int num);

       png_byte png_get_bit_depth (png_const_structp png_ptr,  png_const_infop
       info_ptr);

       png_uint_32    png_get_bKGD   (png_const_structp   png_ptr,   png_infop
       info_ptr, png_color_16p *background);

       png_byte png_get_channels (png_const_structp  png_ptr,  png_const_infop
       info_ptr);

       png_uint_32  png_get_cHRM  (png_const_structp  png_ptr, png_const_infop
       info_ptr, double  *white_x,  double  *white_y,  double  *red_x,  double
       *red_y,  double  *green_x,  double  *green_y,  double  *blue_x,  double
       *blue_y);

       png_uint_32     png_get_cHRM_fixed     (png_const_structp      png_ptr,
       png_const_infop  info_ptr,  png_uint_32 *white_x, png_uint_32 *white_y,
       png_uint_32   *red_x,   png_uint_32   *red_y,   png_uint_32   *green_x,
       png_uint_32 *green_y, png_uint_32 *blue_x, png_uint_32 *blue_y);

       png_uint_32   png_get_cHRM_XYZ  (png_structp  png_ptr,  png_const_infop
       info_ptr, double *red_X, double *red_Y, double *red_Z, double *green_X,
       double  *green_Y, double *green_Z, double *blue_X, double *blue_Y, dou-
       ble *blue_Z);

       png_uint_32      png_get_cHRM_XYZ_fixed      (png_structp      png_ptr,
       png_const_infop  info_ptr,  png_fixed_point *int_red_X, png_fixed_point
       *int_red_Y, png_fixed_point *int_red_Z,  png_fixed_point  *int_green_X,
       png_fixed_point     *int_green_Y,     png_fixed_point     *int_green_Z,
       png_fixed_point     *int_blue_X,      png_fixed_point      *int_blue_Y,
       png_fixed_point *int_blue_Z);

       png_uint_32 png_get_chunk_cache_max (png_const_structp png_ptr);

       png_alloc_size_t png_get_chunk_malloc_max (png_const_structp png_ptr);

       png_byte png_get_color_type (png_const_structp png_ptr, png_const_infop
       info_ptr);

       png_uint_32     png_get_compression_buffer_size      (png_const_structp
       png_ptr);

       png_byte     png_get_compression_type    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_byte png_get_copyright (png_const_structp png_ptr);

       png_uint_32 png_get_current_row_number (png_const_structp);

       png_byte png_get_current_pass_number (png_const_structp);

       png_voidp png_get_error_ptr (png_const_structp png_ptr);

       png_byte      png_get_filter_type      (png_const_structp      png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_gAMA  (png_const_structp  png_ptr, png_const_infop
       info_ptr, double *file_gamma);

       png_uint_32     png_get_gAMA_fixed     (png_const_structp      png_ptr,
       png_const_infop info_ptr, png_uint_32 *int_file_gamma);

       png_byte png_get_header_ver (png_const_structp png_ptr);

       png_byte png_get_header_version (png_const_structp png_ptr);

       png_uint_32  png_get_hIST  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_uint_16p *hist);

       png_uint_32 png_get_iCCP  (png_const_structp  png_ptr,  png_const_infop
       info_ptr,  png_charpp  name, int *compression_type, png_bytepp profile,
       png_uint_32 *proflen);

       png_uint_32  png_get_IHDR  (png_structp  png_ptr,  png_infop  info_ptr,
       png_uint_32   *width,   png_uint_32   *height,   int   *bit_depth,  int
       *color_type, int  *interlace_type,  int  *compression_type,  int  *fil-
       ter_type);

       png_uint_32     png_get_image_height     (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32     png_get_image_width     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_int_32 png_get_int_32 (png_bytep buf);

       png_byte     png_get_interlace_type     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_uint_32 png_get_io_chunk_type (png_const_structp png_ptr);

       png_voidp png_get_io_ptr (png_structp png_ptr);

       png_uint_32 png_get_io_state (png_structp png_ptr);

       png_byte png_get_libpng_ver (png_const_structp png_ptr);

       png_voidp png_get_mem_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_oFFs  (png_const_structp  png_ptr,  png_const_infop
       info_ptr,    png_uint_32    *offset_x,   png_uint_32   *offset_y,   int
       *unit_type);

       png_uint_32 png_get_pCAL  (png_const_structp  png_ptr,  png_const_infop
       info_ptr,  png_charp  *purpose,  png_int_32  *X0,  png_int_32  *X1, int
       *type, int *nparams, png_charp *units, png_charpp *params);

       png_uint_32 png_get_pHYs  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int *unit_type);

       float     png_get_pixel_aspect_ratio     (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32      png_get_pHYs_dpi      (png_const_structp      png_ptr,
       png_const_infop  info_ptr,  png_uint_32 *res_x, png_uint_32 *res_y, int
       *unit_type);

       png_fixed_point   png_get_pixel_aspect_ratio_fixed   (png_const_structp
       png_ptr, png_const_infop info_ptr);

       png_uint_32    png_get_pixels_per_inch    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_pixels_per_meter   (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_voidp png_get_progressive_ptr (png_const_structp png_ptr);

       png_uint_32  png_get_PLTE  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_colorp *palette, int *num_palette);

       png_byte png_get_rgb_to_gray_status (png_const_structp png_ptr);

       png_uint_32      png_get_rowbytes      (png_const_structp      png_ptr,
       png_const_infop info_ptr);

       png_bytepp  png_get_rows  (png_const_structp  png_ptr,  png_const_infop
       info_ptr);

       png_uint_32   png_get_sBIT   (png_const_structp   png_ptr,    png_infop
       info_ptr, png_color_8p *sig_bit);

       void png_get_sCAL (png_const_structp png_ptr, png_const_infop info_ptr,
       int* unit, double* width, double* height);

       void  png_get_sCAL_fixed  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, int* unit, png_fixed_pointp width, png_fixed_pointp height);

       void   png_get_sCAL_s   (png_const_structp   png_ptr,   png_const_infop
       info_ptr, int* unit, png_charpp width, png_charpp height);

       png_bytep  png_get_signature  (png_const_structp   png_ptr,   png_infop
       info_ptr);

       png_uint_32  png_get_sPLT  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_spalette_p *splt_ptr);

       png_uint_32 png_get_sRGB  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, int *file_srgb_intent);

       png_uint_32  png_get_text  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_textp *text_ptr, int *num_text);

       png_uint_32   png_get_tIME   (png_const_structp   png_ptr,    png_infop
       info_ptr, png_timep *mod_time);

       png_uint_32    png_get_tRNS   (png_const_structp   png_ptr,   png_infop
       info_ptr,  png_bytep  *trans_alpha,   int   *num_trans,   png_color_16p
       *trans_color);

       /* This function is really an inline macro. */

       png_uint_16 png_get_uint_16 (png_bytep buf);

       png_uint_32 png_get_uint_31 (png_structp png_ptr, png_bytep buf);

       /* This function is really an inline macro. */

       png_uint_32 png_get_uint_32 (png_bytep buf);

       png_uint_32    png_get_unknown_chunks    (png_const_structp    png_ptr,
       png_const_infop info_ptr, png_unknown_chunkpp unknowns);

       png_voidp png_get_user_chunk_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_user_height_max (png_const_structp png_ptr);

       png_voidp png_get_user_transform_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_user_width_max (png_const_structp png_ptr);

       png_uint_32 png_get_valid (png_const_structp  png_ptr,  png_const_infop
       info_ptr, png_uint_32 flag);

       float      png_get_x_offset_inches      (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_fixed_point  png_get_x_offset_inches_fixed  (png_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_x_offset_microns    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_x_offset_pixels    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_x_pixels_per_inch   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_x_pixels_per_meter   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       float      png_get_y_offset_inches      (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_fixed_point  png_get_y_offset_inches_fixed  (png_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_y_offset_microns    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_y_offset_pixels    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_y_pixels_per_inch   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_y_pixels_per_meter   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

       int   png_image_begin_read_from_file   (png_imagep  image,  const  char
       *file_name);

       int png_image_begin_read_from_stdio (png_imagep image, FILE* file);

       int,      png_image_begin_read_from_memory      (png_imagep      image,
       png_const_voidp memory, png_size_t size);

       int  png_image_finish_read  (png_imagep  image,  png_colorp background,
       void *buffer, png_int_32 row_stride, void *colormap);

       void png_image_free (png_imagep image);

       int png_image_write_to_file (png_imagep image, const  char  *file,  int
       convert_to_8bit,  const void *buffer, png_int_32 row_stride, void *col-
       ormap);

       int png_image_write_to_stdio (png_imagep image, FILE  *file,  int  con-
       vert_to_8_bit,  const  void  *buffer, png_int_32 row_stride, void *col-
       ormap));

       void     png_info_init_3     (png_infopp      info_ptr,      png_size_t
       png_info_struct_size);

       void png_init_io (png_structp png_ptr, FILE *fp);

       void png_longjmp (png_structp png_ptr, int val);

       png_voidp png_malloc (png_structp png_ptr, png_alloc_size_t size);

       png_voidp  png_malloc_default  (png_structp  png_ptr,  png_alloc_size_t
       size);

       png_voidp png_malloc_warn (png_structp png_ptr, png_alloc_size_t size);

       png_uint_32  png_permit_mng_features  (png_structp png_ptr, png_uint_32
       mng_features_permitted);

       void  png_process_data  (png_structp   png_ptr,   png_infop   info_ptr,
       png_bytep buffer, png_size_t buffer_size);

       png_size_t png_process_data_pause (png_structp, int save);

       png_uint_32 png_process_data_skip (png_structp);

       void   png_progressive_combine_row   (png_structp   png_ptr,  png_bytep
       old_row, png_bytep new_row);

       void png_read_end (png_structp png_ptr, png_infop info_ptr);

       void png_read_image (png_structp png_ptr, png_bytepp image);

       void png_read_info (png_structp png_ptr, png_infop info_ptr);

       void png_read_png (png_structp png_ptr, png_infop info_ptr, int  trans-
       forms, png_voidp params);

       void  png_read_row  (png_structp png_ptr, png_bytep row, png_bytep dis-
       play_row);

       void png_read_rows (png_structp  png_ptr,  png_bytepp  row,  png_bytepp
       display_row, png_uint_32 num_rows);

       void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

       int png_reset_zstream (png_structp png_ptr);

       void png_save_int_32 (png_bytep buf, png_int_32 i);

       void png_save_uint_16 (png_bytep buf, unsigned int i);

       void png_save_uint_32 (png_bytep buf, png_uint_32 i);

       void  png_set_add_alpha  (png_structp  png_ptr, png_uint_32 filler, int
       flags);

       void png_set_alpha_mode (png_structp png_ptr,  int  mode,  double  out-
       put_gamma);

       void   png_set_alpha_mode_fixed   (png_structp   png_ptr,   int   mode,
       png_fixed_point output_gamma);

       void  png_set_background  (png_structp  png_ptr,  png_color_16p   back-
       ground_color,  int background_gamma_code, int need_expand, double back-
       ground_gamma);

       void png_set_background_fixed (png_structp png_ptr, png_color_16p back-
       ground_color,  int  background_gamma_code, int need_expand, png_uint_32
       background_gamma);

       void png_set_benign_errors (png_structp png_ptr, int allowed);

       void png_set_bgr (png_structp png_ptr);

       void   png_set_bKGD   (png_structp   png_ptr,    png_infop    info_ptr,
       png_color_16p background);

       void    png_set_check_for_invalid_index(png_structrp    png_ptr,    int
       allowed);

       void png_set_cHRM  (png_structp  png_ptr,  png_infop  info_ptr,  double
       white_x,  double  white_y,  double red_x, double red_y, double green_x,
       double green_y, double blue_x, double blue_y);

       void  png_set_cHRM_fixed  (png_structp  png_ptr,  png_infop   info_ptr,
       png_uint_32    white_x,   png_uint_32   white_y,   png_uint_32   red_x,
       png_uint_32   red_y,   png_uint_32   green_x,   png_uint_32    green_y,
       png_uint_32 blue_x, png_uint_32 blue_y);

       void  png_set_cHRM_XYZ (png_structp png_ptr, png_infop info_ptr, double
       red_X, double red_Y, double red_Z, double green_X, double green_Y, dou-
       ble green_Z, double blue_X, double blue_Y, double blue_Z);

       void  png_set_cHRM_XYZ_fixed  (png_structp png_ptr, png_infop info_ptr,
       png_fixed_point int_red_X, png_fixed_point  int_red_Y,  png_fixed_point
       int_red_Z,  png_fixed_point  int_green_X,  png_fixed_point int_green_Y,
       png_fixed_point      int_green_Z,      png_fixed_point      int_blue_X,
       png_fixed_point int_blue_Y, png_fixed_point int_blue_Z);

       void    png_set_chunk_cache_max   (png_structp   png_ptr,   png_uint_32
       user_chunk_cache_max);

       void png_set_compression_level (png_structp png_ptr, int level);

       void   png_set_compression_mem_level    (png_structp    png_ptr,    int
       mem_level);

       void png_set_compression_method (png_structp png_ptr, int method);

       void png_set_compression_strategy (png_structp png_ptr, int strategy);

       void  png_set_compression_window_bits  (png_structp  png_ptr,  int win-
       dow_bits);

       void png_set_crc_action  (png_structp  png_ptr,  int  crit_action,  int
       ancil_action);

       void   png_set_error_fn   (png_structp  png_ptr,  png_voidp  error_ptr,
       png_error_ptr error_fn, png_error_ptr warning_fn);

       void png_set_expand (png_structp png_ptr);

       void png_set_expand_16 (png_structp png_ptr);

       void png_set_expand_gray_1_2_4_to_8 (png_structp png_ptr);

       void  png_set_filler  (png_structp  png_ptr,  png_uint_32  filler,  int
       flags);

       void png_set_filter (png_structp png_ptr, int method, int filters);

       void   png_set_filter_heuristics   (png_structp  png_ptr,  int  heuris-
       tic_method, int num_weights,  png_doublep  filter_weights,  png_doublep
       filter_costs);

       void  png_set_filter_heuristics_fixed (png_structp png_ptr, int heuris-
       tic_method,   int   num_weights,   png_fixed_point_p    filter_weights,
       png_fixed_point_p filter_costs);

       void png_set_flush (png_structp png_ptr, int nrows);

       void  png_set_gamma  (png_structp  png_ptr, double screen_gamma, double
       default_file_gamma);

       void    png_set_gamma_fixed    (png_structp    png_ptr,     png_uint_32
       screen_gamma, png_uint_32 default_file_gamma);

       void  png_set_gAMA  (png_structp  png_ptr,  png_infop  info_ptr, double
       file_gamma);

       void  png_set_gAMA_fixed  (png_structp  png_ptr,  png_infop   info_ptr,
       png_uint_32 file_gamma);

       void png_set_gray_1_2_4_to_8 (png_structp png_ptr);

       void png_set_gray_to_rgb (png_structp png_ptr);

       void    png_set_hIST    (png_structp   png_ptr,   png_infop   info_ptr,
       png_uint_16p hist);

       void   png_set_iCCP   (png_structp   png_ptr,    png_infop    info_ptr,
       png_const_charp  name,  int  compression_type, png_const_bytep profile,
       png_uint_32 proflen);

       int png_set_interlace_handling (png_structp png_ptr);

       void png_set_invalid  (png_structp  png_ptr,  png_infop  info_ptr,  int
       mask);

       void png_set_invert_alpha (png_structp png_ptr);

       void png_set_invert_mono (png_structp png_ptr);

       void png_set_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32
       width, png_uint_32 height, int bit_depth, int  color_type,  int  inter-
       lace_type, int compression_type, int filter_type);

       void   png_set_keep_unknown_chunks   (png_structp  png_ptr,  int  keep,
       png_bytep chunk_list, int num_chunks);

       jmp_buf*  png_set_longjmp_fn  (png_structp   png_ptr,   png_longjmp_ptr
       longjmp_fn, size_t jmp_buf_size);

       void  png_set_chunk_malloc_max  (png_structp  png_ptr, png_alloc_size_t
       user_chunk_cache_max);

       void png_set_compression_buffer_size (png_structp png_ptr,  png_uint_32
       size);

       void  png_set_mem_fn  (png_structp png_ptr, png_voidp mem_ptr, png_mal-
       loc_ptr malloc_fn, png_free_ptr free_fn);

       void png_set_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32
       offset_x, png_uint_32 offset_y, int unit_type);

       void png_set_packing (png_structp png_ptr);

       void png_set_packswap (png_structp png_ptr);

       void png_set_palette_to_rgb (png_structp png_ptr);

       void  png_set_pCAL  (png_structp png_ptr, png_infop info_ptr, png_charp
       purpose, png_int_32 X0, png_int_32 X1, int type, int nparams, png_charp
       units, png_charpp params);

       void png_set_pHYs (png_structp png_ptr, png_infop info_ptr, png_uint_32
       res_x, png_uint_32 res_y, int unit_type);

       void png_set_progressive_read_fn (png_structp png_ptr,  png_voidp  pro-
       gressive_ptr, png_progressive_info_ptr info_fn, png_progressive_row_ptr
       row_fn, png_progressive_end_ptr end_fn);

       void png_set_PLTE (png_structp png_ptr, png_infop info_ptr,  png_colorp
       palette, int num_palette);

       void  png_set_quantize  (png_structp  png_ptr,  png_colorp palette, int
       num_palette, int maximum_colors, png_uint_16p histogram, int full_quan-
       tize);

       void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr
       read_data_fn);

       void png_set_read_status_fn (png_structp  png_ptr,  png_read_status_ptr
       read_row_fn);

       void   png_set_read_user_chunk_fn   (png_structp   png_ptr,   png_voidp
       user_chunk_ptr, png_user_chunk_ptr read_user_chunk_fn);

       void     png_set_read_user_transform_fn      (png_structp      png_ptr,
       png_user_transform_ptr read_user_transform_fn);

       void png_set_rgb_to_gray (png_structp png_ptr, int error_action, double
       red, double green);

       void png_set_rgb_to_gray_fixed (png_structp png_ptr,  int  error_action
       png_uint_32 red, png_uint_32 green);

       void  png_set_rows (png_structp png_ptr, png_infop info_ptr, png_bytepp
       row_pointers);

       void   png_set_sBIT   (png_structp   png_ptr,    png_infop    info_ptr,
       png_color_8p sig_bit);

       void  png_set_sCAL  (png_structp png_ptr, png_infop info_ptr, int unit,
       double width, double height);

       void png_set_sCAL_fixed (png_structp png_ptr, png_infop  info_ptr,  int
       unit, png_fixed_point width, png_fixed_point height);

       void png_set_sCAL_s (png_structp png_ptr, png_infop info_ptr, int unit,
       png_charp width, png_charp height);

       void png_set_scale_16 (png_structp png_ptr);

       void png_set_shift (png_structp png_ptr, png_color_8p true_bits);

       void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

       void   png_set_sPLT   (png_structp   png_ptr,    png_infop    info_ptr,
       png_spalette_p splt_ptr, int num_spalettes);

       void   png_set_sRGB   (png_structp  png_ptr,  png_infop  info_ptr,  int
       srgb_intent);

       void   png_set_sRGB_gAMA_and_cHRM   (png_structp   png_ptr,   png_infop
       info_ptr, int srgb_intent);

       void png_set_strip_16 (png_structp png_ptr);

       void png_set_strip_alpha (png_structp png_ptr);

       void   png_set_strip_error_numbers  (png_structp  png_ptr,  png_uint_32
       strip_mode);

       void png_set_swap (png_structp png_ptr);

       void png_set_swap_alpha (png_structp png_ptr);

       void png_set_text (png_structp png_ptr, png_infop  info_ptr,  png_textp
       text_ptr, int num_text);

       void png_set_text_compression_level (png_structp png_ptr, int level);

       void   png_set_text_compression_mem_level   (png_structp  png_ptr,  int
       mem_level);

       void png_set_text_compression_strategy (png_structp png_ptr, int strat-
       egy);

       void  png_set_text_compression_window_bits  (png_structp  png_ptr,  int
       window_bits);

       void   png_set_text_compression_method,   (png_structp   png_ptr,   int
       method));

       void  png_set_tIME  (png_structp png_ptr, png_infop info_ptr, png_timep
       mod_time);

       void png_set_tRNS (png_structp png_ptr, png_infop  info_ptr,  png_bytep
       trans_alpha, int num_trans, png_color_16p trans_color);

       void png_set_tRNS_to_alpha (png_structp png_ptr);

       png_uint_32   png_set_unknown_chunks  (png_structp  png_ptr,  png_infop
       info_ptr, png_unknown_chunkp unknowns, int num, int location);

       void  png_set_unknown_chunk_location  (png_structp  png_ptr,  png_infop
       info_ptr, int chunk, int location);

       void     png_set_user_limits    (png_structp    png_ptr,    png_uint_32
       user_width_max, png_uint_32 user_height_max);

       void  png_set_user_transform_info   (png_structp   png_ptr,   png_voidp
       user_transform_ptr,  int user_transform_depth, int user_transform_chan-
       nels);

       void   png_set_write_fn   (png_structp   png_ptr,   png_voidp   io_ptr,
       png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);

       void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
       write_row_fn);

       void     png_set_write_user_transform_fn     (png_structp      png_ptr,
       png_user_transform_ptr write_user_transform_fn);

       int   png_sig_cmp   (png_bytep   sig,   png_size_t   start,  png_size_t
       num_to_check);

       void png_start_read_image (png_structp png_ptr);

       void png_warning (png_structp png_ptr, png_const_charp message);

       void  png_write_chunk  (png_structp  png_ptr,   png_bytep   chunk_name,
       png_bytep data, png_size_t length);

       void   png_write_chunk_data   (png_structp   png_ptr,  png_bytep  data,
       png_size_t length);

       void png_write_chunk_end (png_structp png_ptr);

       void png_write_chunk_start (png_structp png_ptr, png_bytep  chunk_name,
       png_uint_32 length);

       void png_write_end (png_structp png_ptr, png_infop info_ptr);

       void png_write_flush (png_structp png_ptr);

       void png_write_image (png_structp png_ptr, png_bytepp image);

       void png_write_info (png_structp png_ptr, png_infop info_ptr);

       void   png_write_info_before_PLTE   (png_structp   png_ptr,   png_infop
       info_ptr);

       void png_write_png (png_structp png_ptr, png_infop info_ptr, int trans-
       forms, png_voidp params);

       void png_write_row (png_structp png_ptr, png_bytep row);

       void  png_write_rows  (png_structp png_ptr, png_bytepp row, png_uint_32
       num_rows);

       void png_write_sig (png_structp png_ptr);



DESCRIPTION

       The libpng library supports encoding, decoding, and  various  manipula-
       tions  of  the  Portable Network Graphics (PNG) format image files.  It
       uses the zlib(3) compression library.   Following  is  a  copy  of  the
       libpng-manual.txt file that accompanies libpng.


LIBPNG.TXT

       libpng-manual.txt - A description on how to use and modify libpng

        libpng version 1.6.17 - March 26, 2015
        Updated and distributed by Glenn Randers-Pehrson
        <glennrp at users.sourceforge.net>
        Copyright (c) 1998-2015 Glenn Randers-Pehrson

        This document is released under the libpng license.
        For conditions of distribution and use, see the disclaimer
        and license in png.h

        Based on:

        libpng versions 0.97, January 1998, through 1.6.17 - March 26, 2015
        Updated and distributed by Glenn Randers-Pehrson
        Copyright (c) 1998-2015 Glenn Randers-Pehrson

        libpng 1.0 beta 6 - version 0.96 - May 28, 1997
        Updated and distributed by Andreas Dilger
        Copyright (c) 1996, 1997 Andreas Dilger

        libpng 1.0 beta 2 - version 0.88 - January 26, 1996
        For conditions of distribution and use, see copyright
        notice in png.h. Copyright (c) 1995, 1996 Guy Eric
        Schalnat, Group 42, Inc.

        Updated/rewritten per request in the libpng FAQ
        Copyright (c) 1995, 1996 Frank J. T. Wojcik
        December 18, 1995 & January 20, 1996

        TABLE OF CONTENTS

           I. Introduction
          II. Structures
         III. Reading
          IV. Writing
           V. Simplified API
          VI. Modifying/Customizing libpng
         VII. MNG support
        VIII. Changes to Libpng from version 0.88
          IX. Changes to Libpng from version 1.0.x to 1.2.x
           X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
          XI. Changes to Libpng from version 1.4.x to 1.5.x
         XII. Changes to Libpng from version 1.5.x to 1.6.x
        XIII. Detecting libpng
         XIV. Source code repository
          XV. Coding style
         XVI. Y2K Compliance in libpng



I. Introduction

       This  file  describes  how  to use and modify the PNG reference library
       (known as libpng) for your own use.  In addition to  this  file,  exam-
       ple.c  is a good starting point for using the library, as it is heavily
       commented and should include everything  most  people  will  need.   We
       assume  that  libpng  is  already  installed;  see the INSTALL file for
       instructions on how to configure and install libpng.

       For examples of libpng usage, see the files  "example.c",  "pngtest.c",
       and  the files in the "contrib" directory, all of which are included in
       the libpng distribution.

       Libpng was written as a companion to the PNG specification, as a way of
       reducing the amount of time and effort it takes to support the PNG file
       format in application programs.

       The PNG specification (second edition), November 2003, is available  as
       a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at
       http://www.w3.org/TR/2003/REC-PNG-20031110/ The W3C and ISO  documents
       have identical technical content.

       The       PNG-1.2       specification       is       available       at
       <http://www.libpng.org/pub/png/documents/>.  It is technically  equiva-
       lent  to the PNG specification (second edition) but has some additional
       material.

       The    PNG-1.0    specification    is    available    as    RFC    2083
       <http://www.libpng.org/pub/png/documents/>  and as a W3C Recommendation
       <http://www.w3.org/TR/REC.png.html>.

       Some additional chunks are  described  in  the  special-purpose  public
       chunks documents at <http://www.libpng.org/pub/png/documents/>.

       Other  information  about PNG, and the latest version of libpng, can be
       found at the PNG home page, <http://www.libpng.org/pub/png/>.

       Most users will not have to modify the library significantly;  advanced
       users may want to modify it more.  All attempts were made to make it as
       complete as possible, while keeping the code easy to understand.   Cur-
       rently,  this  library only supports C.  Support for other languages is
       being considered.

       Libpng has been designed to handle multiple sessions at one time, to be
       easily  modifiable,  to  be  portable  to the vast majority of machines
       (ANSI, K&R, 16-, 32-, and 64-bit) available, and to  be  easy  to  use.
       The  ultimate  goal  of  libpng is to promote the acceptance of the PNG
       file format in whatever way possible.  While there is still work to  be
       done (see the TODO file), libpng should cover the majority of the needs
       of its users.

       Libpng uses zlib for its compression and decompression  of  PNG  files.
       Further  information about zlib, and the latest version of zlib, can be
       found     at     the     zlib     home     page,      <http://www.info-
       zip.org/pub/infozip/zlib/>.   The zlib compression utility is a general
       purpose utility that is useful for more than PNG files, and can be used
       without  libpng.   See  the  documentation delivered with zlib for more
       details.  You can usually find the source files for  the  zlib  utility
       wherever you find the libpng source files.

       Libpng  is  thread  safe,  provided  the  threads  are  using different
       instances  of  the  structures.   Each  thread  should  have  its   own
       png_struct and png_info instances, and thus its own image.  Libpng does
       not protect itself against two threads using the  same  instance  of  a
       structure.



II. Structures

       There  are two main structures that are important to libpng, png_struct
       and png_info.  Both are internal structures that are no longer  exposed
       in the libpng interface (as of libpng 1.5.0).

       The png_info structure is designed to provide information about the PNG
       file.  At one time, the fields of png_info were intended to be directly
       accessible  to  the  user.  However, this tended to cause problems with
       applications using dynamically loaded libraries, and as a result a  set
       of  interface  functions  for png_info (the png_get_*() and png_set_*()
       functions) was developed, and direct access to the png_info fields  was
       deprecated..

       The  png_struct structure is the object used by the library to decode a
       single image.  As of 1.5.0 this structure is also not exposed.

       Almost all libpng APIs require a pointer to a png_struct as  the  first
       argument.   Many  (in  particular  the  png_set  and png_get APIs) also
       require a pointer to png_info as the second argument.  Some application
       visible macros defined in png.h designed for basic data access (reading
       and writing integers in the PNG format) don't take a png_info  pointer,
       but  it's  almost  always safe to assume that a (png_struct*) has to be
       passed to call an API function.

       You can have more than one png_info structure associated with an image,
       as  illustrated  in  pngtest.c,  one for information valid prior to the
       IDAT chunks and another (called  "end_info"  below)  for  things  after
       them.

       The  png.h  header file is an invaluable reference for programming with
       libpng.  And while I'm on the topic, make sure you include  the  libpng
       header file:

       #include <png.h>

       and also (as of libpng-1.5.0) the zlib header file, if you need it:

       #include <zlib.h>


   Types
       The  png.h  header  file defines a number of integral types used by the
       APIs.  Most of these are fairly obvious; for example types  correspond-
       ing to integers of particular sizes and types for passing color values.

       One exception is how non-integral numbers are handled.  For application
       convenience most APIs that take such numbers have C (double) arguments;
       however, internally PNG, and libpng, use 32  bit  signed  integers  and
       encode  the value by multiplying by 100,000.  As of libpng 1.5.0 a con-
       venience  macro  PNG_FP_1  is  defined  in  png.h  along  with  a  type
       (png_fixed_point) which is simply (png_int_32).

       All  APIs  that  take  (double) arguments also have a matching API that
       takes the corresponding fixed point integer arguments.  The fixed point
       API has the same name as the floating point one with "_fixed" appended.
       The actual range of values permitted in the  APIs  is  frequently  less
       than the full range of (png_fixed_point) (-21474 to +21474).  When APIs
       require a non-negative argument the type  is  recorded  as  png_uint_32
       above.   Consult  the  header file and the text below for more informa-
       tion.

       Special care must be take with sCAL chunk handling  because  the  chunk
       itself  uses  non-integral values encoded as strings containing decimal
       floating point numbers.  See the comments in the header file.


   Configuration
       The main header file function declarations are frequently protected  by
       C preprocessing directives of the form:

           #ifdef PNG_feature_SUPPORTED
           declare-function
           #endif
           ...
           #ifdef PNG_feature_SUPPORTED
           use-function
           #endif

       The  library  can  be  built without support for these APIs, although a
       standard build will have all implemented  APIs.   Application  programs
       should  check the feature macros before using an API for maximum porta-
       bility.  From libpng 1.5.0 the feature macros set during the  build  of
       libpng  are recorded in the header file "pnglibconf.h" and this file is
       always included by png.h.

       If you don't need to change the library configuration from the default,
       skip to the next section ("Reading").

       Notice  that  some  of the makefiles in the 'scripts' directory and (in
       1.5.0) all of the build project files in the 'projects' directory  sim-
       ply  copy  scripts/pnglibconf.h.prebuilt  to  pnglibconf.h.  This means
       that these build systems do not permit easy auto-configuration  of  the
       library - they only support the default configuration.

       The  easiest way to make minor changes to the libpng configuration when
       auto-configuration is supported is to add definitions  to  the  command
       line using (typically) CPPFLAGS.  For example:

       CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC

       will  change  the internal libpng math implementation for gamma correc-
       tion and other arithmetic calculations to  fixed  point,  avoiding  the
       need  for  fast  floating point support.  The result can be seen in the
       generated pnglibconf.h - make sure  it  contains  the  changed  feature
       macro setting.

       If  you  need  to make more extensive configuration changes - more than
       one or two feature macro settings - you can either add  -DPNG_USER_CON-
       FIG  to the build command line and put a list of feature macro settings
       in pngusr.h or you can set DFA_XTRA (a makefile  variable)  to  a  file
       containing the same information in the form of 'option' settings.

       A. Changing pnglibconf.h

       A  variety  of methods exist to build libpng.  Not all of these support
       reconfiguration of pnglibconf.h.  To reconfigure pnglibconf.h  it  must
       either  be  rebuilt from scripts/pnglibconf.dfa using awk or it must be
       edited by hand.

       Hand editing is achieved by  copying  scripts/pnglibconf.h.prebuilt  to
       pnglibconf.h  and  changing  the lines defining the supported features,
       paying  very  close  attention   to   the   'option'   information   in
       scripts/pnglibconf.dfa that describes those features and their require-
       ments.  This is easy to get wrong.

       B. Configuration using DFA_XTRA

       Rebuilding from pnglibconf.dfa is easy if a  functioning  'awk',  or  a
       later  variant  such  as 'nawk' or 'gawk', is available.  The configure
       build will automatically find an  appropriate  awk  and  build  pnglib-
       conf.h.   The  scripts/pnglibconf.mak file contains a set of make rules
       for doing the same thing if configure is not  used,  and  many  of  the
       makefiles in the scripts directory use this approach.

       When  rebuilding simply write a new file containing changed options and
       set DFA_XTRA to the name of this file.  This causes the build to append
       the new file to the end of scripts/pnglibconf.dfa.  The pngusr.dfa file
       should contain lines of the following forms:

       everything = off

       This turns all optional features off.   Include  it  at  the  start  of
       pngusr.dfa  to  make  it  easier to build a minimal configuration.  You
       will need to turn at least some features on afterward to enable  either
       reading or writing code, or both.

       option feature on option feature off

       Enable  or  disable  a  single feature.  This will automatically enable
       other features required by a feature that is turned on or disable other
       features  that require a feature which is turned off.  Conflicting set-
       tings will cause an error message to be emitted by awk.

       setting feature default value

       Changes the default value of setting 'feature' to 'value'.  There are a
       small  number  of  settings listed at the top of pnglibconf.h, they are
       documented in the source code.  Most of these values  have  performance
       implications for the library but most of them have no visible effect on
       the API.  Some can also be overridden from the API.

       This method of building a customized  pnglibconf.h  is  illustrated  in
       contrib/pngminim/*.   See  the "$(PNGCONF):" target in the makefile and
       pngusr.dfa in these directories.

       C. Configuration using PNG_USER_CONFIG

       If -DPNG_USER_CONFIG is added to  the  CPPFLAGS  when  pnglibconf.h  is
       built,  the  file  pngusr.h  will  automatically be included before the
       options in scripts/pnglibconf.dfa are processed.   Your  pngusr.h  file
       should  contain  only  macro  definitions turning features on or off or
       setting settings.

       Apart from the global setting "everything = off" all the options listed
       above can be set using macros in pngusr.h:

       #define PNG_feature_SUPPORTED

       is equivalent to:

       option feature on

       #define PNG_NO_feature

       is equivalent to:

       option feature off

       #define PNG_feature value

       is equivalent to:

       setting feature default value

       Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
       pngusr file you supply override the contents of scripts/pnglibconf.dfa

       If confusing or incomprehensible behavior results  it  is  possible  to
       examine  the  intermediate  file pnglibconf.dfn to find the full set of
       dependency information for each setting and option.  Simply locate  the
       feature in the file and read the C comments that precede it.

       This method is also illustrated in the contrib/pngminim/* makefiles and
       pngusr.h.



III. Reading

       We'll now walk you through the possible functions to call when  reading
       in  a  PNG file sequentially, briefly explaining the syntax and purpose
       of each one.  See example.c and png.h for more detail.  While  progres-
       sive  reading  is covered in the next section, you will still need some
       of the functions discussed in this section to read a PNG file.


   Setup
       You will want to do the  I/O  initialization(*)  before  you  get  into
       libpng, so if it doesn't work, you don't have much to undo.  Of course,
       you will also want to insure that you are, in fact, dealing with a  PNG
       file.   Libpng  provides a simple check to see if a file is a PNG file.
       To use it, pass in the first 1 to 8 bytes of the file to  the  function
       png_sig_cmp(), and it will return 0 (false) if the bytes match the cor-
       responding bytes of the PNG signature, or nonzero (true) otherwise.  Of
       course,  the  more  bytes  you pass in, the greater the accuracy of the
       prediction.

       If you are intending to keep the file pointer open for use  in  libpng,
       you  must ensure you don't read more than 8 bytes from the beginning of
       the file, and you also have to make a call to png_set_sig_bytes()  with
       the number of bytes you read from the beginning.  Libpng will then only
       check the bytes (if any) that your program didn't read.

       (*): If you are not using the standard I/O functions, you will need  to
       replace them with custom functions.  See the discussion under Customiz-
       ing libpng.

           FILE *fp = fopen(file_name, "rb");
           if (!fp)
           {
              return (ERROR);
           }

           if (fread(header, 1, number, fp) != number)
           {
              return (ERROR);
           }

           is_png = !png_sig_cmp(header, 0, number);
           if (!is_png)
           {
              return (NOT_PNG);
           }

       Next, png_struct and png_info need to be allocated and initialized.  In
       order  to ensure that the size of these structures is correct even with
       a dynamically linked libpng, there  are  functions  to  initialize  and
       allocate  the  structures.   We also pass the library version, optional
       pointers to error handling functions, and a pointer to  a  data  struct
       for use by the error functions, if necessary (the pointer and functions
       can be NULL if the default error handlers are to  be  used).   See  the
       section  on  Changes  to  Libpng below regarding the old initialization
       functions.  The structure allocation functions quietly return  NULL  if
       they fail to create the structure, so your application should check for
       that.

           png_structp png_ptr = png_create_read_struct
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn);

           if (!png_ptr)
              return (ERROR);

           png_infop info_ptr = png_create_info_struct(png_ptr);

           if (!info_ptr)
           {
              png_destroy_read_struct(&png_ptr,
                  (png_infopp)NULL, (png_infopp)NULL);
              return (ERROR);
           }

       If you want to use your own memory allocation routines,  use  a  libpng
       that  was  built  with PNG_USER_MEM_SUPPORTED defined, and use png_cre-
       ate_read_struct_2() instead of png_create_read_struct():

           png_structp png_ptr = png_create_read_struct_2
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn, (png_voidp)
               user_mem_ptr, user_malloc_fn, user_free_fn);

       The error handling routines passed to png_create_read_struct() and  the
       memory  alloc/free  routines  passed  to png_create_struct_2() are only
       necessary if you are not using the libpng supplied error  handling  and
       memory alloc/free functions.

       When  libpng  encounters  an  error, it expects to longjmp back to your
       routine.  Therefore, you  will  need  to  call  setjmp  and  pass  your
       png_jmpbuf(png_ptr).  If you read the file from different routines, you
       will need to update the longjmp buffer every time you enter a new  rou-
       tine that will call a png_*() function.

       See  your  documentation  of  setjmp/longjmp for your compiler for more
       information on setjmp/longjmp.  See the discussion on libpng error han-
       dling  in  the Customizing Libpng section below for more information on
       the libpng error handling.  If an error occurs,  and  libpng  longjmp's
       back to your setjmp, you will want to call png_destroy_read_struct() to
       free any memory.

           if (setjmp(png_jmpbuf(png_ptr)))
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                  &end_info);
              fclose(fp);
              return (ERROR);
           }

       Pass (png_infopp)NULL instead of &end_info  if  you  didn't  create  an
       end_info structure.

       If  you would rather avoid the complexity of setjmp/longjmp issues, you
       can compile libpng with PNG_NO_SETJMP, in which case errors will result
       in a call to PNG_ABORT() which defaults to abort().

       You can #define PNG_ABORT() to a function that does something more use-
       ful than abort(), as long as your function does not return.

       Now you need to set up the input code.  The default for  libpng  is  to
       use  the  C function fread().  If you use this, you will need to pass a
       valid FILE * in the function png_init_io().  Be sure that the  file  is
       opened  in  binary mode.  If you wish to handle reading data in another
       way, you need not call the png_init_io() function, but  you  must  then
       implement  the  libpng  I/O methods discussed in the Customizing Libpng
       section below.

           png_init_io(png_ptr, fp);

       If you had previously opened the file and read  any  of  the  signature
       from  the beginning in order to see if this was a PNG file, you need to
       let libpng know that there are some bytes missing from the start of the
       file.

           png_set_sig_bytes(png_ptr, number);

       You  can change the zlib compression buffer size to be used while read-
       ing compressed data with

           png_set_compression_buffer_size(png_ptr, buffer_size);

       where the default size is 8192 bytes.  Note that  the  buffer  size  is
       changed  immediately and the buffer is reallocated immediately, instead
       of setting a flag to be acted upon later.

       If you want CRC errors to be handled in a  different  manner  than  the
       default, use

           png_set_crc_action(png_ptr, crit_action, ancil_action);

       The  values  for  png_set_crc_action()  say how libpng is to handle CRC
       errors in ancillary and critical chunks, and whether to  use  the  data
       contained  therein.   Note that it is impossible to "discard" data in a
       critical chunk.

       Choices for (int) crit_action are
          PNG_CRC_DEFAULT      0  error/quit
          PNG_CRC_ERROR_QUIT   1  error/quit
          PNG_CRC_WARN_USE     3  warn/use data
          PNG_CRC_QUIET_USE    4  quiet/use data
          PNG_CRC_NO_CHANGE    5  use the current value

       Choices for (int) ancil_action are
          PNG_CRC_DEFAULT      0  error/quit
          PNG_CRC_ERROR_QUIT   1  error/quit
          PNG_CRC_WARN_DISCARD 2  warn/discard data
          PNG_CRC_WARN_USE     3  warn/use data
          PNG_CRC_QUIET_USE    4  quiet/use data
          PNG_CRC_NO_CHANGE    5  use the current value


   Setting up callback code
       You can set up a callback function to handle any unknown chunks in  the
       input stream. You must supply the function

           read_chunk_callback(png_structp png_ptr,
                png_unknown_chunkp chunk);
           {
              /* The unknown chunk structure contains your
                 chunk data, along with similar data for any other
                 unknown chunks: */

                  png_byte name[5];
                  png_byte *data;
                  png_size_t size;

              /* Note that libpng has already taken care of
                 the CRC handling */

              /* put your code here.  Search for your chunk in the
                 unknown chunk structure, process it, and return one
                 of the following: */

              return (-n); /* chunk had an error */
              return (0); /* did not recognize */
              return (n); /* success */
           }

       (You  can  give  your  function  another  name that you like instead of
       "read_chunk_callback")

       To inform libpng about your function, use

           png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
               read_chunk_callback);

       This names not only the callback function, but also a user pointer that
       you can retrieve with

           png_get_user_chunk_ptr(png_ptr);

       If you call the png_set_read_user_chunk_fn() function, then all unknown
       chunks which the callback does not handle will be saved when read.  You
       can  cause them to be discarded by returning '1' ("handled") instead of
       '0'.  This behavior will change in libpng 1.7 and the default  handling
       set  by  the  png_set_keep_unknown_chunks()  function, described below,
       will be used when the callback returns 0.  If  you  want  the  existing
       behavior  you should set the global default to PNG_HANDLE_CHUNK_IF_SAFE
       now; this is compatible with all current versions of  libpng  and  with
       1.7.   Libpng 1.6 issues a warning if you keep the default, or PNG_HAN-
       DLE_CHUNK_NEVER, and the callback returns 0.

       At this point, you can set up a callback function that will  be  called
       after  each  row has been read, which you can use to control a progress
       meter or the like.  It's demonstrated in pngtest.c.  You must supply  a
       function

           void read_row_callback(png_structp png_ptr,
              png_uint_32 row, int pass);
           {
             /* put your code here */
           }

       (You  can give it another name that you like instead of "read_row_call-
       back")

       To inform libpng about your function, use

           png_set_read_status_fn(png_ptr, read_row_callback);

       When this function is called the row has already been  completely  pro-
       cessed  and  the  'row' and 'pass' refer to the next row to be handled.
       For the non-interlaced case the row that was just handled is simply one
       less than the passed in row number, and pass will always be 0.  For the
       interlaced case the same applies unless the row value is  0,  in  which
       case  the  row  just handled was the last one from one of the preceding
       passes.  Because interlacing may skip a pass you cannot  be  sure  that
       the  preceding  pass  is just 'pass-1', if you really need to know what
       the last pass is record (row,pass) from the callback and use  the  last
       recorded value each time.

       As  with  the  user  transform  you  can  find the output row using the
       PNG_ROW_FROM_PASS_ROW macro.


   Unknown-chunk handling
       Now you get to set the way the library processes unknown chunks in  the
       input  PNG  stream. Both known and unknown chunks will be read.  Normal
       behavior is that known chunks will be parsed into information in  vari-
       ous  info_ptr  members  while  unknown  chunks  will be discarded. This
       behavior can be wasteful if your application will never use some  known
       chunk types. To change this, you can call:

           png_set_keep_unknown_chunks(png_ptr, keep,
               chunk_list, num_chunks);

           keep       - 0: default unknown chunk handling
                        1: ignore; do not keep
                        2: keep only if safe-to-copy
                        3: keep even if unsafe-to-copy

                      You can use these definitions:
                        PNG_HANDLE_CHUNK_AS_DEFAULT   0
                        PNG_HANDLE_CHUNK_NEVER        1
                        PNG_HANDLE_CHUNK_IF_SAFE      2
                        PNG_HANDLE_CHUNK_ALWAYS       3

           chunk_list - list of chunks affected (a byte string,
                        five bytes per chunk, NULL or ' ' if
                        num_chunks is positive; ignored if
                        numchunks <= 0).

           num_chunks - number of chunks affected; if 0, all
                        unknown chunks are affected.  If positive,
                        only the chunks in the list are affected,
                        and if negative all unknown chunks and
                        all known chunks except for the IHDR,
                        PLTE, tRNS, IDAT, and IEND chunks are
                        affected.

       Unknown  chunks  declared  in this way will be saved as raw data onto a
       list of png_unknown_chunk structures.  If  a  chunk  that  is  normally
       known  to  libpng  is named in the list, it will be handled as unknown,
       according to the "keep" directive.  If a chunk is named  in  successive
       instances  of  png_set_keep_unknown_chunks(),  the  final instance will
       take precedence.  The IHDR and IEND  chunks  should  not  be  named  in
       chunk_list;  if they are, libpng will process them normally anyway.  If
       you know that your application will never make use of  some  particular
       chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.

       Here is an example of the usage of png_set_keep_unknown_chunks(), where
       the private "vpAg" chunk will later be processed by a user chunk  call-
       back function:

           png_byte vpAg[5]={118, 112,  65, 103, (png_byte) ' '};

           #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
             png_byte unused_chunks[]=
             {
               104,  73,  83,  84, (png_byte) ' ',   /* hIST */
               105,  84,  88, 116, (png_byte) ' ',   /* iTXt */
               112,  67,  65,  76, (png_byte) ' ',   /* pCAL */
               115,  67,  65,  76, (png_byte) ' ',   /* sCAL */
               115,  80,  76,  84, (png_byte) ' ',   /* sPLT */
               116,  73,  77,  69, (png_byte) ' ',   /* tIME */
             };
           #endif

           ...

           #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
             /* ignore all unknown chunks
              * (use global setting "2" for libpng16 and earlier):
              */
             png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);

             /* except for vpAg: */
             png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);

             /* also ignore unused known chunks: */
             png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
                (int)(sizeof unused_chunks)/5);
           #endif


   User limits
       The  PNG specification allows the width and height of an image to be as
       large as 2^(31-1 (0x7fffffff), or about 2.147 billion rows and columns.
       Larger  images will be rejected immediately with a png_error() call. If
       you wish to change these limits, you can use

          png_set_user_limits(png_ptr, width_max, height_max);

       to set your own limits (libpng may reject some very wide images  anyway
       because of potential buffer overflow conditions).

       You  should  put  this statement after you create the PNG structure and
       before calling png_read_info(), png_read_png(), or  png_process_data().

       When  writing  a  PNG  datastream,  put  this  statement before calling
       png_write_info() or png_write_png().

       If you need to retrieve the limits that are being applied, use

          width_max = png_get_user_width_max(png_ptr);
          height_max = png_get_user_height_max(png_ptr);

       The PNG specification sets no limit on the number of  ancillary  chunks
       allowed  in a PNG datastream.  You can impose a limit on the total num-
       ber of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be  stored,
       with

          png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

       where 0x7fffffffL means unlimited.  You can retrieve this limit with

          chunk_cache_max = png_get_chunk_cache_max(png_ptr);

       You  can  also  set  a  limit on the amount of memory that a compressed
       chunk other than IDAT can occupy, with

          png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);

       and you can retrieve the limit with

          chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);

       Any chunks that would cause either of these limits to be exceeded  will
       be ignored.


   Information about your system
       If  you  intend  to display the PNG or to incorporate it in other image
       data you need to tell libpng information about your display or  drawing
       surface so that libpng can convert the values in the image to match the
       display.

       From libpng-1.5.4 this information can be set before  reading  the  PNG
       file  header.   In earlier versions png_set_gamma() existed but behaved
       incorrectly if called before the PNG file  header  had  been  read  and
       png_set_alpha_mode() did not exist.

       If  you need to support versions prior to libpng-1.5.4 test the version
       number as illustrated below using "PNG_LIBPNG_VER >= 10504" and  follow
       the procedures described in the appropriate manual page.

       You  give  libpng  the  encoding expected by your system expressed as a
       'gamma' value.  You can also specify a default  encoding  for  the  PNG
       file  in  case  the  required information is missing from the file.  By
       default libpng assumes that the PNG data matches your system,  to  keep
       this default call:

          png_set_gamma(png_ptr, screen_gamma, output_gamma);

       or you can use the fixed point equivalent:

          png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
             PNG_FP_1*output_gamma);

       If you don't know the gamma for your system it is probably 2.2 - a good
       approximation to the IEC  standard  for  display  systems  (sRGB).   If
       images  are too contrasty or washed out you got the value wrong - check
       your system documentation!

       Many systems permit the system gamma to be changed via a  lookup  table
       in  the display driver, a few systems, including older Macs, change the
       response by default.  As of 1.5.4 three special values are available to
       handle common situations:

          PNG_DEFAULT_sRGB: Indicates that the system conforms to the
                            IEC 61966-2-1 standard.  This matches almost
                            all systems.
          PNG_GAMMA_MAC_18: Indicates that the system is an older
                            (pre Mac OS 10.6) Apple Macintosh system with
                            the default settings.
          PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
                            that the system expects data with no gamma
                            encoding.

       You  would  use the linear (unencoded) value if you need to process the
       pixel values further because this avoids the need  to  decode  and  re-
       encode each component value whenever arithmetic is performed.  A lot of
       graphics software uses linear values for this reason, often with higher
       precision component values to preserve overall accuracy.


       The  output_gamma  value expresses how to decode the output values, not
       how they are encoded.  The values used correspond to the normal numbers
       used  to  describe  the overall gamma of a computer display system; for
       example 2.2 for an sRGB conformant system.  The values  are  scaled  by
       100000 in the _fixed version of the API (so 220000 for sRGB.)

       The  inverse  of  the value is always used to provide a default for the
       PNG file encoding if it has no gAMA chunk and  if  png_set_gamma()  has
       not been called to override the PNG gamma information.

       When  the  ALPHA_OPTIMIZED mode is selected the output gamma is used to
       encode opaque pixels however pixels with lower  alpha  values  are  not
       encoded, regardless of the output gamma setting.

       When  the  standard  Porter  Duff handling is requested with mode 1 the
       output encoding is set to be linear and the output_gamma value is  only
       relevant  as  a  default  for input data that has no gamma information.
       The linear output encoding will be  overridden  if  png_set_gamma()  is
       called - the results may be highly unexpected!

       The  following  numbers  are  derived  from  the  sRGB standard and the
       research behind it.  sRGB is defined to be approximated by a  PNG  gAMA
       chunk  value of 0.45455 (1/2.2) for PNG.  The value implicitly includes
       any viewing correction required to take account of any  differences  in
       the  color  environment  of the original scene and the intended display
       environment; the value expresses how to *decode* the image for display,
       not how the original data was *encoded*.

       sRGB provides a peg for the PNG standard by defining a viewing environ-
       ment.  sRGB itself, and earlier TV standards, actually use a more  com-
       plex  transform  (a linear portion then a gamma 2.4 power law) than PNG
       can express.  (PNG is limited to simple power laws.)  By saying that an
       image  for direct display on an sRGB conformant system should be stored
       with a gAMA chunk value of 45455 (11.3.3.2 and 11.3.3.5 of the ISO  PNG
       specification) the PNG specification makes it possible to derive values
       for other display systems and environments.

       The Mac value is deduced from the sRGB based on an assumption that  the
       actual  extra  viewing correction used in early Mac display systems was
       implemented as a power 1.45 lookup table.

       Any system where a programmable lookup  table  is  used  or  where  the
       behavior  of  the  final  display device characteristics can be changed
       requires system specific code to  obtain  the  current  characteristic.
       However  this  can  be  difficult  and  most  PNG gamma correction only
       requires an approximate value.

       By default, if png_set_alpha_mode() is not called, libpng assumes  that
       all  values  are  unencoded,  linear, values and that the output device
       also has a linear characteristic.  This is only very rarely  correct  -
       it   is   invariably   better   to   call   png_set_alpha_mode()   with
       PNG_DEFAULT_sRGB than rely on the default if you don't  know  what  the
       right answer is!

       The  special  value PNG_GAMMA_MAC_18 indicates an older Mac system (pre
       Mac OS 10.6) which used a correction  table  to  implement  a  somewhat
       lower gamma on an otherwise sRGB system.

       Both  these  values  are reserved (not simple gamma values) in order to
       allow more precise correction internally in the future.

       NOTE: the values can be passed to either the fixed  or  floating  point
       APIs,  but  the floating point API will also accept floating point val-
       ues.

       The second thing you may need to tell libpng about is how  your  system
       handles  alpha  channel information.  Some, but not all, PNG files con-
       tain an alpha channel.  To display these files correctly  you  need  to
       compose  the  data  onto a suitable background, as described in the PNG
       specification.

       Libpng only supports composing onto a single color (using png_set_back-
       ground;  see  below).   Otherwise  you must do the composition yourself
       and, in this case, you may need to call png_set_alpha_mode:

          #if PNG_LIBPNG_VER >= 10504
             png_set_alpha_mode(png_ptr, mode, screen_gamma);
          #else
             png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
          #endif

       The screen_gamma value is the same as the  argument  to  png_set_gamma;
       however,   how   it   affects   the   output   depends   on  the  mode.
       png_set_alpha_mode() sets the file gamma default to 1/screen_gamma,  so
       normally  you  don't need to call png_set_gamma.  If you need different
       defaults call png_set_gamma() before png_set_alpha_mode() - if you call
       it after it will override the settings made by png_set_alpha_mode().

       The mode is as follows:

           PNG_ALPHA_PNG:  The data is encoded according to the PNG specifica-
       tion.  Red, green and blue, or gray, components are gamma encoded color
       values  and  are not premultiplied by the alpha value.  The alpha value
       is a linear measure of the contribution of the pixel to the correspond-
       ing final output pixel.

       You should normally use this format if you intend to perform color cor-
       rection on the color values; most, maybe all, color correction software
       has  no  handling for the alpha channel and, anyway, the math to handle
       pre-multiplied component values is unnecessarily complex.

       Before you do any arithmetic on the component values you need to remove
       the  gamma  encoding  and  multiply out the alpha channel.  See the PNG
       specification for more detail.  It is important to note  that  when  an
       image  with  an alpha channel is scaled, linear encoded, pre-multiplied
       component values must be used!

       The remaining modes assume you don't need to do any further color  cor-
       rection  or  that  if  you do, your color correction software knows all
       about alpha (it probably doesn't!).  They 'associate'  the  alpha  with
       the  color  information  by storing color channel values that have been
       scaled by the alpha.  The advantage is that the color channels  can  be
       resampled  (the image can be scaled) in this form.  The disadvantage is
       that normal practice is to store linear, not  (gamma)  encoded,  values
       and  this  requires  16-bit  channels  for still images rather than the
       8-bit channels that are just about  sufficient  if  gamma  encoding  is
       used.   In  addition  all  non-transparent pixel values, including com-
       pletely opaque ones, must be gamma encoded to produce the final  image.
       These   are  the  'STANDARD',  'ASSOCIATED'  or  'PREMULTIPLIED'  modes
       described below (the latter being the two common names  for  associated
       alpha  color  channels). Note that PNG files always contain non-associ-
       ated color channels; png_set_alpha_mode() with one of the modes  causes
       the  decoder to convert the pixels to an associated form before return-
       ing them to your application.

       Since it is not necessary to perform arithmetic on opaque color  values
       so  long  as  they  are  not to be resampled and are in the final color
       space it is possible to optimize the handling of alpha by  storing  the
       opaque  pixels  in the PNG format (adjusted for the output color space)
       while storing partially opaque pixels in the standard, linear,  format.
       The accuracy required for standard alpha composition is relatively low,
       because the pixels are isolated, therefore typically the accuracy  loss
       in storing 8-bit linear values is acceptable.  (This is not true if the
       alpha channel is used to simulate transparency over large areas  -  use
       16  bits  or the PNG mode in this case!)  This is the 'OPTIMIZED' mode.
       For this mode a pixel is treated as opaque only if the alpha  value  is
       equal to the maximum value.

           PNG_ALPHA_STANDARD:   The  data  libpng  produces is encoded in the
       standard way assumed by most correctly written graphics software.   The
       gamma  encoding will be removed by libpng and the linear component val-
       ues will be pre-multiplied by the alpha channel.

       With this format the final image must be re-encoded to match  the  dis-
       play  gamma  before  the image is displayed.  If your system doesn't do
       that, yet still seems to  perform  arithmetic  on  the  pixels  without
       decoding them, it is broken - check out the modes below.

       With PNG_ALPHA_STANDARD libpng always produces linear component values,
       whatever screen_gamma you supply.  The screen_gamma value is,  however,
       used  as  a  default  for  the  file gamma if the PNG file has no gamma
       information.

       If you call png_set_gamma() after png_set_alpha_mode() you  will  over-
       ride the linear encoding.  Instead the pre-multiplied pixel values will
       be gamma encoded but the alpha channel will still be linear.  This  may
       actually  match  the  requirements  of  some broken software, but it is
       unlikely.

       While linear 8-bit data is often used it has insufficient precision for
       any  image  with a reasonable dynamic range.  To avoid problems, and if
       your software supports it, use png_set_expand_16() to force all  compo-
       nents to 16 bits.

           PNG_ALPHA_OPTIMIZED:  This  mode  is the same as PNG_ALPHA_STANDARD
       except that completely opaque pixels are gamma encoded according to the
       screen_gamma  value.   Pixels  with alpha less than 1.0 will still have
       linear components.

       Use this format if you have control over your compositing software  and
       so don't do other arithmetic (such as scaling) on the data you get from
       libpng.  Your compositing software can simply copy opaque pixels to the
       output but still has linear values for the non-opaque pixels.

       In  normal  compositing,  where the alpha channel encodes partial pixel
       coverage (as opposed to broad area translucency), the  inaccuracies  of
       the 8-bit representation of non-opaque pixels are irrelevant.

       You  can also try this format if your software is broken; it might look
       better.

           PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however,  all  compo-
       nent  values,  including  the alpha channel are gamma encoded.  This is
       broken because, in practice, no implementation that  uses  this  choice
       correctly  undoes  the encoding before handling alpha composition.  Use
       this choice only if other serious errors in the  software  or  hardware
       you  use  mandate it.  In most cases of broken software or hardware the
       bug in the final display manifests as a subtle halo  around  composited
       parts of the image.  You may not even perceive this as a halo; the com-
       posited part of the image may simply appear  separate  from  the  back-
       ground, as though it had been cut out of paper and pasted on afterward.

       If you don't have to deal with bugs in software or hardware, or if  you
       can   fix   them,   there   are   three   recommended   ways  of  using
       png_set_alpha_mode():

          png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
              screen_gamma);

       You can do color correction on the result (libpng  does  not  currently
       support  color correction internally).  When you handle the alpha chan-
       nel you need to undo the gamma encoding and multiply out the alpha.

          png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
              screen_gamma);
          png_set_expand_16(png_ptr);

       If   you   are   using   the   high   level   interface,   don't   call
       png_set_expand_16(); instead pass PNG_TRANSFORM_EXPAND_16 to the inter-
       face.

       With this mode you can't do color correction, but  you  can  do  arith-
       metic,  including  composition and scaling, on the data without further
       processing.

          png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
              screen_gamma);

       You can avoid the expansion to 16-bit components with  this  mode,  but
       you  lose the ability to scale the image or perform other linear arith-
       metic.  All you can do is compose the result onto  a  matching  output.
       Since this mode is libpng-specific you also need to write your own com-
       position software.

       The following are examples of calls to  png_set_alpha_mode  to  achieve
       the  required overall gamma correction and, where necessary, alpha pre-
       multiplication.

           png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);

       This is the default libpng handling of the alpha channel -  it  is  not
       pre-multiplied  into the color components.  In addition the call states
       that the output is for a sRGB system and causes all PNG  files  without
       gAMA chunks to be assumed to be encoded using sRGB.

           png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);

       In this case the output is assumed to be something like an sRGB confor-
       mant display preceeded by a power-law lookup table of power 1.45.  This
       is how early Mac systems behaved.

           png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);

       This  is the classic Jim Blinn approach and will work in academic envi-
       ronments where everything is done by the book.  It has the  shortcoming
       of  assuming  that input PNG data with no gamma information is linear -
       this is unlikely to be correct unless the  PNG  files  where  generated
       locally.   Most  of  the time the output precision will be so low as to
       show significant banding in dark areas of the image.

           png_set_expand_16(pp);
           png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);

       This is a somewhat more realistic Jim  Blinn  inspired  approach.   PNG
       files  are assumed to have the sRGB encoding if not marked with a gamma
       value and the output is always 16 bits  per  component.   This  permits
       accurate  scaling  and  processing  of the data.  If you know that your
       input PNG files were  generated  locally  you  might  need  to  replace
       PNG_DEFAULT_sRGB with the correct value for your system.

           png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);

       If you just need to composite the PNG image onto an existing background
       and if you control the code that does this you can use the optimization
       setting.   In  this  case you just copy completely opaque pixels to the
       output.  For pixels that are not completely transparent (you just  skip
       those)  you  do the composition math using png_composite or png_compos-
       ite_16 below then encode the resultant 8-bit or 16-bit values to  match
       the output encoding.

           Other cases

       If  neither  the  PNG  nor  the  standard  linear encoding work for you
       because of the software or hardware you use then you have a  big  prob-
       lem.  The PNG case will probably result in halos around the image.  The
       linear encoding will probably result in a washed out, too bright, image
       (it's  actually  too  contrasty.)  Try the ALPHA_OPTIMIZED mode above -
       this will probably substantially reduce the halos.  Alternatively try:

           png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);

       This option will also reduce the halos, but there will be  slight  dark
       halos  round  the  opaque  parts  of  the image where the background is
       light.  In the OPTIMIZED mode the halos will be light halos  where  the
       background  is dark.  Take your pick - the halos are unavoidable unless
       you can get your hardware/software fixed!  (The OPTIMIZED  approach  is
       slightly faster.)

       When the default gamma of PNG files doesn't match the output gamma.  If
       you have PNG files with no gamma information png_set_alpha_mode  allows
       you to provide a default gamma, but it also sets the ouput gamma to the
       matching value.  If you know your PNG files have a gamma  that  doesn't
       match   the   output   you   can   take  advantage  of  the  fact  that
       png_set_alpha_mode always sets the output gamma but only sets  the  PNG
       default if it is not already set:

           png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
           png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);

       The  first  call sets both the default and the output gamma values, the
       second call overrides the output gamma without  changing  the  default.
       This  is easier than achieving the same effect with png_set_gamma.  You
       must use PNG_ALPHA_PNG for  the  first  call  -  internal  checking  in
       png_set_alpha will fire if more than one call to png_set_alpha_mode and
       png_set_background is made in the same read operation, however multiple
       calls with PNG_ALPHA_PNG are ignored.

       If  you  don't  need,  or  can't handle, the alpha channel you can call
       png_set_background() to remove it by compositing against a fixed color.
       Don't  call  png_set_strip_alpha()  to do this - it will leave spurious
       pixel values in transparent parts of this image.

          png_set_background(png_ptr, &background_color,
              PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);

       The background_color is an RGB or grayscale value according to the data
       format  libpng  will  produce  for you.  Because you don't yet know the
       format of the PNG file, if you call png_set_background  at  this  point
       you must arrange for the format produced by libpng to always have 8-bit
       or 16-bit components and then store the color as  an  8-bit  or  16-bit
       color  as appropriate.  The color contains separate gray and RGB compo-
       nent values, so you can let libpng produce gray or RGB output according
       to  the input format, but low bit depth grayscale images must always be
       converted to at  least  8-bit  format.   (Even  though  low  bit  depth
       grayscale  images can't have an alpha channel they can have a transpar-
       ent color!)

       You set the transforms you need later, either  as  flags  to  the  high
       level  interface  or libpng API calls for the low level interface.  For
       reference the settings and API calls required are:

       8-bit values:
          PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
          png_set_expand(png_ptr); png_set_scale_16(png_ptr);

          If you must get exactly the same inaccurate results
          produced by default in versions prior to libpng-1.5.4,
          use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
          instead.

       16-bit values:
          PNG_TRANSFORM_EXPAND_16
          png_set_expand_16(png_ptr);

       In either case palette image data will be expanded to RGB.  If you just
       want    color   data   you   can   add   PNG_TRANSFORM_GRAY_TO_RGB   or
       png_set_gray_to_rgb(png_ptr) to the list.

       Calling png_set_background before the PNG file header is read will  not
       work  prior  to  libpng-1.5.4.  Because the failure may result in unex-
       pected  warnings  or  errors  it  is  therefore  much  safer  to   call
       png_set_background  after  the  head has been read.  Unfortunately this
       means that prior to libpng-1.5.4 it cannot be used with the high  level
       interface.


   The high-level read interface
       At  this  point  there  are two ways to proceed; through the high-level
       read interface, or through a sequence  of  low-level  read  operations.
       You can use the high-level interface if (a) you are willing to read the
       entire image into memory, and (b) the input transformations you want to
       do are limited to the following set:

           PNG_TRANSFORM_IDENTITY      No transformation
           PNG_TRANSFORM_SCALE_16      Strip 16-bit samples to
                                       8-bit accurately
           PNG_TRANSFORM_STRIP_16      Chop 16-bit samples to
                                       8-bit less accurately
           PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel
           PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit
                                       samples to bytes
           PNG_TRANSFORM_PACKSWAP      Change order of packed
                                       pixels to LSB first
           PNG_TRANSFORM_EXPAND        Perform set_expand()
           PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
           PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                       sBIT depth
           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
           PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                       to transparency
           PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
           PNG_TRANSFORM_GRAY_TO_RGB   Expand grayscale samples
                                       to RGB (or GA to RGBA)
           PNG_TRANSFORM_EXPAND_16     Expand samples to 16 bits

       (This  excludes setting a background color, doing gamma transformation,
       quantizing, and setting filler.)  If this is the case, simply do this:

           png_read_png(png_ptr, info_ptr, png_transforms, NULL)

       where png_transforms is an integer containing the bitwise  OR  of  some
       set   of   transformation   flags.    This   call   is   equivalent  to
       png_read_info(), followed the set of transformations indicated  by  the
       transform mask, then png_read_image(), and finally png_read_end().

       (The  final  parameter  of this call is not yet used.  Someday it might
       point to transformation parameters required by some future input trans-
       form.)

       You  must use png_transforms and not call any png_set_transform() func-
       tions when you use png_read_png().

       After you have called png_read_png(), you can retrieve the  image  data
       with

          row_pointers = png_get_rows(png_ptr, info_ptr);

       where  row_pointers  is an array of pointers to the pixel data for each
       row:

          png_bytep row_pointers[height];

       If you know your image size and pixel size ahead of time, you can allo-
       cate row_pointers prior to calling png_read_png() with

          if (height > PNG_UINT_32_MAX/(sizeof (png_byte)))
             png_error (png_ptr,
                 "Image is too tall to process in memory");

          if (width > PNG_UINT_32_MAX/pixel_size)
             png_error (png_ptr,
                 "Image is too wide to process in memory");

          row_pointers = png_malloc(png_ptr,
              height*(sizeof (png_bytep)));

          for (int i=0; i<height, i++)
             row_pointers[i]=NULL;  /* security precaution */

          for (int i=0; i<height, i++)
             row_pointers[i]=png_malloc(png_ptr,
                 width*pixel_size);

          png_set_rows(png_ptr, info_ptr, &row_pointers);

       Alternatively you could allocate your image in one big block and define
       row_pointers[i] to point into the proper places in your block.

       If you use png_set_rows(), the application is responsible  for  freeing
       row_pointers  (and row_pointers[i], if they were separately allocated).

       If you don't allocate row_pointers ahead of time,  png_read_png()  will
       do it, and it'll be free'ed by libpng when you call png_destroy_*().


   The low-level read interface
       If you are going the low-level route, you are now ready to read all the
       file information up to the actual image data.  You do this with a  call
       to png_read_info().

           png_read_info(png_ptr, info_ptr);

       This will process all chunks up to but not including the image data.

       This  also  copies  some  of the data from the PNG file into the decode
       structure for use  in  later  transformations.   Important  information
       copied in is:

       1) The PNG file gamma from the gAMA chunk.  This overwrites the default
       value   provided   by   an   earlier   call   to    png_set_gamma    or
       png_set_alpha_mode.

       2)  Prior to libpng-1.5.4 the background color from a bKGd chunk.  This
       damages the information provided by an earlier  call  to  png_set_back-
       ground  resulting  in unexpected behavior.  Libpng-1.5.4 no longer does
       this.

       3) The number of significant bits in each component value.  Libpng uses
       this  to  optimize gamma handling by reducing the internal lookup table
       sizes.

       4) The transparent color information from a tRNS chunk.   This  can  be
       modified by a later call to png_set_tRNS.


   Querying the info structure
       Functions are used to get the information from the info_ptr once it has
       been read.  Note that these fields may  not  be  completely  filled  in
       until png_read_end() has read the chunk data following the image.

           png_get_IHDR(png_ptr, info_ptr, &width, &height,
              &bit_depth, &color_type, &interlace_type,
              &compression_type, &filter_method);

           width          - holds the width of the image
                            in pixels (up to 2^31).

           height         - holds the height of the image
                            in pixels (up to 2^31).

           bit_depth      - holds the bit depth of one of the
                            image channels.  (valid values are
                            1, 2, 4, 8, 16 and depend also on
                            the color_type.  See also
                            significant bits (sBIT) below).

           color_type     - describes which color/alpha channels
                                are present.
                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - (PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7)

           compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                            for PNG 1.0)

           filter_method  - (must be PNG_FILTER_TYPE_BASE
                            for PNG 1.0, and can also be
                            PNG_INTRAPIXEL_DIFFERENCING if
                            the PNG datastream is embedded in
                            a MNG-1.0 datastream)

           Any of width, height, color_type, bit_depth,
           interlace_type, compression_type, or filter_method can
           be NULL if you are not interested in their values.

           Note that png_get_IHDR() returns 32-bit data into
           the application's width and height variables.
           This is an unsafe situation if these are not png_uint_32
           variables.  In such situations, the
           png_get_image_width() and png_get_image_height()
           functions described below are safer.

           width            = png_get_image_width(png_ptr,
                                info_ptr);

           height           = png_get_image_height(png_ptr,
                                info_ptr);

           bit_depth        = png_get_bit_depth(png_ptr,
                                info_ptr);

           color_type       = png_get_color_type(png_ptr,
                                info_ptr);

           interlace_type   = png_get_interlace_type(png_ptr,
                                info_ptr);

           compression_type = png_get_compression_type(png_ptr,
                                info_ptr);

           filter_method    = png_get_filter_type(png_ptr,
                                info_ptr);

           channels = png_get_channels(png_ptr, info_ptr);

           channels       - number of channels of info for the
                            color type (valid values are 1 (GRAY,
                            PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
                            4 (RGB_ALPHA or RGB + filler byte))

           rowbytes = png_get_rowbytes(png_ptr, info_ptr);

           rowbytes       - number of bytes needed to hold a row

           signature = png_get_signature(png_ptr, info_ptr);

           signature      - holds the signature read from the
                            file (if any).  The data is kept in
                            the same offset it would be if the
                            whole signature were read (i.e. if an
                            application had already read in 4
                            bytes of signature before starting
                            libpng, the remaining 4 bytes would
                            be in signature[4] through signature[7]
                            (see png_set_sig_bytes())).

       These  are  also  important,  but their validity depends on whether the
       chunk   has   been   read.    The   png_get_valid(png_ptr,    info_ptr,
       PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr, info_ptr, ...) functions
       return non-zero if the data has been read, or zero if  it  is  missing.
       The parameters to the png_get_<chunk> are set directly if they are sim-
       ple data types, or a pointer into the info_ptr is returned for any com-
       plex types.

       The  colorspace  data  from  gAMA, cHRM, sRGB, iCCP, and sBIT chunks is
       simply returned to give the application information about how the image
       was  encoded.   Libpng  itself only does transformations using the file
       gamma when combining semitransparent pixels with the background  color,
       and,  since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit
       linear pixels within the simplified API.  Libpng  also  uses  the  file
       gamma  when converting RGB to gray, beginning with libpng-1.0.5, if the
       application calls png_set_rgb_to_gray()).

           png_get_PLTE(png_ptr, info_ptr, &palette,
                            &num_palette);

           palette        - the palette for the file
                            (array of png_color)

           num_palette    - number of entries in the palette

           png_get_gAMA(png_ptr, info_ptr, &file_gamma);
           png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);

           file_gamma     - the gamma at which the file is
                            written (PNG_INFO_gAMA)

           int_file_gamma - 100,000 times the gamma at which the
                            file is written

           png_get_cHRM(png_ptr, info_ptr,  &white_x, &white_y, &red_x,
                            &red_y, &green_x, &green_y, &blue_x, &blue_y)
           png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z,
                            &green_X, &green_Y, &green_Z, &blue_X, &blue_Y,
                            &blue_Z)
           png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x,
                            &int_white_y, &int_red_x, &int_red_y,
                            &int_green_x, &int_green_y, &int_blue_x,
                            &int_blue_y)
           png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
                            &int_red_Z, &int_green_X, &int_green_Y,
                            &int_green_Z, &int_blue_X, &int_blue_Y,
                            &int_blue_Z)

           {white,red,green,blue}_{x,y}
                            A color space encoding specified using the
                            chromaticities of the end points and the
                            white point. (PNG_INFO_cHRM)

           {red,green,blue}_{X,Y,Z}
                            A color space encoding specified using the
                            encoding end points - the CIE tristimulus
                            specification of the intended color of the red,
                            green and blue channels in the PNG RGB data.
                            The white point is simply the sum of the three
                            end points. (PNG_INFO_cHRM)

           png_get_sRGB(png_ptr, info_ptr, &srgb_intent);

           srgb_intent -    the rendering intent (PNG_INFO_sRGB)
                            The presence of the sRGB chunk
                            means that the pixel data is in the
                            sRGB color space.  This chunk also
                            implies specific values of gAMA and
                            cHRM.

           png_get_iCCP(png_ptr, info_ptr, &name,
              &compression_type, &profile, &proflen);

           name             - The profile name.

           compression_type - The compression type; always
                              PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                              You may give NULL to this argument to
                              ignore it.

           profile          - International Color Consortium color
                              profile data. May contain NULs.

           proflen          - length of profile data in bytes.

           png_get_sBIT(png_ptr, info_ptr, &sig_bit);

           sig_bit        - the number of significant bits for
                            (PNG_INFO_sBIT) each of the gray,
                            red, green, and blue channels,
                            whichever are appropriate for the
                            given color type (png_color_16)

           png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
                            &num_trans, &trans_color);

           trans_alpha    - array of alpha (transparency)
                            entries for palette (PNG_INFO_tRNS)

           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)

           trans_color    - graylevel or color sample values of
                            the single transparent color for
                            non-paletted images (PNG_INFO_tRNS)

           png_get_hIST(png_ptr, info_ptr, &hist);
                            (PNG_INFO_hIST)

           hist           - histogram of palette (array of
                            png_uint_16)

           png_get_tIME(png_ptr, info_ptr, &mod_time);

           mod_time       - time image was last modified
                           (PNG_VALID_tIME)

           png_get_bKGD(png_ptr, info_ptr, &background);

           background     - background color (of type
                            png_color_16p) (PNG_VALID_bKGD)
                            valid 16-bit red, green and blue
                            values, regardless of color_type

           num_comments   = png_get_text(png_ptr, info_ptr,
                            &text_ptr, &num_text);

           num_comments   - number of comments

           text_ptr       - array of png_text holding image
                            comments

           text_ptr[i].compression - type of compression used
                        on "text" PNG_TEXT_COMPRESSION_NONE
                                  PNG_TEXT_COMPRESSION_zTXt
                                  PNG_ITXT_COMPRESSION_NONE
                                  PNG_ITXT_COMPRESSION_zTXt

           text_ptr[i].key   - keyword for comment.  Must contain
                                1-79 characters.

           text_ptr[i].text  - text comments for current
                                keyword.  Can be empty.

           text_ptr[i].text_length - length of text string,
                        after decompression, 0 for iTXt

           text_ptr[i].itxt_length - length of itxt string,
                        after decompression, 0 for tEXt/zTXt

           text_ptr[i].lang  - language of comment (empty
                                string for unknown).

           text_ptr[i].lang_key  - keyword in UTF-8
                                (empty string for unknown).

           Note that the itxt_length, lang, and lang_key
           members of the text_ptr structure only exist when the
           library is built with iTXt chunk support.  Prior to
           libpng-1.4.0 the library was built by default without
           iTXt support. Also note that when iTXt is supported,
           they contain NULL pointers when the "compression"
           field contains PNG_TEXT_COMPRESSION_NONE or
           PNG_TEXT_COMPRESSION_zTXt.

           num_text       - number of comments (same as
                            num_comments; you can put NULL here
                            to avoid the duplication)

           Note while png_set_text() will accept text, language,
           and translated keywords that can be NULL pointers, the
           structure returned by png_get_text will always contain
           regular zero-terminated C strings.  They might be
           empty strings but they will never be NULL pointers.

           num_spalettes = png_get_sPLT(png_ptr, info_ptr,
              &palette_ptr);

           num_spalettes  - number of sPLT chunks read.

           palette_ptr    - array of palette structures holding
                            contents of one or more sPLT chunks
                            read.

           png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
              &unit_type);

           offset_x       - positive offset from the left edge
                            of the screen (can be negative)

           offset_y       - positive offset from the top edge
                            of the screen (can be negative)

           unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

           png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
              &unit_type);

           res_x          - pixels/unit physical resolution in
                            x direction

           res_y          - pixels/unit physical resolution in
                            x direction

           unit_type      - PNG_RESOLUTION_UNKNOWN,
                            PNG_RESOLUTION_METER

           png_get_sCAL(png_ptr, info_ptr, &unit, &width,
              &height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units

           height      - height of a pixel in physical scale units
                        (width and height are doubles)

           png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
              &height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units
                         (expressed as a string)

           height      - height of a pixel in physical scale units
                        (width and height are strings like "2.54")

           num_unknown_chunks = png_get_unknown_chunks(png_ptr,
              info_ptr, &unknowns)

           unknowns          - array of png_unknown_chunk
                               structures holding unknown chunks

           unknowns[i].name  - name of unknown chunk

           unknowns[i].data  - data of unknown chunk

           unknowns[i].size  - size of unknown chunk's data

           unknowns[i].location - position of chunk in file

           The value of "i" corresponds to the order in which the
           chunks were read from the PNG file or inserted with the
           png_set_unknown_chunks() function.

           The value of "location" is a bitwise "or" of

                PNG_HAVE_IHDR  (0x01)
                PNG_HAVE_PLTE  (0x02)
                PNG_AFTER_IDAT (0x08)

       The data from the pHYs chunk can be  retrieved  in  several  convenient
       forms:

           res_x = png_get_x_pixels_per_meter(png_ptr,
              info_ptr)

           res_y = png_get_y_pixels_per_meter(png_ptr,
              info_ptr)

           res_x_and_y = png_get_pixels_per_meter(png_ptr,
              info_ptr)

           res_x = png_get_x_pixels_per_inch(png_ptr,
              info_ptr)

           res_y = png_get_y_pixels_per_inch(png_ptr,
              info_ptr)

           res_x_and_y = png_get_pixels_per_inch(png_ptr,
              info_ptr)

           aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
              info_ptr)

           Each of these returns 0 [signifying "unknown"] if
              the data is not present or if res_x is 0;
              res_x_and_y is 0 if res_x != res_y

           Note that because of the way the resolutions are
              stored internally, the inch conversions won't
              come out to exactly even number.  For example,
              72 dpi is stored as 0.28346 pixels/meter, and
              when this is retrieved it is 71.9988 dpi, so
              be sure to round the returned value appropriately
              if you want to display a reasonable-looking result.

       The  data  from  the  oFFs chunk can be retrieved in several convenient
       forms:

           x_offset = png_get_x_offset_microns(png_ptr, info_ptr);

           y_offset = png_get_y_offset_microns(png_ptr, info_ptr);

           x_offset = png_get_x_offset_inches(png_ptr, info_ptr);

           y_offset = png_get_y_offset_inches(png_ptr, info_ptr);

           Each of these returns 0 [signifying "unknown" if both
              x and y are 0] if the data is not present or if the
              chunk is present but the unit is the pixel.  The
              remark about inexact inch conversions applies here
              as well, because a value in inches can't always be
              converted to microns and back without some loss
              of precision.

       For more information, see the PNG specification for chunk contents.  Be
       careful  with  trusting  rowbytes, as some of the transformations could
       increase the space needed to hold a row (expand,  filler,  gray_to_rgb,
       etc.).  See png_read_update_info(), below.

       A  quick word about text_ptr and num_text.  PNG stores comments in key-
       word/text pairs, one pair per chunk, with no limit  on  the  number  of
       text chunks, and a 2^31 byte limit on their size.  While there are sug-
       gested keywords, there is no requirement to restrict the use  to  these
       strings.   It  is strongly suggested that keywords and text be sensible
       to humans (that's the point), so don't use abbreviations.  Non-printing
       symbols  are  not allowed.  See the PNG specification for more details.
       There is also no requirement to have text after the keyword.

       Keywords should be limited to 79 Latin-1 characters without leading  or
       trailing spaces, but non-consecutive spaces are allowed within the key-
       word.  It is possible to have the same keyword  any  number  of  times.
       The text_ptr is an array of png_text structures, each holding a pointer
       to a language string, a pointer to a keyword and a pointer  to  a  text
       string.   The text string, language code, and translated keyword may be
       empty or NULL pointers.  The keyword/text pairs are put into the  array
       in  the order that they are received.  However, some or all of the text
       chunks may be after the image, so, to make sure you have read  all  the
       text chunks, don't mess with these until after you read the stuff after
       the image.  This will be mentioned again below in the  discussion  that
       goes with png_read_end().


   Input transformations
       After you've read the header information, you can set up the library to
       handle any special transformations of the image data.  The various ways
       to  transform  the data will be described in the order that they should
       occur.  This is important, as some  of  these  change  the  color  type
       and/or  bit  depth  of  the  data, and some others only work on certain
       color types and bit depths.

       Transformations you request are ignored if they don't have any  meaning
       for  a  particular input data format.  However some transformations can
       have an effect as a result of a previous transformation.  If you  spec-
       ify a contradictory set of transformations, for example both adding and
       removing the alpha channel, you cannot predict the final result.

       The color used for the transparency values should be  supplied  in  the
       same  format/depth as the current image data.  It is stored in the same
       format/depth as the image data in a tRNS chunk, so this is what  libpng
       expects for this data.

       The  color  used  for  the  background value depends on the need_expand
       argument as described below.

       Data will be decoded into the supplied row buffers  packed  into  bytes
       unless  the  library has been told to transform it into another format.
       For example, 4 bit/pixel paletted or grayscale data will be returned  2
       pixels/byte with the leftmost pixel in the high-order bits of the byte,
       unless png_set_packing() is called.  8-bit RGB data will be  stored  in
       RGB  RGB  RGB  format unless png_set_filler() or png_set_add_alpha() is
       called to insert filler bytes, either before or after each RGB triplet.
       16-bit  RGB data will be returned RRGGBB RRGGBB, with the most signifi-
       cant byte of the color value first, unless png_set_scale_16() is called
       to  transform  it  to  regular RGB RGB triplets, or png_set_filler() or
       png_set_add alpha() is called to insert filler bytes, either before  or
       after  each  RRGGBB triplet.  Similarly, 8-bit or 16-bit grayscale data
       can   be   modified   with    png_set_filler(),    png_set_add_alpha(),
       png_set_strip_16(), or png_set_scale_16().

       The  following  code  transforms  grayscale  images of less than 8 to 8
       bits, changes paletted images to RGB, and adds a full alpha channel  if
       there is transparency information in a tRNS chunk.  This is most useful
       on grayscale images with bit depths of 2 or 4 or if there is  a  multi-
       ple-image  viewing  application  that wishes to treat all images in the
       same way.

           if (color_type == PNG_COLOR_TYPE_PALETTE)
               png_set_palette_to_rgb(png_ptr);

           if (png_get_valid(png_ptr, info_ptr,
               PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);

           if (color_type == PNG_COLOR_TYPE_GRAY &&
               bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);

       The first two functions  are  actually  aliases  for  png_set_expand(),
       added  in  libpng  version  1.0.4,  with the function names expanded to
       improve code readability.  In some future version they may actually  do
       different things.

       As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was added.
       It expands the sample depth without changing tRNS to alpha.

       As of libpng version 1.5.2, png_set_expand_16() was added.  It  behaves
       as  png_set_expand();  however,  the  resultant  channels  have 16 bits
       rather than 8.  Use this when the output color  or  gray  channels  are
       made linear to avoid fairly severe accuracy loss.

          if (bit_depth < 16)
             png_set_expand_16(png_ptr);

       PNG  can have files with 16 bits per channel.  If you only can handle 8
       bits per channel, this will strip the pixels down to 8-bit.

           if (bit_depth == 16) #if PNG_LIBPNG_VER >= 10504
              png_set_scale_16(png_ptr); #else
              png_set_strip_16(png_ptr); #endif

       (The more accurate "png_set_scale_16()" API became available in  libpng
       version 1.5.4).

       If  you  need to process the alpha channel on the image separately from
       the image data (for example if you convert it to a bitmap mask)  it  is
       possible  to  have  libpng  strip  the channel leaving just RGB or gray
       data:

           if (color_type & PNG_COLOR_MASK_ALPHA)
              png_set_strip_alpha(png_ptr);

       If you strip the alpha channel you need to find some other way of deal-
       ing  with  the information.  If, instead, you want to convert the image
       to an opaque version with no alpha channel use png_set_background;  see
       below.

       As of libpng version 1.5.2, almost all useful expansions are supported,
       the major ommissions are conversion  of  grayscale  to  indexed  images
       (which  can  be  done  trivially  in the application) and conversion of
       indexed to grayscale (which can be done by a  trivial  manipulation  of
       the palette.)

       In  the  following table, the 01 means grayscale with depth<8, 31 means
       indexed with depth<8, other numerals  represent  the  color  type,  "T"
       means  the  tRNS chunk is present, A means an alpha channel is present,
       and O means tRNS or alpha is present but all pixels in  the  image  are
       opaque.

         FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O  6A  6O
          TO
          01    -  [G]  -   -   -   -   -   -   -   -   -   -   -   -   -
          31   [Q]  Q  [Q] [Q] [Q]  Q   Q   Q   Q   Q   Q  [Q] [Q]  Q   Q
           0    1   G   +   .   .   G   G   G   G   G   G   B   B  GB  GB
          0T    lt  Gt  t   +   .   Gt  G   G   Gt  G   G   Bt  Bt GBt GBt
          0O    lt  Gt  t   .   +   Gt  Gt  G   Gt  Gt  G   Bt  Bt GBt GBt
           2    C   P   C   C   C   +   .   .   C   -   -  CB  CB   B   B
          2T    Ct  -   Ct  C   C   t   +   t   -   -   -  CBt CBt  Bt  Bt
          2O    Ct  -   Ct  C   C   t   t   +   -   -   -  CBt CBt  Bt  Bt
           3   [Q]  p  [Q] [Q] [Q]  Q   Q   Q   +   .   .  [Q] [Q]  Q   Q
          3T   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   +   t  [Qt][Qt] Qt  Qt
          3O   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   t   +  [Qt][Qt] Qt  Qt
          4A    lA  G   A   T   T   GA  GT  GT  GA  GT  GT  +   BA  G  GBA
          4O    lA GBA  A   T   T   GA  GT  GT  GA  GT  GT  BA  +  GBA  G
          6A    CA  PA  CA  C   C   A   T  tT   PA  P   P   C  CBA  +   BA
          6O    CA PBA  CA  C   C   A  tT   T   PA  P   P  CBA  C   BA  +

       Within the matrix,
            "+" identifies entries where 'from' and 'to' are the same.
            "-" means the transformation is not supported.
            "." means nothing is necessary (a tRNS chunk can just be ignored).
            "t" means the transformation is obtained by png_set_tRNS.
            "A" means the transformation is obtained by png_set_add_alpha().
            "X" means the transformation is obtained by png_set_expand().
            "1" means the transformation is obtained by
                png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
                if there is no transparency in the original or the final
                format).
            "C" means the transformation is obtained by png_set_gray_to_rgb().
            "G" means the transformation is obtained by png_set_rgb_to_gray().
            "P" means the transformation is obtained by
                png_set_expand_palette_to_rgb().
            "p" means the transformation is obtained by png_set_packing().
            "Q" means the transformation is obtained by png_set_quantize().
            "T" means the transformation is obtained by
                png_set_tRNS_to_alpha().
            "B" means the transformation is obtained by
                png_set_background(), or png_strip_alpha().

       When an entry has multiple transforms listed all are required to  cause
       the right overall transformation.  When two transforms are separated by
       a comma either will do the job.  When transforms are enclosed in [] the
       transform  should  do  the  job but this is currently unimplemented - a
       different format will result if the suggested transformations are used.

       In  PNG  files,  the alpha channel in an image is the level of opacity.
       If you need the alpha channel in an image to be  the  level  of  trans-
       parency  instead  of  opacity, you can invert the alpha channel (or the
       tRNS chunk data) after it's read, so that 0 is fully opaque and 255 (in
       8-bit  or  paletted images) or 65535 (in 16-bit images) is fully trans-
       parent, with

           png_set_invert_alpha(png_ptr);

       PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small  as
       they can, resulting in, for example, 8 pixels per byte for 1 bit files.
       This code expands to 1 pixel per byte without changing  the  values  of
       the pixels:

           if (bit_depth < 8)
              png_set_packing(png_ptr);

       PNG  files  have possible bit depths of 1, 2, 4, 8, and 16.  All pixels
       stored in a PNG image have been "scaled" or "shifted" up  to  the  next
       higher  possible bit depth (e.g. from 5 bits/sample in the range [0,31]
       to 8 bits/sample in the range [0, 255]).  However, it is also  possible
       to  convert  the  PNG  pixel data back to the original bit depth of the
       image.  This call reduces the pixels back  down  to  the  original  bit
       depth:

           png_color_8p sig_bit;

           if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
              png_set_shift(png_ptr, sig_bit);

       PNG  files  store  3-color pixels in red, green, blue order.  This code
       changes the storage of the pixels to blue, green, red:

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_RGB_ALPHA)
              png_set_bgr(png_ptr);

       PNG files store RGB pixels packed into 3 or 6 bytes. This code  expands
       them  into  4  or  8 bytes for windowing systems that need them in this
       format:

           if (color_type == PNG_COLOR_TYPE_RGB)
              png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);

       where "filler" is the 8 or 16-bit number to fill with, and the location
       is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
       you want the filler before the RGB or after.  This transformation  does
       not  affect  images  that  already have full alpha channels.  To add an
       opaque alpha channel, use filler=0xff or  0xffff  and  PNG_FILLER_AFTER
       which will generate RGBA pixels.

       Note that png_set_filler() does not change the color type.  If you want
       to do that, you can add a true alpha channel with

           if (color_type == PNG_COLOR_TYPE_RGB ||
              color_type == PNG_COLOR_TYPE_GRAY)
              png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);

       where "filler" contains the alpha value to assign to each pixel.   This
       function was added in libpng-1.2.7.

       If  you  are  reading  an image with an alpha channel, and you need the
       data as ARGB instead of the normal PNG format RGBA:

           if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
              png_set_swap_alpha(png_ptr);

       For some uses, you may want a grayscale image to be represented as RGB.
       This code will do that conversion:

           if (color_type == PNG_COLOR_TYPE_GRAY ||
               color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
              png_set_gray_to_rgb(png_ptr);

       Conversely,  you  can  convert  an  RGB  or  RGBA image to grayscale or
       grayscale with alpha.

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_RGB_ALPHA)
              png_set_rgb_to_gray(png_ptr, error_action,
                 double red_weight, double green_weight);

           error_action = 1: silently do the conversion

           error_action = 2: issue a warning if the original
                             image has any pixel where
                             red != green or red != blue

           error_action = 3: issue an error and abort the
                             conversion if the original
                             image has any pixel where
                             red != green or red != blue

           red_weight:       weight of red component

           green_weight:     weight of green component
                             If either weight is negative, default
                             weights are used.

       In the corresponding fixed point API the  red_weight  and  green_weight
       values are simply scaled by 100,000:

           png_set_rgb_to_gray(png_ptr, error_action,
              png_fixed_point red_weight,
              png_fixed_point green_weight);

       If  you have set error_action = 1 or 2, you can later check whether the
       image really was gray,  after  processing  the  image  rows,  with  the
       png_get_rgb_to_gray_status(png_ptr)   function.    It   will  return  a
       png_byte that is zero if the image was gray or 1 if there were any non-
       gray  pixels.   Background  and sBIT data will be silently converted to
       grayscale, using the green channel data for  sBIT,  regardless  of  the
       error_action setting.

       The default values come from the PNG file cHRM chunk if present; other-
       wise, the defaults correspond to the ITU-R recommendation 709, and also
       the  sRGB  color  space, as recommended in the Charles Poynton's Colour
       FAQ, <http://www.poynton.com/>, in section 9:

          <http://www.poynton.com/notes/colour_and_gamma/Color-
       FAQ.html#RTFToC9>

           Y = 0.2126 * R + 0.7152 * G + 0.0722 * B

       Previous  versions  of  this document, 1998 through 2002, recommended a
       slightly different formula:

           Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

       Libpng uses an integer approximation:

           Y = (6968 * R + 23434 * G + 2366 * B)/32768

       The calculation is done in a linear colorspace, if the image gamma  can
       be determined.

       The  png_set_background() function has been described already; it tells
       libpng to composite images with alpha or  simple  transparency  against
       the  supplied  background  color.   For  compatibility with versions of
       libpng earlier than libpng-1.5.4 it is recommended that  you  call  the
       function  after  reading the file header, even if you don't want to use
       the color in a bKGD chunk, if one exists.

       If the PNG file contains a bKGD chunk (PNG_INFO_bKGD  valid),  you  may
       use  this  color, or supply another color more suitable for the current
       display (e.g., the background color from a web page).  You need to tell
       libpng  how  the color is represented, both the format of the component
       values in the color (the number of bits) and the gamma encoding of  the
       color.   The  function  takes  two arguments, background_gamma_mode and
       need_expand to convey this information; however, only two  combinations
       are likely to be useful:

           png_color_16 my_background;
           png_color_16p image_background;

           if (png_get_bKGD(png_ptr, info_ptr, &image_background))
              png_set_background(png_ptr, image_background,
                  PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
           else
              png_set_background(png_ptr, &my_background,
                  PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);

       The second call was described above - my_background is in the format of
       the final, display, output produced by libpng.  Because  you  now  know
       the format of the PNG it is possible to avoid the need to choose either
       8-bit or 16-bit output and to retain palette images (the palette colors
       will  be  modified appropriately and the tRNS chunk removed.)  However,
       if you are doing this, take great care not to ask  for  transformations
       without checking first that they apply!

       In  the  first call the background color has the original bit depth and
       color type of the PNG file.  So, for palette images the color  is  sup-
       plied  as a palette index and for low bit greyscale images the color is
       a reduced bit value in image_background->gray.

       If you didn't call png_set_gamma() before reading the file header,  for
       example  if you need your code to remain compatible with older versions
       of libpng prior to libpng-1.5.4, this is the place to call it.

       Do not call it if you called png_set_alpha_mode(); doing so will damage
       the    settings    put   in   place   by   png_set_alpha_mode().    (If
       png_set_alpha_mode()  is  supported   then   you   can   certainly   do
       png_set_gamma() before reading the PNG header.)

       This  API  unconditionally sets the screen and file gamma values, so it
       will override the value in the PNG file unless it is called before  the
       PNG  file reading starts.  For this reason you must always call it with
       the PNG file value when you call it in this position:

          if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
             png_set_gamma(png_ptr, screen_gamma, file_gamma);

          else
             png_set_gamma(png_ptr, screen_gamma, 0.45455);

       If you need to reduce an RGB file to a paletted file, or if a  paletted
       file  has more entries than will fit on your screen, png_set_quantize()
       will do that.  Note that this  is  a  simple  match  quantization  that
       merely finds the closest color available.  This should work fairly well
       with optimized palettes, but fairly badly with linear color cubes.   If
       you  pass  a  palette that is larger than maximum_colors, the file will
       reduce the number of colors in the palette so it will  fit  into  maxi-
       mum_colors.   If  there is a histogram, libpng will use it to make more
       intelligent choices when reducing the palette.  If  there  is  no  his-
       togram, it may not do as good a job.

          if (color_type & PNG_COLOR_MASK_COLOR)
          {
             if (png_get_valid(png_ptr, info_ptr,
                 PNG_INFO_PLTE))
             {
                png_uint_16p histogram = NULL;

                png_get_hIST(png_ptr, info_ptr,
                    &histogram);
                png_set_quantize(png_ptr, palette, num_palette,
                   max_screen_colors, histogram, 1);
             }

             else
             {
                png_color std_color_cube[MAX_SCREEN_COLORS] =
                   { ... colors ... };

                png_set_quantize(png_ptr, std_color_cube,
                   MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
                   NULL,0);
             }
          }

       PNG  files describe monochrome as black being zero and white being one.
       The following code will reverse this (make black be one  and  white  be
       zero):

          if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
             png_set_invert_mono(png_ptr);

       This  function  can  also  be  used  to invert grayscale and gray-alpha
       images:

          if (color_type == PNG_COLOR_TYPE_GRAY ||
              color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
             png_set_invert_mono(png_ptr);

       PNG files store 16-bit pixels in network byte  order  (big-endian,  ie.
       most  significant  bits  first).   This code changes the storage to the
       other way (little-endian, i.e. least significant bits  first,  the  way
       PCs store them):

           if (bit_depth == 16)
              png_set_swap(png_ptr);

       If  you  are using packed-pixel images (1, 2, or 4 bits/pixel), and you
       need to change the order the pixels are packed into bytes, you can use:

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

       Finally,  you can write your own transformation function if none of the
       existing ones meets your needs.  This is done  by  setting  a  callback
       with

           png_set_read_user_transform_fn(png_ptr,
               read_transform_fn);

       You must supply the function

           void read_transform_fn(png_structp png_ptr, png_row_infop
               row_info, png_bytep data)

       See  pngtest.c  for  a  working  example.  Your function will be called
       after all of the other transformations have been processed.  Take  care
       with  interlaced images if you do the interlace yourself - the width of
       the row is the width in 'row_info', not the overall image width.

       If supported, libpng provides two information routines that you can use
       to find where you are in processing the image:

          png_get_current_pass_number(png_structp png_ptr);
          png_get_current_row_number(png_structp png_ptr);

       Don't  try  using these outside a transform callback - firstly they are
       only supported if user transforms are supported, secondly they may well
       return unexpected results unless the row is actually being processed at
       the moment they are called.

       With interlaced images the value returned is the row in the input  sub-
       image     image.      Use    PNG_ROW_FROM_PASS_ROW(row,    pass)    and
       PNG_COL_FROM_PASS_COL(col, pass) to find the output pixel  (x,y)  given
       an interlaced sub-image pixel (row,col,pass).

       The discussion of interlace handling above contains more information on
       how to use these values.

       You can also set up a pointer to a user structure for use by your call-
       back  function,  and you can inform libpng that your transform function
       will change the number of channels or bit depth with the function

           png_set_user_transform_info(png_ptr, user_ptr,
               user_depth, user_channels);

       The user's application, not libpng, is responsible for  allocating  and
       freeing any memory required for the user structure.

       You  can  retrieve  the  pointer  via  the function png_get_user_trans-
       form_ptr().  For example:

           voidp read_user_transform_ptr =
               png_get_user_transform_ptr(png_ptr);

       The last thing to handle is interlacing;  this  is  covered  in  detail
       below, but you must call the function here if you want libpng to handle
       expansion of the interlaced image.

           number_of_passes = png_set_interlace_handling(png_ptr);

       After setting the transformations,  libpng  can  update  your  png_info
       structure  to  reflect  any  transformations you've requested with this
       call.

           png_read_update_info(png_ptr, info_ptr);

       This is most useful to update the info structure's  rowbytes  field  so
       you  can use it to allocate your image memory.  This function will also
       update your palette with the correct  screen_gamma  and  background  if
       these  have  been  given  with  the  calls  above.   You  may only call
       png_read_update_info() once with a particular info_ptr.

       After you call png_read_update_info(), you can allocate any memory  you
       need  to  hold the image.  The row data is simply raw byte data for all
       forms of images.  As the actual allocation varies  among  applications,
       no  example  will be given.  If you are allocating one large chunk, you
       will need to build an array of pointers to each  row,  as  it  will  be
       needed for some of the functions below.

       Remember: Before you call png_read_update_info(), the png_get_*() func-
       tions return the values corresponding to the original PNG image.  After
       you call png_read_update_info the values refer to the image that libpng
       will output.  Consequently you must call  all  the  png_set_  functions
       before you call png_read_update_info().  This is particularly important
       for  png_set_interlace_handling()  -  if  you   are   going   to   call
       png_read_update_info()   you   must  call  png_set_interlace_handling()
       before it unless you want to receive interlaced output.


   Reading image data
       After you've allocated memory, you can read the image data.   The  sim-
       plest  way  to  do this is in one function call.  If you are allocating
       enough  memory  to  hold  the  whole   image,   you   can   just   call
       png_read_image()  and libpng will read in all the image data and put it
       in the memory area supplied.  You will need to  pass  in  an  array  of
       pointers to each row.

       This  function  automatically handles interlacing, so you don't need to
       call      png_set_interlace_handling()      (unless      you       call
       png_read_update_info()) or call this function multiple times, or any of
       that other stuff necessary with png_read_rows().

          png_read_image(png_ptr, row_pointers);

       where row_pointers is:

          png_bytep row_pointers[height];

       You can point to void or char or whatever you use for pixels.

       If you don't want to read in the whole  image  at  once,  you  can  use
       png_read_rows()  instead.   If  there  is  no interlacing (check inter-
       lace_type == PNG_INTERLACE_NONE), this is simple:

           png_read_rows(png_ptr, row_pointers, NULL,
               number_of_rows);

       where row_pointers is the same as in the png_read_image() call.

       If you are doing this just one row at a time, you can do  this  with  a
       single row_pointer instead of an array of row_pointers:

           png_bytep row_pointer = row;
           png_read_row(png_ptr, row_pointer, NULL);

       If  the  file  is  interlaced  (interlace_type != 0 in the IHDR chunk),
       things get somewhat harder.  The only current (PNG  Specification  ver-
       sion  1.2)  interlacing  type  for PNG is (interlace_type == PNG_INTER-
       LACE_ADAM7); a somewhat  complicated  2D  interlace  scheme,  known  as
       Adam7,  that  breaks down an image into seven smaller images of varying
       size, based on an 8x8 grid.  This number is defined (from  libpng  1.5)
       as PNG_INTERLACE_ADAM7_PASSES in png.h

       libpng  can  fill  out those images or it can give them to you "as is".
       It is almost always better to have libpng handle  the  interlacing  for
       you.  If you want the images filled out, there are two ways to do that.
       The one mentioned in the PNG specification is to expand each  pixel  to
       cover  those  pixels  that  have  not  been  read  yet (the "rectangle"
       method).  This results in a blocky image  for  the  first  pass,  which
       gradually smooths out as more pixels are read.  The other method is the
       "sparkle" method, where pixels are drawn only in their final locations,
       with the rest of the image remaining whatever colors they were initial-
       ized to before the start of the read.  The first method  usually  looks
       better,  but tends to be slower, as there are more pixels to put in the
       rows.

       If, as is likely, you want libpng  to  expand  the  images,  call  this
       before calling png_start_read_image() or png_read_update_info():

           if (interlace_type == PNG_INTERLACE_ADAM7)
              number_of_passes
                  = png_set_interlace_handling(png_ptr);

       This  will  return  the  number  of  passes needed.  Currently, this is
       seven, but may change if another interlace type is added.   This  func-
       tion  can  be  called even if the file is not interlaced, where it will
       return one  pass.   You  then  need  to  read  the  whole  image  'num-
       ber_of_passes'  times.   Each  time will distribute the pixels from the
       current pass to the correct place in the output image, so you  need  to
       supply the same rows to png_read_rows in each pass.

       If  you  are  not  going  to display the image after each pass, but are
       going to wait until the entire  image  is  read  in,  use  the  sparkle
       effect.   This  effect is faster and the end result of either method is
       exactly the same.  If you are planning on displaying  the  image  after
       each  pass,  the  "rectangle" effect is generally considered the better
       looking one.

       If you only want the "sparkle" effect,  just  call  png_read_rows()  as
       normal,  with  the  third parameter NULL.  Make sure you make pass over
       the image number_of_passes times, and you don't change the data in  the
       rows between calls.  You can change the locations of the data, just not
       the data.  Each pass only writes the pixels appropriate for that  pass,
       and assumes the data from previous passes is still valid.

           png_read_rows(png_ptr, row_pointers, NULL,
               number_of_rows);

       If  you  only  want  the  first effect (the rectangles), do the same as
       before except pass the row buffer in the third parameter, and leave the
       second parameter NULL.

           png_read_rows(png_ptr, NULL, row_pointers,
               number_of_rows);

       If  you  don't want libpng to handle the interlacing details, just call
       png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to  read  in  all  the
       images.   Each  of  the images is a valid image by itself; however, you
       will almost certainly need to distribute the pixels from each sub-image
       to the correct place.  This is where everything gets very tricky.

       If  you  want to retrieve the separate images you must pass the correct
       number of rows to each successive call of png_read_rows().  The  calcu-
       lation  gets pretty complicated for small images, where some sub-images
       may not even exist because either their width or height ends  up  zero.
       libpng provides two macros to help you in 1.5 and later versions:

          png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
          png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);

       Respectively  these tell you the width and height of the sub-image cor-
       responding to the numbered pass.  'pass' is in in the range 0  to  6  -
       this  can  be  confusing  because  the specification refers to the same
       passes as 1 to 7!  Be careful, you must check both the width and height
       before  calling png_read_rows() and not call it for that pass if either
       is zero.

       You can, of course, read each sub-image row by row.   If  you  want  to
       produce  optimal  code  to  make  a pixel-by-pixel transformation of an
       interlaced image this is the best approach; read each row of each pass,
       transform it, and write it out to a new interlaced image.

       If  you want to de-interlace the image yourself libpng provides further
       macros to help that tell you where to place the pixels  in  the  output
       image.   Because the interlacing scheme is rectangular - sub-image pix-
       els are always arranged on a rectangular grid - all you  need  to  know
       for each pass is the starting column and row in the output image of the
       first pixel plus the spacing between each  pixel.   As  of  libpng  1.5
       there are four macros to retrieve this information:

          png_uint_32 x = PNG_PASS_START_COL(pass);
          png_uint_32 y = PNG_PASS_START_ROW(pass);
          png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
          png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);

       These allow you to write the obvious loop:

          png_uint_32 input_y = 0;
          png_uint_32 output_y = PNG_PASS_START_ROW(pass);

          while (output_y < output_image_height)
          {
             png_uint_32 input_x = 0;
             png_uint_32 output_x = PNG_PASS_START_COL(pass);

             while (output_x < output_image_width)
             {
                image[output_y][output_x] =
                    subimage[pass][input_y][input_x++];

                output_x += xStep;
             }

             ++input_y;
             output_y += yStep;
          }

       Notice  that  the  steps between successive output rows and columns are
       returned as shifts.  This is possible because the pixels in the  subim-
       ages are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the orig-
       inal image.  In practice you may need to directly calculate the  output
       coordinate  given  an  input  coordinate.   libpng provides two further
       macros for this purpose:

          png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
          png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);

       Finally a pair of macros are provided to tell you if a particular image
       row or column appears in a given pass:

          int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
          int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);

       Bear  in  mind  that you will probably also need to check the width and
       height of the pass in addition to the above to be sure  the  pass  even
       exists!

       With  any  luck you are convinced by now that you don't want to do your
       own interlace handling.  In reality normally the only good  reason  for
       doing this is if you are processing PNG files on a pixel-by-pixel basis
       and don't want to load the whole file into memory  when  it  is  inter-
       laced.

       libpng  includes a test program, pngvalid, that illustrates reading and
       writing of interlaced images.  If you can't get interlacing to work  in
       your  code  and  don't  want  to  leave  it  to libpng (the recommended
       approach), see how pngvalid.c does it.


   Finishing a sequential read
       After you are finished reading the image through the  low-level  inter-
       face, you can finish reading the file.

       If  you  want  to use a different crc action for handling CRC errors in
       chunks after the image data, you can call png_set_crc_action() again at
       this point.

       If  you  are interested in comments or time, which may be stored either
       before or after the image data, you should pass the  separate  png_info
       struct if you want to keep the comments from before and after the image
       separate.

           png_infop end_info = png_create_info_struct(png_ptr);

           if (!end_info)
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                  (png_infopp)NULL);
              return (ERROR);
           }

          png_read_end(png_ptr, end_info);

       If you are not interested, you should still call png_read_end() but you
       can  pass  NULL, avoiding the need to create an end_info structure.  If
       you do this, libpng will not process any chunks after IDAT  other  than
       skipping  over  them  and perhaps (depending on whether you have called
       png_set_crc_action) checking their CRCs  while  looking  for  the  IEND
       chunk.

          png_read_end(png_ptr, (png_infop)NULL);

       If  you  don't call png_read_end(), then your file pointer will be left
       pointing to the first chunk after the last IDAT, which is probably  not
       what you want if you expect to read something beyond the end of the PNG
       datastream.

       When you are done, you can free all memory  allocated  by  libpng  like
       this:

          png_destroy_read_struct(&png_ptr, &info_ptr,
              &end_info);

       or, if you didn't create an end_info structure,

          png_destroy_read_struct(&png_ptr, &info_ptr,
              (png_infopp)NULL);

       It  is  also  possible  to  individually free the info_ptr members that
       point to libpng-allocated storage with the following function:

           png_free_data(png_ptr, info_ptr, mask, seq)

           mask - identifies data to be freed, a mask
                  containing the bitwise OR of one or
                  more of
                    PNG_FREE_PLTE, PNG_FREE_TRNS,
                    PNG_FREE_HIST, PNG_FREE_ICCP,
                    PNG_FREE_PCAL, PNG_FREE_ROWS,
                    PNG_FREE_SCAL, PNG_FREE_SPLT,
                    PNG_FREE_TEXT, PNG_FREE_UNKN,
                  or simply PNG_FREE_ALL

           seq  - sequence number of item to be freed
                  (-1 for all items)

       This function may be  safely  called  when  the  relevant  storage  has
       already  been freed, or has not yet been allocated, or was allocated by
       the user and not by libpng,  and will in those cases do  nothing.   The
       "seq"  parameter is ignored if only one item of the selected data type,
       such as PLTE, is allowed.  If "seq" is not -1, and multiple  items  are
       allowed for the data type identified in the mask, such as text or sPLT,
       only the n'th item in the structure is freed, where n is "seq".

       The default behavior is only to free data that was allocated internally
       by libpng.  This can be changed, so that libpng will not free the data,
       or so that it will free data  that  was  allocated  by  the  user  with
       png_malloc()  or png_calloc() and passed in via a png_set_*() function,
       with

           png_data_freer(png_ptr, info_ptr, freer, mask)

           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

           mask   - which data elements are affected
                    same choices as in png_free_data()

       This function only affects data that has already been  allocated.   You
       can  call  this  function after reading the PNG data but before calling
       any  png_set_*()  functions,  to  control  whether  the  user  or   the
       png_set_*()  function is responsible for freeing any existing data that
       might be present, and again after the png_set_*() functions to  control
       whether the user or png_destroy_*() is supposed to free the data.  When
       the user assumes responsibility for libpng-allocated data, the applica-
       tion  must  use  png_free()  to  free  it,  and when the user transfers
       responsibility to libpng for data that the user has allocated, the user
       must have used png_malloc() or png_calloc() to allocate it.

       If  you  allocated  your  row_pointers  in a single block, as suggested
       above in the description of the high level read interface, you must not
       transfer   responsibility   for  freeing  it  to  the  png_set_rows  or
       png_read_destroy function, because they would  also  try  to  free  the
       individual row_pointers[i].

       If  you  allocated  text_ptr.text,  text_ptr.lang,  and text_ptr.trans-
       lated_keyword separately, do not transfer  responsibility  for  freeing
       text_ptr  to  libpng, because when libpng fills a png_text structure it
       combines these members with the key member,  and  png_free_data()  will
       free  only text_ptr.key.  Similarly, if you transfer responsibility for
       free'ing text_ptr from libpng to  your  application,  your  application
       must not separately free those members.

       The  png_free_data()  function  will turn off the "valid" flag for any-
       thing it frees.  If you need to turn the flag off for a chunk that  was
       freed by your application instead of by libpng, you can use

           png_set_invalid(png_ptr, info_ptr, mask);

           mask - identifies the chunks to be made invalid,
                  containing the bitwise OR of one or
                  more of
                    PNG_INFO_gAMA, PNG_INFO_sBIT,
                    PNG_INFO_cHRM, PNG_INFO_PLTE,
                    PNG_INFO_tRNS, PNG_INFO_bKGD,
                    PNG_INFO_hIST, PNG_INFO_pHYs,
                    PNG_INFO_oFFs, PNG_INFO_tIME,
                    PNG_INFO_pCAL, PNG_INFO_sRGB,
                    PNG_INFO_iCCP, PNG_INFO_sPLT,
                    PNG_INFO_sCAL, PNG_INFO_IDAT

       For  a  more compact example of reading a PNG image, see the file exam-
       ple.c.


   Reading PNG files progressively
       The progressive reader is slightly different from  the  non-progressive
       reader.   Instead  of  calling  png_read_info(),  png_read_rows(),  and
       png_read_end(), you make one call to  png_process_data(),  which  calls
       callbacks  when  it  has the info, a row, or the end of the image.  You
       set up these callbacks with png_set_progressive_read_fn().   You  don't
       have  to  worry  about the input/output functions of libpng, as you are
       giving the library the data directly  in  png_process_data().   I  will
       assume  that you have read the section on reading PNG files above, so I
       will only highlight the differences (although I will show  all  of  the
       code).

       png_structp png_ptr; png_infop info_ptr;

        /*  An example code fragment of how you would
            initialize the progressive reader in your
            application. */
        int
        initialize_png_reader()
        {
           png_ptr = png_create_read_struct
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
                user_error_fn, user_warning_fn);

           if (!png_ptr)
               return (ERROR);

           info_ptr = png_create_info_struct(png_ptr);

           if (!info_ptr)
           {
              png_destroy_read_struct(&png_ptr,
                 (png_infopp)NULL, (png_infopp)NULL);
              return (ERROR);
           }

           if (setjmp(png_jmpbuf(png_ptr)))
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                 (png_infopp)NULL);
              return (ERROR);
           }

           /* This one's new.  You can provide functions
              to be called when the header info is valid,
              when each row is completed, and when the image
              is finished.  If you aren't using all functions,
              you can specify NULL parameters.  Even when all
              three functions are NULL, you need to call
              png_set_progressive_read_fn().  You can use
              any struct as the user_ptr (cast to a void pointer
              for the function call), and retrieve the pointer
              from inside the callbacks using the function

                 png_get_progressive_ptr(png_ptr);

              which will return a void pointer, which you have
              to cast appropriately.
            */
           png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
               info_callback, row_callback, end_callback);

           return 0;
        }

        /* A code fragment that you call as you receive blocks
          of data */
        int
        process_data(png_bytep buffer, png_uint_32 length)
        {
           if (setjmp(png_jmpbuf(png_ptr)))
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                  (png_infopp)NULL);
              return (ERROR);
           }

           /* This one's new also.  Simply give it a chunk
              of data from the file stream (in order, of
              course).  On machines with segmented memory
              models machines, don't give it any more than
              64K.  The library seems to run fine with sizes
              of 4K. Although you can give it much less if
              necessary (I assume you can give it chunks of
              1 byte, I haven't tried less than 256 bytes
              yet).  When this function returns, you may
              want to display any rows that were generated
              in the row callback if you don't already do
              so there.
            */
           png_process_data(png_ptr, info_ptr, buffer, length);

           /* At this point you can call png_process_data_skip if
              you want to handle data the library will skip yourself;
              it simply returns the number of bytes to skip (and stops
              libpng skipping that number of bytes on the next
              png_process_data call).
           return 0;
        }

        /* This function is called (as set by
           png_set_progressive_read_fn() above) when enough data
           has been supplied so all of the header has been
           read.
        */
        void
        info_callback(png_structp png_ptr, png_infop info)
        {
           /* Do any setup here, including setting any of
              the transformations mentioned in the Reading
              PNG files section.  For now, you _must_ call
              either png_start_read_image() or
              png_read_update_info() after all the
              transformations are set (even if you don't set
              any).  You may start getting rows before
              png_process_data() returns, so this is your
              last chance to prepare for that.

              This is where you turn on interlace handling,
              assuming you don't want to do it yourself.

              If you need to you can stop the processing of
              your original input data at this point by calling
              png_process_data_pause.  This returns the number
              of unprocessed bytes from the last png_process_data
              call - it is up to you to ensure that the next call
              sees these bytes again.  If you don't want to bother
              with this you can get libpng to cache the unread
              bytes by setting the 'save' parameter (see png.h) but
              then libpng will have to copy the data internally.
            */
        }

        /* This function is called when each row of image
           data is complete */
        void
        row_callback(png_structp png_ptr, png_bytep new_row,
           png_uint_32 row_num, int pass)
        {
           /* If the image is interlaced, and you turned
              on the interlace handler, this function will
              be called for every row in every pass.  Some
              of these rows will not be changed from the
              previous pass.  When the row is not changed,
              the new_row variable will be NULL.  The rows
              and passes are called in order, so you don't
              really need the row_num and pass, but I'm
              supplying them because it may make your life
              easier.

              If you did not turn on interlace handling then
              the callback is called for each row of each
              sub-image when the image is interlaced.  In this
              case 'row_num' is the row in the sub-image, not
              the row in the output image as it is in all other
              cases.

              For the non-NULL rows of interlaced images when
              you have switched on libpng interlace handling,
              you must call png_progressive_combine_row()
              passing in the row and the old row.  You can
              call this function for NULL rows (it will just
              return) and for non-interlaced images (it just
              does the memcpy for you) if it will make the
              code easier.  Thus, you can just do this for
              all cases if you switch on interlace handling;
            */

               png_progressive_combine_row(png_ptr, old_row,
                 new_row);

           /* where old_row is what was displayed
              previously for the row.  Note that the first
              pass (pass == 0, really) will completely cover
              the old row, so the rows do not have to be
              initialized.  After the first pass (and only
              for interlaced images), you will have to pass
              the current row, and the function will combine
              the old row and the new row.

              You can also call png_process_data_pause in this
              callback - see above.
           */
        }

        void
        end_callback(png_structp png_ptr, png_infop info)
        {
           /* This function is called after the whole image
              has been read, including any chunks after the
              image (up to and including the IEND).  You
              will usually have the same info chunk as you
              had in the header, although some data may have
              been added to the comments and time fields.

              Most people won't do much here, perhaps setting
              a flag that marks the image as finished.
            */
        }





IV. Writing

       Much of this is very similar to reading.  However, everything of impor-
       tance is repeated here, so you won't have to constantly look back up in
       the reading section to understand writing.


   Setup
       You  will want to do the I/O initialization before you get into libpng,
       so if it doesn't work, you don't have anything to undo. If you are  not
       using  the  standard  I/O functions, you will need to replace them with
       custom writing functions.  See the discussion under Customizing libpng.

           FILE *fp = fopen(file_name, "wb");

           if (!fp)
              return (ERROR);

       Next, png_struct and png_info need to be allocated and initialized.  As
       these can be both relatively large, you may not want to store these  on
       the  stack,  unless you have stack space to spare.  Of course, you will
       want to check if they return NULL.  If you are also reading, you  won't
       want  to  name  your  read  structure  and  your  write  structure both
       "png_ptr"; you can call them anything you like, such as "read_ptr"  and
       "write_ptr".  Look at pngtest.c, for example.

           png_structp png_ptr = png_create_write_struct
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn);

           if (!png_ptr)
              return (ERROR);

           png_infop info_ptr = png_create_info_struct(png_ptr);
           if (!info_ptr)
           {
              png_destroy_write_struct(&png_ptr,
                  (png_infopp)NULL);
              return (ERROR);
           }

       If  you  want  to  use  your  own  memory  allocation  routines, define
       PNG_USER_MEM_SUPPORTED and use png_create_write_struct_2()  instead  of
       png_create_write_struct():

           png_structp png_ptr = png_create_write_struct_2
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn, (png_voidp)
               user_mem_ptr, user_malloc_fn, user_free_fn);

       After you have these structures, you will need to set up the error han-
       dling.  When libpng encounters an error, it expects to  longjmp()  back
       to  your  routine.   Therefore, you will need to call setjmp() and pass
       the png_jmpbuf(png_ptr).  If you write the  file  from  different  rou-
       tines,  you  will need to update the png_jmpbuf(png_ptr) every time you
       enter a new routine that will call a png_*() function.  See your  docu-
       mentation  of  setjmp/longjmp for your compiler for more information on
       setjmp/longjmp.  See the discussion on libpng  error  handling  in  the
       Customizing  Libpng  section  below  for more information on the libpng
       error handling.

           if (setjmp(png_jmpbuf(png_ptr)))
           {
           png_destroy_write_struct(&png_ptr, &info_ptr);
              fclose(fp);
              return (ERROR);
           }
           ...
           return;

       If you would rather avoid the complexity of setjmp/longjmp issues,  you
       can compile libpng with PNG_NO_SETJMP, in which case errors will result
       in a call to PNG_ABORT() which defaults to abort().

       You can #define PNG_ABORT() to a function that does something more use-
       ful than abort(), as long as your function does not return.

       Checking for invalid palette index on write was added at libpng 1.5.10.
       If a pixel contains an invalid (out-of-range)  index  libpng  issues  a
       benign  error.  This is enabled by default because this condition is an
       error according to the PNG specification, Clause 11.3.2, but the  error
       can be ignored in each png_ptr with

          png_set_check_for_invalid_index(png_ptr, 0);

       If  the error is ignored, or if png_benign_error() treats it as a warn-
       ing, any invalid pixels are written as-is by the encoder, resulting  in
       an  invalid  PNG datastream as output.  In this case the application is
       responsible for ensuring that the pixel indexes are in  range  when  it
       writes  a PLTE chunk with fewer entries than the bit depth would allow.

       Now you need to set up the output code.  The default for libpng  is  to
       use  the C function fwrite().  If you use this, you will need to pass a
       valid FILE * in the function png_init_io().  Be sure that the  file  is
       opened  in  binary  mode.  Again, if you wish to handle writing data in
       another way, see the discussion on libpng I/O handling in the Customiz-
       ing Libpng section below.

           png_init_io(png_ptr, fp);

       If  you are embedding your PNG into a datastream such as MNG, and don't
       want libpng to write the 8-byte signature, or if you have already writ-
       ten the signature in your application, use

           png_set_sig_bytes(png_ptr, 8);

       to inform libpng that it should not write a signature.


   Write callbacks
       At  this  point, you can set up a callback function that will be called
       after each row has been  written,  which  you  can  use  to  control  a
       progress  meter or the like.  It's demonstrated in pngtest.c.  You must
       supply a function

           void write_row_callback(png_structp png_ptr, png_uint_32 row,
              int pass);
           {
             /* put your code here */
           }

       (You can give it another name that you like instead of "write_row_call-
       back")

       To inform libpng about your function, use

           png_set_write_status_fn(png_ptr, write_row_callback);

       When  this  function is called the row has already been completely pro-
       cessed and it has also been written out.  The 'row' and 'pass' refer to
       the  next  row to be handled.  For the non-interlaced case the row that
       was just handled is simply one less than the passed in row number,  and
       pass will always be 0.  For the interlaced case the same applies unless
       the row value is 0, in which case the row just handled was the last one
       from  one of the preceding passes.  Because interlacing may skip a pass
       you cannot be sure that the preceding pass is  just  'pass-1',  if  you
       really  need  to  know what the last pass is record (row,pass) from the
       callback and use the last recorded value each time.

       As with the user transform you  can  find  the  output  row  using  the
       PNG_ROW_FROM_PASS_ROW macro.

       You  now  have the option of modifying how the compression library will
       run.  The following functions are mainly for testing, but may be useful
       in  some  cases, like if you need to write PNG files extremely fast and
       are willing to give up some compression, or if you want to get the max-
       imum  possible  compression  at  the expense of slower writing.  If you
       have no special needs in this area, let the library do what it wants by
       not  calling  this  function  at all, as it has been tuned to deliver a
       good speed/compression ratio. The second parameter to  png_set_filter()
       is  the filter method, for which the only valid values are 0 (as of the
       July 1999 PNG specification, version 1.2) or 64 (if you are  writing  a
       PNG  datastream that is to be embedded in a MNG datastream).  The third
       parameter is a flag that indicates  which  filter  type(s)  are  to  be
       tested for each scanline.  See the PNG specification for details on the
       specific filter types.


           /* turn on or off filtering, and/or choose
              specific filters.  You can use either a single
              PNG_FILTER_VALUE_NAME or the bitwise OR of one
              or more PNG_FILTER_NAME masks.
            */
           png_set_filter(png_ptr, 0,
              PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
              PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
              PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
              PNG_FILTER_AVG   | PNG_FILTER_VALUE_AVG  |
              PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
              PNG_ALL_FILTERS);

       If an application wants to start and stop using particular filters dur-
       ing compression, it should start out with all of the filters (to ensure
       that the previous row of pixels will be  stored  in  case  it's  needed
       later), and then add and remove them after the start of compression.

       If  you  are  writing  a PNG datastream that is to be embedded in a MNG
       datastream, the second parameter can be either 0 or 64.

       The png_set_compression_*() functions interface to the zlib compression
       library,  and  should mostly be ignored unless you really know what you
       are  doing.   The  only  generally  useful  call  is   png_set_compres-
       sion_level()  which changes how much time zlib spends on trying to com-
       press the image data.  See the Compression Library  (zlib.h  and  algo-
       rithm.txt,  distributed  with zlib) for details on the compression lev-
       els.

           #include zlib.h

           /* Set the zlib compression level */
           png_set_compression_level(png_ptr,
               Z_BEST_COMPRESSION);

           /* Set other zlib parameters for compressing IDAT */
           png_set_compression_mem_level(png_ptr, 8);
           png_set_compression_strategy(png_ptr,
               Z_DEFAULT_STRATEGY);
           png_set_compression_window_bits(png_ptr, 15);
           png_set_compression_method(png_ptr, 8);
           png_set_compression_buffer_size(png_ptr, 8192)

           /* Set zlib parameters for text compression
            * If you don't call these, the parameters
            * fall back on those defined for IDAT chunks
            */
           png_set_text_compression_mem_level(png_ptr, 8);
           png_set_text_compression_strategy(png_ptr,
               Z_DEFAULT_STRATEGY);
           png_set_text_compression_window_bits(png_ptr, 15);
           png_set_text_compression_method(png_ptr, 8);


   Setting the contents of info for output
       You now need to fill in the png_info structure with all  the  data  you
       wish  to  write  before the actual image.  Note that the only thing you
       are allowed to write after the image is the text chunks  and  the  time
       chunk  (as  of PNG Specification 1.2, anyway).  See png_write_end() and
       the latest PNG specification for more information on that.  If you wish
       to write them before the image, fill them in now, and flag that data as
       being valid.  If you want to wait until after the data, don't fill them
       until  png_write_end().   For all the fields in png_info and their data
       types, see png.h.  For explanations of what the fields contain, see the
       PNG specification.

       Some of the more important parts of the png_info are:

           png_set_IHDR(png_ptr, info_ptr, width, height,
              bit_depth, color_type, interlace_type,
              compression_type, filter_method)

           width          - holds the width of the image
                            in pixels (up to 2^31).

           height         - holds the height of the image
                            in pixels (up to 2^31).

           bit_depth      - holds the bit depth of one of the
                            image channels.
                            (valid values are 1, 2, 4, 8, 16
                            and depend also on the
                            color_type.  See also significant
                            bits (sBIT) below).

           color_type     - describes which color/alpha
                            channels are present.
                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7

           compression_type - (must be
                            PNG_COMPRESSION_TYPE_DEFAULT)

           filter_method  - (must be PNG_FILTER_TYPE_DEFAULT
                            or, if you are writing a PNG to
                            be embedded in a MNG datastream,
                            can also be
                            PNG_INTRAPIXEL_DIFFERENCING)

       If  you  call  png_set_IHDR(),  the  call must appear before any of the
       other png_set_*() functions, because they might require access to  some
       of  the  IHDR  settings.   The  remaining  png_set_*() functions can be
       called in any order.

       If you wish, you can reset  the  compression_type,  interlace_type,  or
       filter_method  later  by  calling png_set_IHDR() again; if you do this,
       the width, height, bit_depth, and color_type must be the same  in  each
       call.

           png_set_PLTE(png_ptr, info_ptr, palette,
              num_palette);

           palette        - the palette for the file
                            (array of png_color)
           num_palette    - number of entries in the palette

           png_set_gAMA(png_ptr, info_ptr, file_gamma);
           png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);

           file_gamma     - the gamma at which the image was
                            created (PNG_INFO_gAMA)

           int_file_gamma - 100,000 times the gamma at which
                            the image was created

           png_set_cHRM(png_ptr, info_ptr,  white_x, white_y, red_x, red_y,
                            green_x, green_y, blue_x, blue_y)
           png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
                            green_Y, green_Z, blue_X, blue_Y, blue_Z)
           png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
                            int_red_x, int_red_y, int_green_x, int_green_y,
                            int_blue_x, int_blue_y)
           png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
                            int_red_Z, int_green_X, int_green_Y, int_green_Z,
                            int_blue_X, int_blue_Y, int_blue_Z)

           {white,red,green,blue}_{x,y}
                            A  color  space encoding specified using the chro-
       maticities
                            of the end points and the white point.

           {red,green,blue}_{X,Y,Z}
                            A color space encoding specified using the  encod-
       ing end
                            points  - the CIE tristimulus specification of the
       intended
                            color of the red, green and blue channels  in  the
       PNG RGB
                            data.   The  white  point is simply the sum of the
       three end
                            points.

           png_set_sRGB(png_ptr, info_ptr, srgb_intent);

           srgb_intent    - the rendering intent
                            (PNG_INFO_sRGB) The presence of
                            the sRGB chunk means that the pixel
                            data is in the sRGB color space.
                            This chunk also implies specific
                            values of gAMA and cHRM.  Rendering
                            intent is the CSS-1 property that
                            has been defined by the International
                            Color Consortium
                            (http://www.color.org).
                            It can be one of
                            PNG_sRGB_INTENT_SATURATION,
                            PNG_sRGB_INTENT_PERCEPTUAL,
                            PNG_sRGB_INTENT_ABSOLUTE, or
                            PNG_sRGB_INTENT_RELATIVE.


           png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
              srgb_intent);

           srgb_intent    - the rendering intent
                            (PNG_INFO_sRGB) The presence of the
                            sRGB chunk means that the pixel
                            data is in the sRGB color space.
                            This function also causes gAMA and
                            cHRM chunks with the specific values
                            that are consistent with sRGB to be
                            written.

           png_set_iCCP(png_ptr, info_ptr, name, compression_type,
                              profile, proflen);

           name             - The profile name.

           compression_type - The compression type; always
                              PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                              You may give NULL to this argument to
                              ignore it.

           profile          - International Color Consortium color
                              profile data. May contain NULs.

           proflen          - length of profile data in bytes.

           png_set_sBIT(png_ptr, info_ptr, sig_bit);

           sig_bit        - the number of significant bits for
                            (PNG_INFO_sBIT) each of the gray, red,
                            green, and blue channels, whichever are
                            appropriate for the given color type
                            (png_color_16)

           png_set_tRNS(png_ptr, info_ptr, trans_alpha,
              num_trans, trans_color);

           trans_alpha    - array of alpha (transparency)
                            entries for palette (PNG_INFO_tRNS)

           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)

           trans_color    - graylevel or color sample values
                            (in order red, green, blue) of the
                            single transparent color for
                            non-paletted images (PNG_INFO_tRNS)

           png_set_hIST(png_ptr, info_ptr, hist);

           hist           - histogram of palette (array of
                            png_uint_16) (PNG_INFO_hIST)

           png_set_tIME(png_ptr, info_ptr, mod_time);

           mod_time       - time image was last modified
                            (PNG_VALID_tIME)

           png_set_bKGD(png_ptr, info_ptr, background);

           background     - background color (of type
                            png_color_16p) (PNG_VALID_bKGD)

           png_set_text(png_ptr, info_ptr, text_ptr, num_text);

           text_ptr       - array of png_text holding image
                            comments

           text_ptr[i].compression - type of compression used
                        on "text" PNG_TEXT_COMPRESSION_NONE
                                  PNG_TEXT_COMPRESSION_zTXt
                                  PNG_ITXT_COMPRESSION_NONE
                                  PNG_ITXT_COMPRESSION_zTXt
           text_ptr[i].key   - keyword for comment.  Must contain
                        1-79 characters.
           text_ptr[i].text  - text comments for current
                                keyword.  Can be NULL or empty.
           text_ptr[i].text_length - length of text string,
                        after decompression, 0 for iTXt
           text_ptr[i].itxt_length - length of itxt string,
                        after decompression, 0 for tEXt/zTXt
           text_ptr[i].lang  - language of comment (NULL or
                                empty for unknown).
           text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
                                or empty for unknown).

           Note that the itxt_length, lang, and lang_key
           members of the text_ptr structure only exist when the
           library is built with iTXt chunk support.  Prior to
           libpng-1.4.0 the library was built by default without
           iTXt support. Also note that when iTXt is supported,
           they contain NULL pointers when the "compression"
           field contains PNG_TEXT_COMPRESSION_NONE or
           PNG_TEXT_COMPRESSION_zTXt.

           num_text       - number of comments

           png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
              num_spalettes);

           palette_ptr    - array of png_sPLT_struct structures
                            to be added to the list of palettes
                            in the info structure.
           num_spalettes  - number of palette structures to be
                            added.

           png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
               unit_type);

           offset_x  - positive offset from the left
                            edge of the screen

           offset_y  - positive offset from the top
                            edge of the screen

           unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

           png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
               unit_type);

           res_x       - pixels/unit physical resolution
                         in x direction

           res_y       - pixels/unit physical resolution
                         in y direction

           unit_type   - PNG_RESOLUTION_UNKNOWN,
                         PNG_RESOLUTION_METER

           png_set_sCAL(png_ptr, info_ptr, unit, width, height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units

           height      - height of a pixel in physical scale units
                         (width and height are doubles)

           png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units
                         expressed as a string

           height      - height of a pixel in physical scale units
                        (width and height are strings like "2.54")

           png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
              num_unknowns)

           unknowns          - array of png_unknown_chunk
                               structures holding unknown chunks
           unknowns[i].name  - name of unknown chunk
           unknowns[i].data  - data of unknown chunk
           unknowns[i].size  - size of unknown chunk's data
           unknowns[i].location - position to write chunk in file
                                  0: do not write chunk
                                  PNG_HAVE_IHDR: before PLTE
                                  PNG_HAVE_PLTE: before IDAT
                                  PNG_AFTER_IDAT: after IDAT

       The "location" member is set automatically according to  what  part  of
       the  output  file  has  already been written.  You can change its value
       after calling png_set_unknown_chunks() as  demonstrated  in  pngtest.c.
       Within  each  of the "locations", the chunks are sequenced according to
       their position in the structure (that is, the value of  "i",  which  is
       the  order  in  which  the chunk was either read from the input file or
       defined with png_set_unknown_chunks).

       A quick word about text and num_text.  text is  an  array  of  png_text
       structures.   num_text  is the number of valid structures in the array.
       Each png_text structure holds a language code, a keyword, a text value,
       and a compression type.

       The  compression  types  have the same valid numbers as the compression
       types of the image data.  Currently, the only  valid  number  is  zero.
       However,  you  can store text either compressed or uncompressed, unlike
       images, which always have to be compressed.  So if you don't  want  the
       text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
       Because tEXt and zTXt chunks don't have a language field, if you  spec-
       ify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt any language
       code or translated keyword will not be written out.

       Until text gets around a few hundred bytes, it is not worth compressing
       it.   After  the text has been written out to the file, the compression
       type  is  set  to  PNG_TEXT_COMPRESSION_NONE_WR  or   PNG_TEXT_COMPRES-
       SION_zTXt_WR,  so  that  it isn't written out again at the end (in case
       you are calling png_write_end() with the same struct).

       The keywords that are given in the PNG Specification are:

           Title            Short (one line) title or
                            caption for image

           Author           Name of image's creator

           Description      Description of image (possibly long)

           Copyright        Copyright notice

           Creation Time    Time of original image creation
                            (usually RFC 1123 format, see below)

           Software         Software used to create the image

           Disclaimer       Legal disclaimer

           Warning          Warning of nature of content

           Source           Device used to create the image

           Comment          Miscellaneous comment; conversion
                            from other image format

       The keyword-text pairs work like this.  Keywords should be short simple
       descriptions  of  what the comment is about.  Some typical keywords are
       found in the PNG specification, as is some recommendations on keywords.
       You can repeat keywords in a file.  You can even write some text before
       the image and some after.  For example, you may want to put a  descrip-
       tion  of  the  image  before  the image, but leave the disclaimer until
       after, so viewers working over modem connections don't have to wait for
       the disclaimer to go over the modem before they start seeing the image.
       Finally, keywords should be full words,  not  abbreviations.   Keywords
       and  text  are in the ISO 8859-1 (Latin-1) character set (a superset of
       regular ASCII) and can not contain NUL characters, and should not  con-
       tain  control  or  other  unprintable characters.  To make the comments
       widely readable, stick with basic ASCII,  and  avoid  machine  specific
       character  set  extensions  like the IBM-PC character set.  The keyword
       must be present, but you can leave off  the  text  string  on  non-com-
       pressed  pairs.   Compressed pairs must have a text string, as only the
       text string is compressed anyway, so the compression would be  meaning-
       less.

       PNG supports modification time via the png_time structure.  Two conver-
       sion routines are provided, png_convert_from_time_t()  for  time_t  and
       png_convert_from_struct_tm()  for  struct  tm.  The time_t routine uses
       gmtime().  You don't have to use either of these, but if  you  wish  to
       fill in the png_time structure directly, you should provide the time in
       universal time (GMT) if possible instead of your local time.  Note that
       the  year  number  is the full year (e.g. 1998, rather than 98 - PNG is
       year 2000 compliant!), and that months start with 1.

       If you want to store the time  of  the  original  image  creation,  you
       should  use  a plain tEXt chunk with the "Creation Time" keyword.  This
       is necessary because the "creation time" of a  PNG  image  is  somewhat
       vague,  depending  on whether you mean the PNG file, the time the image
       was created in a non-PNG format, a still photo from which the image was
       scanned, or possibly the subject matter itself.  In order to facilitate
       machine-readable dates, it is recommended that the "Creation Time" tEXt
       chunk  use  RFC  1123  format  dates (e.g. "22 May 1997 18:07:10 GMT"),
       although this isn't a requirement.  Unlike the tIME  chunk,  the  "Cre-
       ation  Time"  tEXt chunk is not expected to be automatically changed by
       the software.  To facilitate the use of  RFC  1123  dates,  a  function
       png_convert_to_rfc1123_buffer(buffer, png_timep) is provided to convert
       from PNG time to an RFC 1123 format string.  The caller must provide  a
       writeable buffer of at least 29 bytes.


   Writing unknown chunks
       You  can  use  the  png_set_unknown_chunks function to queue up private
       chunks for writing.  You give it a chunk name, location, raw data,  and
       a size.  You also must use png_set_keep_unknown_chunks() to ensure that
       libpng will handle them.  That's all there is to it.  The  chunks  will
       be   written   by   the   next   following  png_write_info_before_PLTE,
       png_write_info, or png_write_end function, depending upon the specified
       location.   Any  chunks  previously  read  into  the  info  structure's
       unknown-chunk list will also be written out in a sequence  that  satis-
       fies the PNG specification's ordering rules.

       Here is an example of writing two private chunks, prVt and miNE:

           #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
           /* Set unknown chunk data */
           png_unknown_chunk unk_chunk[2];
           strcpy((char *) unk_chunk[0].name, "prVt";
           unk_chunk[0].data = (unsigned char *) "PRIVATE DATA";
           unk_chunk[0].size = strlen(unk_chunk[0].data)+1;
           unk_chunk[0].location = PNG_HAVE_IHDR;
           strcpy((char *) unk_chunk[1].name, "miNE";
           unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA";
           unk_chunk[1].size = strlen(unk_chunk[0].data)+1;
           unk_chunk[1].location = PNG_AFTER_IDAT;
           png_set_unknown_chunks(write_ptr, write_info_ptr,
               unk_chunk, 2);
           /* Needed because miNE is not safe-to-copy */
           png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS,
              (png_bytep) "miNE", 1);
           # if PNG_LIBPNG_VER < 10600
             /* Deal with unknown chunk location bug in 1.5.x and earlier */
             png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR);
             png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT);
           # endif
           # if PNG_LIBPNG_VER < 10500
             /*  PNG_AFTER_IDAT  writes  two  copies  of  the  chunk  prior to
       libpng-1.5.0,
              * one before IDAT and another after IDAT, so don't use it;  only
       use
              *  PNG_HAVE_IHDR location.  This call resets the location previ-
       ously
              * set by  assignment  and  png_set_unknown_chunk_location()  for
       chunk 1.
              */
             png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR);
           # endif
           #endif


   The high-level write interface
       At  this  point  there  are two ways to proceed; through the high-level
       write interface, or through a sequence of low-level  write  operations.
       You  can  use the high-level interface if your image data is present in
       the info structure.  All defined output transformations are  permitted,
       enabled by the following masks.

           PNG_TRANSFORM_IDENTITY      No transformation
           PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit samples
           PNG_TRANSFORM_PACKSWAP      Change order of packed
                                       pixels to LSB first
           PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
           PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                       sBIT depth
           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
           PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                       to transparency
           PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
           PNG_TRANSFORM_STRIP_FILLER        Strip out filler
                                             bytes (deprecated).
           PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
                                             filler bytes
           PNG_TRANSFORM_STRIP_FILLER_AFTER  Strip out trailing
                                             filler bytes

       If  you  have  valid  image  data  in  the  info structure (you can use
       png_set_rows() to put image data in  the  info  structure),  simply  do
       this:

           png_write_png(png_ptr, info_ptr, png_transforms, NULL)

       where  png_transforms  is  an integer containing the bitwise OR of some
       set  of   transformation   flags.    This   call   is   equivalent   to
       png_write_info(),  followed the set of transformations indicated by the
       transform mask, then png_write_image(), and finally png_write_end().

       (The final parameter of this call is not yet used.   Someday  it  might
       point  to  transformation  parameters  required  by  some future output
       transform.)

       You must use png_transforms and not call any png_set_transform()  func-
       tions when you use png_write_png().


   The low-level write interface
       If  you  are  going  the  low-level route instead, you are now ready to
       write all the file information up to the actual  image  data.   You  do
       this with a call to png_write_info().

           png_write_info(png_ptr, info_ptr);

       Note  that  there  is  one  transformation  you  may  need to do before
       png_write_info().  In PNG files, the alpha channel in an image  is  the
       level of opacity.  If your data is supplied as a level of transparency,
       you can invert the alpha channel before you write  it,  so  that  0  is
       fully  transparent  and  255 (in 8-bit or paletted images) or 65535 (in
       16-bit images) is fully opaque, with

           png_set_invert_alpha(png_ptr);

       This must appear before png_write_info()  instead  of  later  with  the
       other  transformations  because in the case of paletted images the tRNS
       chunk data has to be inverted before the tRNS  chunk  is  written.   If
       your  image is not a paletted image, the tRNS data (which in such cases
       represents a single color to be rendered as transparent) won't need  to
       be  changed,  and  you  can  safely  do  this transformation after your
       png_write_info() call.

       If you need to write a private chunk that you want to appear before the
       PLTE  chunk  when  PLTE  is  present, you can write the PNG info in two
       steps, and insert code to write your own chunk between them:

           png_write_info_before_PLTE(png_ptr, info_ptr);
           png_set_unknown_chunks(png_ptr, info_ptr, ...);
           png_write_info(png_ptr, info_ptr);

       After you've written the file information, you can set up  the  library
       to  handle  any special transformations of the image data.  The various
       ways to transform the data will be described in  the  order  that  they
       should  occur.   This  is  important, as some of these change the color
       type and/or bit depth of the data, and some others only work on certain
       color  types and bit depths.  Even though each transformation checks to
       see if it has data that it can do something with, you should make  sure
       to  only enable a transformation if it will be valid for the data.  For
       example, don't swap red and blue on grayscale data.

       PNG files store RGB pixels packed into 3 or 6 bytes.  This  code  tells
       the library to strip input data that has 4 or 8 bytes per pixel down to
       3 or 6 bytes (or strip 2 or 4-byte grayscale+filler  data  to  1  or  2
       bytes per pixel).

           png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

       where  the 0 is unused, and the location is either PNG_FILLER_BEFORE or
       PNG_FILLER_AFTER, depending upon whether the filler byte in  the  pixel
       is stored XRGB or RGBX.

       PNG  files pack pixels of bit depths 1, 2, and 4 into bytes as small as
       they can, resulting in, for example, 8 pixels per byte for 1 bit files.
       If  the data is supplied at 1 pixel per byte, use this code, which will
       correctly pack the pixels into a single byte:

           png_set_packing(png_ptr);

       PNG files reduce possible bit depths to 1, 2, 4, 8, and  16.   If  your
       data is of another bit depth, you can write an sBIT chunk into the file
       so that decoders can recover the original data if desired.

           /* Set the true bit depth of the image data */
           if (color_type & PNG_COLOR_MASK_COLOR)
           {
              sig_bit.red = true_bit_depth;
              sig_bit.green = true_bit_depth;
              sig_bit.blue = true_bit_depth;
           }

           else
           {
              sig_bit.gray = true_bit_depth;
           }

           if (color_type & PNG_COLOR_MASK_ALPHA)
           {
              sig_bit.alpha = true_bit_depth;
           }

           png_set_sBIT(png_ptr, info_ptr, &sig_bit);

       If the data is stored in the row buffer in a bit depth other  than  one
       supported  by  PNG  (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
       this will scale the values to appear to be the correct bit depth as  is
       required by PNG.

           png_set_shift(png_ptr, &sig_bit);

       PNG  files  store  16-bit pixels in network byte order (big-endian, ie.
       most significant bits first).  This code would be used if they are sup-
       plied  the other way (little-endian, i.e. least significant bits first,
       the way PCs store them):

           if (bit_depth > 8)
              png_set_swap(png_ptr);

       If you are using packed-pixel images (1, 2, or 4 bits/pixel),  and  you
       need to change the order the pixels are packed into bytes, you can use:

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

       PNG files store 3 color pixels in red, green, blue  order.   This  code
       would be used if they are supplied as blue, green, red:

           png_set_bgr(png_ptr);

       PNG  files describe monochrome as black being zero and white being one.
       This code would be used if the pixels are supplied with  this  reversed
       (black being one and white being zero):

           png_set_invert_mono(png_ptr);

       Finally,  you can write your own transformation function if none of the
       existing ones meets your needs.  This is done  by  setting  a  callback
       with

           png_set_write_user_transform_fn(png_ptr,
              write_transform_fn);

       You must supply the function

           void write_transform_fn(png_structp png_ptr, png_row_infop
              row_info, png_bytep data)

       See  pngtest.c  for  a  working  example.  Your function will be called
       before any of the other transformations are  processed.   If  supported
       libpng  also  supplies  an  information routine that may be called from
       your callback:

          png_get_current_row_number(png_ptr);
          png_get_current_pass_number(png_ptr);

       This returns the current row passed to the transform.  With  interlaced
       images the value returned is the row in the input sub-image image.  Use
       PNG_ROW_FROM_PASS_ROW(row, pass) and  PNG_COL_FROM_PASS_COL(col,  pass)
       to  find  the  output  pixel  (x,y) given an interlaced sub-image pixel
       (row,col,pass).

       The discussion of interlace handling above contains more information on
       how to use these values.

       You can also set up a pointer to a user structure for use by your call-
       back function.

           png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

       The user_channels  and  user_depth  parameters  of  this  function  are
       ignored when writing; you can set them to zero as shown.

       You  can  retrieve  the  pointer  via  the function png_get_user_trans-
       form_ptr().  For example:

           voidp write_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

       It is possible to have libpng flush any pending  output,  either  manu-
       ally,  or automatically after a certain number of lines have been writ-
       ten.  To flush the output stream a single time call:

           png_write_flush(png_ptr);

       and to have libpng flush the output stream periodically after a certain
       number of scanlines have been written, call:

           png_set_flush(png_ptr, nrows);

       Note   that   the   distance   between  rows  is  from  the  last  time
       png_write_flush() was called, or the first row of the image if  it  has
       never  been  called.   So if you write 50 lines, and then png_set_flush
       25, it will flush the output on the next scanline, and every  25  lines
       thereafter,  unless  png_write_flush()  is  called before 25 more lines
       have been written.  If nrows is too small (less than about 10 lines for
       a  640 pixel wide RGB image) the image compression may decrease notice-
       ably (although this may  be  acceptable  for  real-time  applications).
       Infrequent  flushing will only degrade the compression performance by a
       few percent over images that do not use flushing.


   Writing the image data
       That's it for the transformations.  Now you can write the  image  data.
       The  simplest  way to do this is in one function call.  If you have the
       whole image in memory, you can just call png_write_image()  and  libpng
       will write the image.  You will need to pass in an array of pointers to
       each row.  This function  automatically  handles  interlacing,  so  you
       don't  need  to call png_set_interlace_handling() or call this function
       multiple  times,  or  any  of   that   other   stuff   necessary   with
       png_write_rows().

           png_write_image(png_ptr, row_pointers);

       where row_pointers is:

           png_byte *row_pointers[height];

       You can point to void or char or whatever you use for pixels.

       If  you  don't  want  to  write  the  whole  image at once, you can use
       png_write_rows() instead.  If the file is not interlaced, this is  sim-
       ple:

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

       row_pointers is the same as in the png_write_image() call.

       If  you are just writing one row at a time, you can do this with a sin-
       gle row_pointer instead of an array of row_pointers:

           png_bytep row_pointer = row;

           png_write_row(png_ptr, row_pointer);

       When the file is interlaced, things can get a good  deal  more  compli-
       cated.   The  only  currently (as of the PNG Specification version 1.2,
       dated July 1999) defined  interlacing  scheme  for  PNG  files  is  the
       "Adam7"  interlace scheme, that breaks down an image into seven smaller
       images of varying size.  libpng will build these images for you, or you
       can  do them yourself.  If you want to build them yourself, see the PNG
       specification for details of which pixels to write when.

       If you don't want libpng to handle the interlacing  details,  just  use
       png_set_interlace_handling() and call png_write_rows() the correct num-
       ber of times to write all the sub-images  (png_set_interlace_handling()
       returns the number of sub-images.)

       If  you want libpng to build the sub-images, call this before you start
       writing any rows:

           number_of_passes = png_set_interlace_handling(png_ptr);

       This will return the number  of  passes  needed.   Currently,  this  is
       seven, but may change if another interlace type is added.

       Then write the complete image number_of_passes times.

           png_write_rows(png_ptr, row_pointers, number_of_rows);

       Think  carefully  before you write an interlaced image.  Typically code
       that reads such images reads all the image  data  into  memory,  uncom-
       pressed,  before  doing  any processing.  Only code that can display an
       image on the fly can take advantage of the interlacing  and  even  then
       the  image  has  to  be exactly the correct size for the output device,
       because scaling an image requires adjacent pixels  and  these  are  not
       available until all the passes have been read.

       If you do write an interlaced image you will hardly ever need to handle
       the interlacing yourself.  Call  png_set_interlace_handling()  and  use
       the approach described above.

       The  only  time it is conceivable that you will really need to write an
       interlaced image pass-by-pass is when you have read one  pass  by  pass
       and  made some pixel-by-pixel transformation to it, as described in the
       read code above.  In this case use the PNG_PASS_ROWS and  PNG_PASS_COLS
       macros to determine the size of each sub-image in turn and simply write
       the rows you obtained from the read code.


   Finishing a sequential write
       After you are finished writing the image, you should finish writing the
       file.   If  you  are interested in writing comments or time, you should
       pass an appropriately filled png_info pointer.  If you are  not  inter-
       ested, you can pass NULL.

           png_write_end(png_ptr, info_ptr);

       When you are done, you can free all memory used by libpng like this:

           png_destroy_write_struct(&png_ptr, &info_ptr);

       It  is  also  possible  to  individually free the info_ptr members that
       point to libpng-allocated storage with the following function:

           png_free_data(png_ptr, info_ptr, mask, seq)

           mask  - identifies data to be freed, a mask
                   containing the bitwise OR of one or
                   more of
                     PNG_FREE_PLTE, PNG_FREE_TRNS,
                     PNG_FREE_HIST, PNG_FREE_ICCP,
                     PNG_FREE_PCAL, PNG_FREE_ROWS,
                     PNG_FREE_SCAL, PNG_FREE_SPLT,
                     PNG_FREE_TEXT, PNG_FREE_UNKN,
                   or simply PNG_FREE_ALL

           seq   - sequence number of item to be freed
                   (-1 for all items)

       This function may be  safely  called  when  the  relevant  storage  has
       already  been freed, or has not yet been allocated, or was allocated by
       the user  and not by libpng,  and will in those cases do nothing.   The
       "seq"  parameter is ignored if only one item of the selected data type,
       such as PLTE, is allowed.  If "seq" is not -1, and multiple  items  are
       allowed for the data type identified in the mask, such as text or sPLT,
       only the n'th item in the structure is freed, where n is "seq".

       If you allocated data such as a palette that you passed  in  to  libpng
       with  png_set_*,  you  must  not  free it until just before the call to
       png_destroy_write_struct().

       The default behavior is only to free data that was allocated internally
       by libpng.  This can be changed, so that libpng will not free the data,
       or so that it will free data  that  was  allocated  by  the  user  with
       png_malloc()  or png_calloc() and passed in via a png_set_*() function,
       with

           png_data_freer(png_ptr, info_ptr, freer, mask)

           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

           mask   - which data elements are affected
                    same choices as in png_free_data()

       For example, to transfer responsibility  for  some  data  from  a  read
       structure to a write structure, you could use

           png_data_freer(read_ptr, read_info_ptr,
              PNG_USER_WILL_FREE_DATA,
              PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

           png_data_freer(write_ptr, write_info_ptr,
              PNG_DESTROY_WILL_FREE_DATA,
              PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

       thereby  briefly reassigning responsibility for freeing to the user but
       immediately afterwards reassigning it once more  to  the  write_destroy
       function.   Having done this, it would then be safe to destroy the read
       structure and continue to use the PLTE, tRNS,  and  hIST  data  in  the
       write structure.

       This  function  only affects data that has already been allocated.  You
       can call this function before calling after the  png_set_*()  functions
       to  control whether the user or png_destroy_*() is supposed to free the
       data.  When the user assumes responsibility for libpng-allocated  data,
       the  application  must  use  png_free()  to  free it, and when the user
       transfers responsibility to libpng for data that  the  user  has  allo-
       cated, the user must have used png_malloc() or png_calloc() to allocate
       it.

       If you  allocated  text_ptr.text,  text_ptr.lang,  and  text_ptr.trans-
       lated_keyword  separately,  do  not transfer responsibility for freeing
       text_ptr to libpng, because when libpng fills a png_text  structure  it
       combines  these  members  with the key member, and png_free_data() will
       free only text_ptr.key.  Similarly, if you transfer responsibility  for
       free'ing  text_ptr  from  libpng  to your application, your application
       must not separately free those members.  For a more compact example  of
       writing a PNG image, see the file example.c.



V. Simplified API

       The  simplified  API, which became available in libpng-1.6.0, hides the
       details of both libpng and the PNG file format itself.  It  allows  PNG
       files to be read into a very limited number of in-memory bitmap formats
       or to be written from the same formats.  If these formats do not accom-
       modate  your needs then you can, and should, use the more sophisticated
       APIs above - these support a wide variety of in-memory  formats  and  a
       wide  variety of sophisticated transformations to those formats as well
       as a wide variety of APIs to manipulate ancilliary information.

       To read a PNG file using the simplified API:

         1) Declare a 'png_image' structure (see below) on the
            stack and memset() it to all zero.

         2) Call the appropriate png_image_begin_read... function.

         3) Set the png_image 'format' member to the required
            format and allocate a buffer for the image.

         4) Call png_image_finish_read to read the image into
            your buffer.

       There are no restrictions on the format of the PNG  input  itself;  all
       valid  color  types,  bit depths, and interlace methods are acceptable,
       and the input image is transformed as necessary to  the  requested  in-
       memory format during the png_image_finish_read() step.

       To write a PNG file using the simplified API:

         1) Declare a 'png_image' structure on the stack and memset()
            it to all zero.

         2) Initialize the members of the structure that describe the
            image, setting the 'format' member to the format of the
            image in memory.

         3) Call the appropriate png_image_write... function with a
            pointer to the image to write the PNG data.

       png_image  is  a  structure  that  describes the in-memory format of an
       image when it is being read or define the in-memory format of an  image
       that you need to write.  The "png_image" structure contains the follow-
       ing members:

          png_uint_32  version Set to PNG_IMAGE_VERSION
          png_uint_32  width   Image width in pixels (columns)
          png_uint_32  height  Image height in pixels (rows)
          png_uint_32  format  Image format as defined below
          png_uint_32  flags   A bit mask containing informational flags
          png_controlp opaque  Initialize to NULL, free with png_image_free
          png_uint_32  colormap_entries; Number of entries in the color-map
          png_uint_32  warning_or_error;
          char         message[64];

       In the  event  of  an  error  or  warning  the  following  field  warn-
       ing_or_error  field  will  be set to a non-zero value and the 'message'
       field will contain a ' ' terminated string with  the  libpng  error  or
       warning  message.  If both warnings and an error were encountered, only
       the error is recorded.  If there are multiple warnings, only the  first
       one is recorded.

       The  upper 30 bits of this value are reserved; the low two bits contain
       a two bit code such that a value more than 1 indicates a failure in the
       API just called:

          0 - no warning or error
          1 - warning
          2 - error
          3 - error preceded by warning

       The  pixels (samples) of the image have one to four channels whose com-
       ponents have original values in the range 0 to 1.0:

         1: A single gray or luminance channel (G).
         2: A gray/luminance channel and an alpha channel (GA).
         3: Three red, green, blue color channels (RGB).
         4: Three color channels and an alpha channel (RGBA).

       The channels are encoded in one of two ways:

         a) As a small integer, value 0..255, contained in a single byte.  For
       the  alpha  channel  the  original  value is simply value/255.  For the
       color or luminance channels the value is encoded according to the  sRGB
       specification  and matches the 8-bit format expected by typical display
       devices.

       The color/gray channels are not scaled (pre-multiplied)  by  the  alpha
       channel and are suitable for passing to color management software.

         b)  As  a value in the range 0..65535, contained in a 2-byte integer,
       in the native byte order of the platform on which  the  application  is
       running.  All channels can be converted to the original value by divid-
       ing by 65535; all channels are linear.   Color  channels  use  the  RGB
       encoding  (RGB end-points) of the sRGB specification.  This encoding is
       identified by the PNG_FORMAT_FLAG_LINEAR flag below.

       When an alpha channel is present it is expected to denote pixel  cover-
       age of the color or luminance channels and is returned as an associated
       alpha channel: the color/gray channels are scaled  (pre-multiplied)  by
       the alpha value.

       When   a   color-mapped   image   is   used  as  a  result  of  calling
       png_image_read_colormap or png_image_write_colormap  the  channels  are
       encoded in the color-map and the descriptions above apply to the color-
       map entries.  The image  data  is  encoded  as  small  integers,  value
       0..255,  that  index  the  entries  in the color-map.  One integer (one
       byte) is stored for each pixel.

       PNG_FORMAT_*

       The #defines to be used in png_image::format.  Each #define  identifies
       a  particular  layout  of  channel  data and, if present, alpha values.
       There are separate defines for each of the two channel encodings.

       A format is built up using single bit flag values.   Not  all  combina-
       tions  are  valid:  use  the bit flag values below for testing a format
       returned by the read APIs, but set formats from the derived values.

       When reading or writing color-mapped images the format should be set to
       the    format    of    the    entries    in    the    color-map    then
       png_image_{read,write}_colormap called to read or write  the  color-map
       and  set  the  format  correctly  for  the  image data.  Do not set the
       PNG_FORMAT_FLAG_COLORMAP bit directly!

       NOTE: libpng can be built with particular features disabled, if you see
       compiler  errors  because  the definition of one of the following flags
       has been compiled out it is because libpng does not have  the  required
       support.   It  is  possible,  however,  for the libpng configuration to
       enable the format on just read or just write; in that case you may  see
       an  error  at run time.  You can guard against this by checking for the
       definition of:

          PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED

          PNG_FORMAT_FLAG_ALPHA    0x01 format with an alpha channel
          PNG_FORMAT_FLAG_COLOR    0x02 color format: otherwise grayscale
          PNG_FORMAT_FLAG_LINEAR   0x04 png_uint_16 channels else png_byte
          PNG_FORMAT_FLAG_COLORMAP 0x08 libpng use only
          PNG_FORMAT_FLAG_BGR      0x10 BGR colors, else order is RGB
          PNG_FORMAT_FLAG_AFIRST   0x20 alpha channel comes first

       Supported formats are as follows.  Future versions of libpng  may  sup-
       port  more  formats; for compatibility with older versions simply check
       if the format macro is defined using #ifdef.   These  defines  describe
       the in-memory layout of the components of the pixels of the image.

       First the single byte formats:

          PNG_FORMAT_GRAY 0
          PNG_FORMAT_GA   PNG_FORMAT_FLAG_ALPHA
          PNG_FORMAT_AG   (PNG_FORMAT_GA|PNG_FORMAT_FLAG_AFIRST)
          PNG_FORMAT_RGB  PNG_FORMAT_FLAG_COLOR
          PNG_FORMAT_BGR  (PNG_FORMAT_FLAG_COLOR|PNG_FORMAT_FLAG_BGR)
          PNG_FORMAT_RGBA (PNG_FORMAT_RGB|PNG_FORMAT_FLAG_ALPHA)
          PNG_FORMAT_ARGB (PNG_FORMAT_RGBA|PNG_FORMAT_FLAG_AFIRST)
          PNG_FORMAT_BGRA (PNG_FORMAT_BGR|PNG_FORMAT_FLAG_ALPHA)
          PNG_FORMAT_ABGR (PNG_FORMAT_BGRA|PNG_FORMAT_FLAG_AFIRST)

       Then the linear 2-byte formats.  When naming these "Y" is used to indi-
       cate a luminance (gray) channel.  The component order within the  pixel
       is  always  the  same - there is no provision for swapping the order of
       the components in the linear format.  The components are  16-bit  inte-
       gers in the native byte order for your platform, and there is no provi-
       sion for swapping the bytes to a different endian condition.

          PNG_FORMAT_LINEAR_Y PNG_FORMAT_FLAG_LINEAR
          PNG_FORMAT_LINEAR_Y_ALPHA
             (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_ALPHA)
          PNG_FORMAT_LINEAR_RGB
             (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR)
          PNG_FORMAT_LINEAR_RGB_ALPHA
             (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR|
             PNG_FORMAT_FLAG_ALPHA)

       Color-mapped      formats      are      obtained       by       calling
       png_image_{read,write}_colormap,    as    appropriate   after   setting
       png_image::format to the format of the color-map to be read or written.
       Applications  may check the value of PNG_FORMAT_FLAG_COLORMAP to see if
       they have called the colormap API.  The format of the color-map may  be
       extracted using the following macro.

          PNG_FORMAT_OF_COLORMAP(fmt) ((fmt) & ~PNG_FORMAT_FLAG_COLORMAP)

       PNG_IMAGE macros

       These  are  convenience  macros  to derive information from a png_image
       structure.  The PNG_IMAGE_SAMPLE_ macros return values  appropriate  to
       the actual image sample values - either the entries in the color-map or
       the pixels in the image.  The  PNG_IMAGE_PIXEL_  macros  return  corre-
       sponding values for the pixels and will always return 1 after a call to
       png_image_{read,write}_colormap.  The remaining macros return  informa-
       tion about the rows in the image and the complete image.

       NOTE:  All  the macros that take a png_image::format parameter are com-
       pile time constants if the format parameter  is,  itself,  a  constant.
       Therefore  these  macros  can  be  used  in array declarations and case
       labels where required.  Similarly the  macros  are  also  pre-processor
       constants (sizeof is not used) so they can be used in #if tests.

       First the information about the samples.

         PNG_IMAGE_SAMPLE_CHANNELS(fmt)
           Returns the total number of channels in a given format: 1..4

         PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt)
           Returns  the  size  in  bytes  of  a single component of a pixel or
       color-map
           entry (as appropriate) in the image.

         PNG_IMAGE_SAMPLE_SIZE(fmt)
           This is the size of the sample data for one sample.  If  the  image
       is
           color-mapped  it is the size of one color-map entry (and image pix-
       els are
           one byte in size), otherwise it is the size of one image pixel.

         PNG_IMAGE_COLORMAP_SIZE(fmt)
          The size of the color-map required by the format; this is  the  size
       of the
          color-map buffer passed to the png_image_{read,write}_colormap APIs,
       it is
          a fixed number determined by the format so can easily  be  allocated
       on the
          stack if necessary.

       #define   PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt)    (PNG_IMAGE_SAM-
       PLE_CHANNELS(fmt) * 256)
          /* The  maximum  size  of  the  color-map  required  by  the  format
       expressed in a
           * count of components.  This can be used to compile-time allocate a
           * color-map:
           *
           *  png_uint_16  colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(lin-
       ear_fmt)];
           *
           *        png_byte        colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPO-
       NENTS(sRGB_fmt)];
           *
           * Alternatively, use the PNG_IMAGE_COLORMAP_SIZE macro below to use
       the
           * information from one of the png_image_begin_read_ APIs and dynam-
       ically
           * allocate the required memory.
           */


       Corresponding information about the pixels

         PNG_IMAGE_PIXEL_(test,fmt)

         PNG_IMAGE_PIXEL_CHANNELS(fmt)
          The number of separate channels (components) in a pixel; 1 for a
          color-mapped image.

         PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)   The size,  in  bytes,  of  each
       component in a pixel; 1 for a color-mapped
          image.

         PNG_IMAGE_PIXEL_SIZE(fmt)
          The size, in bytes, of a complete pixel; 1 for a color-mapped image.

       Information about the whole row, or whole image

         PNG_IMAGE_ROW_STRIDE(image)
          Returns the total number of components in a single row of the image;
       this
          is the minimum 'row stride', the minimum count of components between
       each
          row.  For a color-mapped image this is the minimum number  of  bytes
       in a
          row.

          If you need the stride measured in bytes, row_stride_bytes is
          PNG_IMAGE_ROW_STRIDE(image) * PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)
          plus any padding bytes that your application might need, for example
          to start the next row on a 4-byte boundary.

         PNG_IMAGE_BUFFER_SIZE(image, row_stride)
           Returns the size, in bytes, of an image buffer  given  a  png_image
       and a row
           stride  -  the number of components to leave space for in each row.
       This
           macro takes care of multiplying row_stride by PNG_IMAGE_PIXEL_COMO-
       NENT_SIZE
           when the image has 2-byte components.

         PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01
           This indicates the the RGB values of the in-memory bitmap do not
           correspond to the red, green and blue end-points defined by sRGB.

         PNG_IMAGE_FLAG_COLORMAP == 0x02
           The  PNG  is color-mapped.  If this flag is set png_image_read_col-
       ormap
           can be used without further loss of image information.   If  it  is
       not set
           png_image_read_colormap  will  cause  significant loss if the image
       has any

       READ APIs

          The png_image passed to the read APIs must have been initialized  by
       setting
          the  png_controlp  field  'opaque'  to  NULL (or, better, memset the
       whole thing.)

          int png_image_begin_read_from_file( png_imagep image,
            const char *file_name)

            The named file is opened for read and the image header
            is filled in from the PNG header in the file.

          int png_image_begin_read_from_stdio (png_imagep image,
            FILE* file)

             The PNG header is read from the stdio FILE object.

          int png_image_begin_read_from_memory(png_imagep image,
             png_const_voidp memory, png_size_t size)

             The PNG header is read from the given memory buffer.

          int png_image_finish_read(png_imagep image,
             png_colorp background, void *buffer,
             png_int_32 row_stride, void *colormap));

             Finish reading the image into the supplied buffer and
             clean up the png_image structure.

             row_stride is the step, in png_byte or png_uint_16 units
             as appropriate, between adjacent rows.  A positive stride
             indicates that the top-most row is first in the buffer -
             the normal top-down arrangement.  A negative stride
             indicates that the bottom-most row is first in the buffer.

             background need only be supplied if an alpha channel must
             be removed from a png_byte format and the removal is to be
             done by compositing on a solid color; otherwise it may be
             NULL and any composition will be done directly onto the
             buffer.  The value is an sRGB color to use for the
             background, for grayscale output the green channel is used.

             For linear output removing the alpha channel is always done
             by compositing on black.

          void png_image_free(png_imagep image)

             Free any data allocated by libpng in image->opaque,
             setting the pointer to NULL.  May be called at any time
             after the structure is initialized.

       When the simplified API needs to convert between sRGB and  linear  col-
       orspaces, the actual sRGB transfer curve defined in the sRGB specifica-
       tion (see the article at  http://en.wikipedia.org/wiki/SRGB)  is  used,
       not the gamma=1/2.2 approximation used elsewhere in libpng.

       WRITE APIS

       For  write  you  must  initialize a png_image structure to describe the
       image to be written:

          version: must be set to PNG_IMAGE_VERSION
          opaque: must be initialized to NULL
          width: image width in pixels
          height: image height in rows
          format: the format of the data you wish to write
          flags: set to 0 unless one of the defined flags applies; set
             PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images
             where the RGB values do not correspond to the colors in sRGB.
          colormap_entries: set to the number of entries in the  color-map  (0
       to 256)

          int png_image_write_to_file, (png_imagep image,
             const char *file, int convert_to_8bit, const void *buffer,
             png_int_32 row_stride, const void *colormap));

             Write the image to the named file.

          int png_image_write_to_stdio(png_imagep image, FILE *file,
             int convert_to_8_bit, const void *buffer,
             png_int_32 row_stride, const void *colormap)

             Write the image to the given (FILE*).

       With  all  write  APIs  if  image  is in one of the linear formats with
       (png_uint_16) data then setting convert_to_8_bit will cause the  output
       to  be  a (png_byte) PNG gamma encoded according to the sRGB specifica-
       tion, otherwise a 16-bit linear encoded PNG file is written.

       With all APIs row_stride is handled as in the read APIs  -  it  is  the
       spacing  from  one row to the next in component sized units (float) and
       if negative indicates a bottom-up row layout in the buffer.

       Note that the write API does not support interlacing, sub-8-bit pixels,
       and indexed (paletted) images.



VI. Modifying/Customizing libpng

       There are two issues here.  The first is changing how libpng does stan-
       dard things like memory allocation, input/output, and  error  handling.
       The  second  deals with more complicated things like adding new chunks,
       adding new transformations, and generally changing  how  libpng  works.
       Both  of  those  are  compile-time  issues; that is, they are generally
       determined at the time the code is written, and there is rarely a  need
       to provide the user with a means of changing them.

       Memory allocation, input/output, and error handling

       All  of  the  memory  allocation,  input/output,  and error handling in
       libpng goes through callbacks that are user-settable.  The default rou-
       tines  are  in  pngmem.c,  pngrio.c,  pngwio.c, and pngerror.c, respec-
       tively.  To change these functions, call the appropriate png_set_*_fn()
       function.

       Memory  allocation is done through the functions png_malloc(), png_cal-
       loc(), and png_free().  The png_malloc() and png_free() functions  cur-
       rently  just  call  the  standard  C  functions  and png_calloc() calls
       png_malloc() and then clears the newly allocated memory to  zero;  note
       that  png_calloc(png_ptr,  size)  is not the same as the calloc(number,
       size) function provided by stdlib.h.  There is limited support for cer-
       tain  systems  with  segmented  memory  architectures  and the types of
       pointers declared by png.h match this; you will have to use appropriate
       pointers  in your application.  If you prefer to use a different method
       of allocating and freeing data, you can use  png_create_read_struct_2()
       or  png_create_write_struct_2()  to  register  your  own  functions  as
       described above.  These functions also provide a void pointer that  can
       be retrieved via

           mem_ptr=png_get_mem_ptr(png_ptr);

       Your replacement memory functions must have prototypes as follows:

           png_voidp malloc_fn(png_structp png_ptr,
              png_alloc_size_t size);

           void free_fn(png_structp png_ptr, png_voidp ptr);

       Your malloc_fn() must return NULL in case of failure.  The png_malloc()
       function will normally call png_error() if it receives a NULL from  the
       system memory allocator or from your replacement malloc_fn().

       Your  free_fn()  will  never  be called with a NULL ptr, since libpng's
       png_free() checks for NULL before calling free_fn().

       Input/Output in libpng is  done  through  png_read()  and  png_write(),
       which  currently  just call fread() and fwrite().  The FILE * is stored
       in png_struct and is initialized via png_init_io().   If  you  wish  to
       change  the  method of I/O, the library supplies callbacks that you can
       set through the function png_set_read_fn()  and  png_set_write_fn()  at
       run  time,  instead of calling the png_init_io() function.  These func-
       tions also provide a void pointer that can be retrieved via  the  func-
       tion png_get_io_ptr().  For example:

           png_set_read_fn(png_structp read_ptr,
               voidp read_io_ptr, png_rw_ptr read_data_fn)

           png_set_write_fn(png_structp write_ptr,
               voidp write_io_ptr, png_rw_ptr write_data_fn,
               png_flush_ptr output_flush_fn);

           voidp read_io_ptr = png_get_io_ptr(read_ptr);
           voidp write_io_ptr = png_get_io_ptr(write_ptr);

       The replacement I/O functions must have prototypes as follows:

           void user_read_data(png_structp png_ptr,
               png_bytep data, png_size_t length);

           void user_write_data(png_structp png_ptr,
               png_bytep data, png_size_t length);

           void user_flush_data(png_structp png_ptr);

       The user_read_data() function is responsible for detecting and handling
       end-of-data errors.

       Supplying NULL for the read, write, or flush functions sets  them  back
       to  using  the  default  C stream functions, which expect the io_ptr to
       point to a standard *FILE structure.  It is probably a mistake  to  use
       NULL for one of write_data_fn and output_flush_fn but not both of them,
       unless you have built libpng with PNG_NO_WRITE_FLUSH defined.  It is an
       error to read from a write stream, and vice versa.

       Error handling in libpng is done through png_error() and png_warning().
       Errors handled through png_error() are fatal, meaning that  png_error()
       should  never  return  to  its  caller.  Currently, this is handled via
       setjmp()  and  longjmp()  (unless  you  have   compiled   libpng   with
       PNG_NO_SETJMP,  in  which  case it is handled via PNG_ABORT()), but you
       could change this to do things like exit() if you should wish, as  long
       as your function does not return.

       On  non-fatal  errors,  png_warning() is called to print a warning mes-
       sage, and then  control  returns  to  the  calling  code.   By  default
       png_error()  and  png_warning() print a message on stderr via fprintf()
       unless the library is compiled with PNG_NO_CONSOLE_IO defined  (because
       you don't want the messages) or PNG_NO_STDIO defined (because fprintf()
       isn't available).  If you wish to change  the  behavior  of  the  error
       functions,  you  will need to set up your own message callbacks.  These
       functions are normally supplied at the time that the png_struct is cre-
       ated.   It is also possible to redirect errors and warnings to your own
       replacement functions after png_create_*_struct() has  been  called  by
       calling:

           png_set_error_fn(png_structp png_ptr,
               png_voidp error_ptr, png_error_ptr error_fn,
               png_error_ptr warning_fn);

           png_voidp error_ptr = png_get_error_ptr(png_ptr);

       If  NULL is supplied for either error_fn or warning_fn, then the libpng
       default function will be used, calling fprintf() and/or longjmp() if  a
       problem  is  encountered.   The replacement error functions should have
       parameters as follows:

           void user_error_fn(png_structp png_ptr,
               png_const_charp error_msg);

           void user_warning_fn(png_structp png_ptr,
               png_const_charp warning_msg);

       The motivation behind using setjmp() and longjmp() is the C++ throw and
       catch  exception  handling methods.  This makes the code much easier to
       write, as there is no need to check every return code of every function
       call.   However, there are some uncertainties about the status of local
       variables after a longjmp, so the user may want  to  be  careful  about
       doing  anything after setjmp returns non-zero besides returning itself.
       Consult your compiler documentation for more details.  For an  alterna-
       tive  approach,  you  may  wish  to  use  the  "cexcept"  facility (see
       http://cexcept.sourceforge.net), which is illustrated in pngvalid.c and
       in contrib/visupng.

       Beginning  in  libpng-1.4.0,  the  png_set_benign_errors()  API  became
       available.  You can use this to handle certain errors (normally handled
       as errors) as warnings.

           png_set_benign_errors (png_ptr, int allowed);

           allowed: 0: treat png_benign_error() as an error.
                    1: treat png_benign_error() as a warning.

       As  of libpng-1.6.0, the default condition is to treat benign errors as
       warnings while reading and as errors while writing.


   Custom chunks
       If you need to read or write custom chunks, you may need to get  deeper
       into  the  libpng code.  The library now has mechanisms for storing and
       writing chunks of unknown type; you can even declare callbacks for cus-
       tom  chunks.   However, this may not be good enough if the library code
       itself needs to know about interactions between your chunk and existing
       `intrinsic' chunks.

       If you need to write a new intrinsic chunk, first read the PNG specifi-
       cation. Acquire a first level of understanding of how  it  works.   Pay
       particular  attention  to  the  sections that describe chunk names, and
       look at how other chunks were designed, so you can do things similarly.
       Second,  check  out  the sections of libpng that read and write chunks.
       Try to find a chunk that is similar to yours and use it as a  template.
       More  details can be found in the comments inside the code.  It is best
       to handle private or unknown chunks in a generic method,  via  callback
       functions,  instead  of  by  modifying libpng functions. This is illus-
       trated in pngtest.c, which uses a callback function to handle a private
       "vpAg"  chunk  and  the  new  "sTER"  chunk,  which are both unknown to
       libpng.

       If you wish to write your own transformation for the data, look through
       the  part of the code that does the transformations, and check out some
       of the simpler ones to get an idea of how they work.   Try  to  find  a
       similar  transformation  to the one you want to add and copy off of it.
       More details can be found in the comments inside the code itself.


   Configuring for gui/windowing platforms:
       You will need to write new error and warning functions that use the GUI
       interface,  as  described  previously, and set them to be the error and
       warning functions at the time that png_create_*_struct() is called,  in
       order to have them available during the structure initialization.  They
       can be changed later via png_set_error_fn().  On  some  compilers,  you
       may also have to change the memory allocators (png_malloc, etc.).


   Configuring zlib:
       There  are special functions to configure the compression.  Perhaps the
       most useful one changes the compression  level,  which  currently  uses
       input compression values in the range 0 - 9.  The library normally uses
       the default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests  have
       shown  that  for  a large majority of images, compression values in the
       range 3-6 compress nearly as well as higher  levels,  and  do  so  much
       faster.   For  online  applications it may be desirable to have maximum
       speed (Z_BEST_SPEED = 1).  With versions of zlib after v0.99,  you  can
       also specify no compression (Z_NO_COMPRESSION = 0), but this would cre-
       ate files larger than just storing the raw bitmap.  You can specify the
       compression level by calling:

           #include zlib.h
           png_set_compression_level(png_ptr, level);

       Another  useful  one is to reduce the memory level used by the library.
       The memory level defaults to 8, but it can be lowered if you are  short
       on  memory  (running DOS, for example, where you only have 640K).  Note
       that the memory level does have an effect on compression;  among  other
       things,  lower  levels  will  result in sections of incompressible data
       being emitted in smaller stored blocks, with a  correspondingly  larger
       relative overhead of up to 15% in the worst case.

           #include zlib.h
           png_set_compression_mem_level(png_ptr, level);

       The other functions are for configuring zlib.  They are not recommended
       for normal use and may result in writing  an  invalid  PNG  file.   See
       zlib.h for more information on what these mean.

           #include zlib.h
           png_set_compression_strategy(png_ptr,
               strategy);

           png_set_compression_window_bits(png_ptr,
               window_bits);

           png_set_compression_method(png_ptr, method);

       This controls the size of the IDAT chunks (default 8192):

           png_set_compression_buffer_size(png_ptr, size);

       As  of  libpng  version  1.5.4, additional APIs became available to set
       these separately for non-IDAT compressed chunks such as zTXt, iTXt, and
       iCCP:

           #include zlib.h
           #if PNG_LIBPNG_VER >= 10504
           png_set_text_compression_level(png_ptr, level);

           png_set_text_compression_mem_level(png_ptr, level);

           png_set_text_compression_strategy(png_ptr,
               strategy);

           png_set_text_compression_window_bits(png_ptr,
               window_bits);

           png_set_text_compression_method(png_ptr, method);
           #endif


   Controlling row filtering
       If you want to control whether libpng uses filtering or not, which fil-
       ters are used, and how it goes about picking row filters, you can  call
       one of these functions.  The selection and configuration of row filters
       can have a significant impact on the size  and  encoding  speed  and  a
       somewhat lesser impact on the decoding speed of an image.  Filtering is
       enabled by default for RGB  and  grayscale  images  (with  and  without
       alpha),  but not for paletted images nor for any images with bit depths
       less than 8 bits/pixel.

       The 'method' parameter sets the main filtering method,  which  is  cur-
       rently  only '0' in the PNG 1.2 specification.  The 'filters' parameter
       sets which filter(s), if any, should be used for each scanline.  Possi-
       ble  values are PNG_ALL_FILTERS and PNG_NO_FILTERS to turn filtering on
       and off, respectively.

       Individual filter types are PNG_FILTER_NONE,  PNG_FILTER_SUB,  PNG_FIL-
       TER_UP,  PNG_FILTER_AVG,  PNG_FILTER_PAETH,  which  can be bitwise ORed
       together with '|' to specify one or more filters to use.  These filters
       are  described  in more detail in the PNG specification.  If you intend
       to change the filter type during the course of writing the  image,  you
       should start with flags set for all of the filters you intend to use so
       that libpng can initialize its internal  structures  appropriately  for
       all  of  the  filter  types.   (Note that this means the first row must
       always be adaptively filtered, because libpng currently does not  allo-
       cate  the  filter buffers until png_write_row() is called for the first
       time.)

           filters = PNG_FILTER_NONE | PNG_FILTER_SUB
                     PNG_FILTER_UP | PNG_FILTER_AVG |
                     PNG_FILTER_PAETH | PNG_ALL_FILTERS;

           png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
              filters);
                     The second parameter can also be
                     PNG_INTRAPIXEL_DIFFERENCING if you are
                     writing a PNG to be embedded in a MNG
                     datastream.  This parameter must be the
                     same as the value of filter_method used
                     in png_set_IHDR().

       It is also possible to influence how  libpng  chooses  from  among  the
       available  filters.   This  is  done  in  one  or both of two ways - by
       telling it how important it is to keep the same filter  for  successive
       rows,  and  by  telling it the relative computational costs of the fil-
       ters.

           double weights[3] = {1.5, 1.3, 1.1},
              costs[PNG_FILTER_VALUE_LAST] =
              {1.0, 1.3, 1.3, 1.5, 1.7};

           png_set_filter_heuristics(png_ptr,
              PNG_FILTER_HEURISTIC_WEIGHTED, 3,
              weights, costs);

       The weights are multiplying factors that indicate to  libpng  that  the
       row  filter  should  be the same for successive rows unless another row
       filter is that many times better than  the  previous  filter.   In  the
       above  example,  if the previous 3 filters were SUB, SUB, NONE, the SUB
       filter could have a "sum of  absolute  differences"  1.5  x  1.3  times
       higher  than  other  filters and still be chosen, while the NONE filter
       could have a sum 1.1 times higher than other filters and still be  cho-
       sen.   Unspecified  weights  are  taken  to  be  1.0, and the specified
       weights should probably be declining  like  those  above  in  order  to
       emphasize recent filters over older filters.

       The  filter costs specify for each filter type a relative decoding cost
       to be considered when selecting row filters.  This means  that  filters
       with  higher costs are less likely to be chosen over filters with lower
       costs, unless their "sum of absolute differences" is that much smaller.
       The  costs do not necessarily reflect the exact computational speeds of
       the various filters, since this would unduly influence the final  image
       size.

       Note  that  the numbers above were invented purely for this example and
       are given only to help explain the function usage.  Little testing  has
       been done to find optimum values for either the costs or the weights.


   Requesting debug printout
       The  macro definition PNG_DEBUG can be used to request debugging print-
       out.  Set it to an integer value in the range 0 to 3.   Higher  numbers
       result in increasing amounts of debugging information.  The information
       is printed to the "stderr" file, unless another file name is  specified
       in the PNG_DEBUG_FILE macro definition.

       When PNG_DEBUG > 0, the following functions (macros) become available:

          png_debug(level, message)
          png_debug1(level, message, p1)
          png_debug2(level, message, p1, p2)

       in  which  "level"  is compared to PNG_DEBUG to decide whether to print
       the message, "message" is the formatted string to be  printed,  and  p1
       and  p2  are parameters that are to be embedded in the string according
       to printf-style formatting directives.  For example,

          png_debug1(2, "foo=%d", foo);

       is expanded to

          if (PNG_DEBUG > 2)
             fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);

       When PNG_DEBUG is defined but is zero, the macros aren't  defined,  but
       you can still use PNG_DEBUG to control your own debugging:

          #ifdef PNG_DEBUG
              fprintf(stderr, ...
          #endif

       When  PNG_DEBUG  = 1, the macros are defined, but only png_debug state-
       ments having level = 0 will be printed.  There aren't any  such  state-
       ments  in  this  version of libpng, but if you insert some they will be
       printed.



VII. MNG support

       The  MNG  specification  (available  at  http://www.libpng.org/pub/mng)
       allows  certain  extensions  to PNG for PNG images that are embedded in
       MNG datastreams.  Libpng can support  some  of  these  extensions.   To
       enable them, use the png_permit_mng_features() function:

          feature_set = png_permit_mng_features(png_ptr, mask)

          mask is a png_uint_32 containing the bitwise OR of the
               features you want to enable.  These include
               PNG_FLAG_MNG_EMPTY_PLTE
               PNG_FLAG_MNG_FILTER_64
               PNG_ALL_MNG_FEATURES

          feature_set is a png_uint_32 that is the bitwise AND of
             your mask with the set of MNG features that is
             supported by the version of libpng that you are using.

       It  is  an  error to use this function when reading or writing a stand-
       alone PNG file with the PNG 8-byte signature.  The PNG datastream  must
       be  wrapped  in  a  MNG datastream.  As a minimum, it must have the MNG
       8-byte signature and the MHDR and MEND chunks.  Libpng does not provide
       support  for  these or any other MNG chunks; your application must pro-
       vide its own support for them.  You may wish to consider  using  libmng
       (available at http://www.libmng.com) instead.



VIII. Changes to Libpng from version 0.88

       It should be noted that versions of libpng later than 0.96 are not dis-
       tributed by the original libpng author, Guy Schalnat,  nor  by  Andreas
       Dilger,  who had taken over from Guy during 1996 and 1997, and distrib-
       uted versions 0.89 through 0.96, but rather by another  member  of  the
       original  PNG  Group, Glenn Randers-Pehrson.  Guy and Andreas are still
       alive and well, but they have moved on to other things.

       The   old   libpng   functions    png_read_init(),    png_write_init(),
       png_info_init(),  png_read_destroy(), and png_write_destroy() have been
       moved to PNG_INTERNAL in version 0.95 to discourage their  use.   These
       functions will be removed from libpng version 1.4.0.

       The preferred method of creating and initializing the libpng structures
       is via  the  png_create_read_struct(),  png_create_write_struct(),  and
       png_create_info_struct()  because  they  isolate the size of the struc-
       tures from the application, allow  version  error  checking,  and  also
       allow  the use of custom error handling routines during the initializa-
       tion, which the old functions do not.  The functions png_read_destroy()
       and  png_write_destroy()  do  not  actually free the memory that libpng
       allocated for these structs, but just reset  the  data  structures,  so
       they   can   be   used   instead   of   png_destroy_read_struct()   and
       png_destroy_write_struct() if you feel there is too much  system  over-
       head allocating and freeing the png_struct for each image read.

       Setting   the   error   callbacks   via   png_set_message_fn()   before
       png_read_init() as was suggested in libpng-0.88 is no longer  supported
       because this caused applications that do not use custom error functions
       to fail if the png_ptr was not initialized to zero.  It is still possi-
       ble to set the error callbacks AFTER png_read_init(), or to change them
       with png_set_error_fn(), which is essentially the  same  function,  but
       with  a new name to force compilation errors with applications that try
       to use the old method.

       Support for the  sCAL,  iCCP,  iTXt,  and  sPLT  chunks  was  added  at
       libpng-1.0.6; however, iTXt support was not enabled by default.

       Starting  with  version  1.0.7,  you  can find out which version of the
       library you are using at run-time:

          png_uint_32 libpng_vn = png_access_version_number();

       The number libpng_vn is constructed from the major version, minor  ver-
       sion  with  leading  zero, and release number with leading zero, (e.g.,
       libpng_vn for version 1.0.7 is 10007).

       Note that this function does not take a png_ptr, so  you  can  call  it
       before you've created one.

       You  can also check which version of png.h you used when compiling your
       application:

          png_uint_32 application_vn = PNG_LIBPNG_VER;



IX. Changes to Libpng from version 1.0.x to 1.2.x

       Support for user memory management was enabled by default.   To  accom-
       plish   this,   the   functions   png_create_read_struct_2(),  png_cre-
       ate_write_struct_2(),  png_set_mem_fn(),  png_get_mem_ptr(),   png_mal-
       loc_default(), and png_free_default() were added.

       Support  for  the  iTXt chunk has been enabled by default as of version
       1.2.41.

       Support for certain MNG features was enabled.

       Support for numbered error messages was added.  However, we  never  got
       around   to  actually  numbering  the  error  messages.   The  function
       png_set_strip_error_numbers() was added (Note: the prototype  for  this
       function  was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
       builds of libpng-1.2.15.  It was restored in libpng-1.2.36).

       The png_malloc_warn() function was added at libpng-1.2.3.  This  issues
       a  png_warning  and  returns  NULL instead of aborting when it fails to
       acquire the requested memory allocation.

       Support for setting user limits on image width and height  was  enabled
       by       default.        The      functions      png_set_user_limits(),
       png_get_user_width_max(), and png_get_user_height_max() were  added  at
       libpng-1.2.6.

       The png_set_add_alpha() function was added at libpng-1.2.7.

       The    function    png_set_expand_gray_1_2_4_to_8()    was   added   at
       libpng-1.2.9.  Unlike png_set_gray_1_2_4_to_8(), the new function  does
       not expand the tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() func-
       tion is deprecated.

       A number of macro definitions in support of runtime selection of assem-
       bler  code  features  (especially Intel MMX code support) were added at
       libpng-1.2.0:

           PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
           PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
           PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
           PNG_ASM_FLAG_MMX_READ_INTERLACE
           PNG_ASM_FLAG_MMX_READ_FILTER_SUB
           PNG_ASM_FLAG_MMX_READ_FILTER_UP
           PNG_ASM_FLAG_MMX_READ_FILTER_AVG
           PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
           PNG_ASM_FLAGS_INITIALIZED
           PNG_MMX_READ_FLAGS
           PNG_MMX_FLAGS
           PNG_MMX_WRITE_FLAGS
           PNG_MMX_FLAGS

       We added the following functions in support  of  runtime  selection  of
       assembler code features:

           png_get_mmx_flagmask()
           png_set_mmx_thresholds()
           png_get_asm_flags()
           png_get_mmx_bitdepth_threshold()
           png_get_mmx_rowbytes_threshold()
           png_set_asm_flags()

       We  replaced all of these functions with simple stubs in libpng-1.2.20,
       when the Intel assembler code was removed due to a licensing issue.

       These macros are deprecated:

           PNG_READ_TRANSFORMS_NOT_SUPPORTED
           PNG_PROGRESSIVE_READ_NOT_SUPPORTED
           PNG_NO_SEQUENTIAL_READ_SUPPORTED
           PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
           PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
           PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED

       They have been replaced, respectively, by:

           PNG_NO_READ_TRANSFORMS
           PNG_NO_PROGRESSIVE_READ
           PNG_NO_SEQUENTIAL_READ
           PNG_NO_WRITE_TRANSFORMS
           PNG_NO_READ_ANCILLARY_CHUNKS
           PNG_NO_WRITE_ANCILLARY_CHUNKS

       PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.  It has been deprecated
       since libpng-1.0.16 and libpng-1.2.6.

       The function
           png_check_sig(sig, num) was replaced with
           !png_sig_cmp(sig, 0, num) It has been deprecated since libpng-0.90.

       The function
           png_set_gray_1_2_4_to_8() which also  expands  tRNS  to  alpha  was
       replaced with
           png_set_expand_gray_1_2_4_to_8() which does not. It has been depre-
       cated since libpng-1.0.18 and 1.2.9.



X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x

       Private libpng prototypes and macro definitions were moved  from  png.h
       and pngconf.h into a new pngpriv.h header file.

       Functions      png_set_benign_errors(),     png_benign_error(),     and
       png_chunk_benign_error() were added.

       Support for setting the maximum amount of memory that  the  application
       will allocate for reading chunks was added, as a security measure.  The
       functions png_set_chunk_cache_max() and png_get_chunk_cache_max()  were
       added to the library.

       We implemented support for I/O states by adding png_ptr member io_state
       and  functions  png_get_io_chunk_name()   and   png_get_io_state()   in
       pngget.c

       We  added  PNG_TRANSFORM_GRAY_TO_RGB  to the available high-level input
       transforms.

       Checking for and reporting of errors in the IHDR chunk  is  more  thor-
       ough.

       Support for global arrays was removed, to improve thread safety.

       Some obsolete/deprecated macros and functions have been removed.

       Typecasted NULL definitions such as
          #define  png_voidp_NULL             (png_voidp)NULL were eliminated.
       If you used these in your application, just use NULL instead.

       The png_struct and info_struct members "trans" and "trans_values"  were
       changed to "trans_alpha" and "trans_color", respectively.

       The  obsolete,  unused pnggccrd.c and pngvcrd.c files and related make-
       files were removed.

       The PNG_1_0_X and PNG_1_2_X macros were eliminated.

       The PNG_LEGACY_SUPPORTED macro was eliminated.

       Many WIN32_WCE #ifdefs were removed.

       The   functions   png_read_init(info_ptr),    png_write_init(info_ptr),
       png_info_init(info_ptr),  png_read_destroy(),  and  png_write_destroy()
       have been removed.  They have been deprecated since libpng-0.95.

       The png_permit_empty_plte() was removed. It has been  deprecated  since
       libpng-1.0.9.  Use png_permit_mng_features() instead.

       We   removed   the   obsolete  stub  functions  png_get_mmx_flagmask(),
       png_set_mmx_thresholds(),     png_get_asm_flags(),     png_get_mmx_bit-
       depth_threshold(),                    png_get_mmx_rowbytes_threshold(),
       png_set_asm_flags(), and png_mmx_supported()

       We  removed  the  obsolete  png_check_sig(),  png_memcpy_check(),   and
       png_memset_check()  functions.   Instead  use !png_sig_cmp(), memcpy(),
       and memset(), respectively.

       The function png_set_gray_1_2_4_to_8() was removed. It has been  depre-
       cated  since  libpng-1.0.18  and  1.2.9,  when  it  was  replaced  with
       png_set_expand_gray_1_2_4_to_8()  because  the  former  function   also
       expanded any tRNS chunk to an alpha channel.

       Macros  for  png_get_uint_16,  png_get_uint_32, and png_get_int_32 were
       added and are used by default instead of the  corresponding  functions.
       Unfortunately, from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro
       (but  not  the  function)  incorrectly  returned  a   value   of   type
       png_uint_32.

       We changed the prototype for png_malloc() from
           png_malloc(png_structp png_ptr, png_uint_32 size) to
           png_malloc(png_structp png_ptr, png_alloc_size_t size)

       This  also  applies  to  the  prototype  for  the user replacement mal-
       loc_fn().

       The png_calloc() function  was  added  and  is  used  in  place  of  of
       "png_malloc();  memset();"  except  in the case in png_read_png() where
       the array consists of pointers; in this case a "for" loop is used after
       the png_malloc() to set the pointers to NULL, to give robust.  behavior
       in case the  application  runs  out  of  memory  part-way  through  the
       process.

       We  changed  the  prototypes  of  png_get_compression_buffer_size() and
       png_set_compression_buffer_size() to work with  png_size_t  instead  of
       png_uint_32.

       Support  for  numbered  error messages was removed by default, since we
       never got around to actually numbering the error messages. The function
       png_set_strip_error_numbers()  was removed from the library by default.

       The png_zalloc() and png_zfree() functions are no longer exported.  The
       png_zalloc()  function  no  longer  zeroes out the memory that it allo-
       cates.  Applications that called png_zalloc(png_ptr, number, size)  can
       call  png_calloc(png_ptr, number*size) instead, and can call png_free()
       instead of png_zfree().

       Support for dithering was disabled by default in libpng-1.4.0,  because
       it  has  not  been well tested and doesn't actually "dither".  The code
       was not removed, however, and could be enabled by building libpng  with
       PNG_READ_DITHER_SUPPORTED  defined.   In libpng-1.4.2, this support was
       re-enabled, but the function was renamed png_set_quantize() to  reflect
       more  accurately  what  it  actually  does.   At  the  same  time,  the
       PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to  PNG_QUAN-
       TIZE_[RED,GREEN,BLUE]_BITS,  and  PNG_READ_DITHER_SUPPORTED was renamed
       to PNG_READ_QUANTIZE_SUPPORTED.

       We removed the trailing '.' from the warning and error messages.



XI. Changes to Libpng from version 1.4.x to 1.5.x

       From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but  not  the
       function) incorrectly returned a value of type png_uint_32.  The incor-
       rect macro was removed from libpng-1.4.5.

       Checking for invalid palette index on write was added at libpng 1.5.10.
       If  a  pixel  contains  an invalid (out-of-range) index libpng issues a
       benign error.  This is enabled by default because this condition is  an
       error  according to the PNG specification, Clause 11.3.2, but the error
       can be ignored in each png_ptr with

          png_set_check_for_invalid_index(png_ptr, allowed);

             allowed  - one of
                        0: disable benign error (accept the
                           invalid data without warning).
                        1: enable benign error (treat the
                           invalid data as an error or a
                           warning).

       If the error is ignored, or if png_benign_error() treats it as a  warn-
       ing,  any invalid pixels are decoded as opaque black by the decoder and
       written as-is by the encoder.

       Retrieving the maximum palette index found was added at  libpng-1.5.15.
       This  statement  must  appear  after png_read_png() or png_read_image()
       while reading, and after  png_write_png()  or  png_write_image()  while
       writing.

          int max_palette = png_get_palette_max(png_ptr, info_ptr);

       This  will return the maximum palette index found in the image, or "-1"
       if the palette was not checked, or "0" if no palette was  found.   Note
       that  this  does  not  account  for any palette index used by ancillary
       chunks such as the bKGD chunk;  you  must  check  those  separately  to
       determine the maximum palette index actually used.

       There  are  no substantial API changes between the non-deprecated parts
       of the 1.4.5 API and the 1.5.0 API; however, the  ability  to  directly
       access  members  of  the main libpng control structures, png_struct and
       png_info, deprecated in earlier versions of libpng, has been completely
       removed from libpng 1.5.

       We  no  longer include zlib.h in png.h.  The include statement has been
       moved to pngstruct.h, where  it  is  not  accessible  by  applications.
       Applications that need access to information in zlib.h will need to add
       the '#include "zlib.h"' directive.  It does not matter whether this  is
       placed prior to or after the '"#include png.h"' directive.

       The png_sprintf(), png_strcpy(), and png_strncpy() macros are no longer
       used and were removed.

       We moved the png_strlen(), png_memcpy(), png_memset(), and png_memcmp()
       macros into a private header file (pngpriv.h) that is not accessible to
       applications.

       In png_get_iCCP, the type of "profile" was changed from  png_charpp  to
       png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep.

       There are changes of form in png.h, including new and changed macros to
       declare parts of the API.  Some API functions with arguments  that  are
       pointers  to  data not modified within the function have been corrected
       to declare these arguments with PNG_CONST.

       Much of the internal use of C macros to control the library  build  has
       also  changed and some of this is visible in the exported header files,
       in particular the use of macros to control data and API elements  visi-
       ble  during application compilation may require significant revision to
       application code.  (It is extremely  rare  for  an  application  to  do
       this.)

       Any program that compiled against libpng 1.4 and did not use deprecated
       features or access internal library structures should compile and  work
       against  libpng  1.5,  except  for  the  change  in  the  prototype for
       png_get_iCCP() and png_set_iCCP() API functions mentioned above.

       libpng 1.5.0 adds PNG_ PASS macros to help in the reading  and  writing
       of interlaced images.  The macros return the number of rows and columns
       in each pass and information that can be used to de-interlace  and  (if
       absolutely necessary) interlace an image.

       libpng  1.5.0  adds an API png_longjmp(png_ptr, value).  This API calls
       the application-provided png_longjmp_ptr on the internal, but  applica-
       tion  initialized,  longjmp buffer.  It is provided as a convenience to
       avoid the need to use the png_jmpbuf macro, which had  the  unnecessary
       side effect of resetting the internal png_longjmp_ptr value.

       libpng  1.5.0  includes a complete fixed point API.  By default this is
       present along with the corresponding floating point  API.   In  general
       the  fixed  point API is faster and smaller than the floating point one
       because the PNG file format used fixed point, not floating point.  This
       applies  even  if  the library uses floating point in internal calcula-
       tions.  A new macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether
       the library uses floating point arithmetic (the default) or fixed point
       arithmetic internally for performance  critical  calculations  such  as
       gamma  correction.   In  some cases, the gamma calculations may produce
       slightly  different  results.   This  has  changed   the   results   in
       png_rgb_to_gray  and in alpha composition (png_set_background for exam-
       ple). This applies even if the original image was already linear (gamma
       ==  1.0)  and,  therefore,  it is not necessary to linearize the image.
       This is because libpng has *not* been changed  to  optimize  that  case
       correctly, yet.

       Fixed  point support for the sCAL chunk comes with an important caveat;
       the sCAL specification uses a decimal encoding of floating point values
       and  the  accuracy of PNG fixed point values is insufficient for repre-
       sentation of these values. Consequently a "string" API  (png_get_sCAL_s
       and  png_set_sCAL_s) is the only reliable way of reading arbitrary sCAL
       chunks in the absence of either the  floating  point  API  or  internal
       floating point calculations.  Starting with libpng-1.5.0, both of these
       functions are present when PNG_sCAL_SUPPORTED  is  defined.   Prior  to
       libpng-1.5.0,  their  presence  also depended upon PNG_FIXED_POINT_SUP-
       PORTED  being  defined  and  PNG_FLOATING_POINT_SUPPORTED   not   being
       defined.

       Applications no longer need to include the optional distribution header
       file pngusr.h or define the  corresponding  macros  during  application
       build  in  order  to  see  the correct variant of the libpng API.  From
       1.5.0 application code  can  check  for  the  corresponding  _SUPPORTED
       macro:

       #ifdef PNG_INCH_CONVERSIONS_SUPPORTED
          /* code that uses the inch conversion APIs. */ #endif

       This  macro  will only be defined if the inch conversion functions have
       been compiled into libpng.  The full set of macros, and whether or  not
       support  has been compiled in, are available in the header file pnglib-
       conf.h.  This header file is specific to the libpng build.  Notice that
       prior to 1.5.0 the _SUPPORTED macros would always have the default def-
       inition unless reset by pngusr.h or by explicit settings  on  the  com-
       piler  command  line.   These settings may produce compiler warnings or
       errors in 1.5.0 because of macro redefinition.

       Applications can now choose whether to use these macros or to call  the
       corresponding    function    by    defining    PNG_USE_READ_MACROS   or
       PNG_NO_USE_READ_MACROS before including png.h.   Notice  that  this  is
       only  supported  from  1.5.0;  defining PNG_NO_USE_READ_MACROS prior to
       1.5.0 will lead to a link failure.

       Prior to libpng-1.5.4, the zlib compressor used the same set of parame-
       ters  when  compressing the IDAT data and textual data such as zTXt and
       iCCP.  In libpng-1.5.4 we reinitialized the zlib stream for  each  type
       of  data.   We  added  five  png_set_text_*() functions for setting the
       parameters to use with textual data.

       Prior to  libpng-1.5.4,  the  PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
       option  was  off  by default, and slightly inaccurate scaling occurred.
       This option can no longer be turned off, and the choice of accurate  or
       inaccurate  16-to-8 scaling is by using the new png_set_scale_16_to_8()
       API for accurate scaling or the  old  png_set_strip_16_to_8()  API  for
       simple   chopping.    In   libpng-1.5.4,   the   PNG_READ_16_TO_8_ACCU-
       RATE_SCALE_SUPPORTED macro became PNG_READ_SCALE_16_TO_8_SUPPORTED, and
       the  PNG_READ_16_TO_8 macro became PNG_READ_STRIP_16_TO_8_SUPPORTED, to
       enable the two png_set_*_16_to_8() functions separately.

       Prior to libpng-1.5.4, the png_set_user_limits() function could only be
       used  to  reduce  the  width  and  height  limits  from  the  value  of
       PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said
       that  it could be used to override them.  Now this function will reduce
       or increase the limits.

       Starting in libpng-1.5.10, the user limits can be set en masse with the
       configuration  option  PNG_SAFE_LIMITS_SUPPORTED.   If  this  option is
       enabled, a set of "safe" limits is applied in pngpriv.h.  These can  be
       overridden    by    application    calls    to   png_set_user_limits(),
       png_set_user_chunk_cache_max(), and/or  png_set_user_malloc_max()  that
       increase  or  decrease  the limits.  Also, in libpng-1.5.10 the default
       width and height limits were  increased  from  1,000,000  to  0x7ffffff
       (i.e., made unlimited).  Therefore, the limits are now
                                      default      safe
          png_user_width_max        0x7fffffff    1,000,000
          png_user_height_max       0x7fffffff    1,000,000
          png_user_chunk_cache_max  0 (unlimited)   128
          png_user_chunk_malloc_max 0 (unlimited) 8,000,000

       The  png_set_option()  function  (and  the  "options" member of the png
       struct) was added to libpng-1.5.15.

       The library now supports a complete fixed point implementation and  can
       thus  be  used  on  systems that have no floating point support or very
       limited or slow support.  Previously  gamma  correction,  an  essential
       part  of complete PNG support, required reasonably fast floating point.

       As part of this the choice of internal  implementation  has  been  made
       independent  of  the choice of fixed versus floating point APIs and all
       the missing fixed point APIs have been implemented.

       The exact mechanism used to control attributes  of  API  functions  has
       changed, as described in the INSTALL file.

       A new test program, pngvalid, is provided in addition to pngtest.  png-
       valid validates the arithmetic accuracy of the gamma correction  calcu-
       lations  and  includes  a  number of validations of the file format.  A
       subset of the full range of tests is run when "make check" is done  (in
       the  'configure'  build.)   pngvalid also allows total allocated memory
       usage to be evaluated and performs additional memory overwrite  valida-
       tion.

       Many changes to individual feature macros have been made. The following
       are the changes most likely to be noticed by library builders who  con-
       figure libpng:

       1) All feature macros now have consistent naming:

       #define  PNG_NO_feature  turns the feature off #define PNG_feature_SUP-
       PORTED turns the feature on

       pnglibconf.h contains one line for each feature macro which is either:

       #define PNG_feature_SUPPORTED

       if the feature is supported or:

       /*#undef PNG_feature_SUPPORTED*/

       if it is not.  Library code consistently  checks  for  the  'SUPPORTED'
       macro.   It does not, and libpng applications should not, check for the
       'NO' macro which will not normally be defined even if  the  feature  is
       not supported.  The 'NO' macros are only used internally for setting or
       not setting the corresponding 'SUPPORTED' macros.

       Compatibility with the old names is provided as follows:

       PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED

       And the following definitions disable the corresponding feature:

       PNG_SETJMP_NOT_SUPPORTED disables  SETJMP  PNG_READ_TRANSFORMS_NOT_SUP-
       PORTED  disables  READ_TRANSFORMS PNG_NO_READ_COMPOSITED_NODIV disables
       READ_COMPOSITE_NODIV    PNG_WRITE_TRANSFORMS_NOT_SUPPORTED     disables
       WRITE_TRANSFORMS    PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED    disables
       READ_ANCILLARY_CHUNKS PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables
       WRITE_ANCILLARY_CHUNKS

       Library builders should remove use of the above, inconsistent, names.

       2)  Warning  and error message formatting was previously conditional on
       the STDIO feature. The library has been changed to use  the  CONSOLE_IO
       feature  instead. This means that if CONSOLE_IO is disabled the library
       no longer  uses  the  printf(3)  functions,  even  though  the  default
       read/write implementations use (FILE) style stdio.h functions.

       3) Three feature macros now control the fixed/floating point decisions:

       PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs

       PNG_FIXED_POINT_SUPPORTED enables the fixed  point  APIs;  however,  in
       practice these are normally required internally anyway (because the PNG
       file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT
       merely stops the function from being exported.

       PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating
       point implementation or the fixed point one.  Typically the fixed point
       implementation is larger and slower than the floating point implementa-
       tion on a system that supports  floating  point;  however,  it  may  be
       faster  on  a  system which lacks floating point hardware and therefore
       uses a software emulation.

       4) Added  PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED.   This  allows  the
       functions  to  read  and  write  ints  to  be disabled independently of
       PNG_USE_READ_MACROS, which allows libpng to be built with the functions
       even though the default is to use the macros - this allows applications
       to choose at app buildtime whether or not  to  use  macros  (previously
       impossible because the functions weren't in the default build.)



XII. Changes to Libpng from version 1.5.x to 1.6.x

       A  "simplified  API"  has  been added (see documentation in png.h and a
       simple example in contrib/examples/pngtopng.c).  The new publicly visi-
       ble API includes the following:

          macros:
            PNG_FORMAT_*
            PNG_IMAGE_*
          structures:
            png_control
            png_image
          read functions
            png_image_begin_read_from_file()
            png_image_begin_read_from_stdio()
            png_image_begin_read_from_memory()
            png_image_finish_read()
            png_image_free()
          write functions
            png_image_write_to_file()
            png_image_write_to_stdio()

       Starting  with  libpng-1.6.0,  you  can  configure libpng to prefix all
       exported symbols, using the PNG_PREFIX macro.

       We no longer include string.h in png.h.  The include statement has been
       moved to pngpriv.h, where it is not accessible by applications.  Appli-
       cations that need  access  to  information  in  string.h  must  add  an
       '#include  <string.h>'  directive.   It does not matter whether this is
       placed prior to or after the '#include "png.h"' directive.

       The following API are now DEPRECATED:
          png_info_init_3()
          png_convert_to_rfc1123() which has been replaced
            with png_convert_to_rfc1123_buffer()
          png_malloc_default()
          png_free_default()
          png_reset_zstream()

       The following have been removed:
          png_get_io_chunk_name(), which has been replaced
            with png_get_io_chunk_type().  The new
            function returns a 32-bit integer instead of
            a string.
          The png_sizeof(), png_strlen(), png_memcpy(), png_memcmp(), and
            png_memset() macros are no longer used in the libpng sources and
            have been removed.  These  had  already  been  made  invisible  to
       applications
            (i.e.,  defined  in  the  private  pngpriv.h  header  file)  since
       libpng-1.5.0.

       The signatures of many exported functions were changed, such that
          png_structp became png_structrp or png_const_structrp
          png_infop became png_inforp or png_const_inforp where "rp" indicates
       a "restricted pointer".

       The support for FAR/far types has been eliminated and the definition of
       png_alloc_size_t is now controlled by a flag  so  that  'small  size_t'
       systems can select it if necessary.

       Error  detection  in  some  chunks has improved; in particular the iCCP
       chunk reader now does pretty complete validation of the  basic  format.
       Some bad profiles that were previously accepted are now accepted with a
       warning or rejected, depending upon  the  png_set_benign_errors()  set-
       ting,  in  particular  the  very old broken Microsoft/HP 3144-byte sRGB
       profile.  Starting with libpng-1.6.11, recognizing  and  checking  sRGB
       profiles can be avoided by means of

           #if             defined(PNG_SKIP_sRGB_CHECK_PROFILE)             &&
       defined(PNG_SET_OPTION_SUPPORTED)
              png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE,
                  PNG_OPTION_ON);
           #endif

       It's not a good idea to do this if you are using the "simplified  API",
       which needs to be able to recognize sRGB profiles conveyed via the iCCP
       chunk.

       The PNG spec requirement that only grayscale  profiles  may  appear  in
       images  with color type 0 or 4 and that even if the image only contains
       gray pixels, only RGB profiles may appear in images with color type  2,
       3,  or  6,  is  now  enforced.   The sRGB chunk is allowed to appear in
       images with any color type and is interpreted by  libpng  to  convey  a
       one-tracer-curve  gray  profile  or a three-tracer-curve RGB profile as
       appropriate.

       Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk  con-
       tained an empty language field or an empty translated keyword.  Both of
       these are allowed by the PNG specification, so these  warnings  are  no
       longer issued.

       The  library  now  issues an error if the application attempts to set a
       transform after it calls png_read_update_info() or if  it  attempts  to
       call  both png_read_update_info() and png_start_read_image() or to call
       either of them more than once.

       The default condition for benign_errors is now to treat  benign  errors
       as warnings while reading and as errors while writing.

       The  library now issues a warning if both background processing and RGB
       to gray are used when gamma correction happens. As with  previous  ver-
       sions of the library the results are numerically very incorrect in this
       case.

       There are some minor arithmetic changes  in  some  transforms  such  as
       png_set_background(),  that  might  be  detected  by certain regression
       tests.

       Unknown chunk handling has been improved internally,  without  any  API
       change.  This adds more correct option control of the unknown handling,
       corrects a pre-existing bug  where  the  per-chunk  'keep'  setting  is
       ignored,  and  makes  it possible to skip IDAT chunks in the sequential
       reader.

       The  machine-generated  configure  files  are  no  longer  included  in
       branches libpng16 and later of the GIT repository.  They continue to be
       included in the tarball releases, however.

       Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning  of  the
       IDAT  stream  to set the size of the sliding window for reading instead
       of using the default 32-kbyte sliding window size.  It  was  discovered
       that  there  are  hundreds of PNG files in the wild that have incorrect
       CMF bytes that caused zlib to issue the "invalid distance too far back"
       error  and reject the file.  Libpng-1.6.3 and later calculate their own
       safe CMF from the image dimensions, provide a  way  to  revert  to  the
       libpng-1.5.x  behavior  (ignoring  the  CMF  bytes and using a 32-kbyte
       sliding window), by using

           png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW,
               PNG_OPTION_ON);

       and provide a tool (contrib/tools/pngfix)  for  rewriting  a  PNG  file
       while optimizing the CMF bytes in its IDAT chunk correctly.

       Libpng-1.6.0  and  libpng-1.6.1 wrote uncompressed iTXt chunks with the
       wrong length, which resulted in PNG files that cannot  be  read  beyond
       the  bad  iTXt chunk.  This error was fixed in libpng-1.6.3, and a tool
       (called contrib/tools/png-fix-itxt) has been added to the  libpng  dis-
       tribution.



XIII. Detecting libpng

       The  png_get_io_ptr()  function has been present since libpng-0.88, has
       never changed, and is unaffected by conditional compilation macros.  It
       is the best choice for use in configure scripts for detecting the pres-
       ence of any libpng version since 0.88.  In an  autoconf  "configure.in"
       you could use

           AC_CHECK_LIB(png, png_get_io_ptr, ...



XV. Source code repository

       Since  about February 2009, version 1.2.34, libpng has been under "git"
       source  control.   The  git  repository  was  built  from  old  libpng-
       x.y.z.tar.gz  files going back to version 0.70.  You can access the git
       repository (read only) at

           git://git.code.sf.net/p/libpng/code

       or you can browse it with a web browser by selecting the "code"  button
       at

           https://sourceforge.net/projects/libpng

       Patches  can be sent to glennrp at users.sourceforge.net or to png-mng-
       implement at lists.sourceforge.net or you can upload them to the libpng
       bug tracker at

           http://libpng.sourceforge.net

       We  also  accept  patches  built from the tar or zip distributions, and
       simple verbal discriptions of bug fixes, reported either to the Source-
       Forge  bug  tracker,  to  the png-mng-implement at lists.sf.net mailing
       list, or directly to glennrp.



XV. Coding style

       Our  coding   style   is   similar   to   the   "Allman"   style   (See
       http://en.wikipedia.org/wiki/Indent_style#Allman_style),   with   curly
       braces on separate lines:

           if (condition)
           {
              action;
           }

           else if (another condition)
           {
              another action;
           }

       The braces can be omitted from simple one-line actions:

           if (condition)
              return (0);

       We use 3-space indentation, except for continued statements  which  are
       usually  indented the same as the first line of the statement plus four
       more spaces.

       For macro definitions we use 2-space indentation,  always  leaving  the
       "#" in the first column.

           #ifndef PNG_NO_FEATURE
           #  ifndef PNG_FEATURE_SUPPORTED
           #    define PNG_FEATURE_SUPPORTED
           #  endif
           #endif

       Comments  appear  with  the leading "/*" at the same indentation as the
       statement that follows the comment:

           /* Single-line comment */
           statement;

           /* This is a multiple-line
            * comment.
            */
           statement;

       Very short comments can be placed after the end  of  the  statement  to
       which they pertain:

           statement;    /* comment */

       We  don't use C++ style ("//") comments. We have, however, used them in
       the past in some now-abandoned MMX assembler code.

       Functions and their curly braces are not indented, and  exported  func-
       tions are marked with PNGAPI:

        /* This is a public function that is visible to
         * application programmers. It does thus-and-so.
         */
        void PNGAPI
        png_exported_function(png_ptr, png_info, foo)
        {
           body;
        }

       The  return type and decorations are placed on a separate line ahead of
       the function name, as illustrated above.

       The prototypes for all exported functions appear in  png.h,  above  the
       comment that says

           /* Maintainer: Put new public prototypes here ... */

       We mark all non-exported functions with "/* PRIVATE */"":

        void /* PRIVATE */
        png_non_exported_function(png_ptr, png_info, foo)
        {
           body;
        }

       The prototypes for non-exported functions (except for those in pngtest)
       appear in pngpriv.h above the comment that says

         /* Maintainer: Put new private prototypes here ^ */

       To avoid polluting the global namespace,  the  names  of  all  exported
       functions  and  variables begin with "png_", and all publicly visible C
       preprocessor macros begin with "PNG".   We  request  that  applications
       that  use  libpng  *not*  begin any of their own symbols with either of
       these strings.

       We put a space after the "sizeof" operator and  we  omit  the  optional
       parentheses around its argument when the argument is an expression, not
       a type name, and we always enclose the sizeof operator, with its  argu-
       ment, in parentheses:

         (sizeof (png_uint_32))
         (sizeof array)

       Prior  to  libpng-1.6.0  we  used  a "png_sizeof()" macro, formatted as
       though it were a function.

       Control keywords if, for, while, and switch are always  followed  by  a
       space  to  distinguish them from function calls, which have no trailing
       space.

       We put a space after each comma  and  after  each  semicolon  in  "for"
       statements,  and  we put spaces before and after each C binary operator
       and after "for" or "while", and before  "?".   We  don't  put  a  space
       between  a  typecast  and  the expression being cast, nor do we put one
       between a function name and the left parenthesis that follows it:

           for (i = 2; i > 0; --i)
              y[i] = a(x) + (int)b;

       We prefer #ifdef and #ifndef to #if defined() and #if  !defined()  when
       there  is  only one macro being tested.  We always use parentheses with
       "defined".

       We prefer to express integers that are used as bit masks in hex format,
       with  an  even  number  of  lower-case  hex  digits  (e.g., 0x00, 0xff,
       0x0100).

       We prefer to use underscores in variable names rather  than  camelCase,
       except for a few type names that we inherit from zlib.h.

       We  prefer  "if  (something  !=  0)" and "if (something == 0)" over "if
       (something)" and if "(!something)", respectively.

       We do not use the TAB character for indentation in the C sources.

       Lines do not exceed 80 characters.

       Other rules can be inferred by inspecting the libpng source.



XVI. Y2K Compliance in libpng

       March 26, 2015

       Since the PNG Development group is an ad-hoc body,  we  can't  make  an
       official declaration.

       This  is  your  unofficial  assurance that libpng from version 0.71 and
       upward through 1.6.17 are Y2K compliant.  It is my belief that  earlier
       versions were also Y2K compliant.

       Libpng only has two year fields.  One is a 2-byte unsigned integer that
       will hold years up to 65535.  The other, which is deprecated, holds the
       date in text format, and will hold years up to 9999.

       The integer is
           "png_uint_16 year" in png_time_struct.

       The string is
           "char  time_buffer[29]"  in  png_struct.  This is no longer used in
       libpng-1.6.x and will be removed from libpng-1.7.0.

       There are seven time-related functions:

           png_convert_to_rfc_1123_buffer() in png.c
             (formerly png_convert_to_rfc_1152() in error, and
             also formerly png_convert_to_rfc_1123())
           png_convert_from_struct_tm() in pngwrite.c, called
             in pngwrite.c
           png_convert_from_time_t() in pngwrite.c
           png_get_tIME() in pngget.c
           png_handle_tIME() in pngrutil.c, called in pngread.c
           png_set_tIME() in pngset.c
           png_write_tIME() in pngwutil.c, called in pngwrite.c

       All appear to handle dates properly in a Y2K environment.  The png_con-
       vert_from_time_t() function calls gmtime() to convert from system clock
       time, which returns (year - 1900), which we  properly  convert  to  the
       full  4-digit  year.   There  is  a possibility that applications using
       libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
       function,  or  that  they  are  incorrectly passing only a 2-digit year
       instead of "year - 1900" into  the  png_convert_from_struct_tm()  func-
       tion,  but this is not under our control.  The libpng documentation has
       always stated that it works with 4-digit years, and the APIs have  been
       documented as such.

       The tIME chunk itself is also Y2K compliant.  It uses a 2-byte unsigned
       integer to hold the year, and can hold years as large as 65535.

       zlib, upon which libpng depends, is also Y2K compliant.  It contains no
       date-related code.


          Glenn Randers-Pehrson
          libpng maintainer
          PNG Development Group



NOTE

       Note about libpng version numbers:

       Due to various miscommunications, unforeseen code incompatibilities and
       occasional factors outside the authors' control, version  numbering  on
       the  library  has  not always been consistent and straightforward.  The
       following table summarizes matters since version 0.89c, which  was  the
       first widely used release:

        source             png.h  png.h  shared-lib
        version            string   int  version
        -------            ------  ----- ----------
        0.89c ("beta 3")  0.89       89  1.0.89
        0.90  ("beta 4")  0.90       90  0.90
        0.95  ("beta 5")  0.95       95  0.95
        0.96  ("beta 6")  0.96       96  0.96
        0.97b ("beta 7")  1.00.97    97  1.0.1
        0.97c             0.97       97  2.0.97
        0.98              0.98       98  2.0.98
        0.99              0.99       98  2.0.99
        0.99a-m           0.99       99  2.0.99
        1.00              1.00      100  2.1.0
        1.0.0             1.0.0     100  2.1.0
        1.0.0   (from here on, the  100  2.1.0
        1.0.1    png.h string is  10001  2.1.0
        1.0.1a-e identical to the 10002  from here on, the
        1.0.2    source version)  10002  shared library is 2.V
        1.0.2a-b                  10003  where V is the source
        1.0.1                     10001  code version except as
        1.0.1a-e                  10002  2.1.0.1a-e   noted.
        1.0.2                     10002  2.1.0.2
        1.0.2a-b                  10003  2.1.0.2a-b
        1.0.3                     10003  2.1.0.3
        1.0.3a-d                  10004  2.1.0.3a-d
        1.0.4                     10004  2.1.0.4
        1.0.4a-f                  10005  2.1.0.4a-f
        1.0.5 (+ 2 patches)       10005  2.1.0.5
        1.0.5a-d                  10006  2.1.0.5a-d
        1.0.5e-r                  10100  2.1.0.5e-r
        1.0.5s-v                  10006  2.1.0.5s-v
        1.0.6 (+ 3 patches)       10006  2.1.0.6
        1.0.6d-g                  10007  2.1.0.6d-g
        1.0.6h                    10007  10.6h
        1.0.6i                    10007  10.6i
        1.0.6j                    10007  2.1.0.6j
        1.0.7beta11-14    DLLNUM  10007  2.1.0.7beta11-14
        1.0.7beta15-18       1    10007  2.1.0.7beta15-18
        1.0.7rc1-2           1    10007  2.1.0.7rc1-2
        1.0.7                1    10007  2.1.0.7
        1.0.8beta1-4         1    10008  2.1.0.8beta1-4
        1.0.8rc1             1    10008  2.1.0.8rc1
        1.0.8                1    10008  2.1.0.8
        1.0.9beta1-6         1    10009  2.1.0.9beta1-6
        1.0.9rc1             1    10009  2.1.0.9rc1
        1.0.9beta7-10        1    10009  2.1.0.9beta7-10
        1.0.9rc2             1    10009  2.1.0.9rc2
        1.0.9                1    10009  2.1.0.9
        1.0.10beta1          1    10010  2.1.0.10beta1
        1.0.10rc1            1    10010  2.1.0.10rc1
        1.0.10               1    10010  2.1.0.10
        1.0.11beta1-3        1    10011  2.1.0.11beta1-3
        1.0.11rc1            1    10011  2.1.0.11rc1
        1.0.11               1    10011  2.1.0.11
        1.0.12beta1-2        2    10012  2.1.0.12beta1-2
        1.0.12rc1            2    10012  2.1.0.12rc1
        1.0.12               2    10012  2.1.0.12
        1.1.0a-f             -    10100  2.1.1.0a-f abandoned
        1.2.0beta1-2         2    10200  2.1.2.0beta1-2
        1.2.0beta3-5         3    10200  3.1.2.0beta3-5
        1.2.0rc1             3    10200  3.1.2.0rc1
        1.2.0                3    10200  3.1.2.0
        1.2.1beta-4          3    10201  3.1.2.1beta1-4
        1.2.1rc1-2           3    10201  3.1.2.1rc1-2
        1.2.1                3    10201  3.1.2.1
        1.2.2beta1-6        12    10202  12.so.0.1.2.2beta1-6
        1.0.13beta1         10    10013  10.so.0.1.0.13beta1
        1.0.13rc1           10    10013  10.so.0.1.0.13rc1
        1.2.2rc1            12    10202  12.so.0.1.2.2rc1
        1.0.13              10    10013  10.so.0.1.0.13
        1.2.2               12    10202  12.so.0.1.2.2
        1.2.3rc1-6          12    10203  12.so.0.1.2.3rc1-6
        1.2.3               12    10203  12.so.0.1.2.3
        1.2.4beta1-3        13    10204  12.so.0.1.2.4beta1-3
        1.2.4rc1            13    10204  12.so.0.1.2.4rc1
        1.0.14              10    10014  10.so.0.1.0.14
        1.2.4               13    10204  12.so.0.1.2.4
        1.2.5beta1-2        13    10205  12.so.0.1.2.5beta1-2
        1.0.15rc1           10    10015  10.so.0.1.0.15rc1
        1.0.15              10    10015  10.so.0.1.0.15
        1.2.5               13    10205  12.so.0.1.2.5
        1.2.6beta1-4        13    10206  12.so.0.1.2.6beta1-4
        1.2.6rc1-5          13    10206  12.so.0.1.2.6rc1-5
        1.0.16              10    10016  10.so.0.1.0.16
        1.2.6               13    10206  12.so.0.1.2.6
        1.2.7beta1-2        13    10207  12.so.0.1.2.7beta1-2
        1.0.17rc1           10    10017  12.so.0.1.0.17rc1
        1.2.7rc1            13    10207  12.so.0.1.2.7rc1
        1.0.17              10    10017  12.so.0.1.0.17
        1.2.7               13    10207  12.so.0.1.2.7
        1.2.8beta1-5        13    10208  12.so.0.1.2.8beta1-5
        1.0.18rc1-5         10    10018  12.so.0.1.0.18rc1-5
        1.2.8rc1-5          13    10208  12.so.0.1.2.8rc1-5
        1.0.18              10    10018  12.so.0.1.0.18
        1.2.8               13    10208  12.so.0.1.2.8
        1.2.9beta1-3        13    10209  12.so.0.1.2.9beta1-3
        1.2.9beta4-11       13    10209  12.so.0.9[.0]
        1.2.9rc1            13    10209  12.so.0.9[.0]
        1.2.9               13    10209  12.so.0.9[.0]
        1.2.10beta1-7       13    10210  12.so.0.10[.0]
        1.2.10rc1-2         13    10210  12.so.0.10[.0]
        1.2.10              13    10210  12.so.0.10[.0]
        1.4.0beta1-6        14    10400  14.so.0.0[.0]
        1.2.11beta1-4       13    10210  12.so.0.11[.0]
        1.4.0beta7-8        14    10400  14.so.0.0[.0]
        1.2.11              13    10211  12.so.0.11[.0]
        1.2.12              13    10212  12.so.0.12[.0]
        1.4.0beta9-14       14    10400  14.so.0.0[.0]
        1.2.13              13    10213  12.so.0.13[.0]
        1.4.0beta15-36      14    10400  14.so.0.0[.0]
        1.4.0beta37-87      14    10400  14.so.14.0[.0]
        1.4.0rc01           14    10400  14.so.14.0[.0]
        1.4.0beta88-109     14    10400  14.so.14.0[.0]
        1.4.0rc02-08        14    10400  14.so.14.0[.0]
        1.4.0               14    10400  14.so.14.0[.0]
        1.4.1beta01-03      14    10401  14.so.14.1[.0]
        1.4.1rc01           14    10401  14.so.14.1[.0]
        1.4.1beta04-12      14    10401  14.so.14.1[.0]
        1.4.1               14    10401  14.so.14.1[.0]
        1.4.2               14    10402  14.so.14.2[.0]
        1.4.3               14    10403  14.so.14.3[.0]
        1.4.4               14    10404  14.so.14.4[.0]
        1.5.0beta01-58      15    10500  15.so.15.0[.0]
        1.5.0rc01-07        15    10500  15.so.15.0[.0]
        1.5.0               15    10500  15.so.15.0[.0]
        1.5.1beta01-11      15    10501  15.so.15.1[.0]
        1.5.1rc01-02        15    10501  15.so.15.1[.0]
        1.5.1               15    10501  15.so.15.1[.0]
        1.5.2beta01-03      15    10502  15.so.15.2[.0]
        1.5.2rc01-03        15    10502  15.so.15.2[.0]
        1.5.2               15    10502  15.so.15.2[.0]
        1.5.3beta01-10      15    10503  15.so.15.3[.0]
        1.5.3rc01-02        15    10503  15.so.15.3[.0]
        1.5.3beta11         15    10503  15.so.15.3[.0]
        1.5.3 [omitted]
        1.5.4beta01-08      15    10504  15.so.15.4[.0]
        1.5.4rc01           15    10504  15.so.15.4[.0]
        1.5.4               15    10504  15.so.15.4[.0]
        1.5.5beta01-08      15    10505  15.so.15.5[.0]
        1.5.5rc01           15    10505  15.so.15.5[.0]
        1.5.5               15    10505  15.so.15.5[.0]
        1.5.6beta01-07      15    10506  15.so.15.6[.0]
        1.5.6rc01-03        15    10506  15.so.15.6[.0]
        1.5.6               15    10506  15.so.15.6[.0]
        1.5.7beta01-05      15    10507  15.so.15.7[.0]
        1.5.7rc01-03        15    10507  15.so.15.7[.0]
        1.5.7               15    10507  15.so.15.7[.0]
        1.6.0beta01-40      16    10600  16.so.16.0[.0]
        1.6.0rc01-08        16    10600  16.so.16.0[.0]
        1.6.0               16    10600  16.so.16.0[.0]
        1.6.1beta01-09      16    10601  16.so.16.1[.0]
        1.6.1rc01           16    10601  16.so.16.1[.0]
        1.6.1               16    10601  16.so.16.1[.0]
        1.6.2beta01         16    10602  16.so.16.2[.0]
        1.6.2rc01-06        16    10602  16.so.16.2[.0]
        1.6.2               16    10602  16.so.16.2[.0]
        1.6.3beta01-11      16    10603  16.so.16.3[.0]
        1.6.3rc01           16    10603  16.so.16.3[.0]
        1.6.3               16    10603  16.so.16.3[.0]
        1.6.4beta01-02      16    10604  16.so.16.4[.0]
        1.6.4rc01           16    10604  16.so.16.4[.0]
        1.6.4               16    10604  16.so.16.4[.0]
        1.6.5               16    10605  16.so.16.5[.0]
        1.6.6               16    10606  16.so.16.6[.0]
        1.6.7beta01-04      16    10607  16.so.16.7[.0]
        1.6.7rc01-02        16    10607  16.so.16.7[.0]
        1.6.7               16    10607  16.so.16.7[.0]
        1.6.8beta01-02      16    10608  16.so.16.8[.0]
        1.6.8rc01-02        16    10608  16.so.16.8[.0]
        1.6.8               16    10608  16.so.16.8[.0]
        1.6.9beta01-04      16    10609  16.so.16.9[.0]
        1.6.9rc01-02        16    10609  16.so.16.9[.0]
        1.6.9               16    10609  16.so.16.9[.0]
        1.6.10beta01-03     16    10610  16.so.16.10[.0]
        1.6.10rc01-03       16    10610  16.so.16.10[.0]
        1.6.10              16    10610  16.so.16.10[.0]
        1.6.11beta01-06     16    10611  16.so.16.11[.0]
        1.6.11rc01-02       16    10611  16.so.16.11[.0]
        1.6.11              16    10611  16.so.16.11[.0]
        1.6.12rc01          16    10612  16.so.16.12[.0]
        1.6.12              16    10612  16.so.16.12[.0]
        1.6.13beta01-04     16    10613  16.so.16.13[.0]
        1.6.13rc01-02       16    10613  16.so.16.13[.0]
        1.6.13              16    10613  16.so.16.13[.0]
        1.6.14beta01-07     16    10614  16.so.16.14[.0]
        1.6.14rc01-02       16    10614  16.so.16.14[.0]
        1.6.14              16    10614  16.so.16.14[.0]
        1.6.15beta01-08     16    10615  16.so.16.15[.0]
        1.6.15rc01-03       16    10615  16.so.16.15[.0]
        1.6.15              16    10615  16.so.16.15[.0]
        1.6.16beta01-03     16    10616  16.so.16.16[.0]
        1.6.16rc01-02       16    10616  16.so.16.16[.0]
        1.6.16              16    10616  16.so.16.16[.0]
        1.6.17beta01-06     16    10617  16.so.16.17[.0]
        1.6.17rc01-06       16    10617  16.so.16.17[.0]
        1.6.17              16    10617  16.so.16.17[.0]

       Henceforth  the  source version will match the shared-library minor and
       patch numbers; the shared-library major version number will be used for
       changes   in   backward   compatibility,   as   it  is  intended.   The
       PNG_PNGLIB_VER macro, which is not used within libpng but is  available
       for  applications, is an unsigned integer of the form xyyzz correspond-
       ing to the source version x.y.z (leading zeros in y and z).  Beta  ver-
       sions  were  given  the  previous  public release number plus a letter,
       until version 1.0.6j; from then on they were given the upcoming  public
       release number plus "betaNN" or "rcN".



SEE ALSO

       png(5), libpngpf(3), zlib(3), deflate(5), and zlib(5)


       libpng:

              http://libpng.sourceforge.net   (follow   the  [DOWNLOAD]  link)
              http://www.libpng.org/pub/png


       zlib:

              (generally) at the same location as libpng or at
              ftp://ftp.info-zip.org/pub/infozip/zlib


       PNGspecification:RFC2083

              (generally) at the same location as libpng or at
              ftp://ds.internic.net/rfc/rfc2083.txt
              or (as a W3C Recommendation) at
              http://www.w3.org/TR/REC-png.html


       In the case of any inconsistency between the PNG specification and this
       library, the specification takes precedence.



AUTHORS

       This man page: Glenn Randers-Pehrson <glennrp at users.sourceforge.net>

       The contributing authors would like to thank all those who helped  with
       testing,  bug  fixes,  and  patience.  This wouldn't have been possible
       without all of you.

       Thanks to Frank J. T. Wojcik for helping with the documentation.

       Libpng version 1.6.17 - March 26, 2015: Initially created  in  1995  by
       Guy  Eric  Schalnat,  then  of  Group 42, Inc.  Currently maintained by
       Glenn Randers-Pehrson (glennrp at users.sourceforge.net).

       Supported by the PNG development group
       png-mng-implement at lists.sf.net (subscription  required;  visit  png-
       mng-implement  at  lists.sourceforge.net  (subscription required; visit
       https://lists.sourceforge.net/lists/listinfo/png-mng-implement to  sub-
       scribe).



COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:

       (This  copy of the libpng notices is provided for your convenience.  In
       case of any discrepancy between this copy and the notices in  the  file
       png.h  that  is  included  in the libpng distribution, the latter shall
       prevail.)

       If you modify libpng you may insert additional notices immediately fol-
       lowing this sentence.

       This code is released under the libpng license.

       libpng versions 1.2.6, August 15, 2004, through 1.6.17, March 26, 2015,
       are Copyright (c) 2004,2006-2015 Glenn Randers-Pehrson,  and  are  dis-
       tributed  according  to the same disclaimer and license as libpng-1.2.5
       with the following individual added to the list of Contributing Authors

          Cosmin Truta

       libpng  versions  1.0.7, July 1, 2000, through 1.2.5 - October 3, 2002,
       are Copyright (c) 2000-2002 Glenn Randers-Pehrson, and are  distributed
       according  to  the same disclaimer and license as libpng-1.0.6 with the
       following individuals added to the list of Contributing Authors

          Simon-Pierre Cadieux
          Eric S. Raymond
          Gilles Vollant

       and with the following additions to the disclaimer:

          There is no warranty against interference with your
          enjoyment of the library or against infringement.
          There is no warranty that our efforts or the library
          will fulfill any of your particular purposes or needs.
          This library is provided with all faults, and the entire
          risk of satisfactory quality, performance, accuracy, and
          effort is with the user.

       libpng versions 0.97, January 1998, through 1.0.6, March 20, 2000,  are
       Copyright (c) 1998, 1999 Glenn Randers-Pehrson Distributed according to
       the same disclaimer and license  as  libpng-0.96,  with  the  following
       individuals added to the list of Contributing Authors:

          Tom Lane
          Glenn Randers-Pehrson
          Willem van Schaik

       libpng  versions 0.89, June 1996, through 0.96, May 1997, are Copyright
       (c) 1996, 1997 Andreas Dilger Distributed according to  the  same  dis-
       claimer  and  license  as  libpng-0.88,  with the following individuals
       added to the list of Contributing Authors:

          John Bowler
          Kevin Bracey
          Sam Bushell
          Magnus Holmgren
          Greg Roelofs
          Tom Tanner

       libpng versions 0.5, May 1995, through 0.88, January  1996,  are  Copy-
       right (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.

       For  the purposes of this copyright and license, "Contributing Authors"
       is defined as the following set of individuals:

          Andreas Dilger
          Dave Martindale
          Guy Eric Schalnat
          Paul Schmidt
          Tim Wegner

       The PNG Reference  Library  is  supplied  "AS  IS".   The  Contributing
       Authors  and  Group  42,  Inc.  disclaim  all  warranties, expressed or
       implied, including, without limitation, the warranties of merchantabil-
       ity and of fitness for any purpose.  The Contributing Authors and Group
       42, Inc.  assume no liability for direct,  indirect,  incidental,  spe-
       cial,  exemplary,  or  consequential damages, which may result from the
       use of the PNG Reference Library, even if advised of the possibility of
       such damage.

       Permission  is hereby granted to use, copy, modify, and distribute this
       source code, or portions hereof, for any purpose, without fee,  subject
       to the following restrictions:

       1. The origin of this source code must not be misrepresented.

       2. Altered versions must be plainly marked as such and
          must not be misrepresented as being the original source.

       3. This Copyright notice may not be removed or altered from
          any source or altered source distribution.

       The  Contributing Authors and Group 42, Inc. specifically permit, with-
       out fee, and encourage the use of this source code as  a  component  to
       supporting the PNG file format in commercial products.  If you use this
       source code in a product, acknowledgment is not required but  would  be
       appreciated.


       A  "png_get_copyright"  function  is  available,  for convenient use in
       "about" boxes and the like:

          printf("%s",png_get_copyright(NULL));

       Also, the PNG logo (in PNG format, of course) is supplied in the  files
       "pngbar.png" and "pngbar.jpg (88x31) and "pngnow.png" (98x31).

       Libpng  is  OSI  Certified  Open  Source  Software.  OSI Certified Open
       Source is a certification mark of the Open Source Initiative.

       Glenn Randers-Pehrson glennrp at users.sourceforge.net March 26, 2015





                                March 26, 2015                       libpng(3)

libpng 1.6.17 - Generated Sun Apr 12 09:02:54 CDT 2015
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