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libpng(3)                                                            libpng(3)




NAME

       libpng - Portable Network Graphics (PNG) Reference Library 1.6.13


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 1.6.13 - August 21, 2014
        Updated and distributed by Glenn Randers-Pehrson
        <glennrp at users.sourceforge.net>
        Copyright (c) 1998-2014 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 1.6.13 - August 21, 2014
        Updated and distributed by Glenn Randers-Pehrson
        Copyright (c) 1998-2014 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_read()
       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);
           }

           fread(header, 1, number, fp);
           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.
       Since  very  few applications really need to process such large images,
       we have imposed an arbitrary  1-million  limit  on  rows  and  columns.
       Larger  images will be rejected immediately with a png_error() call. If
       you wish to change this limit, you can use

          png_set_user_limits(png_ptr, width_max, height_max);

       to set your own limits, or use width_max = height_max = 0x7fffffffL  to
       allow  all  valid  dimensions (libpng may reject some very large 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 or all of 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 16-bit
           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 then 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 then 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(png_ptr, 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".

       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 an sRGB profile  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 ^ */

       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.

       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  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

       August 21, 2014

       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 1.6.13 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() in png.c
             (formerly png_convert_to_rfc_1152() in error)
           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]

       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  1.6.13 - August 21, 2014: 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  1.6.13,  August  21,
       2014,  are  Copyright (c) 2004,2006-2014 Glenn Randers-Pehrson, and are
       distributed  according  to  the  same   disclaimer   and   license   as
       libpng-1.2.5  with  the  following individual added to the list of Con-
       tributing 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 August 21, 2014





                                August 21, 2014                      libpng(3)

libpng 1.6.13 - Generated Tue Sep 30 05:49:54 CDT 2014