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libpng(3)                                                            libpng(3)




NAME

       libpng - Portable Network Graphics (PNG) Reference Library 1.6.21


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);

       int  png_get_palette_max(png_const_structp   png_ptr,   png_const_infop
       info_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);

       int png_set_option(png_structrp png_ptr, int option, int onoff);

       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 1.6.21 - January 15, 2016
        Updated and distributed by Glenn Randers-Pehrson
        <glennrp at users.sourceforge.net>
        Copyright (c) 1998-2016 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 1.6.21 - January 15, 2016
        Updated and distributed by Glenn Randers-Pehrson
        Copyright (c) 1998-2016 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://png-mng.source-
       forge.net/pub/png/spec/1.2/>.   It is technically equivalent to the PNG
       specification (second edition) but has some additional material.

       The  PNG-1.0  specification  is  available  as  RFC  2083  <http://png-
       mng.sourceforge.net/pub/png/spec/1.0/>  and  as  a  W3C  Recommendation
       <http://www.w3.org/TR/REC-png-961001>.

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

       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://zlib.net/>.  The zlib  compression
       utility  is  a general purpose utility that is useful for more than PNG
       files, and can be used without libpng.  See the documentation delivered
       with  zlib for more details.  You can usually find the source files for
       the zlib utility wherever you find the libpng source files.

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



II. Structures

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

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

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

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

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

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

       #include <png.h>

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

       #include <zlib.h>


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

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

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

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


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

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

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

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

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

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

       CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC

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

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

       A. Changing pnglibconf.h

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

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

       B. Configuration using DFA_XTRA

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

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

       everything = off

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

       option feature on option feature off

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

       setting feature default value

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

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

       C. Configuration using PNG_USER_CONFIG

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

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

       #define PNG_feature_SUPPORTED

       is equivalent to:

       option feature on

       #define PNG_NO_feature

       is equivalent to:

       option feature off

       #define PNG_feature value

       is equivalent to:

       setting feature default value

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

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

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



III. Reading

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


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

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

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

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

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

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

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

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

           if (!png_ptr)
              return (ERROR);

           png_infop info_ptr = png_create_info_struct(png_ptr);

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

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

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

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

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

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

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

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

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

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

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

           png_init_io(png_ptr, fp);

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

           png_set_sig_bytes(png_ptr, number);

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

           png_set_compression_buffer_size(png_ptr, buffer_size);

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

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

           png_set_crc_action(png_ptr, crit_action, ancil_action);

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

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

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


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

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

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

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

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

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

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

       To inform libpng about your function, use

           png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
               read_chunk_callback);

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

           png_get_user_chunk_ptr(png_ptr);

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

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

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

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

       To inform libpng about your function, use

           png_set_read_status_fn(png_ptr, read_row_callback);

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

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


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

           png_set_keep_unknown_chunks(png_ptr, keep,
               chunk_list, num_chunks);

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

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

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

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

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

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

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

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

           ...

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

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

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


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

          png_set_user_limits(png_ptr, width_max, height_max);

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

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

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

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

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

       The PNG specification sets no limit on the number of  ancillary  chunks
       allowed  in  a PNG datastream.  By default, libpng imposes a limit of a
       total of 1000 sPLT, tEXt, iTXt, zTXt, and unknown chunks to be  stored.
       If  you  have  set up both info_ptr and end_info_ptr, the limit applies
       separately to each.  You can change the limit on the  total  number  of
       such 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);

       Libpng  imposes  a limit of 8 Megabytes (8,000,000 bytes) on the amount
       of memory that a compressed chunk other  than  IDAT  can  occupy,  when
       decompressed.  You can change this limit with

          png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);

       and you can retrieve the limit with

          chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);

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


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

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

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

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

          png_set_gamma(png_ptr, screen_gamma, output_gamma);

       or you can use the fixed point equivalent:

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       The mode is as follows:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

          png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
              screen_gamma);

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

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

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

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

          png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
              screen_gamma);

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

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

           png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);

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

           png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);

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

           png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);

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

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

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

           png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);

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

           Other cases

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

           png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);

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

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

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

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

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

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

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

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

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

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

       16-bit values:
          PNG_TRANSFORM_EXPAND_16
          png_set_expand_16(png_ptr);

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

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


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

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

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

           png_read_png(png_ptr, info_ptr, png_transforms, NULL)

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

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

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

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

          row_pointers = png_get_rows(png_ptr, info_ptr);

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

          png_bytep row_pointers[height];

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

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

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

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

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

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

          png_set_rows(png_ptr, info_ptr, &row_pointers);

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

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

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


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

           png_read_info(png_ptr, info_ptr);

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

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

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

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

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

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


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

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

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

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

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

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

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - (PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7)

           compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                            for PNG 1.0)

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

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

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

           width            = png_get_image_width(png_ptr,
                                info_ptr);

           height           = png_get_image_height(png_ptr,
                                info_ptr);

           bit_depth        = png_get_bit_depth(png_ptr,
                                info_ptr);

           color_type       = png_get_color_type(png_ptr,
                                info_ptr);

           interlace_type   = png_get_interlace_type(png_ptr,
                                info_ptr);

           compression_type = png_get_compression_type(png_ptr,
                                info_ptr);

           filter_method    = png_get_filter_type(png_ptr,
                                info_ptr);

           channels = png_get_channels(png_ptr, info_ptr);

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

           rowbytes = png_get_rowbytes(png_ptr, info_ptr);

           rowbytes       - number of bytes needed to hold a row

           signature = png_get_signature(png_ptr, info_ptr);

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

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

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

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

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

           num_palette    - number of entries in the palette

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

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

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

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

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

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

           png_get_sRGB(png_ptr, info_ptr, &srgb_intent);

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

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

           name             - The profile name.

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

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

           proflen          - length of profile data in bytes.

           png_get_sBIT(png_ptr, info_ptr, &sig_bit);

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

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

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

           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)

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

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

           hist           - histogram of palette (array of
                            png_uint_16)

           png_get_tIME(png_ptr, info_ptr, &mod_time);

           mod_time       - time image was last modified
                           (PNG_VALID_tIME)

           png_get_bKGD(png_ptr, info_ptr, &background);

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

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

           num_comments   - number of comments

           text_ptr       - array of png_text holding image
                            comments

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

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

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

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

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

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

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

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

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

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

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

           num_spalettes  - number of sPLT chunks read.

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

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

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

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

           unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

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

           res_x          - pixels/unit physical resolution in
                            x direction

           res_y          - pixels/unit physical resolution in
                            x direction

           unit_type      - PNG_RESOLUTION_UNKNOWN,
                            PNG_RESOLUTION_METER

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

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units

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

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

           unit        - physical scale units (an integer)

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

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

           num_unknown_chunks = png_get_unknown_chunks(png_ptr,
              info_ptr, &unknowns)

           unknowns          - array of png_unknown_chunk
                               structures holding unknown chunks

           unknowns[i].name  - name of unknown chunk

           unknowns[i].data  - data of unknown chunk

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

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

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

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

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

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

           res_x = png_get_x_pixels_per_meter(png_ptr,
              info_ptr)

           res_y = png_get_y_pixels_per_meter(png_ptr,
              info_ptr)

           res_x_and_y = png_get_pixels_per_meter(png_ptr,
              info_ptr)

           res_x = png_get_x_pixels_per_inch(png_ptr,
              info_ptr)

           res_y = png_get_y_pixels_per_inch(png_ptr,
              info_ptr)

           res_x_and_y = png_get_pixels_per_inch(png_ptr,
              info_ptr)

           aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
              info_ptr)

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

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

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

           x_offset = png_get_x_offset_microns(png_ptr, info_ptr);

           y_offset = png_get_y_offset_microns(png_ptr, info_ptr);

           x_offset = png_get_x_offset_inches(png_ptr, info_ptr);

           y_offset = png_get_y_offset_inches(png_ptr, info_ptr);

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

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

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

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


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

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

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

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

       Data  will  be  decoded into the supplied row buffers packed into bytes
       unless the library has been told to transform it into  another  format.
       For  example, 4 bit/pixel paletted or grayscale data will be returned 2
       pixels/byte with the leftmost pixel in the high-order bits of the byte,
       unless  png_set_packing()  is called.  8-bit RGB data will be stored in
       RGB RGB RGB format unless png_set_filler()  or  png_set_add_alpha()  is
       called to insert filler bytes, either before or after each RGB triplet.

       16-bit RGB data will be returned RRGGBB RRGGBB, with the most  signifi-
       cant byte of the color value first, unless png_set_scale_16() is called
       to transform it to regular RGB RGB  triplets,  or  png_set_filler()  or
       png_set_add alpha() is called to insert two 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-bit  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. When filling
       an 8-bit pixel, the least significant 8 bits of the number are used, if
       a  16-bit  number  is  supplied.   This  transformation does not affect
       images that already have full alpha channels.  To add an  opaque  alpha
       channel,  use  filler=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.  The
       png_set_add_alpha() 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, Copyright (c) 2006-11-28 Charles Poynton, in section 9:

       <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>

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

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

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

       Libpng uses an integer approximation:

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

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

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

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

           png_color_16 my_background;
           png_color_16p image_background;

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

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

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

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

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

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

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

          else
             png_set_gamma(png_ptr, screen_gamma, 0.45455);

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

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

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

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

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

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

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

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

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

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

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

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

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

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

           png_set_read_user_transform_fn(png_ptr,
               read_transform_fn);

       You must supply the function

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

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

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

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

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

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

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

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

           png_set_user_transform_info(png_ptr, user_ptr,
               user_depth, user_channels);

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

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

           voidp read_user_transform_ptr =
               png_get_user_transform_ptr(png_ptr);

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

           number_of_passes = png_set_interlace_handling(png_ptr);

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

           png_read_update_info(png_ptr, info_ptr);

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

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

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


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

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

          png_read_image(png_ptr, row_pointers);

       where row_pointers is:

          png_bytep row_pointers[height];

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

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

           png_read_rows(png_ptr, row_pointers, NULL,
               number_of_rows);

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

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

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

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

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

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

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

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

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

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

           png_read_rows(png_ptr, row_pointers, NULL,
               number_of_rows);

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

           png_read_rows(png_ptr, NULL, row_pointers,
               number_of_rows);

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

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

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

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

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

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

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

       These allow you to write the obvious loop:

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

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

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

                output_x += xStep;
             }

             ++input_y;
             output_y += yStep;
          }

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

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

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

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

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

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

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


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

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

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

           png_infop end_info = png_create_info_struct(png_ptr);

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

          png_read_end(png_ptr, end_info);

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

          png_read_end(png_ptr, (png_infop)NULL);

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

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

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

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

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

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

           png_free_data(png_ptr, info_ptr, mask, seq)

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

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

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

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

           png_data_freer(png_ptr, info_ptr, freer, mask)

           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

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

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

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

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

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

           png_set_invalid(png_ptr, info_ptr, mask);

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

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


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

       png_structp png_ptr; png_infop info_ptr;

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

           if (!png_ptr)
               return (ERROR);

           info_ptr = png_create_info_struct(png_ptr);

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

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

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

                 png_get_progressive_ptr(png_ptr);

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

           return 0;
        }

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

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

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

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

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

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

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

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

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

               png_progressive_combine_row(png_ptr, old_row,
                 new_row);

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

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

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

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





IV. Writing

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


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

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

           if (!fp)
              return (ERROR);

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

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

           if (!png_ptr)
              return (ERROR);

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

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

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

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

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

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

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

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

          png_set_check_for_invalid_index(png_ptr, 0);

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

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

           png_init_io(png_ptr, fp);

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

           png_set_sig_bytes(png_ptr, 8);

       to inform libpng that it should not write a signature.


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

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

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

       To inform libpng about your function, use

           png_set_write_status_fn(png_ptr, write_row_callback);

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

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

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


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

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

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

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

           #include zlib.h

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

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

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


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

       Some of the more important parts of the png_info are:

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

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

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

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

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

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7

           compression_type - (must be
                            PNG_COMPRESSION_TYPE_DEFAULT)

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

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

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

           png_set_PLTE(png_ptr, info_ptr, palette,
              num_palette);

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


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

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

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

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

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

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

           png_set_sRGB(png_ptr, info_ptr, srgb_intent);

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


           png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
              srgb_intent);

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

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

           name             - The profile name.

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

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

           proflen          - length of profile data in bytes.

           png_set_sBIT(png_ptr, info_ptr, sig_bit);

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

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

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

           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)

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

           png_set_hIST(png_ptr, info_ptr, hist);

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

           png_set_tIME(png_ptr, info_ptr, mod_time);

           mod_time       - time image was last modified
                            (PNG_VALID_tIME)

           png_set_bKGD(png_ptr, info_ptr, background);

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

           png_set_text(png_ptr, info_ptr, text_ptr, num_text);

           text_ptr       - array of png_text holding image
                            comments

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

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

           num_text       - number of comments

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

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

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

           offset_x  - positive offset from the left
                            edge of the screen

           offset_y  - positive offset from the top
                            edge of the screen

           unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

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

           res_x       - pixels/unit physical resolution
                         in x direction

           res_y       - pixels/unit physical resolution
                         in y direction

           unit_type   - PNG_RESOLUTION_UNKNOWN,
                         PNG_RESOLUTION_METER

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

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units

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

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

           unit        - physical scale units (an integer)

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

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

           png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
              num_unknowns)

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

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

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

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

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

       The keywords that are given in the PNG Specification are:

           Title            Short (one line) title or
                            caption for image

           Author           Name of image's creator

           Description      Description of image (possibly long)

           Copyright        Copyright notice

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

           Software         Software used to create the image

           Disclaimer       Legal disclaimer

           Warning          Warning of nature of content

           Source           Device used to create the image

           Comment          Miscellaneous comment; conversion
                            from other image format

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

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

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


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

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

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


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

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

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

           png_write_png(png_ptr, info_ptr, png_transforms, NULL)

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

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

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


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

           png_write_info(png_ptr, info_ptr);

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

           png_set_invert_alpha(png_ptr);

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

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

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

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

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

           png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

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

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

           png_set_packing(png_ptr);

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

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

           else
           {
              sig_bit.gray = true_bit_depth;
           }

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

           png_set_sBIT(png_ptr, info_ptr, &sig_bit);

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

           png_set_shift(png_ptr, &sig_bit);

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

           if (bit_depth > 8)
              png_set_swap(png_ptr);

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

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

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

           png_set_bgr(png_ptr);

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

           png_set_invert_mono(png_ptr);

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

           png_set_write_user_transform_fn(png_ptr,
              write_transform_fn);

       You must supply the function

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

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

          png_get_current_row_number(png_ptr);
          png_get_current_pass_number(png_ptr);

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

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

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

           png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

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

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

           voidp write_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

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

           png_write_flush(png_ptr);

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

           png_set_flush(png_ptr, nrows);

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


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

           png_write_image(png_ptr, row_pointers);

       where row_pointers is:

           png_byte *row_pointers[height];

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

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

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

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

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

           png_bytep row_pointer = row;

           png_write_row(png_ptr, row_pointer);

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

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

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

           number_of_passes = png_set_interlace_handling(png_ptr);

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

       Then write the complete image number_of_passes times.

           png_write_rows(png_ptr, row_pointers, number_of_rows);

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

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

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


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

           png_write_end(png_ptr, info_ptr);

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

           png_destroy_write_struct(&png_ptr, &info_ptr);

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

           png_free_data(png_ptr, info_ptr, mask, seq)

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

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

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

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

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

           png_data_freer(png_ptr, info_ptr, freer, mask)

           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

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

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

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

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

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

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

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



V. Simplified API

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

       To read a PNG file using the simplified API:

         1) Declare a 'png_image' structure (see below) on the stack, set the
            version field to PNG_IMAGE_VERSION and  the  'opaque'  pointer  to
       NULL
            (this is REQUIRED, your program may crash if you don't do it.)

         2) Call the appropriate png_image_begin_read... function.

         3) Set the png_image 'format' member to the required sample format.

         4) Allocate a buffer for the image and, if required, the color-map.

         5) Call png_image_finish_read to read the image and, if required, the
            color-map into your buffers.

       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.  The only caveat
       is that if you request a color-mapped image from a PNG  that  is  full-
       color  or  makes  complex use of an alpha channel the transformation is
       extremely lossy and the result may look terrible.

       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 samples.

         3) Call the appropriate png_image_write... function with a
            pointer to the image and, if necessary, the color-map 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 defines the in-memory format of an image
       that you need to write.  The "png_image" structure contains the follow-
       ing members:

          png_controlp opaque  Initialize to NULL, free with png_image_free
          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_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 "warning_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 the "warning_or_error" 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 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.

       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.

       The samples are either contained directly in the image data, between  1
       and  8  bytes  per  pixel  according  to the encoding, or are held in a
       color-map indexed by bytes in the image data.  In the case of a  color-
       map the color-map entries are individual samples, encoded as above, and
       the image data has one byte per pixel to  select  the  relevant  sample
       from the color-map.

       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 component encodings.

       A  format  is  built up using single bit flag values.  All combinations
       are valid.  Formats can be built up from the flag values or you can use
       one  of  the  predefined values below.  When testing formats always use
       the FORMAT_FLAG macros to test for individual features  -  future  ver-
       sions of the library may add new flags.

       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 the appropriate "_SUPPORTED" macro, one of:

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

          PNG_FORMAT_FLAG_ALPHA    format with an alpha channel
          PNG_FORMAT_FLAG_COLOR    color format: otherwise grayscale
          PNG_FORMAT_FLAG_LINEAR   2-byte channels else 1-byte
          PNG_FORMAT_FLAG_COLORMAP image data is color-mapped
          PNG_FORMAT_FLAG_BGR      BGR colors, else order is RGB
          PNG_FORMAT_FLAG_AFIRST   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 (sRGB) formats:

          PNG_FORMAT_GRAY
          PNG_FORMAT_GA
          PNG_FORMAT_AG
          PNG_FORMAT_RGB
          PNG_FORMAT_BGR
          PNG_FORMAT_RGBA
          PNG_FORMAT_ARGB
          PNG_FORMAT_BGRA
          PNG_FORMAT_ABGR

       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_LINEAR_Y_ALPHA
          PNG_FORMAT_LINEAR_RGB
          PNG_FORMAT_LINEAR_RGB_ALPHA

       With  color-mapped  formats  the image data is one byte for each pixel.
       The byte is an index into the color-map which is  formatted  as  above.
       To  obtain  a  color-mapped  format  it  is  sufficient just to add the
       PNG_FOMAT_FLAG_COLORMAP to one of the above definitions, or you can use
       one of the definitions below.

          PNG_FORMAT_RGB_COLORMAP
          PNG_FORMAT_BGR_COLORMAP
          PNG_FORMAT_RGBA_COLORMAP
          PNG_FORMAT_ARGB_COLORMAP
          PNG_FORMAT_BGRA_COLORMAP
          PNG_FORMAT_ABGR_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 for color-
       mapped formats.  The remaining macros return information 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.

         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: 1 or 2.

         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_MAXIMUM_COLORMAP_COMPONENTS(fmt)
           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_COMPONENTS(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  dynami-
       cally
           allocate the required memory.

         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.

       Corresponding information about the pixels

         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)
          Return 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.

         PNG_IMAGE_SIZE(image)
          Return  the  size,  in  bytes,  of  the image in memory given just a
       png_image;
          the row stride is the minimum stride required for the image.

         PNG_IMAGE_COLORMAP_SIZE(image)
          Return the size, in bytes, of the color-map of this image.   If  the
       image
          format  is not a color-map format this will return a size sufficient
       for
          256 entries in the given format; check PNG_FORMAT_FLAG_COLORMAP if
          you don't want to allocate a color-map in this case.

       PNG_IMAGE_FLAG_*

       Flags containing additional information about the image are held in the
       'flags' field of png_image.

         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_FAST == 0x02
          On  write  emphasise  speed over compression; the resultant PNG file
       will be
          larger but will be produced  significantly  faster,  particular  for
       large
          images.   Do  not  use this option for images which will be distrib-
       uted, only
          used it when producing intermediate files that will be read back in
          repeatedly.  For a typical 24-bit image the option will  double  the
       read
          speed  at  the cost of increasing the image size by 25%, however for
       many
          more compressible images the PNG file can be 10  times  larger  with
       only a
          slight speed gain.

         PNG_IMAGE_FLAG_16BIT_sRGB == 0x04
           On  read  if the image is a 16-bit per component image and there is
       no gAMA
           or sRGB chunk assume that the components are sRGB encoded.   Notice
       that
           images  output by the simplified API always have gamma information;
       setting
           this flag only affects the interpretation of 16-bit images from an
           external source.  It is recommended  that  the  application  expose
       this flag
           to  the user; the user can normally easily recognize the difference
       between
           linear and sRGB encoding.  This flag has no effect on write  -  the
       data
           passed to the write APIs must have the correct encoding (as defined
           above.)

           If the flag is not set (the default)  input  16-bit  per  component
       data is
           assumed to be linear.

           NOTE:  the  flag  can  only  be set after the png_image_begin_read_
       call,
           because that call initializes the 'flags' field.

       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.   If  you
       pass  zero, libpng will calculate the row_stride for you from the width
       and number of channels.

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



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().


   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 0x7fffffff
       (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, with option PNG_ARM_NEON.

       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".

       Dropped support for 16-bit platforms. The support for FAR/far types has
       been  eliminated  and  the  definition  of png_alloc_size_t is now con-
       trolled by a flag so that 'small size_t' systems can select it if  nec-
       essary.

       Error  detection  in  some  chunks has improved; in particular the iCCP
       chunk reader now does pretty complete validation of the  basic  format.
       Some bad profiles that were previously accepted are now accepted with a
       warning or rejected, depending upon  the  png_set_benign_errors()  set-
       ting,  in  particular  the  very old broken Microsoft/HP 3144-byte sRGB
       profile.  Starting with libpng-1.6.11, recognizing  and  checking  sRGB
       profiles can be avoided by means of

           #if             defined(PNG_SKIP_sRGB_CHECK_PROFILE)             &&
       defined(PNG_SET_OPTION_SUPPORTED)
              png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE,
                  PNG_OPTION_ON);
           #endif

       It's not a good idea to do this if you are using the "simplified  API",
       which needs to be able to recognize sRGB profiles conveyed via the iCCP
       chunk.

       The PNG spec requirement that only grayscale  profiles  may  appear  in
       images  with color type 0 or 4 and that even if the image only contains
       gray pixels, only RGB profiles may appear in images with color type  2,
       3,  or  6,  is  now  enforced.   The sRGB chunk is allowed to appear in
       images with any color type and is interpreted by  libpng  to  convey  a
       one-tracer-curve  gray  profile  or a three-tracer-curve RGB profile as
       appropriate.

       Libpng 1.5.x erroneously used /MD for Debug DLL builds; if you used the
       debug  builds  in your app and you changed your app to use /MD you will
       need to change it back to /MDd for libpng 1.6.x.

       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.

       Starting  with  libpng-1.6.17, the PNG_SAFE_LIMITS macro was eliminated
       and safe limits are used by default (users who need larger  limits  can
       still override them at compile time or run time, as described above).

       The new limits are
                                       default   spec limit
          png_user_width_max         1,000,000  2,147,483,647
          png_user_height_max        1,000,000  2,147,483,647
          png_user_chunk_cache_max         128  unlimited
          png_user_chunk_malloc_max  8,000,000  unlimited

       Starting with libpng-1.6.18, a PNG_RELEASE_BUILD macro was added, which
       allows library builders to control compilation for an installed  system
       (a  release  build).  It can be set for testing debug or beta builds to
       ensure that they will compile when the build type is switched to RC  or
       STABLE.  In essence this overrides the PNG_LIBPNG_BUILD_BASE_TYPE defi-
       nition which is not directly user controllable.

       Starting with libpng-1.6.19, attempting  to  set  an  over-length  PLTE
       chunk is an error. Previously this requirement of the PNG specification
       was not enforced, and the palette was always limited to 256 entries. An
       over-length PLTE chunk found in an input PNG is silently truncated.



XIII. Detecting libpng

       The  png_get_io_ptr()  function has been present since libpng-0.88, has
       never changed, and is unaffected by conditional compilation macros.  It
       is the best choice for use in configure scripts for detecting the pres-
       ence of any libpng version since 0.88.  In an  autoconf  "configure.in"
       you could use

           AC_CHECK_LIB(png, png_get_io_ptr, ...



XV. Source code repository

       Since  about February 2009, version 1.2.34, libpng has been under "git"
       source  control.   The  git  repository  was  built  from  old  libpng-
       x.y.z.tar.gz  files going back to version 0.70.  You can access the git
       repository (read only) at

           git://git.code.sf.net/p/libpng/code

       or you can browse it with a web browser by selecting the "code"  button
       at

           https://sourceforge.net/projects/libpng

       Patches  can be sent to glennrp at users.sourceforge.net or to png-mng-
       implement at lists.sourceforge.net or you can upload them to the libpng
       bug tracker at

           http://libpng.sourceforge.net

       We  also  accept  patches  built from the tar or zip distributions, and
       simple verbal discriptions of bug fixes, reported either to the Source-
       Forge  bug  tracker,  to  the png-mng-implement at lists.sf.net mailing
       list, or directly to glennrp.



XV. Coding style

       Our  coding   style   is   similar   to   the   "Allman"   style   (See
       http://en.wikipedia.org/wiki/Indent_style#Allman_style),   with   curly
       braces on separate lines:

           if (condition)
           {
              action;
           }

           else if (another condition)
           {
              another action;
           }

       The braces can be omitted from simple one-line actions:

           if (condition)
              return (0);

       We use 3-space indentation, except for continued statements  which  are
       usually  indented the same as the first line of the statement plus four
       more spaces.

       For macro definitions we use 2-space indentation,  always  leaving  the
       "#" in the first column.

           #ifndef PNG_NO_FEATURE
           #  ifndef PNG_FEATURE_SUPPORTED
           #    define PNG_FEATURE_SUPPORTED
           #  endif
           #endif

       Comments  appear  with  the leading "/*" at the same indentation as the
       statement that follows the comment:

           /* Single-line comment */
           statement;

           /* This is a multiple-line
            * comment.
            */
           statement;

       Very short comments can be placed after the end  of  the  statement  to
       which they pertain:

           statement;    /* comment */

       We  don't use C++ style ("//") comments. We have, however, used them in
       the past in some now-abandoned MMX assembler code.

       Functions and their curly braces are not indented, and  exported  func-
       tions are marked with PNGAPI:

        /* This is a public function that is visible to
         * application programmers. It does thus-and-so.
         */
        void PNGAPI
        png_exported_function(png_ptr, png_info, foo)
        {
           body;
        }

       The  return type and decorations are placed on a separate line ahead of
       the function name, as illustrated above.

       The prototypes for all exported functions appear in  png.h,  above  the
       comment that says

           /* Maintainer: Put new public prototypes here ... */

       We mark all non-exported functions with "/* PRIVATE */"":

        void /* PRIVATE */
        png_non_exported_function(png_ptr, png_info, foo)
        {
           body;
        }

       The prototypes for non-exported functions (except for those in pngtest)
       appear in pngpriv.h above the comment that says

         /* Maintainer: Put new private prototypes here ^ */

       To avoid polluting the global namespace,  the  names  of  all  exported
       functions  and  variables begin with "png_", and all publicly visible C
       preprocessor macros begin with "PNG".   We  request  that  applications
       that  use  libpng  *not*  begin any of their own symbols with either of
       these strings.

       We put a space after the "sizeof" operator and  we  omit  the  optional
       parentheses around its argument when the argument is an expression, not
       a type name, and we always enclose the sizeof operator, with its  argu-
       ment, in parentheses:

         (sizeof (png_uint_32))
         (sizeof array)

       Prior  to  libpng-1.6.0  we  used  a "png_sizeof()" macro, formatted as
       though it were a function.

       Control keywords if, for, while, and switch are always  followed  by  a
       space  to  distinguish them from function calls, which have no trailing
       space.

       We put a space after each comma  and  after  each  semicolon  in  "for"
       statements,  and  we put spaces before and after each C binary operator
       and after "for" or "while", and before  "?".   We  don't  put  a  space
       between  a  typecast  and  the expression being cast, nor do we put one
       between a function name and the left parenthesis that follows it:

           for (i = 2; i > 0; --i)
              y[i] = a(x) + (int)b;

       We prefer #ifdef and #ifndef to #if defined() and #if  !defined()  when
       there  is  only one macro being tested.  We always use parentheses with
       "defined".

       We express integer constants that are used as bit masks in hex  format,
       with an even number of lower-case hex digits, and to make them unsigned
       (e.g., 0x00U, 0xffU, 0x0100U) and long if they are greater than  0x7fff
       (e.g., 0xffffUL).

       We prefer to use underscores rather than camelCase in names, 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

       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 1.6.21 are Y2K compliant.  It is my belief that earlier
       versions were also Y2K compliant.

       Libpng only has two year fields.  One is a 2-byte unsigned integer that
       will hold years up to 65535.  The other, which is deprecated, holds the
       date in text format, and will hold years up to 9999.

       The integer is
           "png_uint_16 year" in png_time_struct.

       The string is
           "char time_buffer[29]" in png_struct.  This is no  longer  used  in
       libpng-1.6.x and will be removed from libpng-1.7.0.

       There are seven time-related functions:

           png_convert_to_rfc_1123_buffer() in png.c
             (formerly png_convert_to_rfc_1152() in error, and
             also formerly png_convert_to_rfc_1123())
           png_convert_from_struct_tm() in pngwrite.c, called
             in pngwrite.c
           png_convert_from_time_t() in pngwrite.c
           png_get_tIME() in pngget.c
           png_handle_tIME() in pngrutil.c, called in pngread.c
           png_set_tIME() in pngset.c
           png_write_tIME() in pngwutil.c, called in pngwrite.c

       All appear to handle dates properly in a Y2K environment.  The png_con-
       vert_from_time_t() function calls gmtime() to convert from system clock
       time,  which  returns  (year  - 1900), which we properly convert to the
       full 4-digit year.  There is  a  possibility  that  applications  using
       libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
       function, or that they are incorrectly  passing  only  a  2-digit  year
       instead  of  "year  - 1900" into the png_convert_from_struct_tm() func-
       tion, but this is not under our control.  The libpng documentation  has
       always  stated that it works with 4-digit years, and the APIs have been
       documented as such.

       The tIME chunk itself is also Y2K compliant.  It uses a 2-byte unsigned
       integer to hold the year, and can hold years as large as 65535.

       zlib, upon which libpng depends, is also Y2K compliant.  It contains no
       date-related code.


          Glenn Randers-Pehrson
          libpng maintainer
          PNG Development Group



NOTE

       Note about libpng version numbers:

       Due to various miscommunications, unforeseen code incompatibilities and
       occasional  factors  outside the authors' control, version numbering on
       the library has not always been consistent  and  straightforward.   The
       following  table  summarizes matters since version 0.89c, which was the
       first widely used release:

        source             png.h  png.h  shared-lib
        version            string   int  version
        -------            ------  ----- ----------
        0.89c "1.0 beta 3"     0.89      89  1.0.89
        0.90  "1.0 beta 4"     0.90      90  0.90  [should have been 2.0.90]
        0.95  "1.0 beta 5"     0.95      95  0.95  [should have been 2.0.95]
        0.96  "1.0 beta 6"     0.96      96  0.96  [should have been 2.0.96]
        0.97b "1.00.97 beta 7" 1.00.97   97  1.0.1 [should have been 2.0.97]
        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 [100 should be 10000]
        1.0.0      (from here on, the   100  2.1.0 [100 should be 10000]
        1.0.1       png.h string is   10001  2.1.0
        1.0.1a-e    identical to the  10002  from here on, the shared library
        1.0.2       source version)   10002  is 2.V where V is the source code
        1.0.2a-b                      10003  version, except as noted.
        1.0.3                         10003
        1.0.3a-d                      10004
        1.0.4                         10004
        1.0.4a-f                      10005
        1.0.5 (+ 2 patches)           10005
        1.0.5a-d                      10006
        1.0.5e-r                      10100 (not source compatible)
        1.0.5s-v                      10006 (not binary compatible)
        1.0.6 (+ 3 patches)           10006 (still binary incompatible)
        1.0.6d-f                      10007 (still binary incompatible)
        1.0.6g                        10007
        1.0.6h                        10007  10.6h (testing xy.z so-numbering)
        1.0.6i                        10007  10.6i
        1.0.6j                         10007   2.1.0.6j   (incompatible   with
       1.0.0)
        1.0.7beta11-14        DLLNUM  10007  2.1.0.7beta11-14 (binary compati-
       ble)
        1.0.7beta15-18           1    10007  2.1.0.7beta15-18 (binary compati-
       ble)
        1.0.7rc1-2               1    10007  2.1.0.7rc1-2 (binary compatible)
        1.0.7                    1    10007  (still compatible)
        ...
        1.0.19                  10    10019  10.so.0.19[.0]
        ...
        1.2.53                  13    10253  12.so.0.53[.0]
        ...
        1.5.23                  15    10523  15.so.15.23[.0]
        ...
        1.6.21                  16    10621  16.so.16.21[.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 "rcNN".



SEE ALSO

       libpngpf(3), png(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://ftp.rfc-editor.org:/in-notes/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 1.6.21 - January 15, 2016: 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).



NOTICES:

       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.

       COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:

       If you modify libpng you may insert additional notices immediately fol-
       lowing this sentence.

       This code is released under the libpng license.

       libpng  versions 1.0.7, July 1, 2000, through 1.6.21, January 15, 2016,
       are Copyright (c) 2000-2002, 2004, 2006-2016 Glenn Randers-Pehrson, are
       derived  from  libpng-1.0.6,  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
          Mans Rullgard
          Cosmin Truta
          Gilles Vollant
          James Yu

       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)  1998-2000  Glenn  Randers-Pehrson,  are  derived   from
       libpng-0.96,  and  are 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, are derived  from  libpng-0.88,  and  are
       distributed   according   to   the   same  disclaimer  and  license  as
       libpng-0.88, with the following individuals added to the list  of  Con-
       tributing 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.

       END OF COPYRIGHT NOTICE, DISCLAIMER, and LICENSE.

       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. OSI has
       not addressed the additional disclaimers inserted at version 1.0.7.

       Glenn Randers-Pehrson glennrp at users.sourceforge.net January 15, 2016





                               January 15, 2016                      libpng(3)

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