Provided by: libpng12-dev_1.2.46-3ubuntu4_amd64 bug

NAME

       libpng - Portable Network Graphics (PNG) Reference Library 1.2.46

SYNOPSIS


       #include <png.h>

       png_uint_32 png_access_version_number (void);

       int png_check_sig (png_bytep sig, int num);

       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);

       int png_debug(int level, png_const_charp message);

       int png_debug1(int level, png_const_charp message, p1);

       int png_debug2(int level, png_const_charp message, p1, p2);

       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_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_structp png_ptr, png_infop info_ptr);

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

       png_byte png_get_channels (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_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);

       png_uint_32  png_get_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);

       png_byte png_get_color_type (png_structp png_ptr, png_infop info_ptr);

       png_byte png_get_compression_type (png_structp png_ptr, png_infop info_ptr);

       png_byte png_get_copyright (png_structp png_ptr);

       png_voidp png_get_error_ptr (png_structp png_ptr);

       png_byte png_get_filter_type (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_gAMA (png_structp png_ptr, png_infop info_ptr, double *file_gamma);

       png_uint_32  png_get_gAMA_fixed  (png_structp  png_ptr,  png_infop  info_ptr,  png_uint_32
       *int_file_gamma);

       png_byte png_get_header_ver (png_structp png_ptr);

       png_byte png_get_header_version (png_structp png_ptr);

       png_uint_32 png_get_hIST (png_structp png_ptr, png_infop info_ptr, png_uint_16p *hist);

       png_uint_32 png_get_iCCP (png_structp png_ptr, png_infop info_ptr,  png_charpp  name,  int
       *compression_type, png_charpp 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 *filter_type);

       png_uint_32 png_get_image_height (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_image_width (png_structp png_ptr, png_infop info_ptr);

       #if !defined(PNG_1_0_X)

       png_int_32 png_get_int_32 (png_bytep buf);

       #endif

       png_byte png_get_interlace_type (png_structp png_ptr, png_infop info_ptr);

       png_voidp png_get_io_ptr (png_structp png_ptr);

       png_byte png_get_libpng_ver (png_structp png_ptr);

       png_voidp png_get_mem_ptr(png_structp png_ptr);

       png_uint_32  png_get_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32 *offset_x,
       png_uint_32 *offset_y, int *unit_type);

       png_uint_32 png_get_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);

       png_uint_32 png_get_pHYs (png_structp png_ptr,  png_infop  info_ptr,  png_uint_32  *res_x,
       png_uint_32 *res_y, int *unit_type);

       float png_get_pixel_aspect_ratio (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);

       png_voidp png_get_progressive_ptr (png_structp png_ptr);

       png_uint_32  png_get_PLTE  (png_structp  png_ptr, png_infop info_ptr, png_colorp *palette,
       int *num_palette);

       png_byte png_get_rgb_to_gray_status (png_structp png_ptr)

       png_uint_32 png_get_rowbytes (png_structp png_ptr, png_infop info_ptr);

       png_bytepp png_get_rows (png_structp png_ptr, png_infop info_ptr);

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

       png_bytep png_get_signature (png_structp png_ptr, png_infop info_ptr);

       png_uint_32  png_get_sPLT  (png_structp  png_ptr,   png_infop   info_ptr,   png_spalette_p
       *splt_ptr);

       png_uint_32 png_get_sRGB (png_structp png_ptr, png_infop info_ptr, int *intent);

       png_uint_32  png_get_text  (png_structp  png_ptr, png_infop info_ptr, png_textp *text_ptr,
       int *num_text);

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

       png_uint_32 png_get_tRNS (png_structp png_ptr, png_infop info_ptr, png_bytep  *trans,  int
       *num_trans, png_color_16p *trans_values);

       #if !defined(PNG_1_0_X)

       png_uint_16 png_get_uint_16 (png_bytep buf);

       png_uint_32 png_get_uint_31 (png_bytep buf);

       png_uint_32 png_get_uint_32 (png_bytep buf);

       #endif

       png_uint_32    png_get_unknown_chunks    (png_structp    png_ptr,    png_infop   info_ptr,
       png_unknown_chunkpp unknowns);

       png_voidp png_get_user_chunk_ptr (png_structp png_ptr);

       png_uint_32 png_get_user_height_max( png_structp png_ptr);

       png_voidp png_get_user_transform_ptr (png_structp png_ptr);

       png_uint_32 png_get_user_width_max (png_structp png_ptr);

       png_uint_32 png_get_valid (png_structp png_ptr, png_infop info_ptr, png_uint_32 flag);

       png_int_32 png_get_x_offset_microns (png_structp png_ptr, png_infop info_ptr);

       png_int_32 png_get_x_offset_pixels (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_x_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);

       png_int_32 png_get_y_offset_microns (png_structp png_ptr, png_infop info_ptr);

       png_int_32 png_get_y_offset_pixels (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_y_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_compression_buffer_size (png_structp png_ptr);

       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

       void png_init_io (png_structp png_ptr, FILE *fp);

       DEPRECATED: void png_info_init (png_infop info_ptr);

       DEPRECATED: void png_info_init_2 (png_infopp ptr_ptr, png_size_t png_info_struct_size);

       png_voidp png_malloc (png_structp png_ptr, png_uint_32 size);

       png_voidp png_malloc_default(png_structp png_ptr, png_uint_32 size);

       voidp png_memcpy (png_voidp s1, png_voidp s2, png_size_t size);

       png_voidp png_memcpy_check (png_structp png_ptr, png_voidp s1, png_voidp  s2,  png_uint_32
       size);

       voidp png_memset (png_voidp s1, int value, png_size_t size);

       png_voidp  png_memset_check  (png_structp  png_ptr,  png_voidp  s1, int value, png_uint_32
       size);

       DEPRECATED: void png_permit_empty_plte (png_structp png_ptr, int empty_plte_permitted);

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

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

       void png_read_destroy (png_structp png_ptr, png_infop info_ptr, png_infop end_info_ptr);

       void png_read_end (png_structp png_ptr, png_infop info_ptr);

       void png_read_image (png_structp png_ptr, png_bytepp image);

       DEPRECATED: void png_read_init (png_structp png_ptr);

       DEPRECATED: void  png_read_init_2  (png_structpp  ptr_ptr,  png_const_charp  user_png_ver,
       png_size_t png_struct_size, png_size_t png_info_size);

       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 transforms, png_voidp
       params);

       void png_read_row (png_structp png_ptr, png_bytep row, png_bytep display_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);

       #if !defined(PNG_1_0_X)

       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);

       #endif

       void   png_set_background   (png_structp   png_ptr,  png_color_16p  background_color,  int
       background_gamma_code, int need_expand, double background_gamma);

       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_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_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 window_bits);

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

       void  png_set_dither  (png_structp  png_ptr,  png_colorp  palette,  int  num_palette,  int
       maximum_colors, png_uint_16p histogram, int full_dither);

       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_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   heuristic_method,   int
       num_weights, png_doublep filter_weights, png_doublep 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_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_charp  name,  int
       compression_type, png_charp 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  interlace_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);

       void  png_set_mem_fn(png_structp  png_ptr,  png_voidp  mem_ptr,  png_malloc_ptr malloc_fn,
       png_free_ptr free_fn);

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

       void png_set_packing (png_structp png_ptr);

       void png_set_packswap (png_structp png_ptr);

       void png_set_palette_to_rgb(png_structp png_ptr);

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

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

       void   png_set_progressive_read_fn   (png_structp   png_ptr,   png_voidp  progressive_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_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_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_fixed_point red,
       png_fixed_point 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, png_charp unit, double width,
       double height);

       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 intent);

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

       void png_set_strip_16 (png_structp png_ptr);

       void png_set_strip_alpha (png_structp png_ptr);

       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_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,   int
       num_trans, png_color_16p trans_values);

       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_read_user_chunk_fn   (png_structp   png_ptr,   png_voidp   user_chunk_ptr,
       png_user_chunk_ptr read_user_chunk_fn);

       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_channels);

       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);

       void png_set_compression_buffer_size(png_structp png_ptr, png_uint_32 size);

       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_destroy (png_structp png_ptr);

       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);

       DEPRECATED: void png_write_init (png_structp png_ptr);

       DEPRECATED: void png_write_init_2  (png_structpp  ptr_ptr,  png_const_charp  user_png_ver,
       png_size_t png_struct_size, png_size_t png_info_size);

       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 transforms, 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);

       voidpf png_zalloc (voidpf png_ptr, uInt items, uInt size);

       void png_zfree (voidpf png_ptr, voidpf ptr);

DESCRIPTION

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

LIBPNG.TXT

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

        libpng version 1.2.46 - July 9, 2011
        Updated and distributed by Glenn Randers-Pehrson
        <glennrp at users.sourceforge.net>
        Copyright (c) 1998-2009 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 1.2.46 - July 9, 2011
        Updated and distributed by Glenn Randers-Pehrson
        Copyright (c) 1998-2009 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

I. Introduction

       This file describes how to use and modify the PNG reference library (known as libpng)  for
       your  own  use.   There are five sections to this file: introduction, structures, reading,
       writing, and modification and configuration  notes  for  various  special  platforms.   In
       addition  to this file, example.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 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    15948:2003   (E))   at
       <http://www.w3.org/TR/2003/REC-PNG-20031110/ The W3C  and  ISO  documents  have  identical
       technical content.

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

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

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

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

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

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

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

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

II. Structures

       There are two main structures that are important to libpng, png_struct and png_info.   The
       first, png_struct, is an internal structure that will not, for the most part, be used by a
       user except as the first variable passed to every libpng function call.

       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.  The fields of  png_info  are  still  available  for
       older applications, but it is suggested that applications use the new interfaces if at all
       possible.

       Applications that do  make  direct  access  to  the  members  of  png_struct  (except  for
       png_ptr->jmpbuf) must be recompiled whenever the library is updated, and applications that
       make direct access to the members of png_info must be recompiled if they were compiled  or
       loaded  with  libpng  version  1.0.6,  in which the members were in a different order.  In
       version 1.0.7, the members of the png_info structure reverted to the old  order,  as  they
       were  in  versions  0.97c through 1.0.5.  Starting with version 2.0.0, both structures are
       going to be hidden, and the contents of the structures will only be accessible through the
       png_get/png_set functions.

       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>

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 progressive 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 corresponding bytes of
       the PNG signature, or nonzero (true) otherwise.  Of course, the more bytes  you  pass  in,
       the greater the accuracy of the prediction.

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

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

           FILE *fp = fopen(file_name, "rb");
           if (!fp)
           {
               return (ERROR);
           }
           fread(header, 1, number, fp);
           is_png = !png_sig_cmp(header, 0, number);
           if (!is_png)
           {
               return (NOT_PNG);
           }

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

           png_structp png_ptr = png_create_read_struct
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn);
           if (!png_ptr)
               return (ERROR);

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

           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);
           }

       If  you want to use your own memory allocation routines, define PNG_USER_MEM_SUPPORTED and
       use png_create_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 jmpbuf field every time you  enter  a  new
       routine 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 handling  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);
           }

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

       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);

   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_ptr 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 will be
       saved when read, in case your callback function will need  one  or  more  of  them.   This
       behavior can be changed with the png_set_keep_unknown_chunks() function, described below.

       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_ptr ptr, png_uint_32 row,
              int pass);
           {
             /* put your code here */
           }

       (You can give it another name that you like instead of "read_row_callback")

       To inform libpng about your function, use

           png_set_read_status_fn(png_ptr, read_row_callback);

   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  various  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 0)
           num_chunks - number of chunks affected; if 0, all
                        unknown chunks are affected.  If nonzero,
                        only the chunks in the list 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.

       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 callback 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: */
             png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0);
             /* except for vpAg: */
             png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
             /* also ignore unused known chunks: */
             png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
                (int)sizeof(unused_chunks)/5);
           #endif

   User limits
       The  PNG  specification  allows  the width and height of an image to be as large as 2^31-1
       (0x7fffffff), or about 2.147 billion rows and columns.  Since very few applications really
       need  to  process  such large images, we have imposed an arbitrary 1-million limit on rows
       and columns.  Larger images will be rejected immediately with a png_error() call.  If  you
       wish to override this limit, you can use

          png_set_user_limits(png_ptr, width_max, height_max);

       to  set  your  own  limits, or use width_max = height_max = 0x7fffffffL to allow all valid
       dimensions (libpng may reject some very large images anyway because  of  potential  buffer
       overflow conditions).

       You  should  put  this  statement  after  you  create the PNG structure and before calling
       png_read_info(), png_read_png(), or png_process_data().   If  you  need  to  retrieve  the
       limits that are being applied, use

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

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

          png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

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

          chunk_cache_max = png_get_chunk_cache_max(png_ptr);

       This   limit   also   applies   to  the  number  of  buffers  that  can  be  allocated  by
       png_decompress_chunk() while decompressing iTXt, zTXt, and iCCP chunks.

   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_STRIP_16      Strip 16-bit samples to
                                       8 bits
           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)

       (This  excludes  setting  a  background  color, doing gamma transformation, dithering, 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 transform.)

       You  must  use  png_transforms and not call any png_set_transform() functions 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  allocate  row_pointers
       prior to calling png_read_png() with

          if (height > PNG_UINT_32_MAX/png_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*png_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 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.

   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

           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)
           compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                            for PNG 1.0)
           interlace_type - (PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7)

           Any or all of interlace_type, compression_type, or
           filter_method can be NULL if you are
           not interested in their values.

           Note that png_get_IHDR() returns 32-bit data into
           the application's width and height variables.
           This is an unsafe situation if these are 16-bit
           variables.  In such situations, the
           png_get_image_width() and png_get_image_height()
           functions described below are safer.

           width            = png_get_image_width(png_ptr,
                                info_ptr);
           height           = png_get_image_height(png_ptr,
                                info_ptr);
           bit_depth        = png_get_bit_depth(png_ptr,
                                info_ptr);
           color_type       = png_get_color_type(png_ptr,
                                info_ptr);
           filter_method    = png_get_filter_type(png_ptr,
                                info_ptr);
           compression_type = png_get_compression_type(png_ptr,
                                info_ptr);
           interlace_type   = png_get_interlace_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  simple  data
       types, or a pointer into the info_ptr is returned for any complex types.

           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, &gamma);
           gamma          - the gamma the file is written
                            at (PNG_INFO_gAMA)

           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     - 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, &num_trans,
                            &trans_values);
           trans          - array of transparent
                            entries for palette (PNG_INFO_tRNS)
           trans_values   - graylevel or color sample values of
                            the single transparent color for
                            non-paletted images (PNG_INFO_tRNS)
           num_trans      - number of transparent entries
                            (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 (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.

           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);
           palette_ptr    - array of palette structures holding
                            contents of one or more sPLT chunks
                            read.
           num_spalettes  - number of 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
           offset_y       - positive offset from the top edge
                            of the screen
           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
           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 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)

       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)

       For  more  information, see the png_info definition in png.h and 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 keyword/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 suggested 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 keyword.  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.  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.

       The  colors used for the background and transparency values should be supplied in the same
       format/depth as the current image data.  They are stored in the same format/depth  as  the
       image  data  in  a  bKGD or tRNS chunk, so this is what libpng expects for this data.  The
       colors are transformed to keep in sync with the image data when an application  calls  the
       png_read_update_info() routine (see 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  significant  byte  of  the  color  value  first,  unless
       png_set_strip_16()  is  called  to  transform  it  to  regular  RGB   RGB   triplets,   or
       png_set_filler() or png_set_add alpha() is called to insert filler bytes, either before or
       after each RRGGBB triplet.  Similarly, 8-bit or 16-bit grayscale data can be modified with
       png_set_filler(), png_set_add_alpha(), or png_set_strip_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 multiple-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 (color_type == PNG_COLOR_TYPE_GRAY &&
               bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);

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

       These three 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 1.2.46, not all possible expansions are supported.

       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    -
          31        -
           0    1       -
          0T                -
          0O                    -
           2           GX           -
          2T                            -
          2O                                -
           3        1                           -
          3T                                        -
          3O                                            -
          4A                T                               -
          4O                                                    -
          6A               GX         TX           TX               -
          6O                   GX                      TX               -

       Within the matrix,
            "-" means the transformation is not supported.
            "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
            "G" means the transformation is obtained by
                png_set_gray_to_rgb().
            "P" means the transformation is obtained by
                png_set_expand_palette_to_rgb().
            "T" means the transformation is obtained by
                png_set_tRNS_to_alpha().

       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)
               png_set_strip_16(png_ptr);

       If,  for some reason, you don't need the alpha channel on an image, and you want to remove
       it rather than combining it with the background (but the image  author  certainly  had  in
       mind  that  you *would* combine it with the background, so that's what you should probably
       do):

           if (color_type & PNG_COLOR_MASK_ALPHA)
               png_set_strip_alpha(png_ptr);

       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 transparency 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
       transparent, with

           png_set_invert_alpha(png_ptr);

       The PNG format only supports pixels with postmultiplied alpha.  If you want to replace the
       pixels,  after reading them, with pixels that have premultiplied color samples, you can do
       this with

           png_set_premultiply_alpha(png_ptr);

       If you do this, any input with a tRNS chunk will be expanded to have an alpha channel.

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

           if (bit_depth < 8)
               png_set_packing(png_ptr);

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

           png_color_8p sig_bit;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_RGB_ALPHA)
                 png_set_rgb_to_gray_fixed(png_ptr, error_action,
                    int red_weight, int 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 times 100000
           green_weight:     weight of green component times 100000
                             If either weight is negative, default
                             weights (21268, 71514) are used.

       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.   bKGD and sBIT data will be silently converted to grayscale, using
       the green channel data, regardless of the error_action setting.

       With red_weight+green_weight<=100000, the normalized graylevel is computed:

           int rw = red_weight * 65536;
           int gw = green_weight * 65536;
           int bw = 65536 - (rw + gw);
           gray = (rw*red + gw*green + bw*blue)/65536;

       The default values approximate those recommended  in  the  Charles  Poynton's  Color  FAQ,
       <http://www.inforamp.net/~poynton/>  Copyright  (c) 1998-01-04 Charles Poynton <poynton at
       inforamp.net>

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

       Libpng approximates this with

           Y = 0.21268 * R    + 0.7151 * G    + 0.07217 * B

       which can be expressed with integers as

           Y = (6969 * R + 23434 * G + 2365 * B)/32768

       The calculation is done in a linear colorspace, if the image gamma is known.

       If you have a grayscale and you are  using  png_set_expand_depth(),  png_set_expand(),  or
       png_set_gray_to_rgb  to  change  to  truecolor  or  to a higher bit-depth, you must either
       supply the background color as a gray value at the original file bit-depth (need_expand  =
       1)  or else supply the background color as an RGB triplet at the final, expanded bit depth
       (need_expand = 0).  Similarly, if you are reading a paletted image, you must either supply
       the background color as a palette index (need_expand = 1) or as an RGB triplet that may or
       may not be in the palette (need_expand = 0).

           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, 1.0);
           else
               png_set_background(png_ptr, &my_background,
                 PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);

       The png_set_background() function tells libpng to composite images with  alpha  or  simple
       transparency against the supplied background color.  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
       whether the color is in the gamma space of the  display  (PNG_BACKGROUND_GAMMA_SCREEN  for
       colors  you  supply), the file (PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk),
       or one that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE -  I  don't  know  why
       anyone would use this, but it's here).

       To  properly  display PNG images on any kind of system, the application needs to know what
       the display gamma is.  Ideally, the user will know this, and the  application  will  allow
       them  to  set it.  One method of allowing the user to set the display gamma separately for
       each system is to check for a SCREEN_GAMMA or DISPLAY_GAMMA  environment  variable,  which
       will hopefully be correctly set.

       Note  that  display_gamma  is  the  overall  gamma correction required to produce pleasing
       results, which depends on the lighting conditions in the surrounding  environment.   In  a
       dim  or  brightly  lit room, no compensation other than the physical gamma exponent of the
       monitor is needed, while in a dark room a slightly smaller exponent is better.

          double gamma, screen_gamma;

          if (/* We have a user-defined screen
              gamma value */)
          {
             screen_gamma = user_defined_screen_gamma;
          }
          /* One way that applications can share the same
             screen gamma value */
          else if ((gamma_str = getenv("SCREEN_GAMMA"))
             != NULL)
          {
             screen_gamma = (double)atof(gamma_str);
          }
          /* If we don't have another value */
          else
          {
             screen_gamma = 2.2; /* A good guess for a
                  PC monitor in a bright office or a dim room */
             screen_gamma = 2.0; /* A good guess for a
                  PC monitor in a dark room */
             screen_gamma = 1.7 or 1.0;  /* A good
                  guess for Mac systems */
          }

       The png_set_gamma() function handles gamma transformations of the  data.   Pass  both  the
       file gamma and the current screen_gamma.  If the file does not have a gamma value, you can
       pass one anyway if you have an idea what it is (usually 0.45455 is a good  guess  for  GIF
       images  on  PCs).   Note  that  file  gammas  are  inverted  from  screen gammas.  See the
       discussions on gamma in the PNG specification for an excellent description of  what  gamma
       is,  and  why  all  applications  should  support it.  It is strongly recommended that PNG
       viewers support gamma correction.

          if (png_get_gAMA(png_ptr, info_ptr, &gamma))
             png_set_gamma(png_ptr, screen_gamma, gamma);
          else
             png_set_gamma(png_ptr, screen_gamma, 0.45455);

       If you need to reduce an RGB file to a paletted file, or  if  a  paletted  file  has  more
       entries  then will fit on your screen, png_set_dither() will do that.  Note that this is a
       simple match dither that merely finds the  closest  color  available.   This  should  work
       fairly  well  with  optimized  palettes, and fairly badly with linear color cubes.  If you
       pass a palette that is larger then maximum_colors, the file  will  reduce  the  number  of
       colors  in  the  palette  so it will fit into maximum_colors.  If there is a histogram, it
       will use it to make more intelligent choices when reducing the palette.  If  there  is  no
       histogram, 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_dither(png_ptr, palette, num_palette,
                   max_screen_colors, histogram, 1);
             }
             else
             {
                png_color std_color_cube[MAX_SCREEN_COLORS] =
                   { ... colors ... };

                png_set_dither(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_ptr ptr, row_info_ptr
              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.

       You  can  also set up a pointer to a user structure for use by your callback 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_transform_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.  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.

           png_read_update_info(png_ptr, 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.

   Reading image data
       After  you've  allocated memory, you can read the image data.  The simplest 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()  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 interlace_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 version 1.2) interlacing type for PNG is
       (interlace_type == PNG_INTERLACE_ADAM7) is 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.

       libpng can fill out those images or it can give them to you "as is".   If  you  want  them
       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 initialized 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  you  don't  want  libpng  to handle the interlacing details, just call png_read_rows()
       seven times to read in all seven images.  Each of the images is a valid image  by  itself,
       or  they can all be combined on an 8x8 grid to form a single image (although if you intend
       to combine them you would be far better off using the libpng interlace handling).

       The first pass will return an image 1/8 as wide as the  entire  image  (every  8th  column
       starting  in  column 0) and 1/8 as high as the original (every 8th row starting in row 0),
       the second will be 1/8 as wide (starting in column 4) and 1/8 as high  (also  starting  in
       row 0).  The third pass will be 1/4 as wide (every 4th pixel starting in column 0) and 1/8
       as high (every 8th row starting in row 4), and the fourth pass will be 1/4 as wide and 1/4
       as high (every 4th column starting in column 2, and every 4th row starting in row 0).  The
       fifth pass will return an image 1/2 as wide, and 1/4 as high (starting at column 0 and row
       2),  while the sixth pass will be 1/2 as wide and 1/2 as high as the original (starting in
       column 1 and row 0).  The seventh and final pass will be as wide as the original, and  1/2
       as high, containing all of the odd numbered scanlines.  Phew!

       If  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 function can be called even if the file is not
       interlaced, where it will return one 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);

   Finishing a sequential read
       After you are finished reading the image through the low-level interface, you  can  finish
       reading  the  file.  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.  If you are not interested,
       you can pass NULL.

          png_read_end(png_ptr, end_info);

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

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

       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_zalloc()  and  passed  in  via  a
       png_set_*() function, with

           png_data_freer(png_ptr, info_ptr, freer, mask)
           mask   - which data elements are affected
                    same choices as in png_free_data()
           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_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 application 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_zalloc() 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.translated_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 anything 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 example.c.

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

       png_structp png_ptr; png_infop info_ptr;

        /*  An example code fragment of how you would
            initialize the progressive reader in your
            application. */
        int
        initialize_png_reader()
        {
           png_ptr = png_create_read_struct
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
                user_error_fn, user_warning_fn);
           if (!png_ptr)
               return (ERROR);
           info_ptr = png_create_info_struct(png_ptr);
           if (!info_ptr)
           {
               png_destroy_read_struct(&png_ptr, (png_infopp)NULL,
                  (png_infopp)NULL);
               return (ERROR);
           }

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

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

                 png_get_progressive_ptr(png_ptr);

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

           return 0;
        }

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

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

              For the non-NULL rows of interlaced images,
              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:
            */

               png_progressive_combine_row(png_ptr, old_row,
                 new_row);

           /* where old_row is what was displayed for
              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.
           */
        }

        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  importance  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 handling.  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 routines, 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 documentation 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_SETJMP_NOT_SUPPORTED,  in which case errors will result in a call to PNG_ABORT()
       which defaults to abort().

       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
       Customizing 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  written  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_ptr, png_uint_32 row,
              int pass);
           {
             /* put your code here */
           }

       (You can give it another name that you like instead of "write_row_callback")

       To inform libpng about your function, use

           png_set_write_status_fn(png_ptr, write_row_callback);

       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 maximum 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 during 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_compression_level() which changes how  much  time  zlib  spends  on
       trying to compress the image data.  See the Compression Library (zlib.h and algorithm.txt,
       distributed with zlib) for details on the compression levels.

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

           /* set other zlib parameters */
           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)

       extern PNG_EXPORT(void,png_set_zbuf_size)

   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, gamma);
           gamma          - the gamma the image was created
                            at (PNG_INFO_gAMA)

           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     - 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, num_trans,
              trans_values);
           trans          - array of transparent
                            entries for palette (PNG_INFO_tRNS)
           trans_values   - graylevel or color sample values
                            (in order red, green, blue) of the
                            single transparent color for
                            non-paletted images (PNG_INFO_tRNS)
           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)

           png_set_hIST(png_ptr, info_ptr, hist);
                           (PNG_INFO_hIST)
           hist           - histogram of palette (array of
                            png_uint_16)

           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 (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.

           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
           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   specify
       PNG_TEXT_COMPRESSION_NONE  or  PNG_TEXT_COMPRESSION_zTXt  any  language code or translated
       keyword will not be written out.

       Until text gets around 1000 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_COMPRESSION_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
       description  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 contain
       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-compressed
       pairs.   Compressed  pairs  must have a text string, as only the text string is compressed
       anyway, so the compression would be meaningless.

       PNG supports modification time via the png_time structure.  Two  conversion  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
       "Creation  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(png_timep)  is
       provided to convert from PNG time to an RFC 1123 format string.

   Writing unknown chunks
       You  can use the png_set_unknown_chunks function to queue up chunks for writing.  You give
       it a chunk name, raw data, and a size; 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.  Any chunks previously read into the info structure's  unknown-chunk  list  will
       also be written out in a sequence that satisfies the PNG specification's ordering rules.

   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() functions  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 supplied 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_ptr ptr, row_info_ptr
              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.

       You can also set up a pointer to a user structure for use by your callback 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_transform_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 manually, or automatically
       after a certain number of lines have been written.  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  noticeably  (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 simple:

           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 single 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  complicated.   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  number  of  times  to
       write all seven 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);

       As some of these rows are not used, and thus return immediately,  you  may  want  to  read
       about  interlacing  in  the  PNG specification, and only update the rows that are actually
       used.

   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 interested, 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_zalloc()  and  passed  in via a
       png_set_*() function, with

           png_data_freer(png_ptr, info_ptr, freer, mask)
           mask   - which data elements are affected
                    same choices as in png_free_data()
           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_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  allocated,  the  user  must
       have used png_malloc() or png_zalloc() to allocate it.

       If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_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. Modifying/Customizing libpng:

       There are two issues here.  The first is changing how libpng  does  standard  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  routines  are  in  pngmem.c,  pngrio.c,
       pngwio.c,  and  pngerror.c, respectively.  To change these functions, call the appropriate
       png_set_*_fn() function.

       Memory  allocation  is  done  through  the  functions  png_malloc(),   png_calloc(),   and
       png_free().   These  currently  just  call  the  standard C functions.  png_calloc() calls
       png_malloc() and then png_memset() to clear the newly allocated memory to zero.   If  your
       pointers  can't access more then 64K at a time, you will want to set MAXSEG_64K in zlib.h.
       Since it is unlikely that the method of handling memory  allocation  on  a  platform  will
       change  between  applications,  these functions must be modified in the library at compile
       time.  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_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 functions also provide a void
       pointer that can be retrieved via the function 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_SETJMP_NOT_SUPPORTED, in which case it is handled via PNG_ABORT()), but you could
       change this to do things like exit() if you should wish.

       On  non-fatal errors, png_warning() is called to print a warning message, 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  created.  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 alternative approach, you may wish to use the
       "cexcept" facility (see http://cexcept.sourceforge.net).

   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 custom 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  specification.  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 unknown chunks in a
       generic method, via callback functions, instead of by modifying libpng functions.

       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 16 bit platforms
       You  will want to look into zconf.h to tell zlib (and thus libpng) that it cannot allocate
       more then 64K at a time.  Even if you can, the memory won't be accessible.  So limit  zlib
       and libpng to 64K by defining MAXSEG_64K.

   Configuring for DOS
       For DOS users who only have access to the lower 640K, you will have to limit zlib's memory
       usage via a png_set_compression_mem_level() call.  See  zlib.h  or  zconf.h  in  the  zlib
       library for more information.

   Configuring for Medium Model
       Libpng's  support for medium model has been tested on most of the popular compilers.  Make
       sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets defined, and FAR gets defined to far in
       pngconf.h,  and  you  should  be  all  set.   Everything in the library (except for zlib's
       structure) is expecting far data.  You must use the typedefs with the p or pp on  the  end
       for  pointers  (or at least look at them and be careful).  Make note that the rows of data
       are defined as png_bytepp, which is an unsigned char far * far *.

   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 for compiler xxx:
       All includes for libpng are in pngconf.h.  If  you  need  to  add,  change  or  delete  an
       include,  this is the place to do it.  The includes that are not needed outside libpng are
       protected by the PNG_INTERNAL definition, which is only defined for those routines  inside
       libpng itself.  The files in libpng proper only include png.h, which includes pngconf.h.

   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 create files larger  than  just  storing  the  raw  bitmap.   You  can  specify  the
       compression level by calling:

           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.

           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.

           png_set_compression_strategy(png_ptr,
               strategy);
           png_set_compression_window_bits(png_ptr,
               window_bits);
           png_set_compression_method(png_ptr, method);
           png_set_compression_buffer_size(png_ptr, size);

   Controlling row filtering
       If you want to control whether libpng uses filtering or not, which filters 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 currently only '0' in the
       PNG 1.2 specification.  The 'filters' parameter sets which filter(s), if  any,  should  be
       used  for  each  scanline.  Possible 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_FILTER_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  allocate  the filter buffers until png_write_row() is called for the
       first time.)

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

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

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

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

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

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

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

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

   Removing unwanted object code
       There are a bunch of #define's  in  pngconf.h  that  control  what  parts  of  libpng  are
       compiled.  All the defines end in _SUPPORTED.  If you are never going to use a capability,
       you can change the #define to #undef before recompiling libpng and save yourself code  and
       data  space,  or  you  can  turn  off individual capabilities with defines that begin with
       PNG_NO_.

       You can also turn all of the transforms and ancillary chunk capabilities off en masse with
       compiler  directives  that  define  PNG_NO_READ[or  WRITE]_TRANSFORMS,  or  PNG_NO_READ[or
       WRITE]_ANCILLARY_CHUNKS, or all four,  along  with  directives  to  turn  on  any  of  the
       capabilities  that  you  do want.  The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable
       the extra transformations but still leave the library fully capable of reading and writing
       PNG  files with all known public chunks. Use of the PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS
       directive produces a library that is incapable of reading or writing ancillary chunks.  If
       you  are  not  using  the  progressive  reading  capability,  you  can  turn that off with
       PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTERLACING capability, which  you'll
       still have).

       All the reading and writing specific code are in separate files, so the linker should only
       grab the files it needs.  However, if you want to make sure, or  if  you  are  building  a
       stand alone library, all the reading files start with pngr and all the writing files start
       with pngw.  The files that don't match either (like png.c, pngtrans.c, etc.)  are used for
       both  reading  and  writing, and always need to be included.  The progressive reader is in
       pngpread.c

       If you are creating or distributing a dynamically linked library (a .so or DLL file),  you
       should  not  remove  or  disable any parts of the library, as this will cause applications
       linked with different versions of the library to fail if they call functions not available
       in  your  library.   The  size  of the library itself should not be an issue, because only
       those sections that are actually used will be loaded into memory.

   Requesting debug printout
       The macro definition PNG_DEBUG can be used to request debugging printout.  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=%d0, foo);

       is expanded to

          if(PNG_DEBUG > 2)
            fprintf(PNG_DEBUG_FILE, "foo=%d0, 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 statements having level = 0
       will  be  printed.  There aren't any such statements in this version of libpng, but if you
       insert some they will be printed.

VI. 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 standalone 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 provide its
       own  support  for  them.   You  may  wish  to  consider   using   libmng   (available   at
       http://www.libmng.com) instead.

VII. Changes to Libpng from version 0.88

       It  should  be  noted  that  versions of libpng later than 0.96 are not distributed by the
       original libpng author, Guy Schalnat, nor by Andreas Dilger, who had taken over  from  Guy
       during  1996  and  1997, and distributed 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 2.0.0.

       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  structures  from  the  application, allow version error
       checking,  and  also  allow  the  use  of  custom  error  handling  routines  during   the
       initialization,  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 overhead 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 possible 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.

       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 version with leading
       zero, and release number with leading zero, (e.g., libpng_vn for version 1.0.7 is 10007).

       You can also check which version of png.h you used when compiling your application:

          png_uint_32 application_vn = PNG_LIBPNG_VER;

VIII. Changes to Libpng from version 1.0.x to 1.2.x

       Support for user memory management was  enabled  by  default.   To  accomplish  this,  the
       functions   png_create_read_struct_2(),   png_create_write_struct_2(),   png_set_mem_fn(),
       png_get_mem_ptr(), png_malloc_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() function is deprecated.

       A number of macro definitions in support of runtime selection of assembler  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  deprecated   since
       libpng-1.0.18 and 1.2.9.

IX. (Omitted)

X. 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  presence of any libpng version since 0.88.  In an
       autoconf "configure.in" you could use

           AC_CHECK_LIB(png, png_get_io_ptr, ...

XI. 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://libpng.git.sourceforge.net/gitroot/libpng

       or you can browse it via "gitweb" at

           http://libpng.git.sourceforge.net/git/gitweb.cgi?p=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

XII. Coding style

       Our coding style is similar to the "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;

           /* Multiple-line
            * comment
            */
           statement;

       Very short comments can be placed at 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 functions are marked with
       PNGAPI:

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

       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  the
       PNG_INTERNAL section of png.h above the comment that says

         /* Maintainer: Put new private prototypes here ^ and in libpngpf.3 */

       The  names  of  all  exported functions and variables begin with  "png_", and all publicly
       visible C preprocessor macros begin with "PNG_".

       We put a space after each comma and after each semicolon in "for" statments,  and  we  put
       spaces before and after each C binary operator and after "for" or "while".  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 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.

XIII. Y2K Compliance in libpng

       July 9, 2011

       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  1.2.46
       are Y2K compliant.  It is my belief that earlier versions were also Y2K compliant.

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

       The integer is
           "png_uint_16 year" in png_time_struct.

       The strings are
           "png_charp time_buffer" in png_struct and
           "near_time_buffer", which is a local character string in png.c.

       There are seven time-related functions:

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

       All appear to handle dates properly in a Y2K environment.   The  png_convert_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() function, but this is not under our control.
       The libpng documentation has always stated that it works with 4-digit years, and the  APIs
       have been documented as such.

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

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

          Glenn Randers-Pehrson
          libpng maintainer
          PNG Development Group

NOTE

       Note about libpng version numbers:

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

        source             png.h  png.h  shared-lib
        version            string   int  version
        -------            ------  ----- ----------
        0.89c ("beta 3")  0.89       89  1.0.89
        0.90  ("beta 4")  0.90       90  0.90
        0.95  ("beta 5")  0.95       95  0.95
        0.96  ("beta 6")  0.96       96  0.96
        0.97b ("beta 7")  1.00.97    97  1.0.1
        0.97c             0.97       97  2.0.97
        0.98              0.98       98  2.0.98
        0.99              0.99       98  2.0.99
        0.99a-m           0.99       99  2.0.99
        1.00              1.00      100  2.1.0
        1.0.0             1.0.0     100  2.1.0
        1.0.0   (from here on, the  100  2.1.0
        1.0.1    png.h string is  10001  2.1.0
        1.0.1a-e identical to the 10002  from here on, the
        1.0.2    source version)  10002  shared library is 2.V
        1.0.2a-b                  10003  where V is the source
        1.0.1                     10001  code version except as
        1.0.1a-e                  10002  2.1.0.1a-e   noted.
        1.0.2                     10002  2.1.0.2
        1.0.2a-b                  10003  2.1.0.2a-b
        1.0.3                     10003  2.1.0.3
        1.0.3a-d                  10004  2.1.0.3a-d
        1.0.4                     10004  2.1.0.4
        1.0.4a-f                  10005  2.1.0.4a-f
        1.0.5 (+ 2 patches)       10005  2.1.0.5
        1.0.5a-d                  10006  2.1.0.5a-d
        1.0.5e-r                  10100  2.1.0.5e-r
        1.0.5s-v                  10006  2.1.0.5s-v
        1.0.6 (+ 3 patches)       10006  2.1.0.6
        1.0.6d-g                  10007  2.1.0.6d-g
        1.0.6h                    10007  10.6h
        1.0.6i                    10007  10.6i
        1.0.6j                    10007  2.1.0.6j
        1.0.7beta11-14    DLLNUM  10007  2.1.0.7beta11-14
        1.0.7beta15-18       1    10007  2.1.0.7beta15-18
        1.0.7rc1-2           1    10007  2.1.0.7rc1-2
        1.0.7                1    10007  2.1.0.7
        1.0.8beta1-4         1    10008  2.1.0.8beta1-4
        1.0.8rc1             1    10008  2.1.0.8rc1
        1.0.8                1    10008  2.1.0.8
        1.0.9beta1-6         1    10009  2.1.0.9beta1-6
        1.0.9rc1             1    10009  2.1.0.9rc1
        1.0.9beta7-10        1    10009  2.1.0.9beta7-10
        1.0.9rc2             1    10009  2.1.0.9rc2
        1.0.9                1    10009  2.1.0.9
        1.0.10beta1          1    10010  2.1.0.10beta1
        1.0.10rc1            1    10010  2.1.0.10rc1
        1.0.10               1    10010  2.1.0.10
        1.0.11beta1-3        1    10011  2.1.0.11beta1-3
        1.0.11rc1            1    10011  2.1.0.11rc1
        1.0.11               1    10011  2.1.0.11
        1.0.12beta1-2        2    10012  2.1.0.12beta1-2
        1.0.12rc1            2    10012  2.1.0.12rc1
        1.0.12               2    10012  2.1.0.12
        1.1.0a-f             -    10100  2.1.1.0a-f abandoned
        1.2.0beta1-2         2    10200  2.1.2.0beta1-2
        1.2.0beta3-5         3    10200  3.1.2.0beta3-5
        1.2.0rc1             3    10200  3.1.2.0rc1
        1.2.0                3    10200  3.1.2.0
        1.2.1beta-4          3    10201  3.1.2.1beta1-4
        1.2.1rc1-2           3    10201  3.1.2.1rc1-2
        1.2.1                3    10201  3.1.2.1
        1.2.2beta1-6        12    10202  12.so.0.1.2.2beta1-6
        1.0.13beta1         10    10013  10.so.0.1.0.13beta1
        1.0.13rc1           10    10013  10.so.0.1.0.13rc1
        1.2.2rc1            12    10202  12.so.0.1.2.2rc1
        1.0.13              10    10013  10.so.0.1.0.13
        1.2.2               12    10202  12.so.0.1.2.2
        1.2.3rc1-6          12    10203  12.so.0.1.2.3rc1-6
        1.2.3               12    10203  12.so.0.1.2.3
        1.2.4beta1-3        13    10204  12.so.0.1.2.4beta1-3
        1.2.4rc1            13    10204  12.so.0.1.2.4rc1
        1.0.14              10    10014  10.so.0.1.0.14
        1.2.4               13    10204  12.so.0.1.2.4
        1.2.5beta1-2        13    10205  12.so.0.1.2.5beta1-2
        1.0.15rc1           10    10015  10.so.0.1.0.15rc1
        1.0.15              10    10015  10.so.0.1.0.15
        1.2.5               13    10205  12.so.0.1.2.5
        1.2.6beta1-4        13    10206  12.so.0.1.2.6beta1-4
        1.2.6rc1-5          13    10206  12.so.0.1.2.6rc1-5
        1.0.16              10    10016  10.so.0.1.0.16
        1.2.6               13    10206  12.so.0.1.2.6
        1.2.7beta1-2        13    10207  12.so.0.1.2.7beta1-2
        1.0.17rc1           10    10017  10.so.0.1.0.17rc1
        1.2.7rc1            13    10207  12.so.0.1.2.7rc1
        1.0.17              10    10017  10.so.0.1.0.17
        1.2.7               13    10207  12.so.0.1.2.7
        1.2.8beta1-5        13    10208  12.so.0.1.2.8beta1-5
        1.0.18rc1-5         10    10018  10.so.0.1.0.18rc1-5
        1.2.8rc1-5          13    10208  12.so.0.1.2.8rc1-5
        1.0.18              10    10018  10.so.0.1.0.18
        1.2.8               13    10208  12.so.0.1.2.8
        1.2.9beta1-3        13    10209  12.so.0.1.2.9beta1-3
        1.2.9beta4-11       13    10209  12.so.0.9[.0]
        1.2.9rc1            13    10209  12.so.0.9[.0]
        1.2.9               13    10209  12.so.0.9[.0]
        1.2.10beta1-8       13    10210  12.so.0.10[.0]
        1.2.10rc1-3         13    10210  12.so.0.10[.0]
        1.2.10              13    10210  12.so.0.10[.0]
        1.2.11beta1-4       13    10211  12.so.0.11[.0]
        1.0.19rc1-5         10    10019  10.so.0.19[.0]
        1.2.11rc1-5         13    10211  12.so.0.11[.0]
        1.0.19              10    10019  10.so.0.19[.0]
        1.2.11              13    10211  12.so.0.11[.0]
        1.0.20              10    10020  10.so.0.20[.0]
        1.2.12              13    10212  12.so.0.12[.0]
        1.2.13beta1         13    10213  12.so.0.13[.0]
        1.0.21              10    10021  10.so.0.21[.0]
        1.2.13              13    10213  12.so.0.13[.0]
        1.2.14beta1-2       13    10214  12.so.0.14[.0]
        1.0.22rc1           10    10022  10.so.0.22[.0]
        1.2.14rc1           13    10214  12.so.0.14[.0]
        1.2.15beta1-6       13    10215  12.so.0.15[.0]
        1.0.23rc1-5         10    10023  10.so.0.23[.0]
        1.2.15rc1-5         13    10215  12.so.0.15[.0]
        1.0.23              10    10023  10.so.0.23[.0]
        1.2.15              13    10215  12.so.0.15[.0]
        1.2.16beta1-2       13    10216  12.so.0.16[.0]
        1.2.16rc1           13    10216  12.so.0.16[.0]
        1.0.24              10    10024  10.so.0.24[.0]
        1.2.16              13    10216  12.so.0.16[.0]
        1.2.17beta1-2       13    10217  12.so.0.17[.0]
        1.0.25rc1           10    10025  10.so.0.25[.0]
        1.2.17rc1-3         13    10217  12.so.0.17[.0]
        1.0.25              10    10025  10.so.0.25[.0]
        1.2.17              13    10217  12.so.0.17[.0]
        1.0.26              10    10026  10.so.0.26[.0]
        1.2.18              13    10218  12.so.0.18[.0]
        1.2.19beta1-31      13    10219  12.so.0.19[.0]
        1.0.27rc1-6         10    10027  10.so.0.27[.0]
        1.2.19rc1-6         13    10219  12.so.0.19[.0]
        1.0.27              10    10027  10.so.0.27[.0]
        1.2.19              13    10219  12.so.0.19[.0]
        1.2.20beta01-04     13    10220  12.so.0.20[.0]
        1.0.28rc1-6         10    10028  10.so.0.28[.0]
        1.2.20rc1-6         13    10220  12.so.0.20[.0]
        1.0.28              10    10028  10.so.0.28[.0]
        1.2.20              13    10220  12.so.0.20[.0]
        1.2.21beta1-2       13    10221  12.so.0.21[.0]
        1.2.21rc1-3         13    10221  12.so.0.21[.0]
        1.0.29              10    10029  10.so.0.29[.0]
        1.2.21              13    10221  12.so.0.21[.0]
        1.2.22beta1-4       13    10222  12.so.0.22[.0]
        1.0.30rc1           13    10030  10.so.0.30[.0]
        1.2.22rc1           13    10222  12.so.0.22[.0]
        1.0.30              10    10030  10.so.0.30[.0]
        1.2.22              13    10222  12.so.0.22[.0]
        1.2.23beta01-05     13    10223  12.so.0.23[.0]
        1.2.23rc01          13    10223  12.so.0.23[.0]
        1.2.23              13    10223  12.so.0.23[.0]
        1.2.24beta01-02     13    10224  12.so.0.24[.0]
        1.2.24rc01          13    10224  12.so.0.24[.0]
        1.2.24              13    10224  12.so.0.24[.0]
        1.2.25beta01-06     13    10225  12.so.0.25[.0]
        1.2.25rc01-02       13    10225  12.so.0.25[.0]
        1.0.31              10    10031  10.so.0.31[.0]
        1.2.25              13    10225  12.so.0.25[.0]
        1.2.26beta01-06     13    10226  12.so.0.26[.0]
        1.2.26rc01          13    10226  12.so.0.26[.0]
        1.2.26              13    10226  12.so.0.26[.0]
        1.0.32              10    10032  10.so.0.32[.0]
        1.2.27beta01-06     13    10227  12.so.0.27[.0]
        1.2.27rc01          13    10227  12.so.0.27[.0]
        1.0.33              10    10033  10.so.0.33[.0]
        1.2.27              13    10227  12.so.0.27[.0]
        1.0.34              10    10034  10.so.0.34[.0]
        1.2.28              13    10228  12.so.0.28[.0]
        1.2.29beta01-03     13    10229  12.so.0.29[.0]
        1.2.29rc01          13    10229  12.so.0.29[.0]
        1.0.35              10    10035  10.so.0.35[.0]
        1.2.29              13    10229  12.so.0.29[.0]
        1.0.37              10    10037  10.so.0.37[.0]
        1.2.30beta01-04     13    10230  12.so.0.30[.0]
        1.0.38rc01-08       10    10038  10.so.0.38[.0]
        1.2.30rc01-08       13    10230  12.so.0.30[.0]
        1.0.38              10    10038  10.so.0.38[.0]
        1.2.30              13    10230  12.so.0.30[.0]
        1.0.39rc01-03       10    10039  10.so.0.39[.0]
        1.2.31rc01-03       13    10231  12.so.0.31[.0]
        1.0.39              10    10039  10.so.0.39[.0]
        1.2.31              13    10231  12.so.0.31[.0]
        1.2.32beta01-02     13    10232  12.so.0.32[.0]
        1.0.40rc01          10    10040  10.so.0.40[.0]
        1.2.32rc01          13    10232  12.so.0.32[.0]
        1.0.40              10    10040  10.so.0.40[.0]
        1.2.32              13    10232  12.so.0.32[.0]
        1.2.33beta01-02     13    10233  12.so.0.33[.0]
        1.2.33rc01-02       13    10233  12.so.0.33[.0]
        1.0.41rc01          10    10041  10.so.0.41[.0]
        1.2.33              13    10233  12.so.0.33[.0]
        1.0.41              10    10041  10.so.0.41[.0]
        1.2.34beta01-07     13    10234  12.so.0.34[.0]
        1.0.42rc01          10    10042  10.so.0.42[.0]
        1.2.34rc01          13    10234  12.so.0.34[.0]
        1.0.42              10    10042  10.so.0.42[.0]
        1.2.34              13    10234  12.so.0.34[.0]
        1.2.35beta01-03     13    10235  12.so.0.35[.0]
        1.0.43rc01-02       10    10043  10.so.0.43[.0]
        1.2.35rc01-02       13    10235  12.so.0.35[.0]
        1.0.43              10    10043  10.so.0.43[.0]
        1.2.35              13    10235  12.so.0.35[.0]
        1.2.36beta01-05     13    10236  12.so.0.36[.0]
        1.2.36rc01          13    10236  12.so.0.36[.0]
        1.0.44              10    10044  10.so.0.44[.0]
        1.2.36              13    10236  12.so.0.36[.0]
        1.2.37beta01-03     13    10237  12.so.0.37[.0]
        1.2.37rc01          13    10237  12.so.0.37[.0]
        1.2.37              13    10237  12.so.0.37[.0]
        1.0.45              10    10045  12.so.0.45[.0]
        1.0.46              10    10046  10.so.0.46[.0]
        1.2.38beta01        13    10238  12.so.0.38[.0]
        1.2.38rc01-03       13    10238  12.so.0.38[.0]
        1.0.47              10    10047  10.so.0.47[.0]
        1.2.38              13    10238  12.so.0.38[.0]
        1.2.39beta01-05     13    10239  12.so.0.39[.0]
        1.2.39rc01          13    10239  12.so.0.39[.0]
        1.0.48              10    10048  10.so.0.48[.0]
        1.2.39              13    10239  12.so.0.39[.0]
        1.2.40beta01        13    10240  12.so.0.40[.0]
        1.2.40rc01          13    10240  12.so.0.40[.0]
        1.0.49              10    10049  10.so.0.49[.0]
        1.2.40              13    10240  12.so.0.40[.0]
        1.0.50              10    10050  10.so.0.50[.0]
        1.2.41beta01-18     13    10241  12.so.0.41[.0]
        1.0.51rc01          10    10051  10.so.0.51[.0]
        1.2.41rc01-03       13    10241  12.so.0.41[.0]
        1.0.51              10    10051  10.so.0.51[.0]
        1.2.41              13    10241  12.so.0.41[.0]
        1.2.42beta01-02     13    10242  12.so.0.42[.0]
        1.2.42rc01-05       13    10242  12.so.0.42[.0]
        1.0.52              10    10052  10.so.0.52[.0]
        1.2.42              13    10242  12.so.0.42[.0]
        1.2.43beta01-05     13    10243  12.so.0.43[.0]
        1.0.53rc01-02       10    10053  10.so.0.53[.0]
        1.2.43rc01-02       13    10243  12.so.0.43[.0]
        1.0.53              10    10053  10.so.0.53[.0]
        1.2.43              13    10243  12.so.0.43[.0]
        1.2.44beta01-03     13    10244  12.so.0.44[.0]
        1.2.44rc01-03       13    10244  12.so.0.44[.0]
        1.2.44              13    10244  12.so.0.44[.0]
        1.2.45beta01-03     13    10245  12.so.0.45[.0]
        1.0.55rc01          10    10055  10.so.0.55[.0]
        1.2.45rc01          13    10245  12.so.0.45[.0]
        1.0.55              10    10055  10.so.0.55[.0]
        1.2.45              13    10245  12.so.0.45[.0]
        1.2.46rc01-02       13    10246  12.so.0.46[.0]
        1.0.56              10    10056  10.so.0.56[.0]
        1.2.46              13    10246  12.so.0.46[.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 corresponding to  the
       source  version  x.y.z  (leading zeros in y and z).  Beta versions were given the previous
       public release number plus a letter, until version 1.0.6j; from then on  they  were  given
       the upcoming public release number plus "betaNN" or "rcN".

SEE ALSO

       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 1.2.46 - July 9, 2011: 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 subscribe).

COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:

       (This copy of the libpng notices is  provided  for  your  convenience.   In  case  of  any
       discrepancy  between  this  copy and the notices in the file png.h that is included in the
       libpng distribution, the latter shall prevail.)

       If you modify  libpng  you  may  insert  additional  notices  immediately  following  this
       sentence.

       This code is released under the libpng license.

       libpng  versions  1.2.6,  August 15, 2004, through 1.2.46, July 9, 2011, are Copyright (c)
       2004,2006-2008 Glenn Randers-Pehrson, and are distributed according to the same disclaimer
       and  license  as  libpng-1.2.5  with  the  following  individual  added  to  the  list  of
       Contributing Authors

          Cosmin Truta

       libpng versions 1.0.7, July 1, 2000, through 1.2.5 - October 3, 2002,  are  Copyright  (c)
       2000-2002  Glenn Randers-Pehrson, and are distributed according to the same disclaimer and
       license as libpng-1.0.6 with the following individuals added to the list  of  Contributing
       Authors

          Simon-Pierre Cadieux
          Eric S. Raymond
          Gilles Vollant

       and with the following additions to the disclaimer:

          There is no warranty against interference with your
          enjoyment of the library or against infringement.
          There is no warranty that our efforts or the library
          will fulfill any of your particular purposes or needs.
          This library is provided with all faults, and the entire
          risk of satisfactory quality, performance, accuracy, and
          effort is with the user.

       libpng versions 0.97, January 1998, through 1.0.6, March 20, 2000, are Copyright (c) 1998,
       1999 Glenn Randers-Pehrson Distributed according to the same  disclaimer  and  license  as
       libpng-0.96, with the following individuals added to the list of Contributing Authors:

          Tom Lane
          Glenn Randers-Pehrson
          Willem van Schaik

       libpng  versions  0.89,  June  1996,  through 0.96, May 1997, are Copyright (c) 1996, 1997
       Andreas Dilger Distributed according to the same disclaimer and  license  as  libpng-0.88,
       with the following individuals added to the list of Contributing Authors:

          John Bowler
          Kevin Bracey
          Sam Bushell
          Magnus Holmgren
          Greg Roelofs
          Tom Tanner

       libpng  versions  0.5,  May 1995, through 0.88, January 1996, are Copyright (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  merchantability  and of fitness for any purpose.  The Contributing Authors
       and Group 42, Inc.   assume  no  liability  for  direct,  indirect,  incidental,  special,
       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,  without fee, and
       encourage the use of this source code as a component to supporting the PNG file format  in
       commercial  products.   If  you  use  this source code in a product, acknowledgment is not
       required but would be appreciated.

       A "png_get_copyright" function is available, for convenient use in "about" boxes  and  the
       like:

          printf("%s",png_get_copyright(NULL));

       Also,  the  PNG  logo (in PNG format, of course) is supplied in the files "pngbar.png" and
       "pngbar.jpg (88x31) and "pngnow.png" (98x31).

       Libpng  is  OSI  Certified  Open  Source  Software.   OSI  Certified  Open  Source  is   a
       certification mark of the Open Source Initiative.

       Glenn Randers-Pehrson glennrp at users.sourceforge.net July 9, 2011

                                           July 9, 2011                                 LIBPNG(3)