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LIBPNG(3)                                                                                          LIBPNG(3)



NAME
       libpng - Portable Network Graphics (PNG) Reference Library 1.2.41

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 *compres-sion_type, *compression_type,
       sion_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 *fil-ter_type); *filter_type);
       ter_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___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
       *X_, 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___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___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   back-ground_gamma_code, background_gamma_code,
       ground_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_dou-blep png_doublep
       blep 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 off-set_y, offset_y,
       set_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  X_,
       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_progres-sive_info_ptr png_progressive_info_ptr
       sive_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_trans-form_fn); read_user_transform_fn);
       form_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 loca-tion); location);
       tion);



       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_trans-form_depth, user_transform_depth,
       form_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_trans-form_fn); write_user_transform_fn);
       form_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 1.2.41 - December 3, 2009
        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 1.2.41 - December 3, 2009
        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 modifica-tion modification
       tion 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 instruc-tions instructions
       tions 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 techni-cally technically
       cally 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 consid-ered. considered.
       ered.

       Libpng has been designed to handle multiple sessions at one time, to be easily modifiable, to be por-table portable
       table 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 inter-face interface
       face 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 acces-sible accessible
       sible 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 func-tions. functions.
       tions.  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 initial-ize initialize
       ize  and  allocate the structures.  We also pass the library version, optional pointers to error han-dling handling
       dling 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  func-tions functions
       tions  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 informa-tion information
       tion 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  Cus-tomizing Customizing
       tomizing 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 informa-tion information
       tion in various info_ptr members while unknown chunks will be discarded. This behavior can be  waste-ful wasteful
       ful 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 struc-tures. structures.
       tures.  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 parame-ters parameters
       ters 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_point-
       ers[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, ...) func-tions functions
       tions 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  con-tents. contents.
       tents.   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 abbrevia-tions. abbreviations.
       tions.  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-con-secutive non-consecutive
       secutive 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 transfor-mations transformations
       mations 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 for-mat/depth format/depth
       mat/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 1.2.41, 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        -0 310
           0    1       -0T 10T
          0T                -0O 0T0O
          0O                    -2 0O2
           2           GX           -2T GX2T
          2T                            -2O 2T2O
          2O                                -3 2O3
           3        1                           -3T 13T
          3T                                        -3O 3T3O
          3O                                            -4A 3O4A
          4A                T                               -4O T4O
          4O                                                    -6A 4O6A
          6A               GX         TX           TX               -6O TX6O
          6O                   GX                      TX               -Within TXWithin

       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 pix-
       els 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 trans-formation transformation
       formation does not affect images that already have full alpha channels.  To add an opaque alpha chan-nel, channel,
       nel, 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  nor-mal normal
       mal 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 con-version: conversion:
       version:

           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 back-ground background
       ground 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_BACK-GROUND_GAMMA_FILE (PNG_BACKGROUND_GAMMA_FILE
       GROUND_GAMMA_FILE for colors from the bKGD chunk), or one that is neither of these gammas  (PNG_BACK-GROUND_GAMMA_UNIQUE (PNG_BACKGROUND_GAMMA_UNIQUE
       GROUND_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 gam-mas gammas
       mas 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 transforma-
       tions 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 func-tion function
       tion 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 transfor-mations transformations
       mations 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_han-dling() png_set_interlace_handling()
       dling()  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 (start-ing (starting
       ing  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 struc-ture structure
       ture 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 informa-tion information
       tion 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 writ-ten, written,
       ten, 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 possi-ble possible
       ble 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 writ-ing writing
       ing 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_com-pression_level() png_set_compression_level()
       pression_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 compres-sion compression
       sion 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 specifica-tion specification
       tion 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 num-ber number
       ber 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.  Cur-rently, Currently,
       rently, the only valid number is zero.  However, you can  store  text  either  compressed  or  uncom-pressed, uncompressed,
       pressed,  unlike  images,  which  always  have  to be compressed.  So if you don't want the text com-pressed, compressed,
       pressed, 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 lan-guage language
       guage 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_COMPRES-SION_zTXt_WR, PNG_TEXT_COMPRESSION_zTXt_WR,
       SION_zTXt_WR, so that it isn't written out again at the end (in case you are calling  png_write_end()
       with the same struct.

       The keywords that are given in the PNG Specification are:

           Title            Short (one line) title or
                            caption for image
           Author           Name of image's creator
           Description      Description of image (possibly long)
           Copyright        Copyright notice
           Creation Time    Time of original image creation
                            (usually RFC 1123 format, see below)
           Software         Software used to create the image
           Disclaimer       Legal disclaimer
           Warning          Warning of nature of content
           Source           Device used to create the image
           Comment          Miscellaneous comment; conversion
                            from other image format

       The keyword-text pairs work like this.  Keywords should be short simple descriptions of what the com-ment comment
       ment 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 dis-claimer disclaimer
       claimer 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 super-set superset
       set 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 rou-tine routine
       tine 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  some-what somewhat
       what  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  fol-lowing following
       lowing  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  satis-fies satisfies
       fies 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 fol-lowing following
       lowing 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 parame-ters parameters
       ters 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 trans-parency, transparency,
       parency, 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 transfor-mations transformations
       mations 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 sup-
       plied 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  transfor-mations transformations
       mations 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  func-tion function
       tion  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 your-self, yourself,
       self, 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 interlac-ing interlacing
       ing 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  reas-signing reassigning
       signing  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  struc-ture structure
       ture  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 alloca-tion, allocation,
       tion,  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, respec-tively. respectively.
       tively.  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  mal-loc_fn(). malloc_fn().
       loc_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 nor-mally normally
       mally 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 call-ing: calling:
       ing:

           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 call-backs callbacks
       backs  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 han-dle handle
       dle  unknown chunks in a generic method, via callback functions, instead of by modifying libpng func-tions. functions.
       tions.

       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  informa-tion. information.
       tion.


   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  pre-viously, previously,
       viously, 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_INTER-NAL PNG_INTERNAL
       NAL 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  lev-els, levels,
       els,  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 lev-els levels
       els  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 scan-line. scanline.
       line.  Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS to turn filtering on and  off,  respec-tively. respectively.
       tively.

       Individual  filter types are PNG_FILTER_NONE, PNG_FILTER_SUB, PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FIL-TER_PAETH, PNG_FILTER_PAETH,
       TER_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 select-ing selecting
       ing 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 individ-ual individual
       ual 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_cre-ate_read_struct(), png_create_read_struct(),
       ate_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  func-tions functions
       tions  png_read_destroy()  and  png_write_destroy() do not actually free the memory that libpng allo-cated allocated
       cated for these structs, but just reset  the  data  structures,  so  they  can  be  used  instead  of
       png_destroy_read_struct()  and  png_destroy_write_struct() if you feel there is too much system over-head overhead
       head allocating and freeing the png_struct for each image read.

       Setting the error callbacks via png_set_message_fn()  before  png_read_init()  as  was  suggested  in
       libpng-0.88  is  no  longer  supported  because this caused applications that do not use custom error
       functions to fail if the png_ptr was not initialized to zero.  It is still 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 unaf-fected unaffected
       fected 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.source-forge.net lists.sourceforge.net
       forge.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  com-ment: comment:
       ment:

           /* 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 pre-processor preprocessor
       processor 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.

       Other rules can be inferred by inspecting the libpng source.


XIII. Y2K Compliance in libpng
       December 3, 2009

       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  1.2.41  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 con-vert convert
       vert 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  straight-forward. straightforward.
       forward.  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 ---------0.89c
        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]
        1.2.45              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]

       Henceforth the source version will match the shared-library minor  and  patch  numbers;  the  shared-library sharedlibrary
       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  1.2.41  -  December 3, 2009: 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.source-forge.net lists.sourceforge.net
       forge.net  (subscription  required; visit https://lists.sourceforge.net/lists/listinfo/png-mng-imple-ment https://lists.sourceforge.net/lists/listinfo/png-mng-implement
       ment 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  1.2.41,  December  3,  2009,  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 individu-als individuals
       als 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 merchantabil-ity merchantability
       ity 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 December 3, 2009





                                              December 3, 2009                                     LIBPNG(3)

Reporting Problems

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