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LIBPNG(3)                                                            LIBPNG(3)



NAME
       libpng - Portable Network Graphics (PNG) Reference Library 1.2.49

SYNOPSIS


       #include <png.h>



       png_uint_32 png_access_version_number (void);



       int png_check_sig (png_bytep sig, int num);



       void png_chunk_error (png_structp png_ptr, png_const_charp error);



       void png_chunk_warning (png_structp png_ptr, png_const_charp message);



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



       void png_convert_from_time_t (png_timep ptime, time_t ttime);



       png_charp png_convert_to_rfc1123 (png_structp png_ptr, png_timep ptime);



       png_infop png_create_info_struct (png_structp png_ptr);



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



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



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



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



       int png_debug(int level, png_const_charp message);



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



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



       void png_destroy_info_struct (png_structp png_ptr, png_infopp info_ptr_ptr);



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



       void png_destroy_write_struct (png_structpp png_ptr_ptr, png_infopp info_ptr_ptr);



       void png_error (png_structp png_ptr, png_const_charp error);



       void png_free (png_structp png_ptr, png_voidp ptr);



       void png_free_chunk_list (png_structp png_ptr);



       void png_free_default(png_structp png_ptr, png_voidp ptr);



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



       png_byte png_get_bit_depth (png_structp png_ptr, png_infop info_ptr);



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



       png_byte png_get_channels (png_structp png_ptr, png_infop info_ptr);



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



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



       png_byte png_get_color_type (png_structp png_ptr, png_infop info_ptr);



       png_byte png_get_compression_type (png_structp png_ptr, png_infop info_ptr);



       png_byte png_get_copyright (png_structp png_ptr);



       png_voidp png_get_error_ptr (png_structp png_ptr);



       png_byte png_get_filter_type (png_structp png_ptr, png_infop info_ptr);



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



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



       png_byte png_get_header_ver (png_structp png_ptr);



       png_byte png_get_header_version (png_structp png_ptr);



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



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



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



       png_uint_32 png_get_image_height (png_structp png_ptr, png_infop info_ptr);



       png_uint_32 png_get_image_width (png_structp png_ptr, png_infop info_ptr);



       #if !defined(PNG_1_0_X)

       png_int_32 png_get_int_32 (png_bytep buf);

       #endif



       png_byte png_get_interlace_type (png_structp png_ptr, png_infop info_ptr);



       png_voidp png_get_io_ptr (png_structp png_ptr);



       png_byte png_get_libpng_ver (png_structp png_ptr);



       png_voidp png_get_mem_ptr(png_structp png_ptr);



       png_uint_32 png_get_oFFs (png_structp png_ptr, png_infop info_ptr,  png_uint_32  *offset_x,  png_uint_32  *off-
       set_y, int *unit_type);



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



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



       float png_get_pixel_aspect_ratio (png_structp png_ptr, png_infop info_ptr);



       png_uint_32 png_get_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);



       png_voidp png_get_progressive_ptr (png_structp png_ptr);



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



       png_byte png_get_rgb_to_gray_status (png_structp png_ptr)

       png_uint_32 png_get_rowbytes (png_structp png_ptr, png_infop info_ptr);



       png_bytepp png_get_rows (png_structp png_ptr, png_infop info_ptr);



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



       png_bytep png_get_signature (png_structp png_ptr, png_infop info_ptr);



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



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



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



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



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



       #if !defined(PNG_1_0_X)

       png_uint_16 png_get_uint_16 (png_bytep buf);



       png_uint_32 png_get_uint_31 (png_bytep buf);



       png_uint_32 png_get_uint_32 (png_bytep buf);

       #endif



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



       png_voidp png_get_user_chunk_ptr (png_structp png_ptr);



       png_uint_32 png_get_user_height_max( png_structp png_ptr);



       png_voidp png_get_user_transform_ptr (png_structp png_ptr);



       png_uint_32 png_get_user_width_max (png_structp png_ptr);



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



       png_int_32 png_get_x_offset_microns (png_structp png_ptr, png_infop info_ptr);



       png_int_32 png_get_x_offset_pixels (png_structp png_ptr, png_infop info_ptr);



       png_uint_32 png_get_x_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);



       png_int_32 png_get_y_offset_microns (png_structp png_ptr, png_infop info_ptr);



       png_int_32 png_get_y_offset_pixels (png_structp png_ptr, png_infop info_ptr);



       png_uint_32 png_get_y_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);



       png_uint_32 png_get_compression_buffer_size (png_structp png_ptr);



       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);



       void png_init_io (png_structp png_ptr, FILE *fp);



       DEPRECATED: void png_info_init (png_infop info_ptr);



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



       png_voidp png_malloc (png_structp png_ptr, png_uint_32 size);



       png_voidp png_malloc_default(png_structp png_ptr, png_uint_32 size);



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



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



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



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



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



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



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



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



       void png_read_end (png_structp png_ptr, png_infop info_ptr);



       void png_read_image (png_structp png_ptr, png_bytepp image);



       DEPRECATED: void png_read_init (png_structp png_ptr);



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



       void png_read_info (png_structp png_ptr, png_infop info_ptr);



       void png_read_png (png_structp png_ptr, png_infop info_ptr, int transforms, png_voidp params);



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



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



       void png_read_update_info (png_structp png_ptr, png_infop info_ptr);



       #if !defined(PNG_1_0_X)

       png_save_int_32 (png_bytep buf, png_int_32 i);



       void png_save_uint_16 (png_bytep buf, unsigned int i);



       void png_save_uint_32 (png_bytep buf, png_uint_32 i);



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

       #endif



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



       void png_set_bgr (png_structp png_ptr);



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



       void png_set_cHRM (png_structp png_ptr, png_infop info_ptr, double white_x, double white_y, double red_x,  dou-
       ble 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  warn-
       ing_fn);



       void png_set_expand (png_structp png_ptr);



       void png_set_expand_gray_1_2_4_to_8(png_structp png_ptr);



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



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



       void  png_set_filter_heuristics  (png_structp  png_ptr, int heuristic_method, int num_weights, png_doublep fil-
       ter_weights, png_doublep filter_costs);



       void png_set_flush (png_structp png_ptr, int nrows);



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



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



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



       void png_set_gray_1_2_4_to_8(png_structp png_ptr);



       void png_set_gray_to_rgb (png_structp png_ptr);



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



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



       int png_set_interlace_handling (png_structp png_ptr);



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



       void png_set_invert_alpha (png_structp png_ptr);



       void png_set_invert_mono (png_structp png_ptr);



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



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



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



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



       void png_set_packing (png_structp png_ptr);



       void png_set_packswap (png_structp png_ptr);



       void png_set_palette_to_rgb(png_structp png_ptr);



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



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



       void  png_set_progressive_read_fn  (png_structp  png_ptr,  png_voidp  progressive_ptr, png_progressive_info_ptr
       info_fn, png_progressive_row_ptr row_fn, png_progressive_end_ptr end_fn);



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



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



       void png_set_read_status_fn (png_structp png_ptr, png_read_status_ptr read_row_fn);



       void png_set_read_user_transform_fn (png_structp png_ptr, png_user_transform_ptr read_user_transform_fn);



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



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



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



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



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



       void png_set_shift (png_structp png_ptr, png_color_8p true_bits);



       void png_set_sig_bytes (png_structp png_ptr, int num_bytes);



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



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



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



       void png_set_strip_16 (png_structp png_ptr);



       void png_set_strip_alpha (png_structp png_ptr);



       void png_set_swap (png_structp png_ptr);



       void png_set_swap_alpha (png_structp png_ptr);



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



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



       void  png_set_tRNS  (png_structp  png_ptr,  png_infop  info_ptr,  png_bytep trans, int num_trans, png_color_16p
       trans_values);



       void png_set_tRNS_to_alpha(png_structp png_ptr);



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



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



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



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



       void png_set_user_transform_info (png_structp png_ptr, png_voidp user_transform_ptr, int  user_transform_depth,
       int user_transform_channels);



       void  png_set_write_fn  (png_structp  png_ptr,  png_voidp  io_ptr, png_rw_ptr write_data_fn, png_flush_ptr out-
       put_flush_fn);



       void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr write_row_fn);



       void png_set_write_user_transform_fn (png_structp png_ptr, png_user_transform_ptr write_user_transform_fn);



       void png_set_compression_buffer_size(png_structp png_ptr, png_uint_32 size);



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



       void png_start_read_image (png_structp png_ptr);



       void png_warning (png_structp png_ptr, png_const_charp message);



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



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



       void png_write_chunk_end (png_structp png_ptr);



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



       void png_write_destroy (png_structp png_ptr);



       void png_write_end (png_structp png_ptr, png_infop info_ptr);



       void png_write_flush (png_structp png_ptr);



       void png_write_image (png_structp png_ptr, png_bytepp image);



       DEPRECATED: void png_write_init (png_structp png_ptr);



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



       void png_write_info (png_structp png_ptr, png_infop info_ptr);



       void png_write_info_before_PLTE (png_structp png_ptr, png_infop info_ptr);



       void png_write_png (png_structp png_ptr, png_infop info_ptr, int transforms, png_voidp params);



       void png_write_row (png_structp png_ptr, png_bytep row);



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



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



       void png_zfree (voidpf png_ptr, voidpf ptr);




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

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

        libpng version 1.2.49 - March 29, 2012
        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 1.2.49 - March 29, 2012
        Updated and distributed by Glenn Randers-Pehrson
        Copyright (c) 1998-2009 Glenn Randers-Pehrson

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

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

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


I. Introduction
       This  file describes how to use and modify the PNG reference library (known as libpng) for your own use.  There
       are five sections to this file: introduction, structures, reading, writing, and modification and  configuration
       notes  for  various  special platforms.  In addition to this file, example.c is a good starting point for using
       the library, as it is heavily commented and should include everything most people will need.   We  assume  that
       libpng is already installed; see the INSTALL file for instructions on how to install libpng.

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

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

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

       The PNG-1.2 specification is available at <http://www.libpng.org/pub/png/documents/>;.  It is technically equiv-
       alent 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 Rec-
       ommendation <http://www.w3.org/TR/REC.png.html>;.

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

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

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

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

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

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


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

       The png_info structure is designed to provide information about the PNG file.   At  one  time,  the  fields  of
       png_info  were  intended  to  be  directly accessible to the user.  However, this tended to cause problems with
       applications using dynamically loaded libraries, and as a result a set of interface functions for png_info (the
       png_get_*()  and  png_set_*()  functions)  was developed.  The fields of png_info are still available for older
       applications, but it is suggested that applications use the new interfaces if at all possible.

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

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

       #include <png.h>


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


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

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

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


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


       Next, png_struct and png_info need to be allocated and initialized.  In order to ensure that the size of  these
       structures is correct even with a dynamically linked libpng, there are functions to initialize and allocate the
       structures.  We also pass the library version, optional pointers to error handling functions, and a pointer  to
       a  data  struct  for  use  by  the  error functions, if necessary (the pointer and functions can be NULL if the
       default error handlers are to be used).  See the section on Changes to Libpng below regarding the old  initial-
       ization  functions.   The  structure allocation functions quietly return NULL if they fail to create the struc-
       ture, 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_cre-
       ate_read_struct_2() instead of png_create_read_struct():

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

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

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

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

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

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

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

           png_init_io(png_ptr, fp);

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

           png_set_sig_bytes(png_ptr, number);


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

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

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

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

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

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

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

       To inform libpng about your function, use

           png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
               read_chunk_callback);

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

           png_get_user_chunk_ptr(png_ptr);

       If you call the png_set_read_user_chunk_fn() function, then all unknown chunks will be saved when read, in case
       your  callback  function  will  need  one  or  more  of  them.   This  behavior  can  be   changed   with   the
       png_set_keep_unknown_chunks() function, described below.

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

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

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

       To inform libpng about your function, use

           png_set_read_status_fn(png_ptr, read_row_callback);


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

           png_set_keep_unknown_chunks(png_ptr, keep,
               chunk_list, num_chunks);
           keep       - 0: default unknown chunk handling
                        1: ignore; do not keep
                        2: keep only if safe-to-copy
                        3: keep even if unsafe-to-copy
                      You can use these definitions:
                        PNG_HANDLE_CHUNK_AS_DEFAULT   0
                        PNG_HANDLE_CHUNK_NEVER        1
                        PNG_HANDLE_CHUNK_IF_SAFE      2
                        PNG_HANDLE_CHUNK_ALWAYS       3
           chunk_list - list of chunks affected (a byte string,
                        five bytes per chunk, NULL or ' ' if
                        num_chunks is 0)
           num_chunks - number of chunks affected; if 0, all
                        unknown chunks are affected.  If nonzero,
                        only the chunks in the list are affected

       Unknown  chunks declared in this way will be saved as raw data onto a list of png_unknown_chunk structures.  If
       a chunk that is normally known to libpng is named in the list, it will be handled as unknown, according to  the
       "keep"  directive.   If  a  chunk  is named in successive instances of png_set_keep_unknown_chunks(), the final
       instance will take precedence.  The IHDR and IEND chunks should not be named in chunk_list; if they are, libpng
       will process them normally anyway.

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

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

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

           ...

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


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

          png_set_user_limits(png_ptr, width_max, height_max);

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

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

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

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

          png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

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

          chunk_cache_max = png_get_chunk_cache_max(png_ptr);

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


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

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

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

           png_read_png(png_ptr, info_ptr, png_transforms, NULL)

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

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

       You must use png_transforms and not call any png_set_transform() functions when you use png_read_png().

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

          row_pointers = png_get_rows(png_ptr, info_ptr);

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

          png_bytep row_pointers[height];

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

          if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
             png_error (png_ptr,
                "Image is too tall to process in memory");
          if (width > PNG_UINT_32_MAX/pixel_size)
             png_error (png_ptr,
                "Image is too wide to process in memory");
          row_pointers = png_malloc(png_ptr,
             height*png_sizeof(png_bytep));
          for (int i=0; i<height, i++)
             row_pointers[i]=NULL;  /* security precaution */
          for (int i=0; i<height, i++)
             row_pointers[i]=png_malloc(png_ptr,
                width*pixel_size);
          png_set_rows(png_ptr, info_ptr, &row_pointers);

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

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

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


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

           png_read_info(png_ptr, info_ptr);

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


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

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

           width          - holds the width of the image
                            in pixels (up to 2^31).
           height         - holds the height of the image
                            in pixels (up to 2^31).
           bit_depth      - holds the bit depth of one of the
                            image channels.  (valid values are
                            1, 2, 4, 8, 16 and depend also on
                            the color_type.  See also
                            significant bits (sBIT) below).
           color_type     - describes which color/alpha channels
                                are present.
                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           filter_method  - (must be PNG_FILTER_TYPE_BASE
                            for PNG 1.0, and can also be
                            PNG_INTRAPIXEL_DIFFERENCING if
                            the PNG datastream is embedded in
                            a MNG-1.0 datastream)
           compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                            for PNG 1.0)
           interlace_type - (PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7)

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

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

           width            = png_get_image_width(png_ptr,
                                info_ptr);
           height           = png_get_image_height(png_ptr,
                                info_ptr);
           bit_depth        = png_get_bit_depth(png_ptr,
                                info_ptr);
           color_type       = png_get_color_type(png_ptr,
                                info_ptr);
           filter_method    = png_get_filter_type(png_ptr,
                                info_ptr);
           compression_type = png_get_compression_type(png_ptr,
                                info_ptr);
           interlace_type   = png_get_interlace_type(png_ptr,
                                info_ptr);

           channels = png_get_channels(png_ptr, info_ptr);
           channels       - number of channels of info for the
                            color type (valid values are 1 (GRAY,
                            PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
                            4 (RGB_ALPHA or RGB + filler byte))
           rowbytes = png_get_rowbytes(png_ptr, info_ptr);
           rowbytes       - number of bytes needed to hold a row

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

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

           png_get_PLTE(png_ptr, info_ptr, &palette,
                            &num_palette);
           palette        - the palette for the file
                            (array of png_color)
           num_palette    - number of entries in the palette

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

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

           png_get_iCCP(png_ptr, info_ptr, &name,
              &compression_type, &profile, &proflen);
           name            - The profile name.
           compression     - The compression type; always
                             PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                             You may give NULL to this argument to
                             ignore it.
           profile         - International Color Consortium color
                             profile data. May contain NULs.
           proflen         - length of profile data in bytes.

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

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

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

           png_get_tIME(png_ptr, info_ptr, &mod_time);
           mod_time       - time image was last modified
                           (PNG_VALID_tIME)

           png_get_bKGD(png_ptr, info_ptr, &background);
           background     - background color (PNG_VALID_bKGD)
                            valid 16-bit red, green and blue
                            values, regardless of color_type

           num_comments   = png_get_text(png_ptr, info_ptr,
                            &text_ptr, &num_text);
           num_comments   - number of comments
           text_ptr       - array of png_text holding image
                            comments
           text_ptr[i].compression - type of compression used
                        on "text" PNG_TEXT_COMPRESSION_NONE
                                  PNG_TEXT_COMPRESSION_zTXt
                                  PNG_ITXT_COMPRESSION_NONE
                                  PNG_ITXT_COMPRESSION_zTXt
           text_ptr[i].key   - keyword for comment.  Must contain
                                1-79 characters.
           text_ptr[i].text  - text comments for current
                                keyword.  Can be empty.
           text_ptr[i].text_length - length of text string,
                        after decompression, 0 for iTXt
           text_ptr[i].itxt_length - length of itxt string,
                        after decompression, 0 for tEXt/zTXt
           text_ptr[i].lang  - language of comment (empty
                                string for unknown).
           text_ptr[i].lang_key  - keyword in UTF-8
                                (empty string for unknown).
           Note that the itxt_length, lang, and lang_key
           members of the text_ptr structure only exist
           when the library is built with iTXt chunk support.

           num_text       - number of comments (same as
                            num_comments; you can put NULL here
                            to avoid the duplication)
           Note while png_set_text() will accept text, language,
           and translated keywords that can be NULL pointers, the
           structure returned by png_get_text will always contain
           regular zero-terminated C strings.  They might be
           empty strings but they will never be NULL pointers.

           num_spalettes = png_get_sPLT(png_ptr, info_ptr,
              &palette_ptr);
           palette_ptr    - array of palette structures holding
                            contents of one or more sPLT chunks
                            read.
           num_spalettes  - number of sPLT chunks read.

           png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
              &unit_type);
           offset_x       - positive offset from the left edge
                            of the screen
           offset_y       - positive offset from the top edge
                            of the screen
           unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

           png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
              &unit_type);
           res_x          - pixels/unit physical resolution in
                            x direction
           res_y          - pixels/unit physical resolution in
                            x direction
           unit_type      - PNG_RESOLUTION_UNKNOWN,
                            PNG_RESOLUTION_METER

           png_get_sCAL(png_ptr, info_ptr, &unit, &width,
              &height)
           unit        - physical scale units (an integer)
           width       - width of a pixel in physical scale units
           height      - height of a pixel in physical scale units
                        (width and height are doubles)

           png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
              &height)
           unit        - physical scale units (an integer)
           width       - width of a pixel in physical scale units
           height      - height of a pixel in physical scale units
                        (width and height are strings like "2.54")

           num_unknown_chunks = png_get_unknown_chunks(png_ptr,
              info_ptr, &unknowns)
           unknowns          - array of png_unknown_chunk
                               structures holding unknown chunks
           unknowns[i].name  - name of unknown chunk
           unknowns[i].data  - data of unknown chunk
           unknowns[i].size  - size of unknown chunk's data
           unknowns[i].location - position of chunk in file

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

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

           res_x = png_get_x_pixels_per_meter(png_ptr,
              info_ptr)
           res_y = png_get_y_pixels_per_meter(png_ptr,
              info_ptr)
           res_x_and_y = png_get_pixels_per_meter(png_ptr,
              info_ptr)
           res_x = png_get_x_pixels_per_inch(png_ptr,
              info_ptr)
           res_y = png_get_y_pixels_per_inch(png_ptr,
              info_ptr)
           res_x_and_y = png_get_pixels_per_inch(png_ptr,
              info_ptr)
           aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
              info_ptr)

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

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

           x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
           y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
           x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
           y_offset = png_get_y_offset_inches(png_ptr, info_ptr);

          (Each of these returns 0 [signifying "unknown" if both
              x and y are 0] if the data is not present or if the
              chunk is present but the unit is the pixel)

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

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

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


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

       The  colors  used for the background and transparency values should be supplied in the same format/depth as the
       current image data.  They are stored in the same format/depth as the image data in a bKGD  or  tRNS  chunk,  so
       this is what libpng expects for this data.  The colors are transformed to keep in sync with the image data when
       an application calls the png_read_update_info() routine (see below).

       Data will be decoded into the supplied row buffers packed into bytes unless the library has been told to trans-
       form  it  into  another  format.   For  example, 4 bit/pixel paletted or grayscale data will be returned 2 pix-
       els/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 func-
       tion 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 1.2.49, 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 rep-
       resent  the  color  type, "T" means the tRNS chunk is present, A means an alpha channel is present, and O means
       tRNS or alpha is present but all pixels in the image are opaque.

         FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O  6A  6O
          TO
          01    -
          31        -
           0    1       -
          0T                -
          0O                    -
           2           GX           -
          2T                            -
          2O                                -
           3        1                           -
          3T                                        -
          3O                                            -
          4A                T                               -
          4O                                                    -
          6A               GX         TX           TX               -
          6O                   GX                      TX               -

       Within the matrix,
            "-" means the transformation is not supported.
            "X" means the transformation is obtained by png_set_expand().
            "1" means the transformation is obtained by
                png_set_expand_gray_1_2_4_to_8
            "G" means the transformation is obtained by
                png_set_gray_to_rgb().
            "P" means the transformation is obtained by
                png_set_expand_palette_to_rgb().
            "T" means the transformation is obtained by
                png_set_tRNS_to_alpha().

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

           if (bit_depth == 16)
               png_set_strip_16(png_ptr);

       If,  for  some reason, you don't need the alpha channel on an image, and you want to remove it rather than com-
       bining 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  read-
       ing 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 pix-
       els:

           if (bit_depth < 8)
               png_set_packing(png_ptr);

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

           png_color_8p sig_bit;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_RGB_ALPHA)
                 png_set_rgb_to_gray_fixed(png_ptr, error_action,
                    int red_weight, int green_weight);

           error_action = 1: silently do the conversion
           error_action = 2: issue a warning if the original
                             image has any pixel where
                             red != green or red != blue
           error_action = 3: issue an error and abort the
                             conversion if the original
                             image has any pixel where
                             red != green or red != blue

           red_weight:       weight of red component times 100000
           green_weight:     weight of green component times 100000
                             If either weight is negative, default
                             weights (21268, 71514) are used.

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

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

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

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

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

       Libpng approximates this with

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

       which can be expressed with integers as

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

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

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

           png_color_16 my_background;
           png_color_16p image_background;

           if (png_get_bKGD(png_ptr, info_ptr, &image_background))
               png_set_background(png_ptr, image_background,
                 PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
           else
               png_set_background(png_ptr, &my_background,
                 PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);

       The png_set_background() function tells libpng to composite images with alpha or  simple  transparency  against
       the  supplied  background color.  If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid), you may use this
       color, or supply another color more suitable for the current display (e.g., the background  color  from  a  web
       page).   You  need  to  tell  libpng  whether  the  color  is  in  the  gamma  space  of the display (PNG_BACK-
       GROUND_GAMMA_SCREEN for colors you supply), the  file  (PNG_BACKGROUND_GAMMA_FILE  for  colors  from  the  bKGD
       chunk), or one that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't know why anyone would use
       this, but it's here).

       To properly display PNG images on any kind of system, the application needs to know what the display gamma  is.
       Ideally,  the  user  will know this, and the application will allow them to set it.  One method of allowing the
       user to set the display gamma separately for each system is to check for a SCREEN_GAMMA or DISPLAY_GAMMA  envi-
       ronment 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  cur-
       rent  screen_gamma.   If the file does not have a gamma value, you can pass one anyway if you have an idea what
       it is (usually 0.45455 is a good guess for GIF images on PCs).  Note that file gammas are inverted from  screen
       gammas.   See  the discussions on gamma in the PNG specification for an excellent description of what gamma is,
       and why all applications should support it.  It is strongly recommended that PNG viewers support gamma  correc-
       tion.

          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 col-
       ors  in  the  palette so it will fit into maximum_colors.  If there is a histogram, it will use it to make more
       intelligent choices when reducing the palette.  If there is no histogram, it may not do as good a job.

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

                png_get_hIST(png_ptr, info_ptr,
                   &histogram);
                png_set_dither(png_ptr, palette, num_palette,
                   max_screen_colors, histogram, 1);
             }
             else
             {
                png_color std_color_cube[MAX_SCREEN_COLORS] =
                   { ... colors ... };

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

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

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

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

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

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

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

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

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

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

           png_set_read_user_transform_fn(png_ptr,
              read_transform_fn);

       You must supply the function

           void read_transform_fn(png_ptr ptr, row_info_ptr
              row_info, png_bytep data)

       See  pngtest.c for a working example.  Your function will be called after all of the other transformations have
       been processed.

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

           png_set_user_transform_info(png_ptr, user_ptr,
              user_depth, user_channels);

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

       You can retrieve the pointer via the function png_get_user_transform_ptr().  For example:

           voidp read_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

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

           number_of_passes = png_set_interlace_handling(png_ptr);

       After  setting  the  transformations,  libpng can update your png_info structure to reflect any transformations
       you've requested with this call.  This is most useful to update the info structure's rowbytes field so you  can
       use  it  to  allocate  your  image  memory.   This  function  will  also  update  your palette with the correct
       screen_gamma and background if these have been given with the calls above.

           png_read_update_info(png_ptr, info_ptr);

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


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

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

          png_read_image(png_ptr, row_pointers);

       where row_pointers is:

          png_bytep row_pointers[height];

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

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

           png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

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

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

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

       If  the  file is interlaced (interlace_type != 0 in the IHDR chunk), things get somewhat harder.  The only cur-
       rent (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 inter-
       lace 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 col-
       umn  4) and 1/8 as high (also starting in row 0).  The third pass will be 1/4 as wide (every 4th pixel starting
       in column 0) and 1/8 as high (every 8th row starting in row 4), and the fourth pass will be 1/4 as wide and 1/4
       as  high  (every  4th  column  starting in column 2, and every 4th row starting in row 0).  The fifth pass will
       return an image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2), while the sixth pass will be 1/2
       as  wide  and 1/2 as high as the original (starting in column 1 and row 0).  The seventh and final pass will be
       as wide as the original, and 1/2 as high, containing all of the odd numbered scanlines.  Phew!

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

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

       This  will  return  the number of passes needed.  Currently, this is seven, but may change if another interlace
       type is added.  This function can be called even if the file is not interlaced, where it will return one  pass.

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

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

           png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

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

           png_read_rows(png_ptr, NULL, row_pointers,
              number_of_rows);


   Finishing a sequential read
       After you are finished reading the image through the low-level interface, you can finish reading the file.   If
       you  are  interested in comments or time, which may be stored either before or after the image data, you should
       pass the separate png_info struct if you want to keep the comments from before and after  the  image  separate.
       If you are not interested, you can pass NULL.

          png_read_end(png_ptr, end_info);

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

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

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

           png_free_data(png_ptr, info_ptr, mask, seq)
           mask - identifies data to be freed, a mask
                  containing the bitwise OR of one or
                  more of
                    PNG_FREE_PLTE, PNG_FREE_TRNS,
                    PNG_FREE_HIST, PNG_FREE_ICCP,
                    PNG_FREE_PCAL, PNG_FREE_ROWS,
                    PNG_FREE_SCAL, PNG_FREE_SPLT,
                    PNG_FREE_TEXT, PNG_FREE_UNKN,
                  or simply PNG_FREE_ALL
           seq  - sequence number of item to be freed
                  (-1 for all items)

       This function may be safely called when the relevant storage has already been freed, or has not yet been  allo-
       cated,  or was allocated by the user and not by libpng,  and will in those cases do nothing.  The "seq" parame-
       ter 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_mal-
       loc() 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 responsi-
       bility 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_zal-
       loc() to allocate it.

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

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

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

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

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


   Reading PNG files progressively
       The   progressive   reader  is  slightly  different  then  the  non-progressive  reader.   Instead  of  calling
       png_read_info(), png_read_rows(), and png_read_end(), you make one  call  to  png_process_data(),  which  calls
       callbacks  when  it has the info, a row, or the end of the image.  You set up these callbacks with png_set_pro-
       gressive_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_cre-
       ate_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_jmp-
       buf(png_ptr).  If you write the file from different routines, you will need to update  the  png_jmpbuf(png_ptr)
       every time you enter a new routine that will call a png_*() function.  See your documentation of setjmp/longjmp
       for your compiler for more information on setjmp/longjmp.  See the discussion on libpng error handling  in  the
       Customizing Libpng section below for more information on the libpng error handling.

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

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

       Now  you need to set up the output code.  The default for libpng is to use the C function fwrite().  If you use
       this, you will need to pass a valid FILE * in the function png_init_io().  Be sure that the file is  opened  in
       binary  mode.   Again, if you wish to handle writing data in another way, see the discussion on libpng I/O han-
       dling 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  signa-
       ture, or if you have already written the signature in your application, use

           png_set_sig_bytes(png_ptr, 8);

       to inform libpng that it should not write a signature.


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

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

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

       To inform libpng about your function, use

           png_set_write_status_fn(png_ptr, write_row_callback);

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


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

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

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

       The png_set_compression_*() functions interface to the zlib compression library, and should mostly  be  ignored
       unless you really know what you are doing.  The only generally useful call is png_set_compression_level() which
       changes how much time zlib spends on trying to compress the image data.  See the  Compression  Library  (zlib.h
       and algorithm.txt, distributed with zlib) for details on the compression levels.

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

           /* set other zlib parameters */
           png_set_compression_mem_level(png_ptr, 8);
           png_set_compression_strategy(png_ptr,
               Z_DEFAULT_STRATEGY);
           png_set_compression_window_bits(png_ptr, 15);
           png_set_compression_method(png_ptr, 8);
           png_set_compression_buffer_size(png_ptr, 8192)

       extern PNG_EXPORT(void,png_set_zbuf_size)


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

       Some of the more important parts of the png_info are:

           png_set_IHDR(png_ptr, info_ptr, width, height,
              bit_depth, color_type, interlace_type,
              compression_type, filter_method)
           width          - holds the width of the image
                            in pixels (up to 2^31).
           height         - holds the height of the image
                            in pixels (up to 2^31).
           bit_depth      - holds the bit depth of one of the
                            image channels.
                            (valid values are 1, 2, 4, 8, 16
                            and depend also on the
                            color_type.  See also significant
                            bits (sBIT) below).
           color_type     - describes which color/alpha
                            channels are present.
                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7
           compression_type - (must be
                            PNG_COMPRESSION_TYPE_DEFAULT)
           filter_method  - (must be PNG_FILTER_TYPE_DEFAULT
                            or, if you are writing a PNG to
                            be embedded in a MNG datastream,
                            can also be
                            PNG_INTRAPIXEL_DIFFERENCING)

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

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

           png_set_PLTE(png_ptr, info_ptr, palette,
              num_palette);
           palette        - the palette for the file
                            (array of png_color)
           num_palette    - number of entries in the palette

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

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


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

           png_set_iCCP(png_ptr, info_ptr, name, compression_type,
                             profile, proflen);
           name            - The profile name.
           compression     - The compression type; always
                             PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                             You may give NULL to this argument to
                             ignore it.
           profile         - International Color Consortium color
                             profile data. May contain NULs.
           proflen         - length of profile data in bytes.

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

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

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

           png_set_tIME(png_ptr, info_ptr, mod_time);
           mod_time       - time image was last modified
                            (PNG_VALID_tIME)

           png_set_bKGD(png_ptr, info_ptr, background);
           background     - background color (PNG_VALID_bKGD)

           png_set_text(png_ptr, info_ptr, text_ptr, num_text);
           text_ptr       - array of png_text holding image
                            comments
           text_ptr[i].compression - type of compression used
                        on "text" PNG_TEXT_COMPRESSION_NONE
                                  PNG_TEXT_COMPRESSION_zTXt
                                  PNG_ITXT_COMPRESSION_NONE
                                  PNG_ITXT_COMPRESSION_zTXt
           text_ptr[i].key   - keyword for comment.  Must contain
                        1-79 characters.
           text_ptr[i].text  - text comments for current
                                keyword.  Can be NULL or empty.
           text_ptr[i].text_length - length of text string,
                        after decompression, 0 for iTXt
           text_ptr[i].itxt_length - length of itxt string,
                        after decompression, 0 for tEXt/zTXt
           text_ptr[i].lang  - language of comment (NULL or
                                empty for unknown).
           text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
                                or empty for unknown).
           Note that the itxt_length, lang, and lang_key
           members of the text_ptr structure only exist
           when the library is built with iTXt chunk support.

           num_text       - number of comments

           png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
              num_spalettes);
           palette_ptr    - array of png_sPLT_struct structures
                            to be added to the list of palettes
                            in the info structure.
           num_spalettes  - number of palette structures to be
                            added.

           png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
               unit_type);
           offset_x  - positive offset from the left
                            edge of the screen
           offset_y  - positive offset from the top
                            edge of the screen
           unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

           png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
               unit_type);
           res_x       - pixels/unit physical resolution
                         in x direction
           res_y       - pixels/unit physical resolution
                         in y direction
           unit_type   - PNG_RESOLUTION_UNKNOWN,
                         PNG_RESOLUTION_METER

           png_set_sCAL(png_ptr, info_ptr, unit, width, height)
           unit        - physical scale units (an integer)
           width       - width of a pixel in physical scale units
           height      - height of a pixel in physical scale units
                         (width and height are doubles)

           png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
           unit        - physical scale units (an integer)
           width       - width of a pixel in physical scale units
           height      - height of a pixel in physical scale units
                        (width and height are strings like "2.54")

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

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

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

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

       Until  text gets around 1000 bytes, it is not worth compressing it.  After the text has been written out to the
       file, the compression type is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR, so  that  it
       isn't written out again at the end (in case you are calling png_write_end() with the same struct.

       The keywords that are given in the PNG Specification are:

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

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

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

       If you want to store the time of the original image creation, you should use a plain tEXt chunk with the  "Cre-
       ation Time" keyword.  This is necessary because the "creation time" of a PNG image is somewhat vague, depending
       on whether you mean the PNG file, the time the image was created in a non-PNG format, a still photo from  which
       the  image  was scanned, or possibly the subject matter itself.  In order to facilitate machine-readable dates,
       it is recommended that the "Creation Time" tEXt chunk use RFC 1123 format dates (e.g.  "22  May  1997  18:07:10
       GMT"),  although  this  isn't  a  requirement.   Unlike  the  tIME chunk, the "Creation Time" tEXt chunk is not
       expected to be automatically changed by the software.  To facilitate the use of  RFC  1123  dates,  a  function
       png_convert_to_rfc1123(png_timep) is provided to convert from PNG time to an RFC 1123 format string.


   Writing unknown chunks
       You  can use the png_set_unknown_chunks function to queue up chunks for writing.  You give it a chunk name, raw
       data, and  a  size;  that's  all  there  is  to  it.   The  chunks  will  be  written  by  the  next  following
       png_write_info_before_PLTE,  png_write_info,  or  png_write_end  function.  Any chunks previously read into the
       info structure's unknown-chunk list will also be written out in a sequence that satisfies  the  PNG  specifica-
       tion's ordering rules.


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

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

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

           png_write_png(png_ptr, info_ptr, png_transforms, NULL)

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

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

       You must use png_transforms and not call any png_set_transform() functions when you use png_write_png().


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

           png_write_info(png_ptr, info_ptr);

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

           png_set_invert_alpha(png_ptr);

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

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

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

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

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

           png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

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

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

           png_set_packing(png_ptr);

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

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

           png_set_sBIT(png_ptr, info_ptr, &sig_bit);

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

           png_set_shift(png_ptr, &sig_bit);

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

           if (bit_depth > 8)
              png_set_swap(png_ptr);

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

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

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

           png_set_bgr(png_ptr);

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

           png_set_invert_mono(png_ptr);

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

           png_set_write_user_transform_fn(png_ptr,
              write_transform_fn);

       You must supply the function

           void write_transform_fn(png_ptr ptr, row_info_ptr
              row_info, png_bytep data)

       See pngtest.c for a working example.  Your function will be called before any of the other transformations  are
       processed.

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

           png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

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

       You can retrieve the pointer via the function png_get_user_transform_ptr().  For example:

           voidp write_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

       It is possible to have libpng flush any pending output, either manually, or automatically after a certain  num-
       ber 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 out-
       put 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).  Infre-
       quent  flushing  will  only  degrade  the  compression performance by a few percent over images that do not use
       flushing.


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

           png_write_image(png_ptr, row_pointers);

       where row_pointers is:

           png_byte *row_pointers[height];

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

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

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

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

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

           png_bytep row_pointer = row;

           png_write_row(png_ptr, row_pointer);

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

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

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

           number_of_passes =
              png_set_interlace_handling(png_ptr);

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

       Then write the complete image number_of_passes times.

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

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


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

           png_write_end(png_ptr, info_ptr);

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

           png_destroy_write_struct(&png_ptr, &info_ptr);

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

           png_free_data(png_ptr, info_ptr, mask, seq)
           mask  - identifies data to be freed, a mask
                   containing the bitwise OR of one or
                   more of
                     PNG_FREE_PLTE, PNG_FREE_TRNS,
                     PNG_FREE_HIST, PNG_FREE_ICCP,
                     PNG_FREE_PCAL, PNG_FREE_ROWS,
                     PNG_FREE_SCAL, PNG_FREE_SPLT,
                     PNG_FREE_TEXT, PNG_FREE_UNKN,
                   or simply PNG_FREE_ALL
           seq   - sequence number of item to be freed
                   (-1 for all items)

       This function may be safely called when the relevant storage has already been freed, or has not yet been  allo-
       cated, or was allocated by the user  and not by libpng,  and will in those cases do nothing.  The "seq" parame-
       ter 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_mal-
       loc() or png_zalloc() and passed in via a png_set_*() function, with

           png_data_freer(png_ptr, info_ptr, freer, mask)
           mask   - which data elements are affected
                    same choices as in png_free_data()
           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

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

           png_data_freer(read_ptr, read_info_ptr,
              PNG_USER_WILL_FREE_DATA,
              PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
           png_data_freer(write_ptr, write_info_ptr,
              PNG_DESTROY_WILL_FREE_DATA,
              PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

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

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

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


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

       Memory allocation, input/output, and error handling

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

       Memory allocation is done through the functions png_malloc(), png_calloc(), and  png_free().   These  currently
       just  call  the standard C functions.  png_calloc() calls png_malloc() and then png_memset() to clear the newly
       allocated memory to zero.  If your pointers can't access more then  64K  at  a  time,  you  will  want  to  set
       MAXSEG_64K  in  zlib.h.   Since it is unlikely that the method of handling memory allocation on a platform will
       change between applications, these functions must be modified in the library at compile time.  If you prefer to
       use  a  different  method  of  allocating  and freeing data, you can use png_create_read_struct_2() or png_cre-
       ate_write_struct_2() to register your own functions as described above.  These functions also  provide  a  void
       pointer that can be retrieved via

           mem_ptr=png_get_mem_ptr(png_ptr);

       Your replacement memory functions must have prototypes as follows:

           png_voidp malloc_fn(png_structp png_ptr,
              png_size_t size);
           void free_fn(png_structp png_ptr, png_voidp ptr);

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

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

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

           png_set_read_fn(png_structp read_ptr,
               voidp read_io_ptr, png_rw_ptr read_data_fn)

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

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

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

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

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

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

       Error handling in libpng is done through png_error() and png_warning().  Errors handled through png_error() are
       fatal, meaning that png_error() should never return to its caller.  Currently, this is handled via setjmp() and
       longjmp() (unless you have compiled libpng with PNG_SETJMP_NOT_SUPPORTED, in  which  case  it  is  handled  via
       PNG_ABORT()), but you could change this to do things like exit() if you should wish.

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

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

           png_voidp error_ptr = png_get_error_ptr(png_ptr);

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

           void user_error_fn(png_structp png_ptr,
               png_const_charp error_msg);
           void user_warning_fn(png_structp png_ptr,
               png_const_charp warning_msg);

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


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

       If  you  need to write a new intrinsic chunk, first read the PNG specification. Acquire a first level of under-
       standing of how it works.  Pay particular attention to the sections that describe chunk names, and look at  how
       other chunks were designed, so you can do things similarly.  Second, check out the sections of libpng that read
       and write chunks.  Try to find a chunk that is similar to yours and use it as a template.  More details can  be
       found  in  the comments inside the code.  It is best to handle unknown chunks in a generic method, via callback
       functions, instead of by modifying libpng functions.

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


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


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


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


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


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


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

           png_set_compression_level(png_ptr, level);

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

           png_set_compression_mem_level(png_ptr, level);

       The other functions are for configuring zlib.  They are not recommended for normal use and may result in  writ-
       ing 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 specifica-
       tion.  The 'filters' parameter sets which filter(s), if any, should be used for each scanline.  Possible values
       are PNG_ALL_FILTERS and PNG_NO_FILTERS to turn filtering on and off, respectively.

       Individual  filter  types are PNG_FILTER_NONE, PNG_FILTER_SUB, PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH,
       which can be bitwise ORed together with '|' to specify one or more filters to use.  These filters are described
       in  more detail in the PNG specification.  If you intend to change the filter type during the course of writing
       the image, you should start with flags set for all of the filters you intend to use so that libpng can initial-
       ize  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 succes-
       sive rows unless another row filter is that many times better than the previous filter.  In the above  example,
       if  the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a "sum of absolute differences" 1.5 x
       1.3 times higher than other filters and still be chosen, while the NONE filter  could  have  a  sum  1.1  times
       higher  than  other  filters  and  still be chosen.  Unspecified weights are taken to be 1.0, and the specified
       weights should probably be declining like those above in order to emphasize recent filters over older  filters.

       The filter costs specify for each filter type a relative decoding cost to be considered when selecting row fil-
       ters.  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  func-
       tion usage.  Little testing has been done to find optimum values for either the costs or the weights.


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

       You can also turn all of the transforms and ancillary chunk capabilities off en masse with compiler  directives
       that  define  PNG_NO_READ[or  WRITE]_TRANSFORMS,  or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS, or all four, along
       with directives to turn on any of the capabilities that you  do  want.   The  PNG_NO_READ[or  WRITE]_TRANSFORMS
       directives  disable  the extra transformations but still leave the library fully capable of reading and writing
       PNG files with all known public chunks. Use of the PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive produces  a
       library that is incapable of reading or writing ancillary chunks.  If you are not using the progressive reading
       capability, you can turn that off with PNG_NO_PROGRESSIVE_READ (don't confuse this with the  INTERLACING  capa-
       bility, 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, png-
       trans.c, etc.)  are used for both reading and writing, and always need to be included.  The progressive  reader
       is in pngpread.c

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


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

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

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

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

          png_debug1(2, "foo=%d0, foo);

       is expanded to

          if(PNG_DEBUG > 2)
            fprintf(PNG_DEBUG_FILE, "foo=%d0, foo);

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

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

       When  PNG_DEBUG  =  1,  the macros are defined, but only png_debug statements having level = 0 will be printed.
       There aren't any such statements in this version of libpng, but if you insert some they will be printed.


VI. MNG support
       The MNG specification (available at http://www.libpng.org/pub/mng) allows certain extensions  to  PNG  for  PNG
       images that are embedded in MNG datastreams.  Libpng can support some of these extensions.  To enable them, use
       the png_permit_mng_features() function:

          feature_set = png_permit_mng_features(png_ptr, mask)
          mask is a png_uint_32 containing the bitwise OR of the
               features you want to enable.  These include
               PNG_FLAG_MNG_EMPTY_PLTE
               PNG_FLAG_MNG_FILTER_64
               PNG_ALL_MNG_FEATURES
          feature_set is a png_uint_32 that is the bitwise AND of
             your mask with the set of MNG features that is
             supported by the version of libpng that you are using.

       It is an error to use this function when reading  or  writing  a  standalone  PNG  file  with  the  PNG  8-byte
       signature.   The PNG datastream must be wrapped in a MNG datastream.  As a minimum, it must have the MNG 8-byte
       signature and the MHDR and MEND chunks.  Libpng does not provide support for these or  any  other  MNG  chunks;
       your  application  must  provide its own support for them.  You may wish to consider using libmng (available at
       http://www.libmng.com) instead.


VII. Changes to Libpng from version 0.88
       It should be noted that versions of libpng later than 0.96 are not distributed by the original  libpng  author,
       Guy Schalnat, nor by Andreas Dilger, who had taken over from Guy during 1996 and 1997, and distributed versions
       0.89 through 0.96, but rather by another member of the original PNG  Group,  Glenn  Randers-Pehrson.   Guy  and
       Andreas are still alive and well, but they have moved on to other things.

       The   old   libpng   functions  png_read_init(),  png_write_init(),  png_info_init(),  png_read_destroy(),  and
       png_write_destroy() have been moved to PNG_INTERNAL in version 0.95 to discourage their use.   These  functions
       will be removed from libpng version 2.0.0.

       The  preferred  method  of creating and initializing the libpng structures is via the png_create_read_struct(),
       png_create_write_struct(), and png_create_info_struct() because they isolate the size of  the  structures  from
       the  application, allow version error checking, and also allow the use of custom error handling routines during
       the initialization, which the old functions do not.  The functions png_read_destroy()  and  png_write_destroy()
       do not actually free the memory that libpng allocated for these structs, but just reset the data structures, so
       they can be used instead of png_destroy_read_struct() and png_destroy_write_struct() if you feel there  is  too
       much system overhead allocating and freeing the png_struct for each image read.

       Setting  the error callbacks via png_set_message_fn() before png_read_init() as was suggested in libpng-0.88 is
       no longer supported because this caused applications that do not use custom error  functions  to  fail  if  the
       png_ptr was not initialized to zero.  It is still possible to set the error callbacks AFTER png_read_init(), or
       to change them with png_set_error_fn(), which is essentially the same function, but with a new  name  to  force
       compilation errors with applications that try to use the old method.

       Starting with version 1.0.7, you can find out which version of the library you are using at run-time:

          png_uint_32 libpng_vn = png_access_version_number();

       The number libpng_vn is constructed from the major version, minor version with leading zero, and release number
       with leading zero, (e.g., libpng_vn for version 1.0.7 is 10007).

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

          png_uint_32 application_vn = PNG_LIBPNG_VER;


VIII. Changes to Libpng from version 1.0.x to 1.2.x
       Support for user memory management was  enabled  by  default.   To  accomplish  this,  the  functions  png_cre-
       ate_read_struct_2(),  png_create_write_struct_2(),  png_set_mem_fn(),  png_get_mem_ptr(), png_malloc_default(),
       and png_free_default() were added.

       Support for the iTXt chunk has been enabled by default as of version 1.2.41.

       Support for certain MNG features was enabled.

       Support for numbered error messages was added.  However, we never got around to actually  numbering  the  error
       messages.   The  function  png_set_strip_error_numbers()  was  added (Note: the prototype for this function was
       inadvertently removed from png.h  in  PNG_NO_ASSEMBLER_CODE  builds  of  libpng-1.2.15.   It  was  restored  in
       libpng-1.2.36).

       The  png_malloc_warn()  function was added at libpng-1.2.3.  This issues a png_warning and returns NULL instead
       of aborting when it fails to acquire the requested memory allocation.

       Support  for  setting  user  limits  on  image  width  and  height  was  enabled  by  default.   The  functions
       png_set_user_limits(), png_get_user_width_max(), and png_get_user_height_max() were added at libpng-1.2.6.

       The png_set_add_alpha() function was added at libpng-1.2.7.

       The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9.  Unlike png_set_gray_1_2_4_to_8(), the
       new function does not expand the tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is deprecated.

       A number of macro definitions in support of runtime selection of assembler code features (especially Intel  MMX
       code support) were added at libpng-1.2.0:

           PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
           PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
           PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
           PNG_ASM_FLAG_MMX_READ_INTERLACE
           PNG_ASM_FLAG_MMX_READ_FILTER_SUB
           PNG_ASM_FLAG_MMX_READ_FILTER_UP
           PNG_ASM_FLAG_MMX_READ_FILTER_AVG
           PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
           PNG_ASM_FLAGS_INITIALIZED
           PNG_MMX_READ_FLAGS
           PNG_MMX_FLAGS
           PNG_MMX_WRITE_FLAGS
           PNG_MMX_FLAGS

       We added the following functions in support of runtime selection of assembler code features:

           png_get_mmx_flagmask()
           png_set_mmx_thresholds()
           png_get_asm_flags()
           png_get_mmx_bitdepth_threshold()
           png_get_mmx_rowbytes_threshold()
           png_set_asm_flags()

       We  replaced  all  of  these  functions  with  simple stubs in libpng-1.2.20, when the Intel assembler code was
       removed due to a licensing issue.

       These macros are deprecated:

           PNG_READ_TRANSFORMS_NOT_SUPPORTED
           PNG_PROGRESSIVE_READ_NOT_SUPPORTED
           PNG_NO_SEQUENTIAL_READ_SUPPORTED
           PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
           PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
           PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED

       They have been replaced, respectively, by:

           PNG_NO_READ_TRANSFORMS
           PNG_NO_PROGRESSIVE_READ
           PNG_NO_SEQUENTIAL_READ
           PNG_NO_WRITE_TRANSFORMS
           PNG_NO_READ_ANCILLARY_CHUNKS
           PNG_NO_WRITE_ANCILLARY_CHUNKS

       PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.  It has been deprecated since libpng-1.0.16 and libpng-1.2.6.

       The function
           png_check_sig(sig, num) was replaced with
           !png_sig_cmp(sig, 0, num) It has been deprecated since libpng-0.90.

       The function
           png_set_gray_1_2_4_to_8() which also expands tRNS to alpha was replaced with
           png_set_expand_gray_1_2_4_to_8() which does not. It has been deprecated since libpng-1.0.18 and 1.2.9.


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

           AC_CHECK_LIB(png, png_get_io_ptr, ...


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

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

       or you can browse it via "gitweb" at

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

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

           http://libpng.sourceforge.net


XII. Coding style
       Our coding style is similar to the "Allman" style, with curly braces on separate lines:

           if (condition)
           {
              action;
           }

           else if (another condition)
           {
              another action;
           }

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

           if (condition)
              return (0);

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

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

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

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

           /* Single-line comment */
           statement;

           /* Multiple-line
            * comment
            */
           statement;

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

           statement;    /* comment */

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

       Functions and their curly braces are not indented, and exported functions are marked with PNGAPI:

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

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

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

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

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

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

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

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

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

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

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

       We do not use the TAB character for indentation in the C sources.

       Lines do not exceed 80 characters.

       Other rules can be inferred by inspecting the libpng source.


XIII. Y2K Compliance in libpng
       March 29, 2012

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

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

       The integer is
           "png_uint_16 year" in png_time_struct.

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

       There are seven time-related functions:

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

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

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

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


          Glenn Randers-Pehrson
          libpng maintainer
          PNG Development Group


NOTE
       Note about libpng version numbers:

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

        source             png.h  png.h  shared-lib
        version            string   int  version
        -------            ------  ----- ----------
        0.89c ("beta 3")  0.89       89  1.0.89
        0.90  ("beta 4")  0.90       90  0.90
        0.95  ("beta 5")  0.95       95  0.95
        0.96  ("beta 6")  0.96       96  0.96
        0.97b ("beta 7")  1.00.97    97  1.0.1
        0.97c             0.97       97  2.0.97
        0.98              0.98       98  2.0.98
        0.99              0.99       98  2.0.99
        0.99a-m           0.99       99  2.0.99
        1.00              1.00      100  2.1.0
        1.0.0             1.0.0     100  2.1.0
        1.0.0   (from here on, the  100  2.1.0
        1.0.1    png.h string is  10001  2.1.0
        1.0.1a-e identical to the 10002  from here on, the
        1.0.2    source version)  10002  shared library is 2.V
        1.0.2a-b                  10003  where V is the source
        1.0.1                     10001  code version except as
        1.0.1a-e                  10002  2.1.0.1a-e   noted.
        1.0.2                     10002  2.1.0.2
        1.0.2a-b                  10003  2.1.0.2a-b
        1.0.3                     10003  2.1.0.3
        1.0.3a-d                  10004  2.1.0.3a-d
        1.0.4                     10004  2.1.0.4
        1.0.4a-f                  10005  2.1.0.4a-f
        1.0.5 (+ 2 patches)       10005  2.1.0.5
        1.0.5a-d                  10006  2.1.0.5a-d
        1.0.5e-r                  10100  2.1.0.5e-r
        1.0.5s-v                  10006  2.1.0.5s-v
        1.0.6 (+ 3 patches)       10006  2.1.0.6
        1.0.6d-g                  10007  2.1.0.6d-g
        1.0.6h                    10007  10.6h
        1.0.6i                    10007  10.6i
        1.0.6j                    10007  2.1.0.6j
        1.0.7beta11-14    DLLNUM  10007  2.1.0.7beta11-14
        1.0.7beta15-18       1    10007  2.1.0.7beta15-18
        1.0.7rc1-2           1    10007  2.1.0.7rc1-2
        1.0.7                1    10007  2.1.0.7
        1.0.8beta1-4         1    10008  2.1.0.8beta1-4
        1.0.8rc1             1    10008  2.1.0.8rc1
        1.0.8                1    10008  2.1.0.8
        1.0.9beta1-6         1    10009  2.1.0.9beta1-6
        1.0.9rc1             1    10009  2.1.0.9rc1
        1.0.9beta7-10        1    10009  2.1.0.9beta7-10
        1.0.9rc2             1    10009  2.1.0.9rc2
        1.0.9                1    10009  2.1.0.9
        1.0.10beta1          1    10010  2.1.0.10beta1
        1.0.10rc1            1    10010  2.1.0.10rc1
        1.0.10               1    10010  2.1.0.10
        1.0.11beta1-3        1    10011  2.1.0.11beta1-3
        1.0.11rc1            1    10011  2.1.0.11rc1
        1.0.11               1    10011  2.1.0.11
        1.0.12beta1-2        2    10012  2.1.0.12beta1-2
        1.0.12rc1            2    10012  2.1.0.12rc1
        1.0.12               2    10012  2.1.0.12
        1.1.0a-f             -    10100  2.1.1.0a-f abandoned
        1.2.0beta1-2         2    10200  2.1.2.0beta1-2
        1.2.0beta3-5         3    10200  3.1.2.0beta3-5
        1.2.0rc1             3    10200  3.1.2.0rc1
        1.2.0                3    10200  3.1.2.0
        1.2.1beta-4          3    10201  3.1.2.1beta1-4
        1.2.1rc1-2           3    10201  3.1.2.1rc1-2
        1.2.1                3    10201  3.1.2.1
        1.2.2beta1-6        12    10202  12.so.0.1.2.2beta1-6
        1.0.13beta1         10    10013  10.so.0.1.0.13beta1
        1.0.13rc1           10    10013  10.so.0.1.0.13rc1
        1.2.2rc1            12    10202  12.so.0.1.2.2rc1
        1.0.13              10    10013  10.so.0.1.0.13
        1.2.2               12    10202  12.so.0.1.2.2
        1.2.3rc1-6          12    10203  12.so.0.1.2.3rc1-6
        1.2.3               12    10203  12.so.0.1.2.3
        1.2.4beta1-3        13    10204  12.so.0.1.2.4beta1-3
        1.2.4rc1            13    10204  12.so.0.1.2.4rc1
        1.0.14              10    10014  10.so.0.1.0.14
        1.2.4               13    10204  12.so.0.1.2.4
        1.2.5beta1-2        13    10205  12.so.0.1.2.5beta1-2
        1.0.15rc1           10    10015  10.so.0.1.0.15rc1
        1.0.15              10    10015  10.so.0.1.0.15
        1.2.5               13    10205  12.so.0.1.2.5
        1.2.6beta1-4        13    10206  12.so.0.1.2.6beta1-4
        1.2.6rc1-5          13    10206  12.so.0.1.2.6rc1-5
        1.0.16              10    10016  10.so.0.1.0.16
        1.2.6               13    10206  12.so.0.1.2.6
        1.2.7beta1-2        13    10207  12.so.0.1.2.7beta1-2
        1.0.17rc1           10    10017  10.so.0.1.0.17rc1
        1.2.7rc1            13    10207  12.so.0.1.2.7rc1
        1.0.17              10    10017  10.so.0.1.0.17
        1.2.7               13    10207  12.so.0.1.2.7
        1.2.8beta1-5        13    10208  12.so.0.1.2.8beta1-5
        1.0.18rc1-5         10    10018  10.so.0.1.0.18rc1-5
        1.2.8rc1-5          13    10208  12.so.0.1.2.8rc1-5
        1.0.18              10    10018  10.so.0.1.0.18
        1.2.8               13    10208  12.so.0.1.2.8
        1.2.9beta1-3        13    10209  12.so.0.1.2.9beta1-3
        1.2.9beta4-11       13    10209  12.so.0.9[.0]
        1.2.9rc1            13    10209  12.so.0.9[.0]
        1.2.9               13    10209  12.so.0.9[.0]
        1.2.10beta1-8       13    10210  12.so.0.10[.0]
        1.2.10rc1-3         13    10210  12.so.0.10[.0]
        1.2.10              13    10210  12.so.0.10[.0]
        1.2.11beta1-4       13    10211  12.so.0.11[.0]
        1.0.19rc1-5         10    10019  10.so.0.19[.0]
        1.2.11rc1-5         13    10211  12.so.0.11[.0]
        1.0.19              10    10019  10.so.0.19[.0]
        1.2.11              13    10211  12.so.0.11[.0]
        1.0.20              10    10020  10.so.0.20[.0]
        1.2.12              13    10212  12.so.0.12[.0]
        1.2.13beta1         13    10213  12.so.0.13[.0]
        1.0.21              10    10021  10.so.0.21[.0]
        1.2.13              13    10213  12.so.0.13[.0]
        1.2.14beta1-2       13    10214  12.so.0.14[.0]
        1.0.22rc1           10    10022  10.so.0.22[.0]
        1.2.14rc1           13    10214  12.so.0.14[.0]
        1.2.15beta1-6       13    10215  12.so.0.15[.0]
        1.0.23rc1-5         10    10023  10.so.0.23[.0]
        1.2.15rc1-5         13    10215  12.so.0.15[.0]
        1.0.23              10    10023  10.so.0.23[.0]
        1.2.15              13    10215  12.so.0.15[.0]
        1.2.16beta1-2       13    10216  12.so.0.16[.0]
        1.2.16rc1           13    10216  12.so.0.16[.0]
        1.0.24              10    10024  10.so.0.24[.0]
        1.2.16              13    10216  12.so.0.16[.0]
        1.2.17beta1-2       13    10217  12.so.0.17[.0]
        1.0.25rc1           10    10025  10.so.0.25[.0]
        1.2.17rc1-3         13    10217  12.so.0.17[.0]
        1.0.25              10    10025  10.so.0.25[.0]
        1.2.17              13    10217  12.so.0.17[.0]
        1.0.26              10    10026  10.so.0.26[.0]
        1.2.18              13    10218  12.so.0.18[.0]
        1.2.19beta1-31      13    10219  12.so.0.19[.0]
        1.0.27rc1-6         10    10027  10.so.0.27[.0]
        1.2.19rc1-6         13    10219  12.so.0.19[.0]
        1.0.27              10    10027  10.so.0.27[.0]
        1.2.19              13    10219  12.so.0.19[.0]
        1.2.20beta01-04     13    10220  12.so.0.20[.0]
        1.0.28rc1-6         10    10028  10.so.0.28[.0]
        1.2.20rc1-6         13    10220  12.so.0.20[.0]
        1.0.28              10    10028  10.so.0.28[.0]
        1.2.20              13    10220  12.so.0.20[.0]
        1.2.21beta1-2       13    10221  12.so.0.21[.0]
        1.2.21rc1-3         13    10221  12.so.0.21[.0]
        1.0.29              10    10029  10.so.0.29[.0]
        1.2.21              13    10221  12.so.0.21[.0]
        1.2.22beta1-4       13    10222  12.so.0.22[.0]
        1.0.30rc1           13    10030  10.so.0.30[.0]
        1.2.22rc1           13    10222  12.so.0.22[.0]
        1.0.30              10    10030  10.so.0.30[.0]
        1.2.22              13    10222  12.so.0.22[.0]
        1.2.23beta01-05     13    10223  12.so.0.23[.0]
        1.2.23rc01          13    10223  12.so.0.23[.0]
        1.2.23              13    10223  12.so.0.23[.0]
        1.2.24beta01-02     13    10224  12.so.0.24[.0]
        1.2.24rc01          13    10224  12.so.0.24[.0]
        1.2.24              13    10224  12.so.0.24[.0]
        1.2.25beta01-06     13    10225  12.so.0.25[.0]
        1.2.25rc01-02       13    10225  12.so.0.25[.0]
        1.0.31              10    10031  10.so.0.31[.0]
        1.2.25              13    10225  12.so.0.25[.0]
        1.2.26beta01-06     13    10226  12.so.0.26[.0]
        1.2.26rc01          13    10226  12.so.0.26[.0]
        1.2.26              13    10226  12.so.0.26[.0]
        1.0.32              10    10032  10.so.0.32[.0]
        1.2.27beta01-06     13    10227  12.so.0.27[.0]
        1.2.27rc01          13    10227  12.so.0.27[.0]
        1.0.33              10    10033  10.so.0.33[.0]
        1.2.27              13    10227  12.so.0.27[.0]
        1.0.34              10    10034  10.so.0.34[.0]
        1.2.28              13    10228  12.so.0.28[.0]
        1.2.29beta01-03     13    10229  12.so.0.29[.0]
        1.2.29rc01          13    10229  12.so.0.29[.0]
        1.0.35              10    10035  10.so.0.35[.0]
        1.2.29              13    10229  12.so.0.29[.0]
        1.0.37              10    10037  10.so.0.37[.0]
        1.2.30beta01-04     13    10230  12.so.0.30[.0]
        1.0.38rc01-08       10    10038  10.so.0.38[.0]
        1.2.30rc01-08       13    10230  12.so.0.30[.0]
        1.0.38              10    10038  10.so.0.38[.0]
        1.2.30              13    10230  12.so.0.30[.0]
        1.0.39rc01-03       10    10039  10.so.0.39[.0]
        1.2.31rc01-03       13    10231  12.so.0.31[.0]
        1.0.39              10    10039  10.so.0.39[.0]
        1.2.31              13    10231  12.so.0.31[.0]
        1.2.32beta01-02     13    10232  12.so.0.32[.0]
        1.0.40rc01          10    10040  10.so.0.40[.0]
        1.2.32rc01          13    10232  12.so.0.32[.0]
        1.0.40              10    10040  10.so.0.40[.0]
        1.2.32              13    10232  12.so.0.32[.0]
        1.2.33beta01-02     13    10233  12.so.0.33[.0]
        1.2.33rc01-02       13    10233  12.so.0.33[.0]
        1.0.41rc01          10    10041  10.so.0.41[.0]
        1.2.33              13    10233  12.so.0.33[.0]
        1.0.41              10    10041  10.so.0.41[.0]
        1.2.34beta01-07     13    10234  12.so.0.34[.0]
        1.0.42rc01          10    10042  10.so.0.42[.0]
        1.2.34rc01          13    10234  12.so.0.34[.0]
        1.0.42              10    10042  10.so.0.42[.0]
        1.2.34              13    10234  12.so.0.34[.0]
        1.2.35beta01-03     13    10235  12.so.0.35[.0]
        1.0.43rc01-02       10    10043  10.so.0.43[.0]
        1.2.35rc01-02       13    10235  12.so.0.35[.0]
        1.0.43              10    10043  10.so.0.43[.0]
        1.2.35              13    10235  12.so.0.35[.0]
        1.2.36beta01-05     13    10236  12.so.0.36[.0]
        1.2.36rc01          13    10236  12.so.0.36[.0]
        1.0.44              10    10044  10.so.0.44[.0]
        1.2.36              13    10236  12.so.0.36[.0]
        1.2.37beta01-03     13    10237  12.so.0.37[.0]
        1.2.37rc01          13    10237  12.so.0.37[.0]
        1.2.37              13    10237  12.so.0.37[.0]
        1.0.45              10    10045  12.so.0.45[.0]
        1.0.46              10    10046  10.so.0.46[.0]
        1.2.38beta01        13    10238  12.so.0.38[.0]
        1.2.38rc01-03       13    10238  12.so.0.38[.0]
        1.0.47              10    10047  10.so.0.47[.0]
        1.2.38              13    10238  12.so.0.38[.0]
        1.2.39beta01-05     13    10239  12.so.0.39[.0]
        1.2.39rc01          13    10239  12.so.0.39[.0]
        1.0.48              10    10048  10.so.0.48[.0]
        1.2.39              13    10239  12.so.0.39[.0]
        1.2.40beta01        13    10240  12.so.0.40[.0]
        1.2.40rc01          13    10240  12.so.0.40[.0]
        1.0.49              10    10049  10.so.0.49[.0]
        1.2.40              13    10240  12.so.0.40[.0]
        1.0.50              10    10050  10.so.0.50[.0]
        1.2.41beta01-18     13    10241  12.so.0.41[.0]
        1.0.51rc01          10    10051  10.so.0.51[.0]
        1.2.41rc01-03       13    10241  12.so.0.41[.0]
        1.0.51              10    10051  10.so.0.51[.0]
        1.2.41              13    10241  12.so.0.41[.0]
        1.2.42beta01-02     13    10242  12.so.0.42[.0]
        1.2.42rc01-05       13    10242  12.so.0.42[.0]
        1.0.52              10    10052  10.so.0.52[.0]
        1.2.42              13    10242  12.so.0.42[.0]
        1.2.43beta01-05     13    10243  12.so.0.43[.0]
        1.0.53rc01-02       10    10053  10.so.0.53[.0]
        1.2.43rc01-02       13    10243  12.so.0.43[.0]
        1.0.53              10    10053  10.so.0.53[.0]
        1.2.43              13    10243  12.so.0.43[.0]
        1.2.44beta01-03     13    10244  12.so.0.44[.0]
        1.2.44rc01-03       13    10244  12.so.0.44[.0]
        1.2.44              13    10244  12.so.0.44[.0]
        1.2.45beta01-03     13    10245  12.so.0.45[.0]
        1.0.55rc01          10    10055  10.so.0.55[.0]
        1.2.45rc01          13    10245  12.so.0.45[.0]
        1.0.55              10    10055  10.so.0.55[.0]
        1.2.45              13    10245  12.so.0.45[.0]
        1.2.46rc01-02       13    10246  12.so.0.46[.0]
        1.0.56              10    10056  10.so.0.56[.0]
        1.2.46              13    10246  12.so.0.46[.0]
        1.2.47beta01        13    10247  12.so.0.47[.0]
        1.2.47rc01          13    10247  12.so.0.47[.0]
        1.0.57rc01          10    10057  10.so.0.57[.0]
        1.2.47              13    10247  12.so.0.47[.0]
        1.0.57              10    10057  10.so.0.57[.0]
        1.2.48beta01        13    10248  12.so.0.48[.0]
        1.2.48rc01-02       13    10248  12.so.0.48[.0]
        1.0.58              10    10058  10.so.0.58[.0]
        1.2.48              13    10248  12.so.0.48[.0]
        1.2.49rc01          13    10249  12.so.0.49[.0]
        1.0.59              10    10059  10.so.0.59[.0]
        1.2.49              13    10249  12.so.0.49[.0]

       Henceforth  the  source version will match the shared-library minor and patch numbers; the shared-library major
       version number will be used for changes in backward compatibility,  as  it  is  intended.   The  PNG_PNGLIB_VER
       macro,  which  is  not used within libpng but is available for applications, is an unsigned integer of the form
       xyyzz corresponding to the source version x.y.z (leading zeros in y and z).  Beta versions were given the  pre-
       vious  public  release  number  plus  a letter, until version 1.0.6j; from then on they were given the upcoming
       public release number plus "betaNN" or "rcNN".


SEE ALSO
       libpngpf(3), png(5)

       libpng:

              http://libpng.sourceforge.net (follow the [DOWNLOAD] link) http://www.libpng.org/pub/png


       zlib:

              (generally) at the same location as libpng or at
              ftp://ftp.info-zip.org/pub/infozip/zlib


       PNGspecification:RFC2083

              (generally) at the same location as libpng or at
              ftp://ftp.rfc-editor.org:/in-notes/rfc2083.txt
              or (as a W3C Recommendation) at
              http://www.w3.org/TR/REC-png.html


       In the case of any inconsistency between the PNG specification and this library, the specification takes prece-
       dence.


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 1.2.49 - March 29, 2012: Initially created in 1995 by Guy Eric Schalnat, then of Group 42,  Inc.
       Currently maintained by Glenn Randers-Pehrson (glennrp at users.sourceforge.net).

       Supported by the PNG development group
       png-mng-implement  at  lists.sf.net  (subscription  required;  visit png-mng-implement at lists.sourceforge.net
       (subscription required; visit https://lists.sourceforge.net/lists/listinfo/png-mng-implement to subscribe).


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

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

       This code is released under the libpng license.

       libpng versions 1.2.6, August 15, 2004, through 1.2.49, March 29, 2012, 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 fol-
       lowing 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  Ran-
       ders-Pehrson, and are distributed according to the same disclaimer and license as libpng-1.0.6 with the follow-
       ing 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  Dis-
       tributed  according  to the same disclaimer and license as libpng-0.88, with the following individuals added to
       the list of Contributing Authors:

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

       libpng versions 0.5, May 1995, through 0.88, January 1996, are Copyright (c)  1995,  1996  Guy  Eric  Schalnat,
       Group 42, Inc.

       For the purposes of this copyright and license, "Contributing Authors" is defined as the following set of indi-
       viduals:

          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  war-
       ranties,  expressed or implied, including, without limitation, the warranties of merchantability and of fitness
       for any purpose.  The Contributing Authors and Group 42, Inc.  assume no liability for direct, indirect,  inci-
       dental,  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 March 29, 2012





                                March 29, 2012                       LIBPNG(3)