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X11R7.4 - man page for libpng (x11r4 section 3)

LIBPNG(3)			     Library Functions Manual				LIBPNG(3)

NAME
       libpng - Portable Network Graphics (PNG) Reference Library 1.2.35

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 mal-
       loc_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  mal-
       loc_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  *back-
       ground);

       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	*compres-
       sion_type, int *filter_type);

       png_uint_32 png_get_image_height (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_image_width (png_structp png_ptr, png_infop info_ptr);

       #if !defined(PNG_1_0_X)

       png_int_32 png_get_int_32 (png_bytep buf);

       #endif

       png_byte png_get_interlace_type (png_structp png_ptr, png_infop info_ptr);

       png_voidp png_get_io_ptr (png_structp png_ptr);

       png_byte png_get_libpng_ver (png_structp png_ptr);

       png_voidp png_get_mem_ptr(png_structp png_ptr);

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

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

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

       float png_get_pixel_aspect_ratio (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);

       png_voidp png_get_progressive_ptr (png_structp png_ptr);

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

       png_byte png_get_rgb_to_gray_status (png_structp png_ptr)

       png_uint_32 png_get_rowbytes (png_structp png_ptr, png_infop info_ptr);

       png_bytepp png_get_rows (png_structp png_ptr, png_infop info_ptr);

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

       png_bytep png_get_signature (png_structp png_ptr, png_infop info_ptr);

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

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

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

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

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

       #if !defined(PNG_1_0_X)

       png_uint_16 png_get_uint_16 (png_bytep buf);

       png_uint_32 png_get_uint_31 (png_bytep buf);

       png_uint_32 png_get_uint_32 (png_bytep buf);

       #endif

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

       png_voidp png_get_user_chunk_ptr (png_structp png_ptr);

       png_uint_32 png_get_user_height_max( png_structp png_ptr);

       png_voidp png_get_user_transform_ptr (png_structp png_ptr);

       png_uint_32 png_get_user_width_max (png_structp png_ptr);

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

       png_int_32 png_get_x_offset_microns (png_structp png_ptr, png_infop info_ptr);

       png_int_32 png_get_x_offset_pixels (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_x_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);

       png_int_32 png_get_y_offset_microns (png_structp png_ptr, png_infop info_ptr);

       png_int_32 png_get_y_offset_pixels (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_y_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_compression_buffer_size (png_structp png_ptr);

       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

       void png_init_io (png_structp png_ptr, FILE *fp);

       DEPRECATED: void png_info_init (png_infop info_ptr);

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

       png_voidp png_malloc (png_structp png_ptr, png_uint_32 size);

       png_voidp png_malloc_default(png_structp png_ptr, png_uint_32 size);

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

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

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

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

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

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

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

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

       void png_read_end (png_structp png_ptr, png_infop info_ptr);

       void png_read_image (png_structp png_ptr, png_bytepp image);

       DEPRECATED: void png_read_init (png_structp png_ptr);

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

       void png_read_info (png_structp png_ptr, png_infop info_ptr);

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

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

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

       void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

       #if !defined(PNG_1_0_X)

       png_save_int_32 (png_bytep buf, png_int_32 i);

       void png_save_uint_16 (png_bytep buf, unsigned int i);

       void png_save_uint_32 (png_bytep buf, png_uint_32 i);

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

       #endif

       void  png_set_background  (png_structp  png_ptr, png_color_16p background_color, int back-
       ground_gamma_code, int need_expand, double background_gamma);

       void png_set_bgr (png_structp png_ptr);

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

       void  png_set_cHRM  (png_structp  png_ptr,  png_infop  info_ptr,  double  white_x,  double
       white_y, double red_x, double red_y, double green_x, double green_y, double blue_x, double
       blue_y);

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

       void png_set_compression_level (png_structp png_ptr, int level);

       void png_set_compression_mem_level (png_structp png_ptr, int mem_level);

       void png_set_compression_method (png_structp png_ptr, int method);

       void png_set_compression_strategy (png_structp png_ptr, int strategy);

       void png_set_compression_window_bits (png_structp png_ptr, int window_bits);

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

       void png_set_dither (png_structp png_ptr, png_colorp palette, int num_palette,  int  maxi-
       mum_colors, png_uint_16p histogram, int full_dither);

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

       void png_set_expand (png_structp png_ptr);

       void png_set_expand_gray_1_2_4_to_8(png_structp png_ptr);

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

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

       void   png_set_filter_heuristics   (png_structp	 png_ptr,   int   heuristic_method,   int
       num_weights, png_doublep filter_weights, png_doublep filter_costs);

       void png_set_flush (png_structp png_ptr, int nrows);

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

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

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

       void png_set_gray_1_2_4_to_8(png_structp png_ptr);

       void png_set_gray_to_rgb (png_structp png_ptr);

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

       void  png_set_iCCP  (png_structp png_ptr, png_infop info_ptr, png_charp name, int compres-
       sion_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 fil-
       ter_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_pro-
       gressive_info_ptr   info_fn,   png_progressive_row_ptr	row_fn,   png_progressive_end_ptr
       end_fn);

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

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

       void png_set_read_status_fn (png_structp png_ptr, png_read_status_ptr read_row_fn);

       void   png_set_read_user_transform_fn   (png_structp    png_ptr,    png_user_transform_ptr
       read_user_transform_fn);

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

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

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

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

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

       void png_set_shift (png_structp png_ptr, png_color_8p true_bits);

       void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

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

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

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

       void png_set_strip_16 (png_structp png_ptr);

       void png_set_strip_alpha (png_structp png_ptr);

       void png_set_swap (png_structp png_ptr);

       void png_set_swap_alpha (png_structp png_ptr);

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

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

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

       void png_set_tRNS_to_alpha(png_structp png_ptr);

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

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

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

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

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

       void png_set_write_fn (png_structp png_ptr, png_voidp  io_ptr,  png_rw_ptr  write_data_fn,
       png_flush_ptr output_flush_fn);

       void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr write_row_fn);

       void    png_set_write_user_transform_fn	 (png_structp	png_ptr,   png_user_transform_ptr
       write_user_transform_fn);

       void png_set_compression_buffer_size(png_structp png_ptr, png_uint_32 size);

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

       void png_start_read_image (png_structp png_ptr);

       void png_warning (png_structp png_ptr, png_const_charp message);

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

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

       void png_write_chunk_end (png_structp png_ptr);

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

       void png_write_destroy (png_structp png_ptr);

       void png_write_end (png_structp png_ptr, png_infop info_ptr);

       void png_write_flush (png_structp png_ptr);

       void png_write_image (png_structp png_ptr, png_bytepp image);

       DEPRECATED: void png_write_init (png_structp png_ptr);

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

       void png_write_info (png_structp png_ptr, png_infop info_ptr);

       void png_write_info_before_PLTE (png_structp png_ptr, png_infop info_ptr);

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

       void png_write_row (png_structp png_ptr, png_bytep row);

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

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

       void png_zfree (voidpf png_ptr, voidpf ptr);

DESCRIPTION
       The libpng library supports encoding, decoding, and various manipulations of the  Portable
       Network Graphics (PNG) format image files.  It uses the zlib(3) compression library.  Fol-
       lowing 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 1.2.35 - February 14, 2009
	Updated and distributed by Glenn Randers-Pehrson
	<glennrp at users.sourceforge.net>
	Copyright (c) 1998-2008 Glenn Randers-Pehrson
	For conditions of distribution and use, see copyright
	notice in png.h.

	Based on:

	libpng versions 0.97, January 1998, through 1.2.35 - February 14, 2009
	Updated and distributed by Glenn Randers-Pehrson
	Copyright (c) 1998-2008 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 addi-
       tion  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  Recommenda-
       tion  and  as  an ISO Standard (ISO/IEC 15948:2003 (E)) at <http://www.w3.org/TR/2003/REC-
       PNG-20031110/ The W3C and ISO documents have identical technical content.

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

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

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

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

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

       Libpng has been designed to handle multiple sessions at one time, to be easily modifiable,
       to  be  portable to the vast majority of machines (ANSI, K&R, 16-, 32-, and 64-bit) avail-
       able, 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 struc-
       tures.  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.  How-
       ever, 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  applica-
       tions, 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 ver-
       sion  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 if the bytes match the corresponding bytes of the  PNG
       signature,  or  nonzero 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  ver-
       sion,  optional	pointers  to error handling functions, and a pointer to a data struct for
       use by the error functions, if necessary (the pointer and functions can	be  NULL  if  the
       default	error  handlers  are  to  be  used).   See the section on Changes to Libpng below
       regarding the old initialization functions.  The structure  allocation  functions  quietly
       return  NULL  if  they  fail to create the structure, so your application should check for
       that.

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

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

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

       If you want to use your own memory allocation routines, define PNG_USER_MEM_SUPPORTED  and
       use png_create_read_struct_2() instead of png_create_read_struct():

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

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

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

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

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

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

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

	   png_init_io(png_ptr, fp);

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

	   png_set_sig_bytes(png_ptr, number);

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

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

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

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

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

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

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

       To inform libpng about your function, use

	   png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
	       read_chunk_callback);

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

	   png_get_user_chunk_ptr(png_ptr);

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

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

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

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

       To inform libpng about your function, use

	   png_set_read_status_fn(png_ptr, read_row_callback);

   Width and height 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 lim-
       its that are being applied, use

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

   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  dis-
       carded.	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

   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  transforma-
       tions 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

       (This excludes setting a background color, doing gamma transformation, dithering, and set-
       ting 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  transforma-
       tion 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, of
	   filter_method can be NULL if you are
	   not interested in their values.

	   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())).

	   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);

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Data  will  be  decoded into the supplied row buffers packed into bytes unless the library
       has been told to transform it into another format.  For example, 4 bit/pixel  paletted  or
       grayscale  data	will  be returned 2 pixels/byte with the leftmost pixel in the high-order
       bits of the byte, unless png_set_packing() is called.  8-bit RGB data will  be  stored  in
       RGB  RGB  RGB  format  unless  png_set_filler() or png_set_add_alpha() is called to insert
       filler bytes, either before or after each RGB triplet.  16-bit RGB data will  be  returned
       RRGGBB  RRGGBB,	with  the  most  significant  byte  of	the  color  value  first,  unless
       png_set_strip_16()  is  called  to  transform  it  to  regular  RGB   RGB   triplets,   or
       png_set_filler() or png_set_add alpha() is called to insert filler bytes, either before or
       after each RRGGBB triplet.  Similarly, 8-bit or 16-bit grayscale data can be modified with
       png_set_filler(), png_set_add_alpha(), or png_set_strip_16().

       The  following code transforms grayscale images of less than 8 to 8 bits, changes paletted
       images to RGB, and adds a full alpha channel if there is  transparency  information  in	a
       tRNS chunk.  This is most useful on grayscale images with bit depths of 2 or 4 or if there
       is a multiple-image viewing application that wishes to treat all images in the same way.

	   if (color_type == PNG_COLOR_TYPE_PALETTE)
	       png_set_palette_to_rgb(png_ptr);

	   if (color_type == PNG_COLOR_TYPE_GRAY &&
	       bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);

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

       These three functions are actually aliases for png_set_expand(), added in  libpng  version
       1.0.4,  with the function names expanded to improve code readability.  In some future ver-
       sion 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.  At the same time, png_set_gray_1_2_4_to_8()
       was deprecated, and it will be removed from a future version.

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

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

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

	   if (color_type & PNG_COLOR_MASK_ALPHA)
	       png_set_strip_alpha(png_ptr);

       In PNG files, the alpha channel in an image is the level of  opacity.   If  you	need  the
       alpha  channel  in  an  image  to be the level of transparency instead of opacity, you can
       invert the alpha channel (or the tRNS chunk data) after it's read,  so  that  0	is  fully
       opaque  and  255 (in 8-bit or paletted images) or 65535 (in 16-bit images) is fully trans-
       parent, with

	   png_set_invert_alpha(png_ptr);

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

	   if (bit_depth < 8)
	       png_set_packing(png_ptr);

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

	   png_color_8p sig_bit;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	   if (color_type == PNG_COLOR_TYPE_RGB ||
	       color_type == PNG_COLOR_TYPE_RGB_ALPHA)
		 png_set_rgb_to_gray_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) func-
       tion.  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 sup-
       ply the background color as a gray value at the original file bit-depth (need_expand =  1)
       or  else  supply  the  background color as an RGB triplet at the final, expanded bit depth
       (need_expand = 0).  Similarly, if you are reading a paletted image, you must either supply
       the background color as a palette index (need_expand = 1) or as an RGB triplet that may or
       may not be in the palette (need_expand = 0).

	   png_color_16 my_background;
	   png_color_16p image_background;

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

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

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

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

	  double gamma, screen_gamma;

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

       The png_set_gamma() function handles gamma transformations of the  data.   Pass	both  the
       file gamma and the current screen_gamma.  If the file does not have a gamma value, you can
       pass one anyway if you have an idea what it is (usually 0.45455 is a good  guess  for  GIF
       images  on  PCs).  Note that file gammas are inverted from screen gammas.  See the discus-
       sions 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 sup-
       port gamma correction.

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

       If you need to reduce an RGB file to a paletted file, or  if  a	paletted  file	has  more
       entries	then will fit on your screen, png_set_dither() will do that.  Note that this is a
       simple match dither that merely finds the  closest  color  available.   This  should  work
       fairly  well  with  optimized  palettes, and fairly badly with linear color cubes.  If you
       pass a palette that is larger then maximum_colors, the file will reduce the number of 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  his-
       togram, it may not do as good a job.

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

		png_get_hIST(png_ptr, info_ptr,
		   &histogram);
		png_set_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 signif-
       icant 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 allo-
       cation  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_inter-
       lace_handling() or call this function multiple times, or any of that other stuff necessary
       with png_read_rows().

	  png_read_image(png_ptr, row_pointers);

       where row_pointers is:

	  png_bytep row_pointers[height];

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

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

	   png_read_rows(png_ptr, row_pointers, NULL,
	      number_of_rows);

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

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

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

       If the file is interlaced (interlace_type != 0 in the IHDR  chunk),  things  get  somewhat
       harder.	 The  only  current  (PNG  Specification version 1.2) interlacing type for PNG is
       (interlace_type == PNG_INTERLACE_ADAM7) is a somewhat  complicated  2D  interlace  scheme,
       known as Adam7, that breaks down an image into seven smaller images of varying size, based
       on an 8x8 grid.

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

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

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

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

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

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

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

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

	   png_read_rows(png_ptr, row_pointers, NULL,
	      number_of_rows);

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

	   png_read_rows(png_ptr, NULL, row_pointers,
	      number_of_rows);

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

	  png_read_end(png_ptr, end_info);

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

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

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

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

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

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

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

       This  function only affects data that has already been allocated.  You can call this func-
       tion 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 responsibil-
       ity for libpng-allocated data, the application must use png_free() to free  it,	and  when
       the user transfers responsibility to libpng for data that the user has allocated, the user
       must have used png_malloc() or png_zalloc() to allocate it.

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

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

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

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

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

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

       png_structp png_ptr; png_infop info_ptr;

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

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

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

		 png_get_progressive_ptr(png_ptr);

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

	   return 0;
	}

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

	   /* This one's new also.  Simply give it a chunk
	      of data from the file stream (in order, of
	      course).	On machines with segmented memory
	      models machines, don't give it any more than
	      64K.  The library seems to run fine with sizes
	      of 4K. Although you can give it much less if
	      necessary (I assume you can give it chunks of
	      1 byte, I haven't tried less then 256 bytes
	      yet).  When this function returns, you may
	      want to display any rows that were generated
	      in the row callback if you don't already do
	      so there.
	    */
	   png_process_data(png_ptr, info_ptr, buffer, length);
	   return 0;
	}

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

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

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

	       png_progressive_combine_row(png_ptr, old_row,
		 new_row);

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	   png_init_io(png_ptr, fp);

       If you are embedding your PNG into a datastream such as MNG,  and  don't  want  libpng  to
       write  the 8-byte signature, or if you have already written the signature in your applica-
       tion, 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 demon-
       strated in pngtest.c.  You must supply a function

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

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

       To inform libpng about your function, use

	   png_set_write_status_fn(png_ptr, write_row_callback);

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

	   /* turn on or off filtering, and/or choose
	      specific filters.  You can use either a single
	      PNG_FILTER_VALUE_NAME or the bitwise OR of one
	      or more PNG_FILTER_NAME masks. */
	   png_set_filter(png_ptr, 0,
	      PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
	      PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
	      PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
	      PNG_FILTER_AVE   | PNG_FILTER_VALUE_AVE  |
	      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 com-
       pression.

       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 try-
       ing  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_*() func-
       tions, which might require access to some of the IHDR settings.	The remaining png_set_*()
       functions can be called in any order.

	   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 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).
	   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 com-
       pressed 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_COMPRES-
       SION_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 view-
       ers 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  super-
       set  of	regular ASCII) and can not contain NUL characters, and should not contain control
       or other unprintable characters.  To make the comments widely readable, stick  with  basic
       ASCII,  and avoid machine specific character set extensions like the IBM-PC character set.
       The keyword must be present, but you can leave  off  the  text  string  on  non-compressed
       pairs.	Compressed  pairs  must have a text string, as only the text string is compressed
       anyway, so the compression would be meaningless.

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

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

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

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

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

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

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

	   png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

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

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

	   png_set_packing(png_ptr);

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

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

	   png_set_sBIT(png_ptr, info_ptr, &sig_bit);

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

	   png_set_shift(png_ptr, &sig_bit);

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

	   if (bit_depth > 8)
	      png_set_swap(png_ptr);

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

	   if (bit_depth < 8)
	      png_set_packswap(png_ptr);

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

	   png_set_bgr(png_ptr);

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

	   png_set_invert_mono(png_ptr);

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

	   png_set_write_user_transform_fn(png_ptr,
	      write_transform_fn);

       You must supply the function

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

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

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

	   png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

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

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

	   voidp write_user_transform_ptr =
	      png_get_user_transform_ptr(png_ptr);

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

	   png_write_flush(png_ptr);

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

	   png_set_flush(png_ptr, nrows);

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

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

	   png_write_image(png_ptr, row_pointers);

       where row_pointers is:

	   png_byte *row_pointers[height];

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

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

	   png_write_rows(png_ptr, row_pointers,
	      number_of_rows);

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

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

	   png_bytep row_pointer = row;

	   png_write_row(png_ptr, row_pointer);

       When the file is interlaced, things can get a good deal more complicated.  The  only  cur-
       rently  (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_inter-
       lace_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-allo-
       cated storage with the following function:

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

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

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

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

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

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

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

       thereby	briefly reassigning responsibility for freeing to the user but immediately after-
       wards 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  func-
       tion  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 compli-
       cated 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,  png-
       wio.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() and png_free().   These  cur-
       rently just call the standard C functions.  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 func-
       tions 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 nor-
       mally 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);

       Supplying NULL for the read, write, or flush functions sets them back to using the default
       C stream functions.  It is an error to read from a write stream, and vice versa.

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

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

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

	   png_voidp error_ptr = png_get_error_ptr(png_ptr);

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

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

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

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

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

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

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

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

   Configuring for Medium Model
       Libpng's support for medium model has been tested on most of the popular compilers.   Make
       sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets defined, and FAR gets defined to far in
       pngconf.h, and you should be all set.  Everything in the library (except for zlib's struc-
       ture)  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 ini-
       tialization.  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/delete an include,
       this is the place to do it.  The includes that are not needed outside libpng are protected
       by  the	PNG_INTERNAL  definition,  which is only defined for those routines inside libpng
       itself.	The files in libpng proper only include png.h, which includes pngconf.h.

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

	   png_set_compression_level(png_ptr, level);

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

	   png_set_compression_mem_level(png_ptr, level);

       The other functions are for configuring zlib.  They are not recommended for normal use and
       may result in writing an invalid PNG file.  See zlib.h for more information on what  these
       mean.

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

   Controlling row filtering
       If  you	want to control whether libpng uses filtering or not, which filters are used, and
       how it goes about picking row filters, you can call one of these functions.  The selection
       and  configuration  of  row filters can have a significant impact on the size and encoding
       speed and a somewhat lesser impact on the  decoding  speed  of  an  image.   Filtering  is
       enabled	by  default  for  RGB  and grayscale images (with and without alpha), but not for
       paletted images nor for any images with bit depths less than 8 bits/pixel.

       The 'method' parameter sets the main filtering method, which is currently only '0' in  the
       PNG  1.2  specification.   The 'filters' parameter sets which filter(s), if any, should be
       used for each scanline.	Possible values are PNG_ALL_FILTERS and  PNG_NO_FILTERS  to  turn
       filtering on and off, respectively.

       Individual  filter  types  are  PNG_FILTER_NONE,  PNG_FILTER_SUB,  PNG_FILTER_UP, PNG_FIL-
       TER_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_AVE |
		     PNG_FILTER_PAETH | PNG_ALL_FILTERS;

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

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

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

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

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

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

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

   Removing unwanted object code
       There are a bunch of #define's in pngconf.h that control what parts  of	libpng	are  com-
       piled.	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 capabil-
       ities 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_PROGRES-
       SIVE_READ (don't confuse this with the INTERLACING capability, which you'll still have).

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

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

   Requesting debug printout
       The  macro  definition  PNG_DEBUG can be used to request debugging printout.  Set it to an
       integer value in the range 0 to 3.  Higher numbers result in increasing amounts of  debug-
       ging  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,  "mes-
       sage"  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.

VII. MNG support
       The  MNG  specification (available at http://www.libpng.org/pub/mng) allows certain exten-
       sions 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 mini-
       mum,  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.

VIII. 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  check-
       ing,  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  sug-
       gested  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;

IX. Y2K Compliance in libpng
       February 14, 2009

       Since the PNG Development group is an ad-hoc body, we can't make an official declaration.

       This  is your unofficial assurance that libpng from version 0.71 and upward through 1.2.35
       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 applica-
       tions using libpng are not passing 4-digit years into the png_convert_to_rfc_1123()  func-
       tion,  or  that	they are incorrectly passing only a 2-digit year instead of "year - 1900"
       into the png_convert_from_struct_tm() function, but this is not under  our  control.   The
       libpng documentation has always stated that it works with 4-digit years, and the APIs have
       been documented as such.

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

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

	  Glenn Randers-Pehrson
	  libpng maintainer
	  PNG Development Group

NOTE
       Note about libpng version numbers:

       Due to various miscommunications, unforeseen code incompatibilities and occasional factors
       outside the authors' control, version numbering on the library has not always been consis-
       tent  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]

       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  avail-
       able  for  applications,  is  an  unsigned  integer of the form xyyzz corresponding to the
       source version x.y.z (leading zeros in y and z).  Beta versions were  given  the  previous
       public  release	number	plus a letter, until version 1.0.6j; from then on they were given
       the upcoming public release number plus "betaNN" or "rcN".

SEE ALSO
       libpngpf(3), png(5)

       libpng:

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

       zlib:

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

       PNGspecification:RFC2083

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

       In the case of any inconsistency between the PNG specification and this library, the spec-
       ification takes precedence.

AUTHORS
       This man page: Glenn Randers-Pehrson <glennrp at users.sourceforge.net>

       The contributing authors would like to thank all those who helped with testing, bug fixes,
       and patience.  This wouldn't have been possible without all of you.

       Thanks to Frank J. T. Wojcik for helping with the documentation.

       Libpng version 1.2.35 - February 14, 2009: Initially created in 1995 by Guy Eric Schalnat,
       then of Group  42,  Inc.   Currently  maintained  by  Glenn  Randers-Pehrson  (glennrp  at
       users.sourceforge.net).

       Supported by the PNG development group
       png-mng-implement  at  lists.sf.net  (subscription  required;  visit  png-mng-implement at
       lists.sourceforge.net	 (subscription	   required;	 visit	    https://lists.source-
       forge.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 dis-
       crepancy 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 sen-
       tence.

       libpng versions 1.2.6, August 15, 2004, through 1.2.35, February 14, 2009,  are	Copyright
       (c)  2004,2006-2008  Glenn Randers-Pehrson, and are distributed according to the same dis-
       claimer and license as libpng-1.2.5 with the following individual added	to  the  list  of
       Contributing Authors

	  Cosmin Truta

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

	  Simon-Pierre Cadieux
	  Eric S. Raymond
	  Gilles Vollant

       and with the following additions to the disclaimer:

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

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

	  Tom Lane
	  Glenn Randers-Pehrson
	  Willem van Schaik

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

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

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

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

	  Andreas Dilger
	  Dave Martindale
	  Guy Eric Schalnat
	  Paul Schmidt
	  Tim Wegner

       The PNG Reference Library is supplied "AS IS".  The Contributing  Authors  and  Group  42,
       Inc.  disclaim  all  warranties,  expressed or implied, including, without limitation, the
       warranties of merchantability and of fitness for any purpose.   The  Contributing  Authors
       and  Group  42, Inc.  assume no liability for direct, indirect, incidental, special, exem-
       plary, 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  encour-
       age  the  use of this source code as a component to supporting the PNG file format in com-
       mercial 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 certifica-
       tion mark of the Open Source Initiative.

       Glenn Randers-Pehrson glennrp at users.sourceforge.net February 14, 2009

					February 14, 2009				LIBPNG(3)


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