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LIBPNG(3)							     LIBPNG(3)

NAME
       libpng - Portable Network Graphics (PNG) Reference Library 1.4.5

SYNOPSIS

       #include <png.h>

       png_uint_32 png_access_version_number (void);

       void png_benign_error (png_structp png_ptr, png_const_charp error);

       void   png_chunk_benign_error   (png_structp  png_ptr,  png_const_charp
       error);

       void png_chunk_error (png_structp png_ptr, png_const_charp error);

       void png_chunk_warning (png_structp png_ptr, png_const_charp message);

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

       void png_convert_from_time_t (png_timep ptime, time_t ttime);

       png_charp   png_convert_to_rfc1123   (png_structp   png_ptr,  png_timep
       ptime);

       png_infop png_create_info_struct (png_structp png_ptr);

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

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

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

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

       void   png_destroy_info_struct	(png_structp	png_ptr,    png_infopp
       info_ptr_ptr);

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

       void  png_destroy_write_struct  (png_structpp  png_ptr_ptr,  png_infopp
       info_ptr_ptr);

       void png_error (png_structp png_ptr, png_const_charp error);

       void png_free (png_structp png_ptr, png_voidp ptr);

       void png_free_chunk_list (png_structp png_ptr);

       void png_free_default(png_structp png_ptr, png_voidp ptr);

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

       png_byte png_get_bit_depth (png_structp png_ptr, png_infop info_ptr);

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

       png_byte png_get_channels (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_cHRM (png_structp png_ptr, png_infop info_ptr, dou‐
       ble  *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_uint_32 png_get_chunk_cache_max (png_structp png_ptr);

       png_byte png_get_color_type (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_compression_buffer_size (png_structp png_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, dou‐
       ble *file_gamma);

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

       png_byte png_get_header_ver (png_structp png_ptr);

       png_byte png_get_header_version (png_structp png_ptr);

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

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

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

       png_uint_32  png_get_image_height   (png_structp	  png_ptr,   png_infop
       info_ptr);

       png_uint_32   png_get_image_width   (png_structp	  png_ptr,   png_infop
       info_ptr);

       png_int_32 png_get_int_32 (png_bytep buf);

       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_alloc_size_t png_get_chunk_malloc_max (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_color);

       /* This function is really an inline macro. */

       png_uint_16 png_get_uint_16 (png_bytep buf);

       png_uint_32 png_get_uint_31 (png_bytep buf);

       /* This function is really an inline macro. */

       png_uint_32 png_get_uint_32 (png_bytep buf);

       png_uint_32  png_get_unknown_chunks  (png_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);

       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

       void png_init_io (png_structp png_ptr, FILE *fp);

       png_voidp png_malloc (png_structp png_ptr, png_alloc_size_t size);

       png_voidp  png_malloc_default(png_structp   png_ptr,   png_alloc_size_t
       size);

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

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

       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_end (png_structp png_ptr, png_infop info_ptr);

       void png_read_image (png_structp png_ptr, png_bytepp image);

       void png_read_info (png_structp png_ptr, png_infop info_ptr);

       void  png_read_png (png_structp png_ptr, png_infop info_ptr, int trans‐
       forms, png_voidp params);

       void png_read_row (png_structp png_ptr, png_bytep row,  png_bytep  dis‐
       play_row);

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

       void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

       png_save_int_32 (png_bytep buf, png_int_32 i);

       void png_save_uint_16 (png_bytep buf, unsigned int i);

       void png_save_uint_32 (png_bytep buf, png_uint_32 i);

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

       void   png_set_background  (png_structp	png_ptr,  png_color_16p	 back‐
       ground_color, int background_gamma_code, int need_expand, double	 back‐
       ground_gamma);

       void png_set_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_chunk_cache_max	 (png_structp	png_ptr,   png_uint_32
       user_chunk_cache_max);

       void png_set_compression_level (png_structp png_ptr, int level);

       void   png_set_compression_mem_level    (png_structp    png_ptr,	   int
       mem_level);

       void png_set_compression_method (png_structp png_ptr, int method);

       void png_set_compression_strategy (png_structp png_ptr, int strategy);

       void  png_set_compression_window_bits  (png_structp  png_ptr,  int win‐
       dow_bits);

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

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

       void png_set_expand (png_structp png_ptr);

       void png_set_expand_gray_1_2_4_to_8(png_structp png_ptr);

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

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

       void   png_set_filter_heuristics	  (png_structp	png_ptr,  int  heuris‐
       tic_method, int num_weights,  png_doublep  filter_weights,  png_doublep
       filter_costs);

       void png_set_flush (png_structp png_ptr, int nrows);

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

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

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

       void png_set_gray_1_2_4_to_8(png_structp png_ptr);

       void png_set_gray_to_rgb (png_structp png_ptr);

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

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

       int png_set_interlace_handling (png_structp png_ptr);

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

       void png_set_invert_alpha (png_structp png_ptr);

       void png_set_invert_mono (png_structp png_ptr);

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

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

       jmp_buf*	 png_set_longjmp_fn  (png_structp   png_ptr,   png_longjmp_ptr
       longjmp_fn, size_t jmp_buf_size);

       void  png_set_chunk_malloc_max  (png_structp  png_ptr, png_alloc_size_t
       user_chunk_cache_max);

       void png_set_mem_fn(png_structp png_ptr,	 png_voidp  mem_ptr,  png_mal‐
       loc_ptr malloc_fn, png_free_ptr free_fn);

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

       void png_set_packing (png_structp png_ptr);

       void png_set_packswap (png_structp png_ptr);

       void png_set_palette_to_rgb(png_structp png_ptr);

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

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

       void  png_set_progressive_read_fn  (png_structp png_ptr, png_voidp pro‐
       gressive_ptr, png_progressive_info_ptr info_fn, png_progressive_row_ptr
       row_fn, png_progressive_end_ptr end_fn);

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

       void png_set_quantize (png_structp  png_ptr,  png_colorp	 palette,  int
       num_palette, int maximum_colors, png_uint_16p histogram, int full_quan‐
       tize);

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

       void  png_set_read_status_fn  (png_structp png_ptr, png_read_status_ptr
       read_row_fn);

       void	png_set_read_user_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_color);

       void png_set_tRNS_to_alpha(png_structp png_ptr);

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

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

       void   png_set_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_chan‐
       nels);

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

       void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
       write_row_fn);

       void	png_set_write_user_transform_fn	    (png_structp      png_ptr,
       png_user_transform_ptr write_user_transform_fn);

       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_end (png_structp png_ptr, png_infop info_ptr);

       void png_write_flush (png_structp png_ptr);

       void png_write_image (png_structp png_ptr, png_bytepp image);

       void png_write_info (png_structp png_ptr, png_infop info_ptr);

       void   png_write_info_before_PLTE   (png_structp	  png_ptr,   png_infop
       info_ptr);

       void png_write_png (png_structp png_ptr, png_infop info_ptr, int trans‐
       forms, png_voidp params);

       void png_write_row (png_structp png_ptr, png_bytep row);

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

       void png_write_sig (png_structp png_ptr);

       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 manipula‐
       tions of the Portable Network Graphics (PNG) format  image  files.   It
       uses  the  zlib(3)  compression	library.   Following  is a copy of the
       libpng.txt file that accompanies libpng.

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

	libpng version 1.4.5 - December 9, 2010
	Updated and distributed by Glenn Randers-Pehrson
	<glennrp at users.sourceforge.net>
	Copyright (c) 1998-2010 Glenn Randers-Pehrson

	This document is released under the libpng license.
	For conditions of distribution and use, see the disclaimer
	and license in png.h

	Based on:

	libpng versions 0.97, January 1998, through 1.4.5 - December 9, 2010
	Updated and distributed by Glenn Randers-Pehrson
	Copyright (c) 1998-2010 Glenn Randers-Pehrson

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Applications that do make direct access to the  members	of  png_struct
       (except for png_ptr->jmpbuf) must be recompiled whenever the library is
       updated, and applications that make direct access  to  the  members  of
       png_info must be recompiled if they were compiled or loaded with libpng
       version 1.0.6, in which the members were in a different order.  In 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 progres‐
       sive reading is covered in the next section, you will still  need  some
       of the functions discussed in this section to read a PNG file.

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

       If  you	are intending to keep the file pointer open for use in libpng,
       you must ensure you don't read more than 8 bytes from the beginning  of
       the  file, and you also have to make a call to png_set_sig_bytes_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 Customiz‐
       ing libpng.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	   png_init_io(png_ptr, fp);

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

	   png_set_sig_bytes(png_ptr, number);

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

	   png_set_compression_buffer_size(png_ptr, buffer_size);

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

   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_call‐
       back")

       To inform libpng about your function, use

	   png_set_read_status_fn(png_ptr, read_row_callback);

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

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

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

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

	   ...

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

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

	  png_set_user_limits(png_ptr, width_max, height_max);

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

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

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

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

	  png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

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

	  chunk_cache_max = png_get_chunk_cache_max(png_ptr);

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

       You  can	 also  set  a  limit on the amount of memory that a compressed
       chunk other than IDAT can occupy, with

	  png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);

       and you can retrieve the limit with

	  chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);

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

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

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

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

	   png_read_png(png_ptr, info_ptr, png_transforms, NULL)

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

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

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

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

	  row_pointers = png_get_rows(png_ptr, info_ptr);

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

	  png_bytep row_pointers[height];

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

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

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

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

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

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

	   png_read_info(png_ptr, info_ptr);

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

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

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

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

			    PNG_COLOR_MASK_PALETTE
			    PNG_COLOR_MASK_COLOR
			    PNG_COLOR_MASK_ALPHA

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

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

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

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

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

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

       These are also important, but their validity  depends  on  whether  the
       chunk	has   been   read.    The   png_get_valid(png_ptr,   info_ptr,
       PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr, info_ptr, ...) functions
       return  non-zero	 if  the data has been read, or zero if it is missing.
       The parameters to the png_get_<chunk> are set directly if they are sim‐
       ple data types, or a pointer into the info_ptr is returned for any com‐
       plex 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_alpha,
			    &num_trans, &trans_color);
	   trans_alpha	  - array of alpha (transparency)
			    entries for palette (PNG_INFO_tRNS)
	   trans_color	  - graylevel or color sample values of
			    the single transparent color for
			    non-paletted images (PNG_INFO_tRNS)
	   num_trans	  - number of transparent entries
			    (PNG_INFO_tRNS)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       These  three functions are actually aliases for png_set_expand(), added
       in libpng version 1.0.4, with the function names	 expanded  to  improve
       code  readability.  In some future version they may actually do differ‐
       ent things.

       As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was added.
       It expands the sample depth without changing tRNS to alpha.

       As of libpng version 1.4.5, not all possible expansions are supported.

       In  the	following table, the 01 means grayscale with depth<8, 31 means
       indexed with depth<8, other numerals  represent	the  color  type,  "T"
       means  the  tRNS chunk is present, A means an alpha channel is present,
       and O means tRNS or alpha is present but all pixels in  the  image  are
       opaque.

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

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

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

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

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

	   if (color_type & PNG_COLOR_MASK_ALPHA)
	       png_set_strip_alpha(png_ptr);

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

	   png_set_invert_alpha(png_ptr);

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

	   if (bit_depth < 8)
	       png_set_packing(png_ptr);

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

	   png_color_8p sig_bit;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       With red_weight+green_weight<=100000, the normalized graylevel is  com‐
       puted:

	   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 Poyn‐
       ton's  Color  FAQ,  <http://www.inforamp.net/~poynton/>	Copyright  (c)
       1998-01-04 Charles Poynton <poynton at inforamp.net>

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

       Libpng approximates this with

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

       which can be expressed with integers as

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

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

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

	   png_color_16 my_background;
	   png_color_16p image_background;

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

       The png_set_background() function tells libpng to composite images with
       alpha or simple transparency against the supplied background color.  If
       the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid), you  may  use
       this  color, or supply another color more suitable for the current dis‐
       play (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 compen‐
       sation other than the physical gamma exponent of the monitor is needed,
       while in a dark room a slightly smaller exponent is better.

	  double gamma, screen_gamma;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	   if (bit_depth < 8)
	      png_set_packswap(png_ptr);

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

	   png_set_read_user_transform_fn(png_ptr,
	      read_transform_fn);

       You must supply the function

	   void read_transform_fn(png_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 call‐
       back  function,	and you can inform libpng that your transform function
       will change the number of channels or bit depth with the function

	   png_set_user_transform_info(png_ptr, user_ptr,
	      user_depth, user_channels);

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

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

	   voidp read_user_transform_ptr =
	      png_get_user_transform_ptr(png_ptr);

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

	   number_of_passes = png_set_interlace_handling(png_ptr);

       After setting the transformations,  libpng  can	update	your  png_info
       structure  to  reflect  any  transformations you've requested with this
       call.  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	 back‐
       ground if these have been given with the calls above.

	   png_read_update_info(png_ptr, info_ptr);

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

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

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

	  png_read_image(png_ptr, row_pointers);

       where row_pointers is:

	  png_bytep row_pointers[height];

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

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

	   png_read_rows(png_ptr, row_pointers, NULL,
	      number_of_rows);

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

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

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

       If the file is interlaced (interlace_type !=  0	in  the	 IHDR  chunk),
       things  get  somewhat harder.  The only current (PNG Specification ver‐
       sion 1.2) interlacing type for PNG  is  (interlace_type	==  PNG_INTER‐
       LACE_ADAM7)  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  col‐
       umn  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	 func‐
       tion  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	inter‐
       face,  you  can finish reading the file.	 If you are interested in com‐
       ments or time, which may be stored either before	 or  after  the	 image
       data,  you should pass the separate png_info struct if you want to keep
       the comments from before and after the image separate.  If you are  not
       interested, you can pass NULL.

	  png_read_end(png_ptr, end_info);

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

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

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

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

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

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

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

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

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

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

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

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

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

   Reading PNG files progressively
       The  progressive	 reader is slightly different 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 impor‐
       tance is repeated here, so you won't have to constantly look back up in
       the reading section to understand writing.

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

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

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

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

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

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

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

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

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

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

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

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

	   png_init_io(png_ptr, fp);

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

	   png_set_sig_bytes(png_ptr, 8);

       to inform libpng that it should not write a signature.

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

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

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

       To inform libpng about your function, use

	   png_set_write_status_fn(png_ptr, write_row_callback);

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

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

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

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

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

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

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

       extern PNG_EXPORT(void,png_set_zbuf_size)

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

       Some of the more important parts of the png_info are:

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

			    PNG_COLOR_MASK_PALETTE
			    PNG_COLOR_MASK_COLOR
			    PNG_COLOR_MASK_ALPHA

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	   num_text	  - number of comments

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

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

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

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

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

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

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

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

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

       Until text gets around 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  descrip‐
       tion  of	 the  image  before  the image, but leave the disclaimer until
       after, so viewers working over modem connections don't have to wait for
       the disclaimer to go over the modem before they start seeing the image.
       Finally, keywords should be full words,	not  abbreviations.   Keywords
       and  text  are in the ISO 8859-1 (Latin-1) character set (a superset of
       regular ASCII) and can not contain NUL characters, and should not  con‐
       tain  control  or  other	 unprintable characters.  To make the comments
       widely readable, stick with basic ASCII,	 and  avoid  machine  specific
       character  set  extensions  like the IBM-PC character set.  The keyword
       must be present, but you can leave off  the  text  string  on  non-com‐
       pressed	pairs.	 Compressed pairs must have a text string, as only the
       text string is compressed anyway, so the compression would be  meaning‐
       less.

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

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

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

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

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

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

	   png_write_png(png_ptr, info_ptr, png_transforms, NULL)

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

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

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

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

	   png_write_info(png_ptr, info_ptr);

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

	   png_set_invert_alpha(png_ptr);

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

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

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

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

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

	   png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

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

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

	   png_set_packing(png_ptr);

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

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

	   png_set_sBIT(png_ptr, info_ptr, &sig_bit);

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

	   png_set_shift(png_ptr, &sig_bit);

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

	   if (bit_depth > 8)
	      png_set_swap(png_ptr);

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

	   if (bit_depth < 8)
	      png_set_packswap(png_ptr);

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

	   png_set_bgr(png_ptr);

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

	   png_set_invert_mono(png_ptr);

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

	   png_set_write_user_transform_fn(png_ptr,
	      write_transform_fn);

       You must supply the function

	   void write_transform_fn(png_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 call‐
       back function.

	   png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

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

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

	   voidp write_user_transform_ptr =
	      png_get_user_transform_ptr(png_ptr);

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

	   png_write_flush(png_ptr);

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

	   png_set_flush(png_ptr, nrows);

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

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

	   png_write_image(png_ptr, row_pointers);

       where row_pointers is:

	   png_byte *row_pointers[height];

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

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

	   png_write_rows(png_ptr, row_pointers,
	      number_of_rows);

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

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

	   png_bytep row_pointer = row;

	   png_write_row(png_ptr, row_pointer);

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

       If  you	don't  want libpng to handle the interlacing details, just use
       png_set_interlace_handling() and call png_write_rows() the correct num‐
       ber of times to write all 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 inter‐
       ested, you can pass NULL.

	   png_write_end(png_ptr, info_ptr);

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

	   png_destroy_write_struct(&png_ptr, &info_ptr);

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

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

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

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

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

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

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

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

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

       This function only affects data that has already been  allocated.   You
       can  call  this function before calling after the png_set_*() functions
       to control whether the user or png_destroy_*() is supposed to free  the
       data.   When the user assumes responsibility for libpng-allocated data,
       the application must use png_free() to  free  it,  and  when  the  user
       transfers  responsibility  to  libpng  for data that the user has allo‐
       cated, 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.trans‐
       lated_keyword separately, do not transfer  responsibility  for  freeing
       text_ptr	 to  libpng, because when libpng fills a png_text structure it
       combines these members with the key member,  and	 png_free_data()  will
       free  only text_ptr.key.	 Similarly, if you transfer responsibility for
       free'ing text_ptr from libpng to	 your  application,  your  application
       must  not separately free those members.	 For a more compact example of
       writing a PNG image, see the file example.c.

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

       Memory allocation, input/output, and error handling

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

       Memory allocation is done through the functions png_malloc(),  png_cal‐
       loc(),  and png_free().	These currently just call the standard C func‐
       tions.  png_calloc() calls png_malloc() and then png_memset() to	 clear
       the newly allocated memory to zero.  If your pointers can't access more
       then 64K at a time, you will want to set MAXSEG_64K in  zlib.h.	 Since
       it is unlikely that the method of handling memory allocation on a plat‐
       form will change between applications, these functions must be modified
       in  the	library	 at  compile  time.   If you prefer to use a different
       method  of  allocating  and  freeing  data,  you	  can	use   png_cre‐
       ate_read_struct_2() or png_create_write_struct_2() to register your own
       functions as described above.  These  functions	also  provide  a  void
       pointer that can be retrieved via

	   mem_ptr=png_get_mem_ptr(png_ptr);

       Your replacement memory functions must have prototypes as follows:

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

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

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

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

	   png_set_read_fn(png_structp read_ptr,
	       voidp read_io_ptr, png_rw_ptr read_data_fn)

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

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

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

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

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

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

       Error handling in libpng is done through png_error() and png_warning().
       Errors handled through png_error() are fatal, meaning that  png_error()
       should  never  return  to  its  caller.	Currently, this is handled via
       setjmp()	 and  longjmp()	 (unless  you  have   compiled	 libpng	  with
       PNG_NO_SETJMP,  in  which  case it is handled via PNG_ABORT()), but you
       could change this to do things like exit() if you should wish, as  long
       as your function does not return.

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

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

	   png_voidp error_ptr = png_get_error_ptr(png_ptr);

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

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

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

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

       If you need to write a new intrinsic chunk, first read the PNG specifi‐
       cation. Acquire a first level of understanding of how  it  works.   Pay
       particular  attention  to  the  sections that describe chunk names, and
       look at how other chunks were designed, so you can do things similarly.
       Second,	check  out  the sections of libpng that read and write chunks.
       Try to find a chunk that is similar to yours and use it as a  template.
       More  details can be found in the comments inside the code.  It is best
       to handle 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  mem‐
       ory  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	 popu‐
       lar compilers.  Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
       defined, and FAR gets defined to far in pngconf.h, and  you  should  be
       all  set.   Everything  in the library (except for zlib's structure) is
       expecting far data.  You must use the typedefs with the p or pp on  the
       end  for pointers (or at least look at them and be careful).  Make note
       that the rows of data are defined as png_bytepp, which is  an  unsigned
       char far * far *.

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

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

   Configuring zlib:
       There are special functions to configure the compression.  Perhaps  the
       most  useful  one  changes  the compression level, which currently uses
       input compression values in the range 0 - 9.  The library normally uses
       the  default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests have
       shown that for a large majority of images, compression  values  in  the
       range  3-6  compress  nearly  as	 well as higher levels, and do so much
       faster.	For online applications it may be desirable  to	 have  maximum
       speed  (Z_BEST_SPEED  = 1).  With versions of zlib after v0.99, you can
       also specify no compression (Z_NO_COMPRESSION = 0), but this would cre‐
       ate 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 fil‐
       ters  are used, and how it goes about picking row filters, you can call
       one of these functions.	The selection and configuration of row filters
       can  have  a  significant  impact  on the size and encoding speed and a
       somewhat lesser impact on the decoding speed of an image.  Filtering is
       enabled	by  default  for  RGB  and  grayscale images (with and without
       alpha), but not for paletted images nor for any images with bit	depths
       less than 8 bits/pixel.

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

       Individual  filter  types are PNG_FILTER_NONE, PNG_FILTER_SUB, PNG_FIL‐
       TER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which	can  be	 bitwise  ORed
       together with '|' to specify one or more filters to use.	 These filters
       are described in more detail in the PNG specification.  If  you	intend
       to  change  the filter type during the course of writing the image, you
       should start with flags set for all of the filters you intend to use so
       that  libpng  can  initialize its internal structures appropriately for
       all of the filter types.	 (Note that this  means	 the  first  row  must
       always  be adaptively filtered, because libpng currently does not allo‐
       cate the filter buffers until png_write_row() is called for  the	 first
       time.)

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

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

       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  fil‐
       ters.

	   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 cho‐
       sen.  Unspecified weights are  taken  to	 be  1.0,  and	the  specified
       weights	should	probably  be  declining	 like  those above in order to
       emphasize recent filters over older filters.

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

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

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

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

       All the reading and writing specific code are in separate files, so the
       linker  should  only  grab the files it needs.  However, if you want to
       make sure, or if you are building a stand alone library, all the	 read‐
       ing  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	print‐
       out.   Set  it to an integer value in the range 0 to 3.	Higher numbers
       result in increasing amounts of debugging information.  The information
       is  printed to the "stderr" file, unless another file name is specified
       in the PNG_DEBUG_FILE macro definition.

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

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

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

	  png_debug1(2, "foo=%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	state‐
       ments  having  level = 0 will be printed.  There aren't any such state‐
       ments in this version of libpng, but if you insert some	they  will  be
       printed.

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

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

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

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

       The    old    libpng   functions	  png_read_init(),   png_write_init(),
       png_info_init(), png_read_destroy(), and png_write_destroy() have  been
       moved  to  PNG_INTERNAL in version 0.95 to discourage their use.	 These
       functions will be removed from libpng version 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	struc‐
       tures  from  the	 application,  allow  version error checking, and also
       allow the use of custom error handling routines during the  initializa‐
       tion, which the old functions do not.  The functions png_read_destroy()
       and png_write_destroy() do not actually free  the  memory  that	libpng
       allocated  for  these  structs,	but just reset the data structures, so
       they   can   be	 used	instead	  of   png_destroy_read_struct()   and
       png_destroy_write_struct()  if  you feel there is too much system over‐
       head allocating and freeing the png_struct for each image read.

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

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

	  png_uint_32 libpng_vn = png_access_version_number();

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

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

	  png_uint_32 application_vn = PNG_LIBPNG_VER;

VIII. Changes to Libpng from version 1.0.x to 1.2.x
       Support	for  user memory management was enabled by default.  To accom‐
       plish  this,   the   functions	png_create_read_struct_2(),   png_cre‐
       ate_write_struct_2(),   png_set_mem_fn(),  png_get_mem_ptr(),  png_mal‐
       loc_default(), and png_free_default() were added.

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

       Support for certain MNG features was enabled.

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

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

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

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

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

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

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

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

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

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

       These macros are deprecated:

	   PNG_READ_TRANSFORMS_NOT_SUPPORTED
	   PNG_PROGRESSIVE_READ_NOT_SUPPORTED
	   PNG_NO_SEQUENTIAL_READ_SUPPORTED
	   PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
	   PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
	   PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED

       They have been replaced, respectively, by:

	   PNG_NO_READ_TRANSFORMS
	   PNG_NO_PROGRESSIVE_READ
	   PNG_NO_SEQUENTIAL_READ
	   PNG_NO_WRITE_TRANSFORMS
	   PNG_NO_READ_ANCILLARY_CHUNKS
	   PNG_NO_WRITE_ANCILLARY_CHUNKS

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

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

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

IX. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
       Private libpng prototypes and macro definitions were moved  from	 png.h
       and pngconf.h into a new pngpriv.h header file.

       Functions      png_set_benign_errors(),	   png_benign_error(),	   and
       png_chunk_benign_error() were added.

       Support for setting the maximum amount of memory that  the  application
       will allocate for reading chunks was added, as a security measure.  The
       functions png_set_chunk_cache_max() and png_get_chunk_cache_max()  were
       added to the library.

       We implemented support for I/O states by adding png_ptr member io_state
       and  functions  png_get_io_chunk_name()	 and   png_get_io_state()   in
       pngget.c

       We  added  PNG_TRANSFORM_GRAY_TO_RGB  to the available high-level input
       transforms.

       Checking for and reporting of errors in the IHDR chunk  is  more	 thor‐
       ough.

       Support for global arrays was removed, to improve thread safety.

       Some obsolete/deprecated macros and functions have been removed.

       Typecasted NULL definitions such as
	  #define  png_voidp_NULL	      (png_voidp)NULL were eliminated.
       If you used these in your application, just use NULL instead.

       The png_struct and info_struct members "trans" and "trans_values"  were
       changed to "trans_alpha" and "trans_color", respectively.

       The  obsolete,  unused pnggccrd.c and pngvcrd.c files and related make‐
       files were removed.

       The PNG_1_0_X and PNG_1_2_X macros were eliminated.

       The PNG_LEGACY_SUPPORTED macro was eliminated.

       Many WIN32_WCE #ifdefs were removed.

       The   functions	 png_read_init(info_ptr),    png_write_init(info_ptr),
       png_info_init(info_ptr),	 png_read_destroy(),  and  png_write_destroy()
       have been removed.  They have been deprecated since libpng-0.95.

       The png_permit_empty_plte() was removed. It has been  deprecated	 since
       libpng-1.0.9.  Use png_permit_mng_features() instead.

       We   removed   the   obsolete  stub  functions  png_get_mmx_flagmask(),
       png_set_mmx_thresholds(),     png_get_asm_flags(),     png_get_mmx_bit‐
       depth_threshold(),		     png_get_mmx_rowbytes_threshold(),
       png_set_asm_flags(), and png_mmx_supported()

       We  removed  the	 obsolete  png_check_sig(),  png_memcpy_check(),   and
       png_memset_check()  functions.	Instead	 use  !png_sig_cmp(), png_mem‐
       cpy(), and png_memset(), respectively.

       The function png_set_gray_1_2_4_to_8() was removed. It has been	depre‐
       cated  since  libpng-1.0.18  and	 1.2.9,	 when  it  was	replaced  with
       png_set_expand_gray_1_2_4_to_8()	 because  the  former  function	  also
       expanded palette images.

       Macros  for  png_get_uint_16,  png_get_uint_32, and png_get_int_32 were
       added and are used by default instead of the  corresponding  functions.
       Unfortunately, from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro
       (but  not  the  function)  incorrectly  returned	 a   value   of	  type
       png_uint_32.

       We changed the prototype for png_malloc() from
	   png_malloc(png_structp png_ptr, png_uint_32 size) to
	   png_malloc(png_structp png_ptr, png_alloc_size_t size)

       This  also  applies  to	the  prototype	for  the user replacement mal‐
       loc_fn().

       The png_calloc() function  was  added  and  is  used  in	 place	of  of
       "png_malloc();  png_memset();"  except  in  the	case in png_read_png()
       where the array consists of pointers; in this case a "for" loop is used
       after  the  png_malloc()	 to  set the pointers to NULL, to give robust.
       behavior in case the application runs out of  memory  part-way  through
       the process.

       We  changed  the	 prototypes  of	 png_get_compression_buffer_size() and
       png_set_compression_buffer_size() to work with  png_size_t  instead  of
       png_uint_32.

       Support	for  numbered  error messages was removed by default, since we
       never got around to actually numbering the error messages. The function
       png_set_strip_error_numbers() was removed from the library by default.

       The png_zalloc() and png_zfree() functions are no longer exported.  The
       png_zalloc() function no longer zeroes out the  memory  that  it	 allo‐
       cates.

       Support	for dithering was disabled by default in libpng-1.4.0, because
       been well tested and doesn't  actually  "dither".   The	code  was  not
       removed,	 however,  and	could  be  enabled  by	building  libpng  with
       PNG_READ_DITHER_SUPPORTED defined.  In libpng-1.4.2, this  support  was
       reenabled,  but	the function was renamed png_set_quantize() to reflect
       more  accurately	 what  it  actually  does.   At	 the  same  time,  the
       PNG_DITHER_[RED,GREEN_BLUE]_BITS	 macros were also renamed to PNG_QUAN‐
       TIZE_[RED,GREEN,BLUE]_BITS.

       We removed the trailing '.' from the warning and error messages.

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

	   AC_CHECK_LIB(png, png_get_io_ptr, ...

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

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

       or you can browse it via "gitweb" at

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

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

	   http://libpng.sourceforge.net

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

	   if (condition)
	   {
	      action;
	   }

	   else if (another condition)
	   {
	      another action;
	   }

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

	   if (condition)
	      return (0);

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

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

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

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

	   /* Single-line comment */
	   statement;

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

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

	   statement;	 /* comment */

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Lines do not exceed 80 characters.

       Other rules can be inferred by inspecting the libpng source.

XIII. Y2K Compliance in libpng
       December 9, 2010

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

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

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

	  Glenn Randers-Pehrson
	  libpng maintainer
	  PNG Development Group

NOTE
       Note about libpng version numbers:

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

	source		   png.h  png.h	 shared-lib
	version		   string   int	 version
	-------		   ------  ----- ----------
	0.89c ("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	 12.so.0.1.0.17rc1
	1.2.7rc1	    13	  10207	 12.so.0.1.2.7rc1
	1.0.17		    10	  10017	 12.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	 12.so.0.1.0.18rc1-5
	1.2.8rc1-5	    13	  10208	 12.so.0.1.2.8rc1-5
	1.0.18		    10	  10018	 12.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]
	1.2.10beta1-7	    13	  10210	 12.so.0.10[.0]
	1.2.10rc1-2	    13	  10210	 12.so.0.10[.0]
	1.2.10		    13	  10210	 12.so.0.10[.0]
	1.4.0beta1-6	    14	  10400	 14.so.0.0[.0]
	1.2.11beta1-4	    13	  10210	 12.so.0.11[.0]
	1.4.0beta7-8	    14	  10400	 14.so.0.0[.0]
	1.2.11		    13	  10211	 12.so.0.11[.0]
	1.2.12		    13	  10212	 12.so.0.12[.0]
	1.4.0beta9-14	    14	  10400	 14.so.0.0[.0]
	1.2.13		    13	  10213	 12.so.0.13[.0]
	1.4.0beta15-36	    14	  10400	 14.so.0.0[.0]
	1.4.0beta37-87	    14	  10400	 14.so.14.0[.0]
	1.4.0rc01	    14	  10400	 14.so.14.0[.0]
	1.4.0beta88-109	    14	  10400	 14.so.14.0[.0]
	1.4.0rc02-08	    14	  10400	 14.so.14.0[.0]
	1.4.0		    14	  10400	 14.so.14.0[.0]
	1.4.1beta01-03	    14	  10401	 14.so.14.1[.0]
	1.4.1rc01	    14	  10401	 14.so.14.1[.0]
	1.4.1beta04-12	    14	  10401	 14.so.14.1[.0]
	1.4.1rc02-04	    14	  10401	 14.so.14.1[.0]
	1.4.1		    14	  10401	 14.so.14.1[.0]
	1.4.2beta01	    14	  10402	 14.so.14.2[.0]
	1.4.2rc02-06	    14	  10402	 14.so.14.2[.0]
	1.4.2		    14	  10402	 14.so.14.2[.0]
	1.4.3beta01-05	    14	  10403	 14.so.14.3[.0]
	1.4.3rc01-03	    14	  10403	 14.so.14.3[.0]
	1.4.3		    14	  10403	 14.so.14.3[.0]
	1.4.4beta01-08	    14	  10404	 14.so.14.4[.0]
	1.4.4rc01-05	    14	  10404	 14.so.14.4[.0]
	1.4.4		    14	  10404	 14.so.14.4[.0]
	1.4.5beta01-04	    14	  10405	 14.so.14.5[.0]
	1.4.5rc01	    14	  10405	 14.so.14.5[.0]
	1.4.5beta05-07	    14	  10405	 14.so.14.5[.0]
	1.4.5rc02-03	    14	  10405	 14.so.14.5[.0]
	1.4.5		    14	  10405	 14.so.14.5[.0]

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

SEE ALSO
       png(5), libpngpf(3), zlib(3), deflate(5), and zlib(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://ds.internic.net/rfc/rfc2083.txt
	      or (as a W3C Recommendation) at
	      http://www.w3.org/TR/REC-png.html

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

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

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

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

       Libpng  version	1.4.5 - December 9, 2010: Initially created in 1995 by
       Guy Eric Schalnat, then of Group	 42,  Inc.   Currently	maintained  by
       Glenn Randers-Pehrson (glennrp at users.sourceforge.net).

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

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

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

       This code is released under the libpng license.

       libpng versions 1.2.6, August 15,  2004,	 through  1.4.5,  December  9,
       2010,  are  Copyright (c) 2004,2006-2007 Glenn Randers-Pehrson, and are
       distributed  according  to  the	same   disclaimer   and	  license   as
       libpng-1.2.5  with  the	following individual added to the list of Con‐
       tributing 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 dis‐
       claimer 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 Copy‐
       right (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.

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

	  Andreas Dilger
	  Dave Martindale
	  Guy Eric Schalnat
	  Paul Schmidt
	  Tim Wegner

       The  PNG	 Reference  Library  is	 supplied  "AS	IS".  The Contributing
       Authors and Group  42,  Inc.  disclaim  all  warranties,	 expressed  or
       implied, including, without limitation, the warranties of merchantabil‐
       ity and of fitness for any purpose.  The Contributing Authors and Group
       42,  Inc.   assume  no liability for direct, indirect, incidental, spe‐
       cial, exemplary, or consequential damages, which may  result  from  the
       use of the PNG Reference Library, even if advised of the possibility of
       such damage.

       Permission is hereby granted to use, copy, modify, and distribute  this
       source  code, or portions hereof, for any purpose, without fee, subject
       to the following restrictions:

       1. The origin of this source code must not be misrepresented.

       2. Altered versions must be plainly marked as such and
	  must not be misrepresented as being the original source.

       3. This Copyright notice may not be removed or altered from
	  any source or altered source distribution.

       The Contributing Authors and Group 42, Inc. specifically permit,	 with‐
       out  fee,  and  encourage the use of this source code as a component to
       supporting the PNG file format in commercial products.  If you use this
       source  code  in a product, acknowledgment is not required but would be
       appreciated.

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

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

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

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

       Glenn Randers-Pehrson glennrp at users.sourceforge.net December 9, 2010

			       December 9, 2010			     LIBPNG(3)
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