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LIBPNG(3)							     LIBPNG(3)

NAME
       libpng - Portable Network Graphics (PNG) Reference Library 1.0.8

SYNOPSIS

       #include <png.h>

       png_uint_32 png_access_version_number (void);

       int png_check_sig (png_bytep sig, int num);

       void png_chunk_error (png_structp png_ptr, png_const_charp error);

       void png_chunk_warning (png_structp png_ptr, png_const_charp message);

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

       void png_convert_from_time_t (png_timep ptime, time_t ttime);

       png_charp  png_convert_to_rfc1123   (png_structp	  png_ptr,   png_timep
       ptime);

       png_infop png_create_info_struct (png_structp png_ptr);

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

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

       int png_debug(int level, png_const_charp message);

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

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

       void    png_destroy_info_struct	  (png_structp	 png_ptr,   png_infopp
       info_ptr_ptr);

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

       void  png_destroy_write_struct  (png_structpp  png_ptr_ptr,  png_infopp
       info_ptr_ptr);

       void png_error (png_structp png_ptr, png_const_charp error);

       void png_free (png_structp png_ptr, png_voidp ptr);

       void png_free_chunk_list (png_structp png_ptr);

       void png_free_default(png_structp png_ptr, png_voidp ptr);

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

       png_byte png_get_bit_depth (png_structp png_ptr, png_infop info_ptr);

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

       png_byte png_get_channels (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_cHRM (png_structp png_ptr, png_infop info_ptr, 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_byte png_get_color_type (png_structp png_ptr, png_infop info_ptr);

       png_byte	 png_get_compression_type  (png_structp	  png_ptr,   png_infop
       info_ptr);

       png_byte png_get_copyright (png_structp png_ptr);

       png_voidp png_get_error_ptr (png_structp png_ptr);

       png_byte png_get_filter_type (png_structp png_ptr, png_infop info_ptr);

       png_uint_32 png_get_gAMA (png_structp png_ptr, png_infop info_ptr, 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_byte	  png_get_interlace_type   (png_structp	  png_ptr,   png_infop
       info_ptr);

       png_voidp png_get_io_ptr (png_structp png_ptr);

       png_byte png_get_libpng_ver (png_structp png_ptr);

       png_voidp png_get_mem_ptr(png_structp png_ptr);

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

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

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

       float  png_get_pixel_aspect_ratio   (png_structp	  png_ptr,   png_infop
       info_ptr);

       png_uint_32  png_get_pixels_per_meter  (png_structp  png_ptr, png_infop
       info_ptr);

       png_voidp png_get_progressive_ptr (png_structp png_ptr);

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

       png_byte png_get_rgb_to_gray_status (png_structp png_ptr)

       png_uint_32 png_get_rowbytes (png_structp png_ptr, png_infop info_ptr);

       png_bytepp png_get_rows (png_structp png_ptr, png_infop info_ptr);

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

       png_bytep png_get_signature (png_structp png_ptr, png_infop info_ptr);

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

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

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

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

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

       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_voidp png_get_user_transform_ptr (png_structp png_ptr);

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

       png_int_32  png_get_x_offset_microns  (png_structp  png_ptr,  png_infop
       info_ptr);

       png_int_32  png_get_x_offset_pixels  (png_structp  png_ptr,   png_infop
       info_ptr);

       png_uint_32  png_get_x_pixels_per_meter (png_structp png_ptr, png_infop
       info_ptr);

       png_int_32  png_get_y_offset_microns  (png_structp  png_ptr,  png_infop
       info_ptr);

       png_int_32   png_get_y_offset_pixels  (png_structp  png_ptr,  png_infop
       info_ptr);

       png_uint_32 png_get_y_pixels_per_meter (png_structp png_ptr,  png_infop
       info_ptr);

       png_uint_32 png_get_compression_buffer_size (png_structp png_ptr);

       void png_info_init (png_infop info_ptr);

       void png_init_io (png_structp png_ptr, FILE *fp);

       png_voidp png_malloc (png_structp png_ptr, png_uint_32 size);

       png_voidp png_malloc_default(png_structp png_ptr, png_uint_32 size);

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

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

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

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

       void png_permit_empty_plte (png_structp png_ptr, int empty_plte_permit‐
       ted);

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

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

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

       void png_read_end (png_structp png_ptr, png_infop info_ptr);

       void png_read_image (png_structp png_ptr, png_bytepp image);

       DEPRECATED: void png_read_init (png_structp png_ptr);

       DEPRECATED:  void png_read_init_2 (png_structp png_ptr, png_const_charp
       user_png_ver, png_size_t png_struct_size, png_size_t png_info_size);

       void png_read_info (png_structp png_ptr, png_infop info_ptr);

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

       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_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_dither  (png_structp  png_ptr,  png_colorp	 palette,  int
       num_palette,    int   maximum_colors,   png_uint_16p   histogram,   int
       full_dither);

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

       void png_set_expand (png_structp png_ptr);

       void  png_set_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);

       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_read_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr
       read_data_fn);

       void png_set_read_status_fn (png_structp	 png_ptr,  png_read_status_ptr
       read_row_fn);

       void	 png_set_read_user_transform_fn	     (png_structp     png_ptr,
       png_user_transform_ptr read_user_transform_fn);

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

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

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

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

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

       void png_set_shift (png_structp png_ptr, png_color_8p true_bits);

       void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

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

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

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

       void png_set_strip_16 (png_structp png_ptr);

       void png_set_strip_alpha (png_structp png_ptr);

       void png_set_swap (png_structp png_ptr);

       void png_set_swap_alpha (png_structp png_ptr);

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

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

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

       void png_set_tRNS_to_alpha(png_structp png_ptr);

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

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

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

       void  png_set_user_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_destroy (png_structp png_ptr);

       void png_write_destroy_info (png_infop info_ptr);

       void png_write_end (png_structp png_ptr, png_infop info_ptr);

       void png_write_flush (png_structp png_ptr);

       void png_write_image (png_structp png_ptr, png_bytepp image);

       DEPRECATED: void png_write_init (png_structp png_ptr);

       DEPRECATED: void png_write_init_2 (png_structp png_ptr, png_const_charp
       user_png_ver, png_size_t png_struct_size, png_size_t png_info_size);

       void png_write_info (png_structp png_ptr, png_infop info_ptr);

       void   png_write_info_before_PLTE   (png_structp	  png_ptr,   png_infop
       info_ptr);

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

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 1.0.8 - July 24, 2000
	Updated and distributed by Glenn Randers-Pehrson
	<randeg@alum.rpi.edu>
	Copyright (c) 1998, 1999, 2000 Glenn Randers-Pehrson
	For conditions of distribution and use, see copyright
	notice in png.h.

	based on:

	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.

       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-1.2       specification	   is	    available	    at
       <http://www.cdrom.com/pub/png>	    (will	be	moving	    to
       <http://www.libpng.org>)	 and  at  <ftp://ftp.uu.net/graphics/png/docu‐
       ments/>.

       The    PNG-1.0	 specification	  is	available    as	   RFC	  2083
       <ftp://ftp.uu.net/graphics/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
       <ftp://ftp.uu.net/graphics/png/documents/>.

       Other  information  about PNG, and the latest version of libpng, can be
       found at the PNG home page,  <http://www.cdrom.com/pub/png/>  (will  be
       moving  to  <http://www.libpng.org>)  and  at  <ftp://ftp.uu.net/graph‐
       ics/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,	  <ftp://ftp.freesoft‐
       ware.com/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 if the bytes match the correspond‐
       ing bytes of the PNG signature, or nonzero otherwise.  Of  course,  the
       more bytes you pass in, the greater the accuracy of the prediction.

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

       (*):  If you are not using the standard I/O functions, you will need to
       replace them with custom functions.  See the discussion under 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_SETJMP_NOT_SUPPORTED, in which case	errors
       will result in a call to PNG_ABORT() which defaults to abort().

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

	   png_init_io(png_ptr, fp);

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

	   png_set_sig_bytes(png_ptr, number);

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

	   read_chunk_callback(png_ptr ptr,
		png_unknown_chunkp chunk);
	   {
	      /* The unknown chunk structure contains your
		 chunk data: */
		  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.  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);

       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; unknown chunks will be discarded. To change this,
       you can call:

	   png_set_keep_unknown_chunks(png_ptr, info_ptr, keep,
	       chunk_list, num_chunks);
	   keep	      - 0: do not keep
			1: keep only if safe-to-copy
			2: keep even if unsafe-to-copy
	   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

       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 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 trans‐
       parency
	   PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples

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

	   png_read_png(png_ptr, info_ptr, png_transforms, NULL)

       where  png_transforms  is  an integer containing the logical 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.)

       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

	  row_pointers = png_malloc(png_ptr, height*sizeof(png_bytep));
	  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_type);

	   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_type	  - (must be PNG_FILTER_TYPE_BASE
			    for PNG 1.0)
	   compression_type - (must be PNG_COMPRESSION_TYPE_BASE
			    for PNG 1.0)
	   interlace_type - (PNG_INTERLACE_NONE or
			    PNG_INTERLACE_ADAM7)
	   Any or all of interlace_type, compression_type, of
			    filter_type can be NULL if you are not
			    interested in their values.

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

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

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

       These are also important, but their validity  depends  on  whether  the
       chunk	has   been   read.    The   png_get_valid(png_ptr,   info_ptr,
       PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr, info_ptr, ...) functions
       return  non-zero	 if  the data has been read, or zero if it is 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_COMPRES‐
       SION_TYPE_BASE
			     for PNG 1.0.  You may give NULL to this argument
			     to ignore it.
	   profile	   - International Color Consortium color profile
			     data. May contain NULs.
	   proflen	   - length of profile data in bytes.

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

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

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

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

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

	   num_comments	  = png_get_text(png_ptr, info_ptr,
			    &text_ptr, &num_text);
	   num_comments	  - number of comments
	   text_ptr	  - array of png_text holding image
			    comments
	   text_ptr[i].compression - type of compression used
			on "text" PNG_TEXT_COMPRESSION_NONE
				  PNG_TEXT_COMPRESSION_zTXt
				  PNG_ITXT_COMPRESSION_NONE
				  PNG_ITXT_COMPRESSION_zTXt
	   text_ptr[i].key   - keyword for comment.  Must contain
				1-79 characters.
	   text_ptr[i].text  - text comments for current
				keyword.  Can be empty.
	   text_ptr[i].text_length - length of text string,
			after decompression, 0 for iTXt
	   text_ptr[i].itxt_length - length of itxt string,
			after decompression, 0 for tEXt/zTXt
	   text_ptr[i].lang  - language of comment (empty
				string for unknown).
	   text_ptr[i].translated_keyword  - keyword in UTF-8
				(empty string for unknown).
	   num_text	  - number of comments (same as num_comments;
			    you can put NULL here to avoid the duplication)
	   Note while png_set_text() will accept text, language, and
	   translated keywords that can be NULL pointers, the structure
	   returned by png_get_text will always contain regular
	   zero-terminated C strings.  They might be empty strings but
	   they will never be NULL pointers.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       A  quick word about text_ptr and num_text.  PNG stores comments in 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() is called to insert filler
       bytes, either before or after each RGB triplet.	16-bit RGB  data  will
       be  returned RRGGBB RRGGBB, with the most significant byte of the color
       value first, unless png_set_strip_16() is called	 to  transform	it  to
       regular	RGB  RGB  triplets,  or	 png_set_filler()  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() 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_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.

       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_16p 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 bytes. This code expands them
       into 4 bytes for windowing systems that need them in this format:

	   if (bit_depth == 8 && 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.

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

	   png_color_16 my_background;
	   png_color_16p image_background;

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

       The png_set_background() function tells libpng to composite images with
       alpha or simple transparency against the supplied background color.  If
       the  PNG	 file contains a bKGD chunk (PNG_INFO_bKGD valid), you may use
       this color, or supply another color more suitable for the current  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_dither()
       will do that.  Note that this is a  simple  match  dither  that	merely
       finds  the  closest color available.  This should work fairly well with
       optimized palettes, and fairly badly with linear color cubes.   If  you
       pass a palette that is larger then maximum_colors, the file will reduce
       the number of colors in the palette so it will fit into maximum_colors.
       If  there  is  a	 histogram,  it	 will  use it to make more intelligent
       choices when reducing the palette.  If there is no  histogram,  it  may
       not do as good a job.

	  if (color_type & PNG_COLOR_MASK_COLOR)
	  {
	     if (png_get_valid(png_ptr, info_ptr,
		PNG_INFO_PLTE))
	     {
		png_color_16p histogram;

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

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

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

	  if (bit_depth == 1 && color_type == PNG_COLOR_GRAY)
	     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 either	the  high-  or
       low-level  interfaces,  you  can	 finish	 reading the file.  If you are
       interested in comments or time, which may be stored  either  before  or
       after  the  image data, you should pass the separate png_info struct if
       you want to keep the comments from before and after the image separate.
       If you are not interested, you can pass NULL.

	  png_read_end(png_ptr, end_info);

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

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

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

	   png_free_data(png_ptr, info_ptr, mask, n)
	   mask - identifies data to be freed, a mask
		  containing the logical 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
	   n	- 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
       "n" parameter is ignored if only one item of the	 selected  data	 type,
       such  as	 PLTE,	is  allowed.  If "n" 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 is freed.

       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  (the
       png_zalloc()  function  is the same as png_malloc() except that it also
       zeroes the newly-allocated memory).

       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 logical 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_SETJMP_NOT_SUPPORTED, in which case errors
       will result in a call to PNG_ABORT() which defaults to abort().

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

	   png_init_io(png_ptr, fp);

   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 value is '0' (as of the
       July 1999 PNG specification, version 1.2).  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_FIL‐
       TER_VALUE_NAME
	      or the logical OR of one or more PNG_FILTER_NAME masks. */
	   png_set_filter(png_ptr, 0,
	      PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
	      PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
	      PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
	      PNG_FILTER_AVE   | PNG_FILTER_VALUE_AVE  |
	      PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
	      PNG_ALL_FILTERS);

       If an application wants to start and stop using particular filters 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.

       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_type)
	   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_type	  - (must be PNG_FILTER_TYPE_DEFAULT)

	   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_COMPRES‐
       SION_TYPE_BASE
			     for PNG 1.0.  You may give NULL to this argument
			     to ignore it.
	   profile	   - International Color Consortium color profile
			     data. May contain NULs.
	   proflen	   - length of profile data in bytes.

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

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

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

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

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

	   png_set_text(png_ptr, info_ptr, text_ptr, num_text);
	   text_ptr	  - array of png_text holding image
			    comments
	   text_ptr[i].compression - type of compression used
			on "text" PNG_TEXT_COMPRESSION_NONE
				  PNG_TEXT_COMPRESSION_zTXt
				  PNG_ITXT_COMPRESSION_NONE
				  PNG_ITXT_COMPRESSION_zTXt
	   text_ptr[i].key   - keyword for comment.  Must contain
			1-79 characters.
	   text_ptr[i].text  - text comments for current
				keyword.  Can be NULL or empty.
	   text_ptr[i].text_length - length of text string,
			after decompression, 0 for iTXt
	   text_ptr[i].itxt_length - length of itxt string,
			after decompression, 0 for tEXt/zTXt
	   text_ptr[i].lang  - language of comment (NULL or
				empty for unknown).
	   text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
				or empty for unknown).
	   num_text	  - number of comments

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

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

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

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

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

	   png_set_unknown_chunks(png_ptr, info_ptr, &unknowns, num_unknowns)
	   unknowns	     - array of png_unknown_chunk structures holding
			       unknown chunks
	   unknowns[i].name  - name of unknown chunk
	   unknowns[i].data  - data of unknown chunk
	   unknowns[i].size  - size of unknown chunk's data
	   unknowns[i].location - position to write chunk in file
				  0: do not write chunk
				  PNG_HAVE_IHDR: before PLTE
				  PNG_HAVE_PLTE: before IDAT
				  PNG_AFTER_IDAT: after IDAT
	   The "location" member is set automatically according to
	   what part of the output file has already been written.
	   You can change its value after calling png_set_unknown_chunks()
	   as demonstrated in pngtest.c.  Within each of the "locations",
	   the chunks are sequenced according to their position in the
	   structure (that is, the value of "i", which is the order in which
	   the chunk was either read from the input file or defined with
	   png_set_unknown_chunks).

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

       The compression types have the same valid numbers  as  the  compression
       types  of  the  image  data.  Currently, the only valid number is zero.
       However, you can store text either 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	trans‐
       parency
	   PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
	   PNG_TRANSFORM_STRIP_FILLER  Strip out filler bytes.

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

	   png_write_png(png_ptr, info_ptr, png_transforms, NULL)

       where  png_transforms  is  an integer containing the logical 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.)

   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, n)
	   mask	 - identifies data to be freed, a mask
		   containing the logical 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
	   n	 - 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
       "n" parameter is ignored if only one item of the	 selected  data	 type,
       such  as	 PLTE,	is  allowed.  If "n" 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 is freed.

       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.

       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_zal‐
       loc(), and png_free().  These currently just call the standard C	 func‐
       tions.  If your pointers can't access more then 64K at a time, you will
       want to set MAXSEG_64K in zlib.h.  Since it is unlikely that the method
       of  handling memory allocation on a platform will change between appli‐
       cations, these functions must be modified in  the  library  at  compile
       time.   If you prefer to use a different method of allocating and free‐
       ing data, you can use

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

       This function also provides 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_uint_32 size);
	   void free_fn(png_structp png_ptr, png_voidp ptr);

       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_uint_32 length);
	   void user_write_data(png_structp png_ptr,
	       png_bytep data, png_uint_32 length);
	   void user_flush_data(png_structp png_ptr);

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

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

       On  non-fatal  errors,  png_warning() is called to print a warning 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.   Hoewver, 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/delete an include, this is the place to do it.  The includes
       that are not needed outside libpng are protected	 by  the  PNG_INTERNAL
       definition,  which  is  only  defined  for those routines inside libpng
       itself.	The files in libpng proper only include png.h, which  includes
       pngconf.h.

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

	   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.

	   filters = PNG_FILTER_NONE | PNG_FILTER_SUB
		     PNG_FILTER_UP | PNG_FILTER_AVE |
		     PNG_FILTER_PAETH | PNG_ALL_FILTERS;
	   or
	   filters = one of PNG_FILTER_VALUE_NONE,
		     PNG_FILTER_VALUE_SUB, PNG_FILTER_VALUE_UP,
		     PNG_FILTER_VALUE_AVE, PNG_FILTER_VALUE_PAETH

	   png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
	      filters);

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

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

	   png_set_filter_selection(png_ptr,
	      PNG_FILTER_SELECTION_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. 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_destory() 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;

VII. Y2K Compliance in libpng
       July 24, 2000

       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	1.0.8 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 ("1.0 beta 3")	   0.89	       89  1.0.89
	  0.90	 ("1.0	beta  4")      0.90	   90  0.90  [should have been
       2.0.90]
	  0.95	("1.0 beta 5")	   0.95	       95   0.95   [should  have  been
       2.0.95]
	  0.96	 ("1.0	beta  6")      0.96	   96  0.96  [should have been
       2.0.96]
	  0.97b ("1.00.97 beta 7") 1.00.97     97   1.0.1  [should  have  been
       2.0.97]
	  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 [100 should be 10000]
	  1.0.0			   1.0.0      100  2.1.0 [100 should be 10000]
	  1.0.1			   1.0.1    10001  2.1.0
	  1.0.1a-e		   1.0.1a-e 10002  2.1.0.1a-e
	  1.0.2			   1.0.2    10002  2.1.0.2
	  1.0.2a-b		   1.0.2a-b 10003  2.1.0.2a-b
	  1.0.3			   1.0.3    10003  2.1.0.3
	  1.0.3a-d		   1.0.3a-d 10004  2.1.0.3a-d
	  1.0.4			   1.0.4    10004  2.1.0.4
	  1.0.4a-f		   1.0.4a-f 10005  2.1.0.4a-f
	  1.0.5 (+ 2 patches)	   1.0.5    10005  2.1.0.5
	  1.0.5a-d		   1.0.5a-d 10006  2.1.0.5a-d
	  1.0.5e-r		   1.0.5e-r 10100  2.1.0.5e-r (not compatible)
	  1.0.5s-v		   1.0.5s-v 10006  2.1.0.5s-v (compatible)
	  1.0.6 (+ 3 patches)	   1.0.6    10006  2.1.0.6
	  1.0.6d		   1.0.6d   10007  2.1.0.6d
	  1.0.7			    1.0.7    10007  2.1.0.7    (still compati‐
       ble)

	  Henceforth the source version will match the shared-library minor
	  and patch numbers; the shared-library major version number will be
	  used for changes in backward compatibility, as it is intended.  The
	  PNG_PNGLIB_VER macro, which is not used within libpng but is	avail‐
       able
	  for  applications,  is  an unsigned integer of the form xyyzz corre‐
       sponding
	  to the source version x.y.z (leading zeros in y and z).   Beta  ver‐
       sions
	  are given the previous public release number plus a letter or two.

SEE ALSO
       libpngpf(3), png(5)

       libpng:

	      ftp://ftp.uu.net/graphics/png http://www.cdrom.com/pub/png

       zlib:

	      (generally) at the same location as libpng or at
	      ftp://ftp.uu.net/pub/archiving/zip/zlib
	      ftp://ftp.freesoftware.com/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 <randeg@alum.rpi.edu>

       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 1.0.8 - July 24, 2000: Initially created in 1995 by  Guy
       Eric  Schalnat,	then  of Group 42, Inc.	 Currently maintained by Glenn
       Randers-Pehrson (randeg@alum.rpi.edu).

       Supported by the PNG development group
       (png-implement@ccrc.wustl.edu).

COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:
       If you modify libpng you may insert additional notices immediately fol‐
       lowing this sentence.

       libpng versions 1.0.7, July 1, 2000, through  1.0.8, July 24, 2000, are
       Copyright (c) 2000 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 randeg@alum.rpi.edu July 24, 2000

				 July 24, 2000			     LIBPNG(3)
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