libpng - Portable Network Graphics (PNG) Reference Library
       1.0.12


SYNOPSIS

       #include <png.h>

       png_uint_32 png_access_version_number (void);

       int png_check_sig (png_bytep sig, int num);

       void png_chunk_error (png_structp png_ptr, png_const_charp
       error);

       void      png_chunk_warning      (png_structp     png_ptr,
       png_const_charp message);

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

       void   png_convert_from_time_t  (png_timep  ptime,  time_t
       ttime);

       png_charp  png_convert_to_rfc1123  (png_structp   png_ptr,
       png_timep ptime);

       png_infop png_create_info_struct (png_structp png_ptr);

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

       png_structp       png_create_read_struct_2(png_const_charp
       user_png_ver, png_voidp error_ptr, png_error_ptr error_fn,

       malloc_fn, png_free_ptr free_fn);

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

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

       int png_debug(int level, png_const_charp message);

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

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

       void   png_destroy_info_struct    (png_structp    png_ptr,
       png_infopp info_ptr_ptr);

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

       void png_destroy_write_struct  (png_structpp  png_ptr_ptr,
       png_infopp info_ptr_ptr);

       void   png_error   (png_structp  png_ptr,  png_const_charp
       error);

       void png_free (png_structp png_ptr, png_voidp ptr);


       void png_free_default(png_structp png_ptr, png_voidp ptr);

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

       png_byte png_get_bit_depth (png_structp png_ptr, png_infop
       info_ptr);

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

       png_byte png_get_channels (png_structp png_ptr,  png_infop
       info_ptr);

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

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

       png_byte    png_get_color_type    (png_structp    png_ptr,
       png_infop info_ptr);

       png_byte  png_get_compression_type  (png_structp  png_ptr,
       png_infop info_ptr);

       png_byte png_get_copyright (png_structp png_ptr);


       png_byte   png_get_filter_type    (png_structp    png_ptr,
       png_infop info_ptr);

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

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

       png_byte png_get_header_ver (png_structp png_ptr);

       png_byte png_get_header_version (png_structp png_ptr);

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

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

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

       png_uint_32   png_get_image_height  (png_structp  png_ptr,
       png_infop info_ptr);

       png_uint_32  png_get_image_width   (png_structp   png_ptr,
       png_infop info_ptr);

       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_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_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


       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_permitted);

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

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

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

       void   png_read_end   (png_structp   png_ptr,    png_infop
       info_ptr);

       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 transforms, png_voidp params);

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

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

       void png_read_update_info (png_structp png_ptr,  png_infop
       info_ptr);

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

       void png_set_bgr (png_structp png_ptr);

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

       void   png_set_cHRM   (png_structp   png_ptr,    png_infop

       double  red_y,  double  green_x,  double  green_y,  double
       blue_x, double blue_y);

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

       void  png_set_compression_level  (png_structp png_ptr, int
       level);

       void png_set_compression_mem_level  (png_structp  png_ptr,
       int mem_level);

       void  png_set_compression_method (png_structp png_ptr, int
       method);

       void  png_set_compression_strategy  (png_structp  png_ptr,
       int strategy);

       void png_set_compression_window_bits (png_structp png_ptr,
       int window_bits);

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

       void   png_set_dither   (png_structp  png_ptr,  png_colorp
       palette, int num_palette, int maximum_colors, png_uint_16p
       histogram, int full_dither);

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


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

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

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

       void png_set_flush (png_structp png_ptr, int nrows);

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

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

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

       void png_set_gray_1_2_4_to_8(png_structp png_ptr);

       void png_set_gray_to_rgb (png_structp png_ptr);

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

       void    png_set_iCCP   (png_structp   png_ptr,   png_infop
       info_ptr, png_charp name, int compression_type,  png_charp


       int png_set_interlace_handling (png_structp png_ptr);

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

       void png_set_invert_alpha (png_structp png_ptr);

       void png_set_invert_mono (png_structp png_ptr);

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

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

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

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

       void png_set_packing (png_structp png_ptr);

       void png_set_packswap (png_structp png_ptr);

       void png_set_palette_to_rgb(png_structp png_ptr);

       info_ptr, png_charp purpose, png_int_32 X0, png_int_32 X1,
       int   type,   int  nparams,  png_charp  units,  png_charpp
       params);

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

       void  png_set_progressive_read_fn  (png_structp   png_ptr,
       png_voidp     progressive_ptr,    png_progressive_info_ptr
       info_fn,  png_progressive_row_ptr   row_fn,   png_progres­
       sive_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);

       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);

       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_channels);

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

       void   png_set_write_status_fn    (png_structp    png_ptr,
       png_write_status_ptr write_row_fn);

       void png_set_write_user_transform_fn (png_structp png_ptr,
       png_user_transform_ptr write_user_transform_fn);

       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);

       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 transforms, png_voidp params);

       void png_write_row (png_structp png_ptr, png_bytep row);

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


DESCRIPTION

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


LIBPNG.TXT

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

        libpng version 1.0.12 - June 8, 2001
        Updated and distributed by Glenn Randers-Pehrson
        <randeg@alum.rpi.edu>
        Copyright (c) 1998-2001 Glenn Randers-Pehrson

        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 refer­
       ence 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  specifica­
       tion,  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.libpng.org/pub/png>           and           at
       <ftp://ftp.uu.net/graphics/png/documents/>.

       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/graph­
       ics/png/documents/>.

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

       Most  users  will  not have to modify the library signifi­
       cantly; advanced users may want to modify  it  more.   All

       while keeping the code  easy  to  understand.   Currently,
       this library only supports C.  Support for other languages
       is being considered.

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

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

       Libpng is thread safe, provided the threads are using dif­
       ferent  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.  How­
       ever,  this  tended  to  cause  problems with applications
       using dynamically loaded libraries, and as a result a  set
       of  interface  functions for png_info (the png_get_*() and
       png_set_*()  functions)  was  developed.   The  fields  of
       png_info  are  still available for older 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

       recompiled  if  they  were  compiled or loaded with libpng
       version 1.0.6, in which the members were  in  a  different
       order.   In  version  1.0.7,  the  members of the png_info
       structure reverted to the old order, as they were in  ver­
       sions  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  pro­
       gramming  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 explain­
       ing the syntax and purpose of each one.  See example.c and
       png.h  for more detail.  While progressive reading is cov­
       ered in the next section, you will still need some of  the
       functions discussed in this section to read a PNG file.

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

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

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

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

           fread(header, 1, number, fp);
           is_png = !png_sig_cmp(header, 0, number);
           if (!is_png)
           {
               return (NOT_PNG);
           }

       Next,  png_struct  and  png_info  need to be allocated and
       initialized.  In order to ensure that the  size  of  these
       structures  is  correct  even  with  a  dynamically linked
       libpng, there are functions to initialize and allocate the
       structures.   We  also  pass the library 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_cre­
       ate_read_struct_2() instead of png_create_read_struct():

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

       ate_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  mem­
       ory 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 discus­
       sion on libpng error handling in  the  Customizing  Libpng
       section  below  for  more  information on the libpng error
       handling.  If an error occurs, and libpng  longjmp's  back
       to    your    setjmp,    you    will    want    to    call
       png_destroy_read_struct() to free any memory.

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

       If you would rather avoid the complexity of setjmp/longjmp
       issues,  you  can  compile libpng with PNG_SETJMP_NOT_SUP­
       PORTED, 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);

       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  demon­
       strated in pngtest.c.  You must supply a function

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

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

       To inform libpng about your function, use

           png_set_read_status_fn(png_ptr, read_row_callback);

       Now you get to set the way the library  processes  unknown
       chunks  in  the  input  PNG stream. Both known and unknown
       chunks will be read.  Normal behavior is that known chunks
       will  be  parsed into information in various info_ptr mem­
       bers; 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 inter­
       face if (a) you are willing to read the entire image  into
       memory,  and  (b) the input transformations you want to do
       are limited to the following set:

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


       (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  trans­
       formations   indicated   by   the   transform  mask,  then
       png_read_image(), and finally png_read_end().

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

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

          row_pointers = png_get_rows(png_ptr, info_ptr);

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

          png_bytep row_pointers[height];

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

          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

       You do this with a call to png_read_info().

           png_read_info(png_ptr, info_ptr);

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

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

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

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

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

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

           filter_method can be NULL if you are
           not interested in their values.

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

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

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

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

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


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

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

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

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

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

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

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

           png_get_bKGD(png_ptr, info_ptr, &background);

                            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_MICROME­
       TER

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

           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)

              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 com­
       ments  in  keyword/text pairs, one pair per chunk, with no
       limit on the number of text chunks, and a 2^31 byte  limit
       on  their size.  While there are suggested keywords, there
       is no requirement to restrict the use  to  these  strings.
       It  is strongly suggested that keywords and text be sensi­
       ble to humans (that's the point), so don't  use  abbrevia­
       tions.  Non-printing symbols are not allowed.  See the PNG
       specification for more details.  There is also no require­
       ment to have text after the keyword.

       Keywords  should be limited to 79 Latin-1 characters with­
       out leading or trailing spaces, but non-consecutive spaces
       are  allowed  within  the keyword.  It is possible to have
       the same keyword any number of times.  The text_ptr is  an
       array  of png_text structures, each holding a pointer to a
       language string, a pointer to a keyword and a pointer to a
       text  string.   The text string, language code, and trans­
       lated keyword may be empty or  NULL  pointers.   The  key­
       word/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().

       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  cer­
       tain  color types and bit depths.  Even though each trans­
       formation 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 multiple-image
       viewing application that wishes to treat all images in the
       same way.

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

           if (color_type == PNG_COLOR_TYPE_GRAY &&
               bit_depth < 8) png_set_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);

       png_set_expand(), added in libpng version 1.0.4, with  the
       function  names  expanded to improve code readability.  In
       some future version they may actually do different things.

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

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

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

           if (color_type & PNG_COLOR_MASK_ALPHA)
               png_set_strip_alpha(png_ptr);

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

           png_set_invert_alpha(png_ptr);

       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 con­
       vert  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.

       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 repre­
       sented 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

                             If  either   weight   is   negative,
       default
                             weights (21268, 71514) are used.

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

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

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

       The  default  values  approximate those recommended in the
       Charles           Poynton's           Color           FAQ,
       <http://www.inforamp.net/~poynton/>      Copyright     (c)
       1998-01-04 Charles Poynton poynton@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 supply the background
       color as a gray  value  at  the  original  file  bit-depth
       (need_expand  =  1) or else supply the background color as
       an  RGB  triplet  at  the  final,   expanded   bit   depth
       (need_expand = 0).  Similarly, if you are reading a palet­
       ted 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;

       ground))
               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 compos­
       ite 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 col­
       ors  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.  Ide­
       ally, the user will know this, and  the  application  will
       allow  them to set it.  One method of allowing the user to
       set the display gamma separately for  each  system  is  to
       check  for  a  SCREEN_GAMMA  or  DISPLAY_GAMMA environment
       variable, which will hopefully be correctly set.

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

          double gamma, screen_gamma;

          if (/* We have a user-defined screen
              gamma value */)
          {
             screen_gamma = user_defined_screen_gamma;
          }
          /* One way that applications can share the same
             screen gamma value */
          else if ((gamma_str = getenv("SCREEN_GAMMA"))
             != NULL)
          {
             screen_gamma = (double)atof(gamma_str);
          }
          /* If we don't have another value */
          else
          {
             screen_gamma = 2.2; /* A good guess for a

             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 description of what gamma  is,  and  why  all
       applications  should  support  it.   It is strongly recom­
       mended that PNG viewers support gamma correction.

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

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

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

                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,

                   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 callback function, and you can inform libpng that
       your transform function will change the number of channels
       or bit depth with the function

           png_set_user_transform_info(png_ptr, user_ptr,
              user_depth, user_channels);

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

       png_get_user_transform_ptr().  For example:

           voidp read_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

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

           number_of_passes        =       png_set_interlace_han­
       dling(png_ptr);

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

           png_read_update_info(png_ptr, info_ptr);

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

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

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

          png_read_image(png_ptr, row_pointers);

       where row_pointers is:

          png_bytep row_pointers[height];

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


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

           png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

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

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

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

       If the file is interlaced (interlace_type != 0 in the IHDR
       chunk), things get somewhat harder.  The only current (PNG
       Specification  version  1.2)  interlacing  type for PNG is
       (interlace_type == PNG_INTERLACE_ADAM7) is a somewhat com­
       plicated  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 specifica­
       tion 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 gradu­
       ally  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  han­
       dling).

       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)

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

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

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

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

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

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

           png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

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

           png_read_rows(png_ptr, NULL, row_pointers,
              number_of_rows);

   Finishing a sequential read
       After  you  are  finished reading the image through either

       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 sepa­
       rate.  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 mem­
       bers  that point to libpng-allocated storage with the fol­
       lowing function:

           png_free_data(png_ptr, info_ptr, mask, seq)
           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
           seq  - sequence number of item to be freed
                  (-1 for all items)

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

       The default behavior is only to free data that  was  allo­
       cated  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_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 con­
       trol  whether  the  user or png_destroy_*() is supposed to
       free the data.  When the user assumes  responsibility  for
       libpng-allocated data, the application must use png_free()
       to free it, and when the user transfers responsibility  to
       libpng for data that the user has allocated, the user must
       have used png_malloc() or png_zalloc() to allocate it.

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

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

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

           png_set_invalid(png_ptr, info_ptr, mask);
           mask - identifies the chunks to be made invalid,
                  containing the 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 example.c.

       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  high­
       light  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


              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

              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

              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, every­
       thing of importance is repeated here, so you won't have to
       constantly  look  back up in the reading section to under­
       stand 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 any­
       thing to undo. If you are not using the standard I/O func­
       tions,  you  will need to replace them with custom writing
       functions.  See the discussion under Customizing libpng.

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

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

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

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


       If you want to use your own  memory  allocation  routines,
       define    PNG_USER_MEM_SUPPORTED    and    use    png_cre­
       ate_write_struct_2() instead of png_create_write_struct():

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

       After  you  have these structures, you will need to set up
       the error handling.  When libpng encounters an  error,  it
       expects to longjmp() back to your routine.  Therefore, you
       will  need  to  call  setjmp()  and  pass   the   png_jmp­
       buf(png_ptr).   If  you write the file from different 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    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 (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_SUP­
       PORTED, in which case errors will  result  in  a  call  to
       PNG_ABORT() which defaults to abort().

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

           png_init_io(png_ptr, fp);

   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

              int pass);
           {
             /* put your code here */
           }

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

       To inform libpng about your function, use

           png_set_write_status_fn(png_ptr, write_row_callback);

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

           /* turn on or off filtering, and/or choose
              specific filters.  You can use either a single
              PNG_FILTER_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 during compression, it should start out  with  all
       of  the filters (to ensure that the previous row of pixels
       will be stored in case it's needed later),  and  then  add
       and remove them after the start of compression.

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

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

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

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

       extern PNG_EXPORT(void,png_set_zbuf_size)

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

       Some of the more important parts of the png_info are:

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

                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

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

           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.

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

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

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

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

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

           png_set_bKGD(png_ptr, info_ptr, background);


           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

                         (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 com­
       pressed, set the  compression  type  to  PNG_TEXT_COMPRES­
       SION_NONE.  Because tEXt and zTXt chunks don't have a lan­
       guage field, if you specify  PNG_TEXT_COMPRESSION_NONE  or
       PNG_TEXT_COMPRESSION_zTXt  any language code or translated
       keyword will not be written out.

       Until text gets around 1000 bytes, it is  not  worth  com­
       pressing  it.   After the text has been written out to the
       file,    the    compression     type     is     set     to

       SION_zTXt_WR, so that it isn't written out  again  at  the
       end (in case you are calling png_write_end() with the same
       struct.

       The keywords that are given in the PNG Specification are:

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

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

       PNG supports modification time via the png_time structure.
       Two    conversion    routines   are   provided,   png_con­
       vert_from_time_t()     for     time_t     and     png_con­
       vert_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.

       ation, you should use a plain tEXt chunk  with  the  "Cre­
       ation  Time" keyword.  This is necessary because the "cre­
       ation time" of a PNG image is somewhat vague, depending on
       whether you mean the PNG file, the time the image was cre­
       ated 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  rec­
       ommended  that the "Creation Time" tEXt chunk use RFC 1123
       format dates (e.g. "22 May 1997 18:07:10  GMT"),  although
       this  isn't  a  requirement.   Unlike  the tIME chunk, the
       "Creation Time" tEXt chunk is not expected to be automati­
       cally  changed  by the software.  To facilitate the use of
       RFC     1123     dates,      a      function      png_con­
       vert_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 writ­
       ten  out  in  a sequence that satisfies the PNG specifica­
       tion's ordering rules.

   The high-level write interface
       At this point there are two ways to proceed;  through  the
       high-level  write interface, or through a sequence of low-
       level write operations.  You can use the high-level inter­
       face  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 sam­
       ples
           PNG_TRANSFORM_PACKSWAP      Change order of packed
                                       pixels to LSB first
           PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
           PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                       sBIT depth
           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
           PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                       to transparency
           PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
           PNG_TRANSFORM_STRIP_FILLER  Strip out filler bytes.

       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 trans­
       formations  indicated  by   the   transform   mask,   then
       png_write_image(), and finally png_write_end().

       (The  final parameter of this call is not yet used.  Some­
       day 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 sup­
       plied as a level of transparency, you can invert the alpha
       channel before you write it, so that 0 is fully  transpar­
       ent  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  repre­
       sents  a single color to be rendered as transparent) won't
       need to be changed, and you can safely do this transforma­
       tion 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

       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 cer­
       tain color types and bit depths.  Even though each  trans­
       formation  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

       range  0-7 for a 4-bit PNG), this will scale the values to
       appear to be the correct bit depth as is required by  PNG.

           png_set_shift(png_ptr, &sig_bit);

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

           if (bit_depth > 8)
              png_set_swap(png_ptr);

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

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

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

           png_set_bgr(png_ptr);

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

           png_set_invert_mono(png_ptr);

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

           png_set_write_user_transform_fn(png_ptr,
              write_transform_fn);

       You must supply the function

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

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

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


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

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

           voidp write_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

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

           png_write_flush(png_ptr);

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

           png_set_flush(png_ptr, nrows);

       Note  that the distance between rows is from the last time
       png_write_flush() was called, or  the  first  row  of  the
       image  if  it  has  never been called.  So if you write 50
       lines, and then png_set_flush 25, it will flush the output
       on  the  next  scanline,  and  every  25 lines thereafter,
       unless png_write_flush() is called before  25  more  lines
       have been written.  If nrows is too small (less than about
       10 lines for a 640 pixel wide RGB image)  the  image  com­
       pression  may  decrease  noticeably  (although this may be
       acceptable for real-time applications).  Infrequent flush­
       ing 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 func­
       tion 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 interlac­
       ing, so you  don't  need  to  call  png_set_interlace_han­
       dling()  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:


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

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

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

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

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

           png_bytep row_pointer = row;

           png_write_row(png_ptr, row_pointer);

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

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

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

           number_of_passes =
              png_set_interlace_handling(png_ptr);

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

       Then write the complete image number_of_passes times.

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

       As some of these rows are not used, and thus return  imme­
       diately, you may want to read about interlacing in the PNG

       used.

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

           png_write_end(png_ptr, info_ptr);

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

           png_destroy_write_struct(&png_ptr, &info_ptr);

       It is also possible to individually free the info_ptr mem­
       bers  that point to libpng-allocated storage with the fol­
       lowing function:

           png_free_data(png_ptr, info_ptr, mask, seq)
           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
           seq   - sequence number of item to be freed
                   (-1 for all items)

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

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

       The default behavior is only to free data that  was  allo­
       cated  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,


           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 con­
       tinue to use the PLTE, tRNS, and hIST data  in  the  write
       structure.

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

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


V. Modifying/Customizing libpng:

       There are three issues here.  The first  is  changing  how
       libpng   does  standard  things  like  memory  allocation,
       input/output, and error handling.  The second  deals  with
       more complicated things like adding new chunks, adding new

       Both  of  those are compile-time issues; that is, they are
       generally determined at the time the code is written,  and
       there is rarely a need to provide the user with a means of
       changing them.  The third is a run-time  issue:   choosing
       between  and/or  tuning  one or more alternate versions of
       computationally intensive  routines;  specifically,  opti­
       mized assembly-language (and therefore compiler- and plat­
       form-dependent) versions.

       Memory allocation, input/output, and error handling

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

       Memory allocation is done through the  functions  png_mal­
       loc(), png_zalloc(), and png_free().  These currently just
       call the standard C functions.   If  your  pointers  can't
       access  more  then  64K  at  a  time, you will want to set
       MAXSEG_64K in zlib.h.   Since  it  is  unlikely  that  the
       method  of  handling  memory allocation on a platform will
       change between applications, these functions must be modi­
       fied in the library at compile time.  If you prefer to use
       a different method of allocating and freeing data, you can
       use

           png_set_mem_fn(png_structp png_ptr, png_voidp mem_ptr,
              png_malloc_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);

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

       Input/Output  in  libpng  is  done  through png_read() and
       png_write(),  which  currently  just  call   fread()   and
       fwrite().   The FILE * is stored in png_struct and is ini­
       tialized via png_init_io().  If you  wish  to  change  the
       method of I/O, the library supplies callbacks that you can

       png_set_write_fn()  at  run  time,  instead of calling the
       png_init_io() function.  These functions  also  provide  a
       void  pointer  that  can  be  retrieved  via  the function
       png_get_io_ptr().  For example:

           png_set_read_fn(png_structp read_ptr,
               voidp read_io_ptr, png_rw_ptr read_data_fn)

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

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

       The replacement I/O functions must have prototypes as fol­
       lows:

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

           png_set_error_fn(png_structp png_ptr,

               png_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.   How­
       ever,  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 doc­
       umentation for more details.  For an alternative approach,
       you  may  wish  to  use  the   "cexcept"   facility   (see
       http://cexcept.sourceforge.net).

   Custom chunks
       If  you  need to read or write custom chunks, you may need
       to get deeper into the libpng code.  The library  now  has
       mechanisms for storing and writing chunks of unknown type;
       you  can  even  declare  callbacks  for   custom   chunks.
       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 specification. Acquire a first level of  understanding
       of how it works.  Pay particular attention to the sections
       that describe chunk names, and look at  how  other  chunks
       were  designed,  so  you can do things similarly.  Second,
       check out the sections  of  libpng  that  read  and  write
       chunks.   Try to find a chunk that is similar to yours and
       use it as a template.  More details can be  found  in  the
       comments  inside  the  code.  It is best to handle unknown
       chunks  in  a  generic  method,  via  callback  functions,
       instead of by modifying libpng functions.

       If you wish to write your own transformation for the data,
       look through the part of the code that does the  transfor­
       mations,  and check out some of the simpler ones to get an
       idea  of  how  they  work.   Try   to   find   a   similar

       it.  More details can be found in the comments inside  the
       code itself.

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

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

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

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

       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  desir­
       able  to have maximum speed (Z_BEST_SPEED = 1).  With ver­
       sions of zlib after v0.99, you can also  specify  no  com­
       pression  (Z_NO_COMPRESSION  =  0),  but this would create
       files larger than just storing the raw  bitmap.   You  can
       specify the compression level by calling:

           png_set_compression_level(png_ptr, level);

       Another  useful  one is to reduce the memory level used by
       the library.  The memory level defaults to 8, but  it  can
       be  lowered  if  you are short on memory (running DOS, for
       example, where you only have 640K).

           png_set_compression_mem_level(png_ptr, level);

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

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

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

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


       Individual  filter  types  are  PNG_FILTER_NONE,  PNG_FIL­
       TER_SUB, PNG_FILTER_UP, PNG_FILTER_AVG,  PNG_FILTER_PAETH,
       which can be bitwise ORed together with '|' to specify one
       or more filters to use.  These filters  are  described  in
       more  detail  in  the PNG specification.  If you intend to
       change the filter type during the course  of  writing  the
       image, you should start with flags set for all of the fil­
       ters 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);
                     The second parameter can also be
                     PNG_INTRAPIXEL_DIFFERENCING if you are
                     writing a PNG to be embedded in a MNG
                     datastream.  This parameter must be the
                     same as the value of filter_method used
                     in png_set_IHDR().

       It  is  also possible to influence how libpng chooses from
       among the available filters.  This is done in two  ways  -
       by  telling it how important it is to keep the same filter
       for successive rows, and by telling it the relative compu­
       tational 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  succes­
       sive  rows  unless  another  row filter is that many times
       better than the previous filter.  In the above example, if
       the previous 3 filters were SUB, SUB, NONE, the SUB filter
       could have a "sum of absolute differences" 1.5 x 1.3 times
       higher  than  other filters and still be chosen, while the
       NONE filter could have a sum 1.1 times higher  than  other
       filters  and  still  be  chosen.   Unspecified weights are
       taken to be 1.0, and the specified weights should probably

       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 val­
       ues for either the costs or the weights.

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

       You can also turn all  of  the  transforms  and  ancillary
       chunk  capabilities  off en masse with compiler directives
       that   define   PNG_NO_READ[or    WRITE]_TRANSFORMS,    or
       PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS, or all four, along
       with directives to turn on any of  the  capabilities  that
       you  do want.  The PNG_NO_READ[or WRITE]_TRANSFORMS direc­
       tives 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 con­
       fuse  this  with  the INTERLACING capability, which you'll
       still have).

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

       If  you  are creating or distributing a dynamically linked
       library (a .so or DLL file),  you  should  not  remove  or

       applications linked with different versions of the library
       to  fail  if  they  call  functions  not available in your
       library.  The size of the library itself should not be  an
       issue,  because only those sections that are actually used
       will be loaded into memory.

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

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

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

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

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

       is expanded to

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

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

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

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


VI. MNG support

       The       MNG       specification       (available      at
       http://www.libpng.org/pub/mng) allows  certain  extensions

       treams.  Libpng can support some of these extensions.   To
       enable them, use the png_permit_mng_features() function:

          feature_set = png_permit_mng_features(png_ptr, mask)
          mask is a png_uint_32 containing the logical OR of the
               features you want to enable.  These include
               PNG_FLAG_MNG_EMPTY_PLTE
               PNG_FLAG_MNG_FILTER_64
               PNG_ALL_MNG_FEATURES
          feature_set is a png_32_uint that is the logical AND of
             your mask with the set of MNG features that is
             supported by the version  of  libpng  that  you  are
       using.

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


VII. Changes to Libpng from version 0.88

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

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

       The preferred method  of  creating  and  initializing  the
       libpng  structures  is  via  the png_create_read_struct(),
       png_create_write_struct(),  and   png_create_info_struct()
       because  they  isolate the size of the structures from the
       application, allow version error checking, and also  allow
       the  use of custom error handling routines during the ini­
       tialization, which the old functions do  not.   The  func­
       tions  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

       each image read.

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

       Starting  with  version 1.0.7, you can find out which ver­
       sion 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  ver­
       sion,  minor version with leading zero, and release number
       with leading zero, (e.g., libpng_vn for version  1.0.7  is
       10007).

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

          png_uint_32 application_vn = PNG_LIBPNG_VER;


VIII. Y2K Compliance in libpng

       June 8, 2001

       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 1.0.12 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:

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

       All  appear to handle dates properly in a Y2K environment.
       The png_convert_from_time_t() function calls  gmtime()  to
       convert  from  system  clock  time,  which returns (year -
       1900), which we properly convert to the full 4-digit year.
       There  is a possibility that applications using libpng are
       not   passing   4-digit   years    into    the    png_con­
       vert_to_rfc_1123()  function, or that they are incorrectly
       passing only a 2-digit year instead of "year - 1900"  into
       the png_convert_from_struct_tm() function, but this is not
       under our control.  The libpng  documentation  has  always
       stated that it works with 4-digit years, and the APIs have
       been documented as such.

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

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

          Glenn Randers-Pehrson
          libpng maintainer
          PNG Development Group


NOTE

       Note about libpng version numbers:

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

        source             png.h  png.h  shared-lib
        version            string   int  version
        -------            ------  ----- ----------
        0.89c ("beta 3")  0.89       89  1.0.89
        0.90  ("beta 4")  0.90       90  0.90
        0.95  ("beta 5")  0.95       95  0.95
        0.96  ("beta 6")  0.96       96  0.96
        0.97b ("beta 7")  1.00.97    97  1.0.1

        0.98              0.98       98  2.0.98
        0.99              0.99       98  2.0.99
        0.99a-m           0.99       99  2.0.99
        1.00              1.00      100  2.1.0
        1.0.0             1.0.0     100  2.1.0
        1.0.0   (from here on, the  100  2.1.0
        1.0.1    png.h string is  10001  2.1.0
        1.0.1a-e identical to the 10002  from here on, the
        1.0.2    source version)  10002  shared library is 2.V
        1.0.2a-b                  10003  where V is the source
        1.0.1                     10001  code version except as
        1.0.1a-e                  10002  2.1.0.1a-e   noted.
        1.0.2                     10002  2.1.0.2
        1.0.2a-b                  10003  2.1.0.2a-b
        1.0.3                     10003  2.1.0.3
        1.0.3a-d                  10004  2.1.0.3a-d
        1.0.4                     10004  2.1.0.4
        1.0.4a-f                  10005  2.1.0.4a-f
        1.0.5 (+ 2 patches)       10005  2.1.0.5
        1.0.5a-d                  10006  2.1.0.5a-d
        1.0.5e-r                  10100  2.1.0.5e-r
        1.0.5s-v                  10006  2.1.0.5s-v
        1.0.6 (+ 3 patches)       10006  2.1.0.6
        1.0.6d-g                  10007  2.1.0.6d-g
        1.0.6h                    10007  10.6h
        1.0.6i                    10007  10.6i
        1.0.6j                    10007  2.1.0.6j
        1.0.7beta11-14    DLLNUM  10007  2.1.0.7beta11-14
        1.0.7beta15-18       1    10007  2.1.0.7beta15-18
        1.0.7rc1-2           1    10007  2.1.0.7rc1-2
        1.0.7                1    10007  2.1.0.7
        1.0.8beta1-4         1    10008  2.1.0.8beta1-4
        1.0.8rc1             1    10008  2.1.0.8rc1
        1.0.8                1    10008  2.1.0.8
        1.0.9beta1-6         1    10009  2.1.0.9beta1-6
        1.0.9rc1             1    10009  2.1.0.9rc1
        1.0.9beta7-10        1    10009  2.1.0.9beta7-10
        1.0.9rc2             1    10009  2.1.0.9rc2
        1.0.9                1    10009  2.1.0.9
        1.0.10beta1          1    10010  2.1.0.10beta1
        1.0.10rc1            1    10010  2.1.0.10rc1
        1.0.10               1    10010  2.1.0.10
        1.0.11beta1-3        1    10011  2.1.0.11beta1-3
        1.0.11rc1            1    10011  2.1.0.11rc1
        1.0.11               1    10011  2.1.0.11
        1.0.12beta1-2        1    10012  2.1.0.12beta1-2

       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  com­
       patibility,  as it is intended.  The PNG_PNGLIB_VER macro,
       which is not used  within  libpng  but  is  available  for
       applications,  is  an  unsigned  integer of the form xyyzz

       in y and z).  Beta versions were given the previous public
       release number plus a letter, until version  1.0.6j;  from
       then on they were given the upcoming public release number
       plus "betaNN" or "rcN".


SEE ALSO

       libpngpf(3), png(5)

       libpng:

              ftp://ftp.uu.net/graphics/png
              http://www.libpng.org/pub/png

       zlib:

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

       PNGspecification:RFC2083

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

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


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 documen­
       tation.

       Libpng version 1.0.12 - June 8, 2001: Initially created in
       1995 by Guy Eric Schalnat, then of Group  42,  Inc.   Cur­
       rently   maintained   by   Glenn   Randers-Pehrson   (ran­
       deg@alum.rpi.edu).

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

       (This copy of the libpng notices is provided for your con­
       venience.   In  case  of any discrepancy between this copy
       and the notices in the file png.h that is included in  the
       libpng distribution, the latter shall prevail.)

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

       libpng versions 1.0.7, July 1, 2000, through  1.0.12, June
       8,  2001,  are  Copyright  (c)  2000-2001  Glenn  Randers-
       Pehrson, and are distributed according to  the  same  dis­
       claimer  and  license  as  libpng-1.0.6 with the following
       individuals added to the list of Contributing Authors

          Simon-Pierre Cadieux
          Eric S. Raymond
          Gilles Vollant

       and with the following additions to the disclaimer:

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

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

          Tom Lane
          Glenn Randers-Pehrson
          Willem van Schaik

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

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

       are Copyright (c) 1995, 1996 Guy Eric Schalnat, Group  42,
       Inc.

       For  the  purposes  of  this  copyright and license, "Con­
       tributing 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 Con­
       tributing Authors and Group 42,  Inc.  disclaim  all  war­
       ranties,  expressed or implied, including, without limita­
       tion, the warranties of merchantability 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 restric­
       tions:

       1. The origin of this source code must  not  be  misrepre­
       sented.

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

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

       The  Contributing  Authors and Group 42, Inc. specifically
       permit, without fee, and encourage the use of this  source
       code  as  a component to supporting the PNG file format in
       commercial products.  If you use this  source  code  in  a
       product,  acknowledgment  is  not  required  but  would be
       appreciated.

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

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

       Also,  the PNG logo (in PNG format, of course) is supplied

       "pngnow.png" (98x31).

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

       Glenn Randers-Pehrson randeg@alum.rpi.edu June 8, 2001


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