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libpng

LIBPNG(3)                                                            LIBPNG(3)



NAME
       libpng - Portable Network Graphics (PNG) Reference Library 1.2.2

SYNOPSIS


       #include <png.h>



       png_uint_32 png_access_version_number (void);



       int png_check_sig (png_bytep sig, int num);



       void png_chunk_error (png_structp png_ptr, png_const_charp error);



       void png_chunk_warning (png_structp png_ptr, png_const_charp message);



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



       void png_convert_from_time_t (png_timep ptime, time_t ttime);



       png_charp  png_convert_to_rfc1123   (png_structp   png_ptr,   png_timep
       ptime);



       png_infop png_create_info_struct (png_structp png_ptr);



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



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



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



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



       int png_debug(int level, png_const_charp message);



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



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



       void    png_destroy_info_struct    (png_structp   png_ptr,   png_infopp
       info_ptr_ptr);



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



       void  png_destroy_write_struct  (png_structpp  png_ptr_ptr,  png_infopp
       info_ptr_ptr);



       void png_error (png_structp png_ptr, png_const_charp error);



       void png_free (png_structp png_ptr, png_voidp ptr);



       void png_free_chunk_list (png_structp png_ptr);



       void png_free_default(png_structp png_ptr, png_voidp ptr);



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



       png_uint_32 png_get_asm_flags (png_structp png_ptr);



       png_byte png_get_bit_depth (png_structp png_ptr, png_infop info_ptr);



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



       png_byte png_get_channels (png_structp png_ptr, png_infop info_ptr);



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



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



       png_byte png_get_color_type (png_structp png_ptr, png_infop info_ptr);



       png_byte  png_get_compression_type  (png_structp   png_ptr,   png_infop
       info_ptr);



       png_byte png_get_copyright (png_structp png_ptr);



       png_voidp png_get_error_ptr (png_structp png_ptr);



       png_byte png_get_filter_type (png_structp png_ptr, png_infop info_ptr);



       png_uint_32 png_get_gAMA (png_structp png_ptr, png_infop info_ptr, dou-
       ble *file_gamma);



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



       png_byte png_get_header_ver (png_structp png_ptr);



       png_byte png_get_header_version (png_structp png_ptr);



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



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



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



       png_uint_32  png_get_image_height   (png_structp   png_ptr,   png_infop
       info_ptr);



       png_uint_32   png_get_image_width   (png_structp   png_ptr,   png_infop
       info_ptr);



       png_byte   png_get_interlace_type   (png_structp   png_ptr,   png_infop
       info_ptr);



       png_voidp png_get_io_ptr (png_structp png_ptr);



       png_byte png_get_libpng_ver (png_structp png_ptr);



       png_voidp png_get_mem_ptr(png_structp png_ptr);



       png_byte png_get_mmx_bitdepth_threshold (png_structp png_ptr);



       png_uint_32 png_get_mmx_flagmask (int flag_select, int *compilerID);



       png_uint_32 png_get_mmx_rowbytes_threshold (png_structp png_ptr);



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



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



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



       float   png_get_pixel_aspect_ratio   (png_structp   png_ptr,  png_infop
       info_ptr);



       png_uint_32 png_get_pixels_per_meter  (png_structp  png_ptr,  png_infop
       info_ptr);



       png_voidp png_get_progressive_ptr (png_structp png_ptr);



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



       png_byte png_get_rgb_to_gray_status (png_structp png_ptr)

       png_uint_32 png_get_rowbytes (png_structp png_ptr, png_infop info_ptr);



       png_bytepp png_get_rows (png_structp png_ptr, png_infop info_ptr);



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



       png_bytep png_get_signature (png_structp png_ptr, png_infop info_ptr);



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



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



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



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



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



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



       png_voidp png_get_user_chunk_ptr (png_structp png_ptr);



       png_voidp png_get_user_transform_ptr (png_structp png_ptr);



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



       png_int_32  png_get_x_offset_microns  (png_structp  png_ptr,  png_infop
       info_ptr);



       png_int_32  png_get_x_offset_pixels  (png_structp  png_ptr,   png_infop
       info_ptr);



       png_uint_32  png_get_x_pixels_per_meter (png_structp png_ptr, png_infop
       info_ptr);



       png_int_32  png_get_y_offset_microns  (png_structp  png_ptr,  png_infop
       info_ptr);



       png_int_32   png_get_y_offset_pixels  (png_structp  png_ptr,  png_infop
       info_ptr);



       png_uint_32 png_get_y_pixels_per_meter (png_structp png_ptr,  png_infop
       info_ptr);



       png_uint_32 png_get_compression_buffer_size (png_structp png_ptr);



       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);



       void png_init_io (png_structp png_ptr, FILE *fp);



       DEPRECATED: void png_info_init (png_infop info_ptr);



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



       png_voidp png_malloc (png_structp png_ptr, png_uint_32 size);



       png_voidp png_malloc_default(png_structp png_ptr, png_uint_32 size);



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



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



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



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



       int png_mmx_support (void);



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



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



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



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



       void png_read_end (png_structp png_ptr, png_infop info_ptr);



       void png_read_image (png_structp png_ptr, png_bytepp image);



       DEPRECATED: void png_read_init (png_structp png_ptr);



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



       void png_read_info (png_structp png_ptr, png_infop info_ptr);



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



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



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



       void png_read_update_info (png_structp png_ptr, png_infop info_ptr);



       png_set_asm_flags (png_structp png_ptr, png_uint_32 asm_flags);



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



       void png_set_bgr (png_structp png_ptr);



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



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



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



       void png_set_compression_level (png_structp png_ptr, int level);



       void   png_set_compression_mem_level    (png_structp    png_ptr,    int
       mem_level);



       void png_set_compression_method (png_structp png_ptr, int method);



       void png_set_compression_strategy (png_structp png_ptr, int strategy);



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



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



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



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



       void png_set_expand (png_structp png_ptr);



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



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



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



       void png_set_flush (png_structp png_ptr, int nrows);



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



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



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



       void png_set_gray_1_2_4_to_8(png_structp png_ptr);



       void png_set_gray_to_rgb (png_structp png_ptr);



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



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



       int png_set_interlace_handling (png_structp png_ptr);



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



       void png_set_invert_alpha (png_structp png_ptr);



       void png_set_invert_mono (png_structp png_ptr);



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



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



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



       png_set_mmx_thresholds    (png_structp   png_ptr,   png_byte   mmx_bit-
       depth_threshold, png_uint_32 mmx_rowbytes_threshold);



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



       void png_set_packing (png_structp png_ptr);



       void png_set_packswap (png_structp png_ptr);



       void png_set_palette_to_rgb(png_structp png_ptr);



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



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



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



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



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



       void png_set_read_status_fn (png_structp  png_ptr,  png_read_status_ptr
       read_row_fn);



       void      png_set_read_user_transform_fn      (png_structp     png_ptr,
       png_user_transform_ptr read_user_transform_fn);



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



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



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



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



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



       void png_set_shift (png_structp png_ptr, png_color_8p true_bits);



       void png_set_sig_bytes (png_structp png_ptr, int num_bytes);



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



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



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



       void png_set_strip_16 (png_structp png_ptr);



       void png_set_strip_alpha (png_structp png_ptr);



       void png_set_strip_error_numbers (png_structp png_ptr,

       png_uint_32 strip_mode);



       void png_set_swap (png_structp png_ptr);



       void png_set_swap_alpha (png_structp png_ptr);



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



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



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



       void png_set_tRNS_to_alpha(png_structp png_ptr);



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



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



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



       void  png_set_user_transform_info   (png_structp   png_ptr,   png_voidp
       user_transform_ptr,  int user_transform_depth, int user_transform_chan-
       nels);



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



       void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
       write_row_fn);



       void     png_set_write_user_transform_fn     (png_structp      png_ptr,
       png_user_transform_ptr write_user_transform_fn);



       void  png_set_compression_buffer_size(png_structp  png_ptr, png_uint_32
       size);



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



       void png_start_read_image (png_structp png_ptr);



       void png_warning (png_structp png_ptr, png_const_charp message);



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



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



       void png_write_chunk_end (png_structp png_ptr);



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



       void png_write_destroy (png_structp png_ptr);



       void png_write_end (png_structp png_ptr, png_infop info_ptr);



       void png_write_flush (png_structp png_ptr);



       void png_write_image (png_structp png_ptr, png_bytepp image);



       DEPRECATED: void png_write_init (png_structp png_ptr);



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



       void png_write_info (png_structp png_ptr, png_infop info_ptr);



       void   png_write_info_before_PLTE   (png_structp   png_ptr,   png_infop
       info_ptr);



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



       void png_write_row (png_structp png_ptr, png_bytep row);



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



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



       void png_zfree (voidpf png_ptr, voidpf ptr);




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

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

        libpng version 1.2.2 - April 15, 2002
        Updated and distributed by Glenn Randers-Pehrson
        <randeg@alum.rpi.edu>
        Copyright (c) 1998-2002 Glenn Randers-Pehrson
        For conditions of distribution and use, see copyright
        notice in png.h.

        based on:

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

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

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


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

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

       The       PNG-1.2       specification       is       available       at
       <http://www.libpng.org/pub/png>   and    at    <ftp://ftp.uu.net/graph-
       ics/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/graphics/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 significantly; advanced
       users may want to modify it more.  All attempts were made to make it as
       complete  as possible, while keeping the code easy to understand.  Cur-
       rently, this library only supports C.  Support for other  languages  is
       being considered.

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

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

       Libpng is  thread  safe,  provided  the  threads  are  using  different
       instances   of  the  structures.   Each  thread  should  have  its  own
       png_struct and png_info instances, and thus its own image.  Libpng does
       not  protect  itself  against  two threads using the same instance of a
       structure.  Note: thread safety may be defeated by use of some  of  the
       MMX  assembler code in pnggccrd.c, which is only compiled when the user
       defines PNG_THREAD_UNSAFE_OK.



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

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

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

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

       #include <png.h>


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


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

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

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


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


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

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

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

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

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

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

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

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

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

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

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

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

           png_init_io(png_ptr, fp);

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

           png_set_sig_bytes(png_ptr, number);


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

           read_chunk_callback(png_ptr ptr,
                png_unknown_chunkp chunk);
           {
              /* The unknown chunk structure contains your
                 chunk data: */
                  png_byte name[5];
                  png_byte *data;
                  png_size_t size;
              /* Note that libpng has already taken care of
                 the CRC handling */

              /* put your code here.  Return one of the
                 following: */

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

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

       To inform libpng about your function, use

           png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
               read_chunk_callback);

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

           png_get_user_chunk_ptr(png_ptr);

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

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

       (You  can give it another name that you like instead of "read_row_call-
       back")

       To inform libpng about your function, use

           png_set_read_status_fn(png_ptr, read_row_callback);


   Unknown-chunk handling
       Now you get to set the way the library processes unknown chunks in  the
       input  PNG  stream. Both known and unknown chunks will be read.  Normal
       behavior is that known chunks will be parsed into information in  vari-
       ous info_ptr members; unknown chunks will be discarded. To change this,
       you can call:

           png_set_keep_unknown_chunks(png_ptr, info_ptr, keep,
               chunk_list, num_chunks);
           keep       - 0: do not keep
                        1: keep only if safe-to-copy
                        2: keep even if unsafe-to-copy
           chunk_list - list of chunks affected (a byte string,
                        five bytes per chunk, NULL or ' ' if
                        num_chunks is 0)
           num_chunks - number of chunks affected; if 0, all
                        unknown chunks are affected

       Unknown chunks declared in this way will be saved as raw  data  onto  a
       list  of  png_unknown_chunk  structures.   If  a chunk that is normally
       known to libpng is named in the list, it will be  handled  as  unknown,
       according  to  the "keep" directive.  If a chunk is named in successive
       instances of png_set_keep_unknown_chunks(),  the  final  instance  will
       take precedence.


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

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

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

           png_read_png(png_ptr, info_ptr, png_transforms, NULL)

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

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

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

          row_pointers = png_get_rows(png_ptr, info_ptr);

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

          png_bytep row_pointers[height];

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

          row_pointers = png_malloc(png_ptr,
             height*sizeof(png_bytep));
          for (int i=0; i<height, i++)
             row_pointers[i]=png_malloc(png_ptr,
                width*pixel_size);
          png_set_rows(png_ptr, info_ptr, &row_pointers);

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

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

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


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

           png_read_info(png_ptr, info_ptr);

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


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

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

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

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           filter_method  - (must be PNG_FILTER_TYPE_BASE
                            for PNG 1.0, and can also be
                            PNG_INTRAPIXEL_DIFFERENCING if
                            the PNG datastream is embedded in
                            a MNG-1.0 datastream)
           compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                            for PNG 1.0)
           interlace_type - (PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7)
           Any or all of interlace_type, compression_type, of
           filter_method can be NULL if you are
           not interested in their values.

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

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


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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

           if (color_type == PNG_COLOR_TYPE_GRAY &&
               bit_depth < 8) png_set_gray_1_2_4_to_8(png_ptr);

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

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

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

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

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

           if (color_type & PNG_COLOR_MASK_ALPHA)
               png_set_strip_alpha(png_ptr);

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

           png_set_invert_alpha(png_ptr);

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

           if (bit_depth < 8)
               png_set_packing(png_ptr);

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

           png_color_8p sig_bit;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Libpng approximates this with

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

       which can be expressed with integers as

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

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

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

           png_color_16 my_background;
           png_color_16p image_background;

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

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

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

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

          double gamma, screen_gamma;

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

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

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

       If  you need to reduce an RGB file to a paletted file, or if a paletted
       file has more entries then will fit on  your  screen,  png_set_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,
                   MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
                   NULL,0);
             }
          }

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

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

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

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

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

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

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

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

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

           png_set_read_user_transform_fn(png_ptr,
              read_transform_fn);

       You must supply the function

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

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

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

           png_set_user_transform_info(png_ptr, user_ptr,
              user_depth, user_channels);

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

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

           voidp read_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

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

           number_of_passes = png_set_interlace_handling(png_ptr);

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

           png_read_update_info(png_ptr, info_ptr);

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


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

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

          png_read_image(png_ptr, row_pointers);

       where row_pointers is:

          png_bytep row_pointers[height];

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

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

           png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

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

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

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

       If  the  file  is  interlaced  (interlace_type != 0 in the IHDR chunk),
       things get somewhat harder.  The only current (PNG  Specification  ver-
       sion  1.2)  interlacing  type  for PNG is (interlace_type == PNG_INTER-
       LACE_ADAM7) is a somewhat complicated 2D  interlace  scheme,  known  as
       Adam7,  that  breaks down an image into seven smaller images of varying
       size, based on an 8x8 grid.

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

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

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

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

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

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

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

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

           png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

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

           png_read_rows(png_ptr, NULL, row_pointers,
              number_of_rows);


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

          png_read_end(png_ptr, end_info);

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

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

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

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

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

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

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

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

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

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

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

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


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

       png_structp png_ptr; png_infop info_ptr;

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

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

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

                 png_get_progressive_ptr(png_ptr);

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

           return 0;
        }

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

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

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

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

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

               png_progressive_combine_row(png_ptr, old_row,
                 new_row);

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

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

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




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


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

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

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

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

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

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

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

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

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

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

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

           png_init_io(png_ptr, fp);


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

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

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

       To inform libpng about your function, use

           png_set_write_status_fn(png_ptr, write_row_callback);

       You  now  have the option of modifying how the compression library will
       run.  The following functions are mainly for testing, but may be useful
       in  some  cases, like if you need to write PNG files extremely fast and
       are willing to give up some compression, or if  you  want  to  get  the
       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 indicates  which  filter  type(s)  are  to  be
       tested for each scanline.  See the PNG specification for details on the
       specific filter types.


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

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

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

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

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

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

       extern PNG_EXPORT(void,png_set_zbuf_size)


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

       Some of the more important parts of the png_info are:

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

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       The keywords that are given in the PNG Specification are:

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

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

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

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


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


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

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

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

           png_write_png(png_ptr, info_ptr, png_transforms, NULL)

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

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


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

           png_write_info(png_ptr, info_ptr);

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

           png_set_invert_alpha(png_ptr);

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

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

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

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

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

           png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

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

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

           png_set_packing(png_ptr);

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

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

           png_set_sBIT(png_ptr, info_ptr, &sig_bit);

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

           png_set_shift(png_ptr, &sig_bit);

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

           if (bit_depth > 8)
              png_set_swap(png_ptr);

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

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

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

           png_set_bgr(png_ptr);

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

           png_set_invert_mono(png_ptr);

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

           png_set_write_user_transform_fn(png_ptr,
              write_transform_fn);

       You must supply the function

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

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

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

           png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

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

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

           voidp write_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

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

           png_write_flush(png_ptr);

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

           png_set_flush(png_ptr, nrows);

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


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

           png_write_image(png_ptr, row_pointers);

       where row_pointers is:

           png_byte *row_pointers[height];

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

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

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

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

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

           png_bytep row_pointer = row;

           png_write_row(png_ptr, row_pointer);

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

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

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

           number_of_passes =
              png_set_interlace_handling(png_ptr);

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

       Then write the complete image number_of_passes times.

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

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


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

           png_write_end(png_ptr, info_ptr);

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

           png_destroy_write_struct(&png_ptr, &info_ptr);

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

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

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

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

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

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

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

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

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

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


V. Modifying/Customizing libpng:
       There  are  three  issues  here.  The first is changing how libpng does
       standard things like memory allocation, input/output,  and  error  han-
       dling.   The  second deals with more complicated things like adding new
       chunks, adding new transformations, and generally changing  how  libpng
       works.  Both of those are compile-time issues; that is, they are gener-
       ally 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, optimized
       assembly-language (and therefore compiler- and platform-dependent) ver-
       sions.

       Memory allocation, input/output, and error handling

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

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

           png_set_mem_fn(png_structp png_ptr, png_voidp mem_ptr,
              png_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_size_t 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 initialized via png_init_io().   If  you  wish  to
       change  the  method of I/O, the library supplies callbacks that you can
       set through the function png_set_read_fn()  and  png_set_write_fn()  at
       run  time,  instead of calling the png_init_io() function.  These func-
       tions also provide a void pointer that can be retrieved via  the  func-
       tion png_get_io_ptr().  For example:

           png_set_read_fn(png_structp read_ptr,
               voidp read_io_ptr, png_rw_ptr read_data_fn)

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

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

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

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

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

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

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

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

           png_voidp error_ptr = png_get_error_ptr(png_ptr);

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

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

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


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

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

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


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


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


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


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


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


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

           png_set_compression_level(png_ptr, level);

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

           png_set_compression_mem_level(png_ptr, level);

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

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


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

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

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

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

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

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

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

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

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

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

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


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

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

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

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


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

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

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

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

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

       is expanded to

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

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

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

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


VI. Runtime optimization
       A new feature in libpng 1.2.0 is  the  ability  to  dynamically  switch
       between  standard  and  optimized versions of some routines.  Currently
       these are limited to three computationally intensive tasks when reading
       PNG  files:  decoding row filters, expanding interlacing, and combining
       interlaced or transparent row data with previous row  data.   Currently
       the  optimized  versions  are available only for x86 (Intel, AMD, etc.)
       platforms with MMX support, though this may change in future  versions.
       (For example, the non-MMX assembler optimizations for zlib might become
       similarly runtime-selectable in future releases, in which  case  libpng
       could  be  extended  to  support them.  Alternatively, the compile-time
       choice of floating-point versus integer routines for  gamma  correction
       might become runtime-selectable.)

       Because  such  optimizations  tend  to  be very platform- and compiler-
       dependent, both in how they are written and in how  they  perform,  the
       new runtime code in libpng has been written to allow programs to query,
       enable, and disable either specific optimizations or all such optimiza-
       tions.   For  example, to enable all possible optimizations (bearing in
       mind that some "optimizations" may actually run  more  slowly  in  rare
       cases):

           #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
              png_uint_32 mask, flags;

              flags = png_get_asm_flags(png_ptr);
              mask = png_get_asm_flagmask(PNG_SELECT_READ | PNG_SELECT_WRITE);
              png_set_asm_flags(png_ptr, flags | mask);
           #endif

       To  enable  only  optimizations   relevant   to   reading   PNGs,   use
       PNG_SELECT_READ  by  itself  when calling png_get_asm_flagmask(); simi-
       larly for optimizing only writing.  To disable all optimizations:

           #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
              flags = png_get_asm_flags(png_ptr);
              mask = png_get_asm_flagmask(PNG_SELECT_READ | PNG_SELECT_WRITE);
              png_set_asm_flags(png_ptr, flags & ~mask);
           #endif

       To  enable  or disable only MMX-related features, use png_get_mmx_flag-
       mask() in place of png_get_asm_flagmask().  The mmx version  takes  one
       additional parameter:

           #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
              int selection = PNG_SELECT_READ | PNG_SELECT_WRITE;
              int compilerID;

              mask = png_get_mmx_flagmask(selection, &compilerID);
           #endif

       On  return, compilerID will indicate which version of the MMX assembler
       optimizations was compiled.  Currently two  flavors  exist:   Microsoft
       Visual  C++  (compilerID == 1) and GNU C (a.k.a. gcc/gas, compilerID ==
       2).  On non-x86 platforms or on systems compiled without MMX  optimiza-
       tions, a value of -1 is used.

       Note that both png_get_asm_flagmask() and png_get_mmx_flagmask() return
       all valid, settable optimization bits for the version  of  the  library
       that's  currently  in  use.  In the case of shared (dynamically linked)
       libraries, this may include optimizations that did  not  exist  at  the
       time  the  code  was  written  and  compiled.   It is also possible, of
       course, to enable only known, specific optimizations; for example:

           #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
              flags   =   PNG_ASM_FLAG_MMX_READ_COMBINE_ROW                  |
       PNG_ASM_FLAG_MMX_READ_INTERLACE                                       |
       PNG_ASM_FLAG_MMX_READ_FILTER_SUB                                      |
       PNG_ASM_FLAG_MMX_READ_FILTER_UP                                       |
       PNG_ASM_FLAG_MMX_READ_FILTER_AVG                                      |
       PNG_ASM_FLAG_MMX_READ_FILTER_PAETH ;
              png_set_asm_flags(png_ptr, flags);
           #endif

       This  method  would enable only the MMX read-optimizations available at
       the time of libpng 1.2.0's release, regardless of whether a later  ver-
       sion  of the DLL were actually being used.  (Also note that these func-
       tions did not exist in versions older than 1.2.0, so any attempt to run
       a dynamically linked app on such an older version would fail.)

       To  determine  whether  the processor supports MMX instructions at all,
       use the png_mmx_support() function:

           #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
              mmxsupport = png_mmx_support();
           #endif

       It returns -1 if MMX support is not compiled into libpng, 0 if MMX code
       is  compiled  but  MMX  is  not supported by the processor, or 1 if MMX
       support   is   fully   available.    Note    that    png_mmx_support(),
       png_get_mmx_flagmask(),  and  png_get_asm_flagmask()  all may be called
       without allocating and ini- tializing any PNG structures (for  example,
       as part of a usage screen or "about" box).

       The following code can be used to prevent an application from using the
       thread_unsafe   features,   even   if    libpng    was    built    with
       PNG_THREAD_UNSAFE_OK defined:

       #if  defined(PNG_USE_PNGGCCRD) && defined(PNG_ASSEMBLER_CODE_SUPPORTED)
       && defined(PNG_THREAD_UNSAFE_OK)
           /* Disable thread-unsafe features of pnggccrd */
           if (png_access_version() >= 10200)
           {
             png_uint_32 mmx_disable_mask = 0;
             png_uint_32 asm_flags;

             mmx_disable_mask    |=    (     PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
       |     PNG_ASM_FLAG_MMX_READ_FILTER_SUB                                |
       PNG_ASM_FLAG_MMX_READ_FILTER_AVG                                      |
       PNG_ASM_FLAG_MMX_READ_FILTER_PAETH );
             asm_flags = png_get_asm_flags(png_ptr);
             png_set_asm_flags(png_ptr, asm_flags & ~mmx_disable_mask);
           } #endif

       For more extensive examples of runtime querying, enabling and disabling
       of optimized features, see contrib/gregbook/readpng2.c  in  the  libpng
       source-code distribution.



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

          feature_set = png_permit_mng_features(png_ptr, mask)
          mask is a png_uint_32 containing the 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  writing  a  stan-
       dalone PNG file with the PNG 8-byte signature.  The PNG datastream must
       be wrapped in a MNG datastream.  As a minimum, it  must  have  the  MNG
       8-byte signature and the MHDR and MEND chunks.  Libpng does not provide
       support for these or any other MNG chunks; your application  must  pro-
       vide  its  own support for them.  You may wish to consider using libmng
       (available at http://www.libmng.com) instead.


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

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

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

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

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

          png_uint_32 libpng_vn = png_access_version_number();

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

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

          png_uint_32 application_vn = PNG_LIBPNG_VER;


IX. Y2K Compliance in libpng
       April 15, 2002

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

       This is your unofficial assurance that libpng  from  version  0.71  and
       upward  through  1.2.2 are Y2K compliant.  It is my belief that earlier
       versions were also Y2K compliant.

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

       The integer is
           "png_uint_16 year" in png_time_struct.

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

       There are seven time-related functions:

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

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

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

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


          Glenn Randers-Pehrson
          libpng maintainer
          PNG Development Group


NOTE
       Note about libpng version numbers:

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

        source             png.h  png.h  shared-lib
        version            string   int  version
        -------            ------  ----- ----------
        0.89c ("beta 3")  0.89       89  1.0.89
        0.90  ("beta 4")  0.90       90  0.90
        0.95  ("beta 5")  0.95       95  0.95
        0.96  ("beta 6")  0.96       96  0.96
        0.97b ("beta 7")  1.00.97    97  1.0.1
        0.97c             0.97       97  2.0.97
        0.98              0.98       98  2.0.98
        0.99              0.99       98  2.0.99
        0.99a-m           0.99       99  2.0.99
        1.00              1.00      100  2.1.0
        1.0.0             1.0.0     100  2.1.0
        1.0.0   (from here on, the  100  2.1.0
        1.0.1    png.h string is  10001  2.1.0
        1.0.1a-e identical to the 10002  from here on, the
        1.0.2    source version)  10002  shared library is 2.V
        1.0.2a-b                  10003  where V is the source
        1.0.1                     10001  code version except as
        1.0.1a-e                  10002  2.1.0.1a-e   noted.
        1.0.2                     10002  2.1.0.2
        1.0.2a-b                  10003  2.1.0.2a-b
        1.0.3                     10003  2.1.0.3
        1.0.3a-d                  10004  2.1.0.3a-d
        1.0.4                     10004  2.1.0.4
        1.0.4a-f                  10005  2.1.0.4a-f
        1.0.5 (+ 2 patches)       10005  2.1.0.5
        1.0.5a-d                  10006  2.1.0.5a-d
        1.0.5e-r                  10100  2.1.0.5e-r
        1.0.5s-v                  10006  2.1.0.5s-v
        1.0.6 (+ 3 patches)       10006  2.1.0.6
        1.0.6d-g                  10007  2.1.0.6d-g
        1.0.6h                    10007  10.6h
        1.0.6i                    10007  10.6i
        1.0.6j                    10007  2.1.0.6j
        1.0.7beta11-14    DLLNUM  10007  2.1.0.7beta11-14
        1.0.7beta15-18       1    10007  2.1.0.7beta15-18
        1.0.7rc1-2           1    10007  2.1.0.7rc1-2
        1.0.7                1    10007  2.1.0.7
        1.0.8beta1-4         1    10008  2.1.0.8beta1-4
        1.0.8rc1             1    10008  2.1.0.8rc1
        1.0.8                1    10008  2.1.0.8
        1.0.9beta1-6         1    10009  2.1.0.9beta1-6
        1.0.9rc1             1    10009  2.1.0.9rc1
        1.0.9beta7-10        1    10009  2.1.0.9beta7-10
        1.0.9rc2             1    10009  2.1.0.9rc2
        1.0.9                1    10009  2.1.0.9
        1.0.10beta1          1    10010  2.1.0.10beta1
        1.0.10rc1            1    10010  2.1.0.10rc1
        1.0.10               1    10010  2.1.0.10
        1.0.11beta1-3        1    10011  2.1.0.11beta1-3
        1.0.11rc1            1    10011  2.1.0.11rc1
        1.0.11               1    10011  2.1.0.11
        1.0.12beta1-2        2    10012  2.1.0.12beta1-2
        1.0.12rc1            2    10012  2.1.0.12rc1
        1.0.12               2    10012  2.1.0.12
        1.1.0a-f             -    10100  2.1.1.0a-f abandoned
        1.2.0beta1-2         2    10200  2.1.2.0beta1-2
        1.2.0beta3-5         3    10200  3.1.2.0beta3-5
        1.2.0rc1             3    10200  3.1.2.0rc1
        1.2.0                3    10200  3.1.2.0
        1.2.1beta-4          3    10201  3.1.2.1beta1-4
        1.2.1rc1-2           3    10201  3.1.2.1rc1-2
        1.2.1                3    10201  3.1.2.1
        1.2.2beta1-6        12    10202  12.so.0.1.2.2beta1-6
        1.0.13beta1         10    10013  10.so.0.1.0.13beta1
        1.0.13rc1           10    10013  10.so.0.1.0.13rc1
        1.2.2rc1            12    10202  12.so.0.1.2.2rc1
        1.0.13              10    10013  10.so.0.1.0.13
        1.2.2               12    10202  12.so.0.1.2.2

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


SEE ALSO
       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 specification and this
       library, the specification takes precedence.


AUTHORS
       This man page: Glenn Randers-Pehrson <randeg@alum.rpi.edu>

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

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

       Libpng version 1.2.2 - April 15, 2002: Initially created in 1995 by Guy
       Eric  Schalnat,  then  of Group 42, Inc.  Currently maintained by Glenn
       Randers-Pehrson (randeg@alum.rpi.edu).

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


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

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

       libpng versions 1.0.7, July 1, 2000, through 1.2.2, April 15, 2002, are
       Copyright  (c)  2000-2002  Glenn  Randers-Pehrson,  and are distributed
       according to the same disclaimer and license as libpng-1.0.6  with  the
       following individuals added to the list of Contributing Authors

          Simon-Pierre Cadieux
          Eric S. Raymond
          Gilles Vollant

       and with the following additions to the disclaimer:

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

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

          Tom Lane
          Glenn Randers-Pehrson
          Willem van Schaik

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

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

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

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

          Andreas Dilger
          Dave Martindale
          Guy Eric Schalnat
          Paul Schmidt
          Tim Wegner

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

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

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

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

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

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


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

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

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

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

       Glenn Randers-Pehrson randeg@alum.rpi.edu April 15, 2002





                                April 15, 2002                       LIBPNG(3)