Provided by: libpng12-dev_1.2.27-2ubuntu2_i386 bug

NAME

       libpng - Portable Network Graphics (PNG) Reference Library 1.2.27

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_byte png_get_bit_depth (png_structp png_ptr, png_infop info_ptr);

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

       png_byte png_get_channels (png_structp png_ptr, png_infop info_ptr);

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

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

       png_byte png_get_color_type (png_structp png_ptr, png_infop info_ptr);

       png_byte  png_get_compression_type  (png_structp   png_ptr,   png_infop
       info_ptr);

       png_byte png_get_copyright (png_structp png_ptr);

       png_voidp png_get_error_ptr (png_structp png_ptr);

       png_byte png_get_filter_type (png_structp png_ptr, png_infop info_ptr);

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

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

       png_byte png_get_header_ver (png_structp png_ptr);

       png_byte png_get_header_version (png_structp png_ptr);

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

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

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

       png_uint_32  png_get_image_height   (png_structp   png_ptr,   png_infop
       info_ptr);

       png_uint_32   png_get_image_width   (png_structp   png_ptr,   png_infop
       info_ptr);

       #if  !defined(PNG_1_0_X)  png_int_32  png_get_int_32  (png_bytep  buf);
       #endif

       png_byte   png_get_interlace_type   (png_structp   png_ptr,   png_infop
       info_ptr);

       png_voidp png_get_io_ptr (png_structp png_ptr);

       png_byte png_get_libpng_ver (png_structp png_ptr);

       png_voidp png_get_mem_ptr(png_structp png_ptr);

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

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

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

       float  png_get_pixel_aspect_ratio   (png_structp   png_ptr,   png_infop
       info_ptr);

       png_uint_32  png_get_pixels_per_meter  (png_structp  png_ptr, png_infop
       info_ptr);

       png_voidp png_get_progressive_ptr (png_structp png_ptr);

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

       png_byte  png_get_rgb_to_gray_status  (png_structp png_ptr) png_uint_32
       png_get_rowbytes (png_structp png_ptr, png_infop info_ptr);

       png_bytepp png_get_rows (png_structp png_ptr, png_infop info_ptr);

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

       png_bytep png_get_signature (png_structp png_ptr, png_infop info_ptr);

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

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

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

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

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

       #if !defined(PNG_1_0_X) png_uint_16 png_get_uint_16 (png_bytep buf);

       png_uint_32 png_get_uint_31 (png_bytep buf);

       png_uint_32 png_get_uint_32 (png_bytep buf); #endif

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

       png_voidp png_get_user_chunk_ptr (png_structp png_ptr);

       png_uint_32 png_get_user_height_max( png_structp png_ptr);

       png_voidp png_get_user_transform_ptr (png_structp png_ptr);

       png_uint_32 png_get_user_width_max (png_structp png_ptr);

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

       png_int_32  png_get_x_offset_microns  (png_structp  png_ptr,  png_infop
       info_ptr);

       png_int_32   png_get_x_offset_pixels  (png_structp  png_ptr,  png_infop
       info_ptr);

       png_uint_32 png_get_x_pixels_per_meter (png_structp png_ptr,  png_infop
       info_ptr);

       png_int_32  png_get_y_offset_microns  (png_structp  png_ptr,  png_infop
       info_ptr);

       png_int_32  png_get_y_offset_pixels  (png_structp  png_ptr,   png_infop
       info_ptr);

       png_uint_32  png_get_y_pixels_per_meter (png_structp png_ptr, png_infop
       info_ptr);

       png_uint_32 png_get_compression_buffer_size (png_structp png_ptr);

       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

       void png_init_io (png_structp png_ptr, FILE *fp);

       DEPRECATED: void png_info_init (png_infop info_ptr);

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

       png_voidp png_malloc (png_structp png_ptr, png_uint_32 size);

       png_voidp png_malloc_default(png_structp png_ptr, png_uint_32 size);

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

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

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

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

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

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

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

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

       void png_read_end (png_structp png_ptr, png_infop info_ptr);

       void png_read_image (png_structp png_ptr, png_bytepp image);

       DEPRECATED: void png_read_init (png_structp png_ptr);

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

       void png_read_info (png_structp png_ptr, png_infop info_ptr);

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

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

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

       void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

       #if !defined(PNG_1_0_X) png_save_int_32 (png_bytep buf, png_int_32 i);

       void png_save_uint_16 (png_bytep buf, unsigned int i);

       void png_save_uint_32 (png_bytep buf, png_uint_32 i);

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

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

       void png_set_bgr (png_structp png_ptr);

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

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

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

       void png_set_compression_level (png_structp png_ptr, int level);

       void   png_set_compression_mem_level    (png_structp    png_ptr,    int
       mem_level);

       void png_set_compression_method (png_structp png_ptr, int method);

       void png_set_compression_strategy (png_structp png_ptr, int strategy);

       void    png_set_compression_window_bits   (png_structp   png_ptr,   int
       window_bits);

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

       void  png_set_dither  (png_structp  png_ptr,  png_colorp  palette,  int
       num_palette,   int   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_expand_gray_1_2_4_to_8(png_structp png_ptr);

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

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

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

       void png_set_flush (png_structp png_ptr, int nrows);

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

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

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

       void png_set_gray_1_2_4_to_8(png_structp png_ptr);

       void png_set_gray_to_rgb (png_structp png_ptr);

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

       void  png_set_iCCP  (png_structp png_ptr, png_infop info_ptr, png_charp
       name, int 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
       interlace_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_malloc_ptr malloc_fn, png_free_ptr free_fn);

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

       void png_set_packing (png_structp png_ptr);

       void png_set_packswap (png_structp png_ptr);

       void png_set_palette_to_rgb(png_structp png_ptr);

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

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

       void   png_set_progressive_read_fn   (png_structp   png_ptr,  png_voidp
       progressive_ptr,           png_progressive_info_ptr            info_fn,
       png_progressive_row_ptr row_fn, png_progressive_end_ptr end_fn);

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

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

       void  png_set_read_status_fn  (png_structp png_ptr, png_read_status_ptr
       read_row_fn);

       void     png_set_read_user_transform_fn      (png_structp      png_ptr,
       png_user_transform_ptr read_user_transform_fn);

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

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

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

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

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

       void png_set_shift (png_structp png_ptr, png_color_8p true_bits);

       void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

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

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

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

       void png_set_strip_16 (png_structp png_ptr);

       void png_set_strip_alpha (png_structp png_ptr);

       void png_set_swap (png_structp png_ptr);

       void png_set_swap_alpha (png_structp png_ptr);

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

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

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

       void png_set_tRNS_to_alpha(png_structp png_ptr);

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

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

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

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

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

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

       void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
       write_row_fn);

       void     png_set_write_user_transform_fn     (png_structp      png_ptr,
       png_user_transform_ptr write_user_transform_fn);

       void  png_set_compression_buffer_size(png_structp  png_ptr, png_uint_32
       size);

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

       void png_start_read_image (png_structp png_ptr);

       void png_warning (png_structp png_ptr, png_const_charp message);

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

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

       void png_write_chunk_end (png_structp png_ptr);

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

       void png_write_destroy (png_structp png_ptr);

       void png_write_end (png_structp png_ptr, png_infop info_ptr);

       void png_write_flush (png_structp png_ptr);

       void png_write_image (png_structp png_ptr, png_bytepp image);

       DEPRECATED: void png_write_init (png_structp png_ptr);

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

       void png_write_info (png_structp png_ptr, png_infop info_ptr);

       void   png_write_info_before_PLTE   (png_structp   png_ptr,   png_infop
       info_ptr);

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

       void png_write_row (png_structp png_ptr, png_bytep row);

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

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

       void png_zfree (voidpf png_ptr, voidpf ptr);

DESCRIPTION

       The   libpng   library   supports   encoding,   decoding,  and  various
       manipulations of the  Portable  Network  Graphics  (PNG)  format  image
       files.   It  uses the zlib(3) compression library.  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 1.2.27 - April 29, 2008
        Updated and distributed by Glenn Randers-Pehrson
        <glennrp at users.sourceforge.net>
        Copyright (c) 1998-2008 Glenn Randers-Pehrson
        For conditions of distribution and use, see copyright
        notice in png.h.

        Based on:

        libpng versions 0.97, January 1998, through 1.2.27 - April 29, 2008
        Updated and distributed by Glenn Randers-Pehrson
        Copyright (c) 1998-2008 Glenn Randers-Pehrson

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

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

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

I. Introduction

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

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

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

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

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

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

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

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

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

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

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

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

II. Structures

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

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

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

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

       #include <png.h>

III. Reading

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

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

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

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

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

       Next, png_struct and png_info need to be allocated and initialized.  In
       order to ensure that the size of these structures is correct even  with
       a  dynamically  linked  libpng,  there  are functions to initialize and
       allocate the structures.  We also pass the  library  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
       handling in the Customizing Libpng section below for  more  information
       on the libpng error handling.  If an error occurs, and libpng longjmp’s
       back to your setjmp, you will want to call png_destroy_read_struct() to
       free any memory.

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

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

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

           png_init_io(png_ptr, fp);

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

           png_set_sig_bytes(png_ptr, number);

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

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

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

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

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

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

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

       To inform libpng about your function, use

           png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
               read_chunk_callback);

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

           png_get_user_chunk_ptr(png_ptr);

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

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

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

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

       To inform libpng about your function, use

           png_set_read_status_fn(png_ptr, read_row_callback);

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

          png_set_user_limits(png_ptr, width_max, height_max);

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

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

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

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

           png_set_keep_unknown_chunks(png_ptr, keep,
               chunk_list, num_chunks);
           keep       - 0: default unknown chunk handling
                        1: ignore; do not keep
                        2: keep only if safe-to-copy
                        3: keep even if unsafe-to-copy
                      You can use these definitions:
                        PNG_HANDLE_CHUNK_AS_DEFAULT   0
                        PNG_HANDLE_CHUNK_NEVER        1
                        PNG_HANDLE_CHUNK_IF_SAFE      2
                        PNG_HANDLE_CHUNK_ALWAYS       3
           chunk_list - list of chunks affected (a byte string,
                        five bytes per chunk, NULL or ’ ’ if
                        num_chunks is 0)
           num_chunks - number of chunks affected; if 0, all
                        unknown chunks are affected.  If nonzero,
                        only the chunks in the list are affected

       Unknown  chunks  declared  in this way will be saved as raw data onto a
       list of png_unknown_chunk structures.  If  a  chunk  that  is  normally
       known  to  libpng  is named in the list, it will be handled as unknown,
       according to the "keep" directive.  If a chunk is named  in  successive
       instances  of  png_set_keep_unknown_chunks(),  the  final instance will
       take precedence.  The IHDR and IEND  chunks  should  not  be  named  in
       chunk_list; if they are, libpng will process them normally anyway.

       Here is an example of the usage of png_set_keep_unknown_chunks(), where
       the private "vpAg" chunk will  later  be  processed  by  a  user  chunk
       callback function:

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

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

           ...

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

   The high-level read interface
       At  this  point  there  are two ways to proceed; through the high-level
       read interface, or through a sequence  of  low-level  read  operations.
       You can use the high-level interface if (a) you are willing to read the
       entire image into memory, and (b) the input 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 bitwise  OR  of  some
       set   of   transformation   flags.    This   call   is   equivalent  to
       png_read_info(), followed the set of transformations indicated  by  the
       transform mask, then png_read_image(), and finally png_read_end().

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

       You  must  use  png_transforms  and  not  call  any png_set_transform()
       functions when you use png_read_png().

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

          row_pointers = png_get_rows(png_ptr, info_ptr);

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

          png_bytep row_pointers[height];

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

          if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
             png_error (png_ptr,
                "Image is too tall to process in memory");
          if (width > PNG_UINT_32_MAX/pixel_size)
             png_error (png_ptr,
                "Image is too wide to process in memory");
          row_pointers = png_malloc(png_ptr,
             height*png_sizeof(png_bytep));
          for (int i=0; i<height, i++)
             row_pointers[i]=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
       simple data types, or a pointer into the info_ptr is returned  for  any
       complex types.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was added.
       It  expands  the  sample  depth without changing tRNS to alpha.  At the
       same time, png_set_gray_1_2_4_to_8() was deprecated,  and  it  will  be
       removed from a future version.

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

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

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

           if (color_type & PNG_COLOR_MASK_ALPHA)
               png_set_strip_alpha(png_ptr);

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

           png_set_invert_alpha(png_ptr);

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

           if (bit_depth < 8)
               png_set_packing(png_ptr);

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

           png_color_8p sig_bit;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Libpng approximates this with

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

       which can be expressed with integers as

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

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

       If you have a  grayscale  and  you  are  using  png_set_expand_depth(),
       png_set_expand(), or png_set_gray_to_rgb to change to truecolor or to a
       higher bit-depth, you must either 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
       display (e.g., the background color from a web page).  You need to tell
       libpng  whether  the  color  is  in  the  gamma  space  of  the display
       (PNG_BACKGROUND_GAMMA_SCREEN  for  colors   you   supply),   the   file
       (PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one that
       is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don’t  know
       why anyone would use this, but it’s here).

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

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

          double gamma, screen_gamma;

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

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

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

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

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

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

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

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

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

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

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

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

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

           png_set_read_user_transform_fn(png_ptr,
              read_transform_fn);

       You must supply the function

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

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

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

           png_set_user_transform_info(png_ptr, user_ptr,
              user_depth, user_channels);

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

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

           voidp read_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

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

           number_of_passes = png_set_interlace_handling(png_ptr);

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

           png_read_update_info(png_ptr, info_ptr);

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

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

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

          png_read_image(png_ptr, row_pointers);

       where row_pointers is:

          png_bytep row_pointers[height];

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

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

           png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

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

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

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

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

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

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

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

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

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

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

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

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

           png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

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

           png_read_rows(png_ptr, NULL, row_pointers,
              number_of_rows);

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

          png_read_end(png_ptr, end_info);

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       png_structp png_ptr; png_infop info_ptr;

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

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

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

                 png_get_progressive_ptr(png_ptr);

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

           return 0;
        }

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

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

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

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

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

               png_progressive_combine_row(png_ptr, old_row,
                 new_row);

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

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

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

IV. Writing

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

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

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

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

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

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

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

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

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

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

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

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

           png_init_io(png_ptr, fp);

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

           png_set_sig_bytes(png_ptr, 8);

       to inform libpng that it should not write a signature.

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

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

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

       To inform libpng about your function, use

           png_set_write_status_fn(png_ptr, write_row_callback);

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

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

       If an application wants to start  and  stop  using  particular  filters
       during  compression,  it  should  start out with all of the filters (to
       ensure that the previous row of pixels will  be  stored  in  case  it’s
       needed  later),  and  then  add  and  remove  them  after  the start of
       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_compression_level() which changes how much time zlib spends  on
       trying to compress the image data.  See the Compression Library (zlib.h
       and  algorithm.txt,  distributed  with  zlib)  for   details   on   the
       compression levels.

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

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

       extern PNG_EXPORT(void,png_set_zbuf_size)

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

       Some of the more important parts of the png_info are:

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

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       The compression types have the same valid numbers  as  the  compression
       types  of  the  image  data.  Currently, the only valid number is zero.
       However, you can store text either 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
       specify  PNG_TEXT_COMPRESSION_NONE  or  PNG_TEXT_COMPRESSION_zTXt   any
       language code or translated keyword will not be written out.

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

       The keywords that are given in the PNG Specification are:

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

       The keyword-text pairs work like this.  Keywords should be short simple
       descriptions of what the comment is about.  Some typical  keywords  are
       found in the PNG specification, as is some recommendations on keywords.
       You can repeat keywords in a file.  You can even write some text before
       the  image  and  some  after.   For  example,  you  may  want  to put a
       description of the image before the image,  but  leave  the  disclaimer
       until  after,  so  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 contain control or other unprintable  characters.   To  make
       the comments widely readable, stick with basic ASCII, and avoid machine
       specific character set extensions like the IBM-PC character  set.   The
       keyword  must be present, but you can leave off the text string on non-
       compressed pairs.  Compressed pairs must have a text  string,  as  only
       the  text  string  is  compressed  anyway,  so the compression would be
       meaningless.

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

       If  you  want  to  store  the  time of the original image creation, you
       should use a plain tEXt chunk with the "Creation Time"  keyword.   This
       is  necessary  because  the  "creation time" of a PNG image is somewhat
       vague, depending on whether you mean the PNG file, the time  the  image
       was created in a non-PNG format, a still photo from which the image was
       scanned, or possibly the subject matter itself.  In order to facilitate
       machine-readable dates, it is recommended that the "Creation Time" tEXt
       chunk use RFC 1123 format dates (e.g.  "22  May  1997  18:07:10  GMT"),
       although  this  isn’t  a  requirement.   Unlike  the  tIME  chunk,  the
       "Creation 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
       specification’s ordering rules.

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

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

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

           png_write_png(png_ptr, info_ptr, png_transforms, NULL)

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

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

       You must  use  png_transforms  and  not  call  any  png_set_transform()
       functions when you use png_write_png().

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

           png_write_info(png_ptr, info_ptr);

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

           png_set_invert_alpha(png_ptr);

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

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

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

       After you’ve written the file information, you can set up  the  library
       to  handle  any 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
       supplied  the  other  way  (little-endian,  i.e. least significant bits
       first, the way PCs store them):

           if (bit_depth > 8)
              png_set_swap(png_ptr);

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

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

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

           png_set_bgr(png_ptr);

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

           png_set_invert_mono(png_ptr);

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

           png_set_write_user_transform_fn(png_ptr,
              write_transform_fn);

       You must supply the function

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

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

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

           png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

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

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

           voidp write_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

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

           png_write_flush(png_ptr);

       and to have libpng flush the output stream periodically after a certain
       number of 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
       noticeably   (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
       simple:

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

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

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

           png_bytep row_pointer = row;

           png_write_row(png_ptr, row_pointer);

       When  the  file  is  interlaced,  things  can  get  a  good  deal  more
       complicated.  The only currently (as of the PNG  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
       number of times to write all seven sub-images.

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

           number_of_passes =
              png_set_interlace_handling(png_ptr);

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

       Then write the complete image number_of_passes times.

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

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

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

           png_write_end(png_ptr, info_ptr);

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

           png_destroy_write_struct(&png_ptr, &info_ptr);

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

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

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

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

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

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

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

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

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

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

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

V. Modifying/Customizing libpng:

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

       Memory allocation, input/output, and error handling

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

       Memory  allocation  is  done  through  the  functions  png_malloc() and
       png_free().  These currently just call the standard  C  functions.   If
       your  pointers  can’t  access more then 64K at a time, you will want to
       set MAXSEG_64K in zlib.h.  Since it is  unlikely  that  the  method  of
       handling   memory   allocation   on  a  platform  will  change  between
       applications, these functions  must  be  modified  in  the  library  at
       compile  time.   If  you prefer to use a different method of allocating
       and  freeing  data,   you   can   use   png_create_read_struct_2()   or
       png_create_write_struct_2() to register your own functions as described
       above.  These functions  also  provide  a  void  pointer  that  can  be
       retrieved via

           mem_ptr=png_get_mem_ptr(png_ptr);

       Your replacement memory functions must have prototypes as follows:

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

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

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

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

           png_set_read_fn(png_structp read_ptr,
               voidp read_io_ptr, png_rw_ptr read_data_fn)

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

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

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

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

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

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

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

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

           png_voidp error_ptr = png_get_error_ptr(png_ptr);

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

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

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

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

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

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

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

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

   Configuring for Medium Model
       Libpng’s support for medium model  has  been  tested  on  most  of  the
       popular  compilers.  Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD
       gets defined, and FAR gets defined to far in pngconf.h, and you  should
       be all set.  Everything in the library (except for zlib’s 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
       create files larger than just storing the raw bitmap.  You can  specify
       the compression level by calling:

           png_set_compression_level(png_ptr, level);

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

           png_set_compression_mem_level(png_ptr, level);

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

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

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

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

       Individual   filter   types   are   PNG_FILTER_NONE,    PNG_FILTER_SUB,
       PNG_FILTER_UP,  PNG_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  allocate  the filter buffers until png_write_row() is called
       for the first time.)

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

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

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

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

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

       The weights are multiplying factors that indicate to  libpng  that  the
       row  filter  should  be the same for successive rows unless another row
       filter is that many times better than  the  previous  filter.   In  the
       above  example,  if the previous 3 filters were SUB, SUB, NONE, the SUB
       filter could have a "sum of  absolute  differences"  1.5  x  1.3  times
       higher  than  other  filters and still be chosen, while the NONE filter
       could have a sum 1.1 times higher  than  other  filters  and  still  be
       chosen.   Unspecified  weights  are  taken to be 1.0, and the 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
       capabilities   off  en  masse  with  compiler  directives  that  define
       PNG_NO_READ[or       WRITE]_TRANSFORMS,        or        PNG_NO_READ[or
       WRITE]_ANCILLARY_CHUNKS,  or all four, along with directives to turn on
       any  of  the  capabilities  that  you  do  want.   The   PNG_NO_READ[or
       WRITE]_TRANSFORMS  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
       reading  files  start  with  pngr  and all the writing files start with
       pngw.  The files that don’t match either (like png.c, pngtrans.c, etc.)
       are  used for both reading and writing, and always need to be included.
       The progressive reader is in pngpread.c

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

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

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

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

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

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

       is expanded to

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

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

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

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

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

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

VIII. Changes to Libpng from version 0.88

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

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

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

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

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

          png_uint_32 libpng_vn = png_access_version_number();

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

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

          png_uint_32 application_vn = PNG_LIBPNG_VER;

IX. Y2K Compliance in libpng

       April 29, 2008

       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 1.2.27 are Y2K compliant.  It is my belief that earlier
       versions were also Y2K compliant.

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

       The integer is
           "png_uint_16 year" in png_time_struct.

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

       There are seven time-related functions:

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

       All appear  to  handle  dates  properly  in  a  Y2K  environment.   The
       png_convert_from_time_t()  function  calls  gmtime()  to  convert  from
       system clock time, which returns  (year  -  1900),  which  we  properly
       convert  to  the  full  4-digit  year.   There  is  a  possibility that
       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()  function,  but  this  is  not  under  our
       control.  The libpng documentation has always stated that it works with
       4-digit years, and the APIs have been documented as such.

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

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

          Glenn Randers-Pehrson
          libpng maintainer
          PNG Development Group

NOTE

       Note about libpng version numbers:

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

        source             png.h  png.h  shared-lib
        version            string   int  version
        -------            ------  ----- ----------
        0.89c ("beta 3")  0.89       89  1.0.89
        0.90  ("beta 4")  0.90       90  0.90
        0.95  ("beta 5")  0.95       95  0.95
        0.96  ("beta 6")  0.96       96  0.96
        0.97b ("beta 7")  1.00.97    97  1.0.1
        0.97c             0.97       97  2.0.97
        0.98              0.98       98  2.0.98
        0.99              0.99       98  2.0.99
        0.99a-m           0.99       99  2.0.99
        1.00              1.00      100  2.1.0
        1.0.0             1.0.0     100  2.1.0
        1.0.0   (from here on, the  100  2.1.0
        1.0.1    png.h string is  10001  2.1.0
        1.0.1a-e identical to the 10002  from here on, the
        1.0.2    source version)  10002  shared library is 2.V
        1.0.2a-b                  10003  where V is the source
        1.0.1                     10001  code version except as
        1.0.1a-e                  10002  2.1.0.1a-e   noted.
        1.0.2                     10002  2.1.0.2
        1.0.2a-b                  10003  2.1.0.2a-b
        1.0.3                     10003  2.1.0.3
        1.0.3a-d                  10004  2.1.0.3a-d
        1.0.4                     10004  2.1.0.4
        1.0.4a-f                  10005  2.1.0.4a-f
        1.0.5 (+ 2 patches)       10005  2.1.0.5
        1.0.5a-d                  10006  2.1.0.5a-d
        1.0.5e-r                  10100  2.1.0.5e-r
        1.0.5s-v                  10006  2.1.0.5s-v
        1.0.6 (+ 3 patches)       10006  2.1.0.6
        1.0.6d-g                  10007  2.1.0.6d-g
        1.0.6h                    10007  10.6h
        1.0.6i                    10007  10.6i
        1.0.6j                    10007  2.1.0.6j
        1.0.7beta11-14    DLLNUM  10007  2.1.0.7beta11-14
        1.0.7beta15-18       1    10007  2.1.0.7beta15-18
        1.0.7rc1-2           1    10007  2.1.0.7rc1-2
        1.0.7                1    10007  2.1.0.7
        1.0.8beta1-4         1    10008  2.1.0.8beta1-4
        1.0.8rc1             1    10008  2.1.0.8rc1
        1.0.8                1    10008  2.1.0.8
        1.0.9beta1-6         1    10009  2.1.0.9beta1-6
        1.0.9rc1             1    10009  2.1.0.9rc1
        1.0.9beta7-10        1    10009  2.1.0.9beta7-10
        1.0.9rc2             1    10009  2.1.0.9rc2
        1.0.9                1    10009  2.1.0.9
        1.0.10beta1          1    10010  2.1.0.10beta1
        1.0.10rc1            1    10010  2.1.0.10rc1
        1.0.10               1    10010  2.1.0.10
        1.0.11beta1-3        1    10011  2.1.0.11beta1-3
        1.0.11rc1            1    10011  2.1.0.11rc1
        1.0.11               1    10011  2.1.0.11
        1.0.12beta1-2        2    10012  2.1.0.12beta1-2
        1.0.12rc1            2    10012  2.1.0.12rc1
        1.0.12               2    10012  2.1.0.12
        1.1.0a-f             -    10100  2.1.1.0a-f abandoned
        1.2.0beta1-2         2    10200  2.1.2.0beta1-2
        1.2.0beta3-5         3    10200  3.1.2.0beta3-5
        1.2.0rc1             3    10200  3.1.2.0rc1
        1.2.0                3    10200  3.1.2.0
        1.2.1beta-4          3    10201  3.1.2.1beta1-4
        1.2.1rc1-2           3    10201  3.1.2.1rc1-2
        1.2.1                3    10201  3.1.2.1
        1.2.2beta1-6        12    10202  12.so.0.1.2.2beta1-6
        1.0.13beta1         10    10013  10.so.0.1.0.13beta1
        1.0.13rc1           10    10013  10.so.0.1.0.13rc1
        1.2.2rc1            12    10202  12.so.0.1.2.2rc1
        1.0.13              10    10013  10.so.0.1.0.13
        1.2.2               12    10202  12.so.0.1.2.2
        1.2.3rc1-6          12    10203  12.so.0.1.2.3rc1-6
        1.2.3               12    10203  12.so.0.1.2.3
        1.2.4beta1-3        13    10204  12.so.0.1.2.4beta1-3
        1.2.4rc1            13    10204  12.so.0.1.2.4rc1
        1.0.14              10    10014  10.so.0.1.0.14
        1.2.4               13    10204  12.so.0.1.2.4
        1.2.5beta1-2        13    10205  12.so.0.1.2.5beta1-2
        1.0.15rc1           10    10015  10.so.0.1.0.15rc1
        1.0.15              10    10015  10.so.0.1.0.15
        1.2.5               13    10205  12.so.0.1.2.5
        1.2.6beta1-4        13    10206  12.so.0.1.2.6beta1-4
        1.2.6rc1-5          13    10206  12.so.0.1.2.6rc1-5
        1.0.16              10    10016  10.so.0.1.0.16
        1.2.6               13    10206  12.so.0.1.2.6
        1.2.7beta1-2        13    10207  12.so.0.1.2.7beta1-2
        1.0.17rc1           10    10017  10.so.0.1.0.17rc1
        1.2.7rc1            13    10207  12.so.0.1.2.7rc1
        1.0.17              10    10017  10.so.0.1.0.17
        1.2.7               13    10207  12.so.0.1.2.7
        1.2.8beta1-5        13    10208  12.so.0.1.2.8beta1-5
        1.0.18rc1-5         10    10018  10.so.0.1.0.18rc1-5
        1.2.8rc1-5          13    10208  12.so.0.1.2.8rc1-5
        1.0.18              10    10018  10.so.0.1.0.18
        1.2.8               13    10208  12.so.0.1.2.8
        1.2.9beta1-3        13    10209  12.so.0.1.2.9beta1-3
        1.2.9beta4-11       13    10209  12.so.0.9[.0]
        1.2.9rc1            13    10209  12.so.0.9[.0]
        1.2.9               13    10209  12.so.0.9[.0]
        1.2.10beta1-8       13    10210  12.so.0.10[.0]
        1.2.10rc1-3         13    10210  12.so.0.10[.0]
        1.2.10              13    10210  12.so.0.10[.0]
        1.2.11beta1-4       13    10211  12.so.0.11[.0]
        1.0.19rc1-5         10    10019  10.so.0.19[.0]
        1.2.11rc1-5         13    10211  12.so.0.11[.0]
        1.0.19              10    10019  10.so.0.19[.0]
        1.2.11              13    10211  12.so.0.11[.0]
        1.0.20              10    10020  10.so.0.20[.0]
        1.2.12              13    10212  12.so.0.12[.0]
        1.2.13beta1         13    10213  12.so.0.13[.0]
        1.0.21              10    10021  10.so.0.21[.0]
        1.2.13              13    10213  12.so.0.13[.0]
        1.2.14beta1-2       13    10214  12.so.0.14[.0]
        1.0.22rc1           10    10022  10.so.0.22[.0]
        1.2.14rc1           13    10214  12.so.0.14[.0]
        1.2.15beta1-6       13    10215  12.so.0.15[.0]
        1.0.23rc1-5         10    10023  10.so.0.23[.0]
        1.2.15rc1-5         13    10215  12.so.0.15[.0]
        1.0.23              10    10023  10.so.0.23[.0]
        1.2.15              13    10215  12.so.0.15[.0]
        1.2.16beta1-2       13    10216  12.so.0.16[.0]
        1.2.16rc1           13    10216  12.so.0.16[.0]
        1.0.24              10    10024  10.so.0.24[.0]
        1.2.16              13    10216  12.so.0.16[.0]
        1.2.17beta1-2       13    10217  12.so.0.17[.0]
        1.0.25rc1           10    10025  10.so.0.25[.0]
        1.2.17rc1-3         13    10217  12.so.0.17[.0]
        1.0.25              10    10025  10.so.0.25[.0]
        1.2.17              13    10217  12.so.0.17[.0]
        1.0.26              10    10026  10.so.0.26[.0]
        1.2.18              13    10218  12.so.0.18[.0]
        1.2.19beta1-31      13    10219  12.so.0.19[.0]
        1.0.27rc1-6         10    10027  10.so.0.27[.0]
        1.2.19rc1-6         13    10219  12.so.0.19[.0]
        1.0.27              10    10027  10.so.0.27[.0]
        1.2.19              13    10219  12.so.0.19[.0]
        1.2.20beta01-04     13    10220  12.so.0.20[.0]
        1.0.28rc1-6         10    10028  10.so.0.28[.0]
        1.2.20rc1-6         13    10220  12.so.0.20[.0]
        1.0.28              10    10028  10.so.0.28[.0]
        1.2.20              13    10220  12.so.0.20[.0]
        1.2.21beta1-2       13    10221  12.so.0.21[.0]
        1.2.21rc1-3         13    10221  12.so.0.21[.0]
        1.0.29              10    10029  10.so.0.29[.0]
        1.2.21              13    10221  12.so.0.21[.0]
        1.2.22beta1-4       13    10222  12.so.0.22[.0]
        1.0.30rc1           13    10030  10.so.0.30[.0]
        1.2.22rc1           13    10222  12.so.0.22[.0]
        1.0.30              10    10030  10.so.0.30[.0]
        1.2.22              13    10222  12.so.0.22[.0]
        1.2.23beta01-05     13    10223  12.so.0.23[.0]
        1.2.23rc01          13    10223  12.so.0.23[.0]
        1.2.23              13    10223  12.so.0.23[.0]
        1.2.24beta01-02     13    10224  12.so.0.24[.0]
        1.2.24rc01          13    10224  12.so.0.24[.0]
        1.2.24              13    10224  12.so.0.24[.0]
        1.2.25beta01-06     13    10225  12.so.0.25[.0]
        1.2.25rc01-02       13    10225  12.so.0.25[.0]
        1.0.31              10    10031  10.so.0.31[.0]
        1.2.25              13    10225  12.so.0.25[.0]
        1.2.26beta01-06     13    10226  12.so.0.26[.0]
        1.2.26rc01          13    10226  12.so.0.26[.0]
        1.2.26              13    10226  12.so.0.26[.0]
        1.0.32              10    10032  10.so.0.32[.0]
        1.2.27beta01-06     13    10227  12.so.0.27[.0]

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

SEE ALSO

       libpngpf(3), png(5)

       libpng:

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

       zlib:

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

       PNGspecification:RFC2083

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

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

AUTHORS

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

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

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

       Libpng version 1.2.27 - April 29, 2008: Initially created  in  1995  by
       Guy  Eric  Schalnat,  then  of  Group 42, Inc.  Currently maintained by
       Glenn Randers-Pehrson (glennrp at users.sourceforge.net).

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

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
       following this sentence.

       libpng versions 1.2.6, August 15, 2004, through 1.2.27, April 29, 2008,
       are  Copyright  (c)  2004,2006-2008  Glenn  Randers-Pehrson,  and   are
       distributed   according   to   the   same  disclaimer  and  license  as
       libpng-1.2.5 with  the  following  individual  added  to  the  list  of
       Contributing Authors

          Cosmin Truta

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

          Simon-Pierre Cadieux
          Eric S. Raymond
          Gilles Vollant

       and with the following additions to the disclaimer:

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

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

          Tom Lane
          Glenn Randers-Pehrson
          Willem van Schaik

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

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

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

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

          Andreas Dilger
          Dave Martindale
          Guy Eric Schalnat
          Paul Schmidt
          Tim Wegner

       The PNG Reference  Library  is  supplied  "AS  IS".   The  Contributing
       Authors  and  Group  42,  Inc.  disclaim  all  warranties, expressed or
       implied,   including,   without   limitation,   the    warranties    of
       merchantability  and  of  fitness  for  any  purpose.  The Contributing
       Authors and Group 42, Inc.  assume no liability for  direct,  indirect,
       incidental,  special,  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,
       without fee, and encourage the use of this source code as  a  component
       to  supporting  the PNG file format in commercial products.  If you use
       this source code in a product, acknowledgment is not required but would
       be appreciated.

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

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

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

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

       Glenn Randers-Pehrson glennrp at users.sourceforge.net April 29, 2008

                                April 29, 2008                       LIBPNG(3)