Provided by: libpng12-dev_1.2.37-1_i386 bug

NAME

       libpng - Portable Network Graphics (PNG) Reference Library 1.2.37

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 1.2.37 - June 4, 2009
        Updated and distributed by Glenn Randers-Pehrson
        <glennrp at users.sourceforge.net>
        Copyright (c) 1998-2009 Glenn Randers-Pehrson
        For conditions of distribution and use, see copyright
        notice in png.h.

        Based on:

        libpng versions 0.97, January 1998, through 1.2.37 - June 4, 2009
        Updated and distributed by Glenn Randers-Pehrson
        Copyright (c) 1998-2009 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 (false) if the bytes match the
       corresponding bytes of the PNG signature, or nonzero (true)  otherwise.
       Of  course, the more bytes you pass in, the greater the accuracy of the
       prediction.

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

       (*): If you are not using the standard I/O functions, you will need  to
       replace   them   with  custom  functions.   See  the  discussion  under
       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. This
       behavior can be wasteful if your application will never use some  known
       chunk types. To change this, you can call:

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

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

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

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

          png_set_user_limits(png_ptr, width_max, height_max);

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

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

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

   The 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]=NULL;  /* security precaution */
          for (int i=0; i<height, i++)
             row_pointers[i]=png_malloc(png_ptr,
                width*pixel_size);
          png_set_rows(png_ptr, info_ptr, &row_pointers);

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

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

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

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

           png_read_info(png_ptr, info_ptr);

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

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

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

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

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

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

       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_AVG   | PNG_FILTER_VALUE_AVG  |
              PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
              PNG_ALL_FILTERS);

       If  an  application  wants  to  start and stop using particular filters
       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 (deprecated).
           PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
                                             filler bytes
           PNG_TRANSFORM_STRIP_FILLER_AFTER  Strip out trailing
                                             filler bytes

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

           png_write_png(png_ptr, info_ptr, png_transforms, NULL)

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

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

       You  must  use  png_transforms  and  not  call  any png_set_transform()
       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);

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

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

       Error handling in libpng is done through png_error() and png_warning().
       Errors  handled through png_error() are fatal, meaning that png_error()
       should never return to its caller.   Currently,  this  is  handled  via
       setjmp()   and   longjmp()   (unless  you  have  compiled  libpng  with
       PNG_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
       or 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_AVG |
                     PNG_FILTER_PAETH | PNG_ALL_FILTERS;

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

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

VI. MNG support

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

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

       It  is  an  error  to  use  this  function  when  reading  or writing a
       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.

VII. Changes to Libpng from version 0.88

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

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

       The preferred method of creating and initializing the libpng structures
       is via  the  png_create_read_struct(),  png_create_write_struct(),  and
       png_create_info_struct()   because   they   isolate  the  size  of  the
       structures from the application, allow version error checking, and also
       allow   the   use   of   custom  error  handling  routines  during  the
       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;

VIII. Changes to Libpng from version 1.0.x to 1.2.x

       Support  for  user  memory  management  was  enabled  by  default.   To
       accomplish     this,    the    functions    png_create_read_struct_2(),
       png_create_write_struct_2(),    png_set_mem_fn(),    png_get_mem_ptr(),
       png_malloc_default(), and png_free_default() were added.

       Support for certain MNG features was enabled.

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

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

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

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

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

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

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

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

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

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

IX. (Omitted)

X. Detecting libpng

       The  png_get_io_ptr()  function has been present since libpng-0.88, has
       never changed, and is unaffected by conditional compilation macros.  It
       is  the  best  choice  for  use  in configure scripts for detecting the
       presence of any libpng version since 0.88.

XI. Source code repository

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

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

       or you can browse it via "gitweb" at

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

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

           http://libpng.sourceforge.net

XII. Coding style

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

           if (condition)
           {
              action;
           }

           else if (another condition)
           {
              another action;
           }

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

           if (condition)
              return (0);

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

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

           /* Single-line comment */
           statement;

           /* Multiple-line
            * comment
            */
           statement;

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

           statement;    /* comment */

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

       Functions  and  their  curly  brackets  are  not indented, and exported
       functions are marked with PNGAPI:

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

       The prototypes for all exported functions appear in png.h.

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

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

       The prototypes for non-exported functions can appear in pngpriv.h or in
       the file where the function is located.

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

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

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

       Other rules can be inferred by inspecting the libpng source.

XIII. Y2K Compliance in libpng

       June 4, 2009

       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 1.2.37 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]
        1.2.27rc01          13    10227  12.so.0.27[.0]
        1.0.33              10    10033  10.so.0.33[.0]
        1.2.27              13    10227  12.so.0.27[.0]
        1.0.34              10    10034  10.so.0.34[.0]
        1.2.28              13    10228  12.so.0.28[.0]
        1.2.29beta01-03     13    10229  12.so.0.29[.0]
        1.2.29rc01          13    10229  12.so.0.29[.0]
        1.0.35              10    10035  10.so.0.35[.0]
        1.2.29              13    10229  12.so.0.29[.0]
        1.0.37              10    10037  10.so.0.37[.0]
        1.2.30beta01-04     13    10230  12.so.0.30[.0]
        1.0.38rc01-08       10    10038  10.so.0.38[.0]
        1.2.30rc01-08       13    10230  12.so.0.30[.0]
        1.0.38              10    10038  10.so.0.38[.0]
        1.2.30              13    10230  12.so.0.30[.0]
        1.0.39rc01-03       10    10039  10.so.0.39[.0]
        1.2.31rc01-03       13    10231  12.so.0.31[.0]
        1.0.39              10    10039  10.so.0.39[.0]
        1.2.31              13    10231  12.so.0.31[.0]
        1.2.32beta01-02     13    10232  12.so.0.32[.0]
        1.0.40rc01          10    10040  10.so.0.40[.0]
        1.2.32rc01          13    10232  12.so.0.32[.0]
        1.0.40              10    10040  10.so.0.40[.0]
        1.2.32              13    10232  12.so.0.32[.0]
        1.2.33beta01-02     13    10233  12.so.0.33[.0]
        1.2.33rc01-02       13    10233  12.so.0.33[.0]
        1.0.41rc01          10    10041  10.so.0.41[.0]
        1.2.33              13    10233  12.so.0.33[.0]
        1.0.41              10    10041  10.so.0.41[.0]
        1.2.34beta01-07     13    10234  12.so.0.34[.0]
        1.0.42rc01          10    10042  10.so.0.42[.0]
        1.2.34rc01          13    10234  12.so.0.34[.0]
        1.0.42              10    10042  10.so.0.42[.0]
        1.2.34              13    10234  12.so.0.34[.0]
        1.2.35beta01-03     13    10235  12.so.0.35[.0]
        1.0.43rc01-02       10    10043  10.so.0.43[.0]
        1.2.35rc01-02       13    10235  12.so.0.35[.0]
        1.0.43              10    10043  10.so.0.43[.0]
        1.2.35              13    10235  12.so.0.35[.0]
        1.2.36beta01-05     13    10236  12.so.0.36[.0]
        1.2.36rc01          13    10236  12.so.0.36[.0]
        1.0.44              10    10044  10.so.0.44[.0]
        1.2.36              13    10236  12.so.0.36[.0]
        1.2.37beta01-03     13    10237  12.so.0.37[.0]
        1.2.37rc01          13    10237  12.so.0.37[.0]
        1.2.37              13    10237  12.so.0.37[.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 1.2.37 - June 4, 2009: 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 1.2.37, June  4,  2009,
       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 June 4, 2009

                                 June 4, 2009                        LIBPNG(3)