LIBPNG(3)                                             LIBPNG(3)



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      back-
       ground_gamma_code,   int   need_expand,   double   back-
       ground_gamma);



       void png_set_bgr (png_structp png_ptr);



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



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



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



       void png_set_compression_level (png_structp png_ptr, int
       level);



       void png_set_compression_mem_level (png_structp png_ptr,
       int mem_level);



       void  png_set_compression_method  (png_structp  png_ptr,
       int method);



       void png_set_compression_strategy (png_structp  png_ptr,
       int strategy);



       void     png_set_compression_window_bits    (png_structp
       png_ptr, int window_bits);



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



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



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



       void png_set_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  fil-
       ter_weights, png_doublep filter_costs);



       void png_set_flush (png_structp png_ptr, int nrows);



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



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



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



       void png_set_gray_1_2_4_to_8(png_structp png_ptr);



       void png_set_gray_to_rgb (png_structp png_ptr);



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



       void  png_set_iCCP   (png_structp   png_ptr,   png_infop
       info_ptr,    png_charp   name,   int   compression_type,
       png_charp 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 com-
       pression_type, int filter_type);



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



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



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



       void png_set_packing (png_structp png_ptr);



       void png_set_packswap (png_structp png_ptr);



       void png_set_palette_to_rgb(png_structp png_ptr);



       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_progres-
       sive_end_ptr end_fn);



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



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



       void    png_set_read_status_fn   (png_structp   png_ptr,
       png_read_status_ptr read_row_fn);



       void     png_set_read_user_transform_fn     (png_structp
       png_ptr, png_user_transform_ptr read_user_transform_fn);



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



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



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



       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  out-
       put_flush_fn);



       void   png_set_write_status_fn   (png_structp   png_ptr,
       png_write_status_ptr write_row_fn);



       void     png_set_write_user_transform_fn    (png_structp
       png_ptr,    png_user_transform_ptr     write_user_trans-
       form_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 var-
       ious manipulations  of  the  Portable  Network  Graphics
       (PNG)  format image files.  It uses the zlib(3) compres-
       sion 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 refer-
       ence library (known as libpng) for your own use.   There
       are  five  sections  to  this file: introduction, struc-
       tures, reading, writing, and modification and configura-
       tion  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  specifica-
       tion, as a way of reducing the amount of time and effort
       it takes to support the PNG file format  in  application
       programs.

       The  PNG  specification (second edition), November 2003,
       is available as a W3C Recommendation and as an ISO Stan-
       dard       (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 tech-
       nically equivalent to the PNG specification (second edi-
       tion) 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-pur-
       pose        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  signifi-
       cantly;  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  lan-
       guages 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 struc-
       ture.


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  informa-
       tion  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 applica-
       tions 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 accessi-
       ble 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 pro-
       gram 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_cre-
       ate_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_cre-
       ate_info_struct(png_ptr);
           if (!end_info)
           {
               png_destroy_read_struct(&png_ptr, &info_ptr,
                 (png_infopp)NULL);
               return (ERROR);
           }

       If  you want to use your own memory allocation routines,
       define   PNG_USER_MEM_SUPPORTED   and    use    png_cre-
       ate_read_struct_2() instead of png_create_read_struct():

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

       The   error   handling   routines   passed  to  png_cre-
       ate_read_struct() and  the  memory  alloc/free  routines
       passed  to  png_create_struct_2()  are only necessary if
       you are not using the libpng supplied error handling and
       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 rou-
       tine that will call a png_*() function.

       See your documentation of setjmp/longjmp for  your  com-
       piler  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  func-
       tion  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() func-
       tion, 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 applica-
       tions really need to process such large images, we  have
       imposed  an  arbitrary  1-million limit on rows and col-
       umns.  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  poten-
       tial 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 applica-
       tions really need to process such large images, we  have
       imposed  an  arbitrary  1-million limit on rows and col-
       umns.  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  poten-
       tial 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 chan-
       nel
           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 opac-
       ity
                                       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  bit-
       wise  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  responsi-
       ble  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  chan-
       nels
                                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_COMPRES-
       SION_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  signa-
       ture[7]
                            (see png_set_sig_bytes())).


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


       These are also important, but their validity depends  on
       whether     the    chunk    has    been    read.     The
       png_get_valid(png_ptr, info_ptr,  PNG_INFO_<chunk>)  and
       png_get_<chunk>(png_ptr, info_ptr, ...) functions return
       non-zero if the data has been read, or  zero  if  it  is
       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 con-
       tain
                                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, lan-
       guage,
           and translated keywords that can be  NULL  pointers,
       the
           structure  returned by png_get_text will always con-
       tain
           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,  &off-
       set_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_OFF-
       SET_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 transfor-
       mations  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  key-
       words,  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 key-
       word.

       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 hold-
       ing a pointer to a language string, a pointer to a  key-
       word  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 val-
       ues should be supplied in the same format/depth  as  the
       current  image  data.   They are stored in the same for-
       mat/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 combin-
       ing it with the background (but the  image  author  cer-
       tainly  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 pix-
       els:

           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 orig-
       inal bit depth of the image.  This call reduces the pix-
       els 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 for-
       mat 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 rep-
       resented 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  process-
       ing  the  image  rows, with the png_get_rgb_to_gray_sta-
       tus(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_back-
       ground))
               png_set_background(png_ptr, image_background,
                 PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
           else
               png_set_background(png_ptr, &my_background,
                 PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);

       The  png_set_background()  function tells libpng to com-
       posite 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_BACK-
       GROUND_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 environ-
       ment 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  transforma-
       tions  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  speci-
       fication  for an excellent description of what gamma is,
       and why all  applications  should  support  it.   It  is
       strongly recommended that PNG viewers support gamma cor-
       rection.

          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 col-
       ors 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_han-
       dling(png_ptr);

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

           png_read_update_info(png_ptr, info_ptr);

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


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

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

          png_read_image(png_ptr, row_pointers);

       where row_pointers is:

          png_bytep row_pointers[height];

       You  can  point  to void or char or whatever you use for
       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_INTER-
       LACE_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 cur-
       rent (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  specifi-
       cation  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 initial-
       ized 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 col-
       umn 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.  Cur-
       rently, this is seven, but may change if another  inter-
       lace 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 num-
       ber_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 allo-
       cated internally by libpng.  This  can  be  changed,  so
       that  libpng  will not free the data, or so that it will
       free data that was allocated by the user  with  png_mal-
       loc()  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 func-
       tion, 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_progres-
       sive_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_cre-
       ate_info_struct(png_ptr);
           if (!info_ptr)
           {
              png_destroy_write_struct(&png_ptr,
                (png_infopp)NULL);
              return (ERROR);
           }

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

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

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

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

       If   you   would   rather   avoid   the   complexity  of
       setjmp/longjmp  issues,  you  can  compile  libpng  with
       PNG_SETJMP_NOT_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 func-
       tion 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  han-
       dling 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  signa-
       ture,  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 func-
       tion

           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_call-
       back);

       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 will-
       ing 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 speci-
       fication, 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  fil-
       ter 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 particu-
       lar 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 embed-
       ded  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  Interna-
       tional
                            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  val-
       ues
                            that are consistent with sRGB to be
                            written.

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

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

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

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

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

           png_set_bKGD(png_ptr, info_ptr, background);
           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  con-
       tain
                        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 compres-
       sion 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 com-
       pressing it.  After the text has been written out to the
       file,  the  compression type is set to PNG_TEXT_COMPRES-
       SION_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 spec-
       ification, 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 regu-
       lar ASCII) and  can  not  contain  NUL  characters,  and
       should  not contain control or other unprintable charac-
       ters.  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 any-
       way, so the compression would be meaningless.

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

       If you want to store the time of the original image cre-
       ation,  you should use a plain tEXt chunk with the "Cre-
       ation 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  sub-
       ject  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  soft-
       ware.   To facilitate the use of RFC 1123 dates, a func-
       tion 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  speci-
       fication'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 per-
       mitted, enabled by the following masks.

           PNG_TRANSFORM_IDENTITY      No transformation
           PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit sam-
       ples
           PNG_TRANSFORM_PACKSWAP      Change order of packed
                                       pixels to LSB first
           PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
           PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                       sBIT depth
           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
           PNG_TRANSFORM_INVERT_ALPHA  Change alpha from  opac-
       ity
                                       to transparency
           PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
           PNG_TRANSFORM_STRIP_FILLER        Strip out filler
                                             bytes      (depre-
       cated).
           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 bit-
       wise 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 chan-
       nel 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 sup-
       plied 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 there-
       after, 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).   Infre-
       quent flushing will only degrade the compression perfor-
       mance 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  han-
       dles   interlacing,   so   you   don't   need   to  call
       png_set_interlace_handling() or call this function  mul-
       tiple  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" inter-
       lace 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.   Cur-
       rently,  this is seven, but may change if another inter-
       lace 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 writ-
       ing  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 allo-
       cated  internally  by  libpng.   This can be changed, so
       that libpng will not free the data, or so that  it  will
       free  data  that was allocated by the user with png_mal-
       loc() 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  struc-
       ture  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-allo-
       cated  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_mal-
       loc()  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 han-
       dling  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_cre-
       ate_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 detect-
       ing 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 func-
       tions  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  com-
       piler  documentation  for more details.  For an alterna-
       tive approach, you may wish to use the "cexcept"  facil-
       ity (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  under-
       standing  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 simi-
       larly.  Second, check out the sections  of  libpng  that
       read and write chunks.  Try to find a chunk that is sim-
       ilar 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 func-
       tions.

       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 compres-
       sion.  Perhaps the most useful one changes the  compres-
       sion  level, which currently uses input compression val-
       ues 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_COM-
       PRESSION = 0), but this would create files  larger  than
       just  storing  the raw bitmap.  You can specify the com-
       pression 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 mem-
       ory 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 pick-
       ing  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 specifica-
       tion.  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_FIL-
       TER_SUB,   PNG_FILTER_UP,    PNG_FILTER_AVG,    PNG_FIL-
       TER_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 appropri-
       ately 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 suc-
       cessive 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 differ-
       ences" 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 speci-
       fied 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 fil-
       ters.  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 capabil-
       ity,  you can change the #define to #undef before recom-
       piling 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  pro-
       duces 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  sepa-
       rate  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  writ-
       ing,  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  actu-
       ally 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  datas-
       treams.   Libpng  can  support some of these extensions.
       To enable them, use the png_permit_mng_features()  func-
       tion:

          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 signa-
       ture.  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  func-
       tions 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 possi-
       ble to set the error callbacks AFTER png_read_init(), or
       to change them with png_set_error_fn(), which is  essen-
       tially  the  same function, but with a new name to force
       compilation errors with applications that try to use the
       old method.

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

          png_uint_32 libpng_vn = png_access_version_number();

       The number libpng_vn is constructed from the major  ver-
       sion,  minor version with leading zero, and release num-
       ber 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_cre-
       ate_read_struct_2(),        png_create_write_struct_2(),
       png_set_mem_fn(),       png_get_mem_ptr(),      png_mal-
       loc_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 mes-
       sages.  The function  png_set_strip_error_numbers()  was
       added  (Note:  the prototype for this function was inad-
       vertently 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.source-
       forge.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 state-
       ments 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 inden-
       tation 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, how-
       ever, 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 ver-
       sion 0.71 and upward through 1.2.37 are  Y2K  compliant.
       It is my belief that earlier versions were also Y2K com-
       pliant.

       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  environ-
       ment.    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 applica-
       tions  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 documen-
       tation has always stated  that  it  works  with  4-digit
       years, and the APIs have been documented as such.

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

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


          Glenn Randers-Pehrson
          libpng maintainer
          PNG Development Group


NOTE
       Note about libpng version numbers:

       Due to various miscommunications, unforeseen code incom-
       patibilities and occasional factors outside the authors'
       control, version numbering on the library has not always
       been  consistent and straightforward.  The following ta-
       ble 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  back-
       ward    compatibility,   as   it   is   intended.    The
       PNG_PNGLIB_VER macro, which is not  used  within  libpng
       but  is available for applications, is an unsigned inte-
       ger of the form xyyzz corresponding to the  source  ver-
       sion  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 [DOWN-
              LOAD] 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  speci-
       fication  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 docu-
       mentation.

       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.source-
       forge.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 Ran-
       ders-Pehrson, and are distributed according to the  same
       disclaimer  and license as libpng-1.0.6 with the follow-
       ing  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,  "Con-
       tributing  Authors"  is  defined as the following set of
       individuals:

          Andreas Dilger
          Dave Martindale
          Guy Eric Schalnat
          Paul Schmidt
          Tim Wegner

       The PNG Reference Library is supplied "AS IS".  The Con-
       tributing  Authors  and Group 42, Inc. disclaim all war-
       ranties, expressed or implied, including, without  limi-
       tation, the warranties of merchantability and of fitness
       for any purpose.  The Contributing Authors and Group 42,
       Inc.  assume no liability for direct, indirect, inciden-
       tal, 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  restric-
       tions:

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

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

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

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


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

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

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

       Libpng is OSI Certified Open Source Software.  OSI  Cer-
       tified  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)
