MirOS Manual: libpng(3)


LIBPNG(3)           UNIX Programmer's Manual            LIBPNG(3)

NAME

     libpng - Portable Network Graphics (PNG) Reference Library
     1.2.50

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);

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     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);

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     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);

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     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);

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     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);

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     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

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     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);

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     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);

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     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);

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     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);

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

     #endif

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

     void png_set_bgr (png_structp png_ptr);

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

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

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

     void png_set_compression_level (png_structp png_ptr, int
     level);

     void png_set_compression_mem_level (png_structp png_ptr, int
     mem_level);

     void png_set_compression_method (png_structp png_ptr, int
     method);

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     void png_set_compression_strategy (png_structp png_ptr, int
     strategy);

     void png_set_compression_window_bits (png_structp png_ptr,
     int window_bits);

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

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

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

     void png_set_expand (png_structp png_ptr);

     void png_set_expand_gray_1_2_4_to_8(png_structp png_ptr);

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

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

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

     void png_set_flush (png_structp png_ptr, int nrows);

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     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,

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     png_uint_32 width, png_uint_32 height, int bit_depth, int
     color_type, int interlace_type, int compression_type, int
     filter_type);

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

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

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

     void png_set_packing (png_structp png_ptr);

     void png_set_packswap (png_structp png_ptr);

     void png_set_palette_to_rgb(png_structp png_ptr);

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

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

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

     void png_set_PLTE (png_structp png_ptr, png_infop info_ptr,

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     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);

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     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,

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     int location);

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

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

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

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

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

     void png_set_write_status_fn (png_structp png_ptr,
     png_write_status_ptr write_row_fn);

     void png_set_write_user_transform_fn (png_structp png_ptr,
     png_user_transform_ptr write_user_transform_fn);

     void png_set_compression_buffer_size(png_structp png_ptr,
     png_uint_32 size);

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

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     void png_start_read_image (png_structp png_ptr);

     void png_warning (png_structp png_ptr, png_const_charp mes-
     sage);

     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,

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     png_size_t png_info_size);

     void png_write_info (png_structp png_ptr, png_infop
     info_ptr);

     void png_write_info_before_PLTE (png_structp png_ptr,
     png_infop info_ptr);

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

     void png_write_row (png_structp png_ptr, png_bytep row);

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

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

     void png_zfree (voidpf png_ptr, voidpf ptr);

DESCRIPTION

     The libpng library supports encoding, decoding, and various
     manipulations of the Portable Network Graphics (PNG) format
     image files.  It uses the zlib(3) compression library. Fol-
     lowing 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 1.2.50 - July 10, 2012
      Updated and distributed by Glenn Randers-Pehrson
      <glennrp@users.sourceforge.net>
      Copyright (c) 1998-2009 Glenn Randers-Pehrson

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      This document is released under the libpng license.
      For conditions of distribution and use, see the disclaimer
      and license in png.h

      Based on:

      libpng versions 0.97, January 1998, through 1.2.50 - July
     10, 2012
      Updated and distributed by Glenn Randers-Pehrson
      Copyright (c) 1998-2009 Glenn Randers-Pehrson

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

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

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

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

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

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

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

     The PNG-1.2 specification is available at
     <http://www.libpng.org/pub/png/documents/>.  It is

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     technically 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-purpose
     public chunks documents at
     <http://www.libpng.org/pub/png/documents/>.

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

     Most users will not have to modify the library 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 languages is being con-
     sidered.

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

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

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

II. Structures
     There are two main structures that are important to libpng,
     png_struct and png_info.  The first, png_struct, is an
     internal structure that will not, for the most part, be used

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     by a user except as the first variable passed to every
     libpng function call.

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

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

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

     #include <png.h>

III. Reading
     We'll now walk you through the possible functions to call
     when reading in a PNG file sequentially, briefly 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

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     nonzero (true) otherwise. Of course, the more bytes you pass
     in, the greater the accuracy of the prediction.

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

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

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

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

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

         png_infop info_ptr = png_create_info_struct(png_ptr);
         if (!info_ptr)
         {
             png_destroy_read_struct(&png_ptr,

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                (png_infopp)NULL, (png_infopp)NULL);
             return (ERROR);
         }

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

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

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

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

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

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

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

     If you would rather avoid the complexity of setjmp/longjmp

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     issues, you can compile libpng with
     PNG_SETJMP_NOT_SUPPORTED, in which case errors will result
     in a call to PNG_ABORT() which defaults to abort().

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

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     (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);

     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

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                      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 func-
     tion:

         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,

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     unused_chunks,
              (int)sizeof(unused_chunks)/5);
         #endif

     User limits

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

        png_set_user_limits(png_ptr, width_max, height_max);

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

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

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

     The PNG specification sets no limit on the number of ancil-
     lary chunks allowed in a PNG datastream.  You can impose a
     limit on the total number of sPLT, tEXt, iTXt, zTXt, and
     unknown chunks that will be stored, with

        png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

     where 0x7fffffffL means unlimited.  You can retrieve this
     limit with

        chunk_cache_max = png_get_chunk_cache_max(png_ptr);

     This limit also applies to the number of buffers that can be
     allocated by png_decompress_chunk() while decompressing
     iTXt, zTXt, and iCCP chunks.

     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

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     if (a) you are willing to read the entire image into memory,
     and (b) the input transformations you want to do are limited
     to the following set:

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

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

         png_read_png(png_ptr, info_ptr, png_transforms, NULL)

     where png_transforms is an integer containing the bitwise OR
     of some set of transformation flags.  This call is
     equivalent to png_read_info(), followed the set of transfor-
     mations 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:

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        png_bytep row_pointers[height];

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

        if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
           png_error (png_ptr,
              "Image is too tall to process in memory");
        if (width > PNG_UINT_32_MAX/pixel_size)
           png_error (png_ptr,
              "Image is too wide to process in memory");
        row_pointers = png_malloc(png_ptr,
           height*png_sizeof(png_bytep));
        for (int i=0; i<height, i++)
           row_pointers[i]=NULL;  /* security precaution */
        for (int i=0; i<height, i++)
           row_pointers[i]=png_malloc(png_ptr,
              width*pixel_size);
        png_set_rows(png_ptr, info_ptr, &row_pointers);

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

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

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

     The low-level read interface

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

         png_read_info(png_ptr, info_ptr);

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

     Querying the info structure

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

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

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

                          PNG_COLOR_MASK_PALETTE
                          PNG_COLOR_MASK_COLOR
                          PNG_COLOR_MASK_ALPHA

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

         Any or all of interlace_type, compression_type, or
         filter_method can be NULL if you are
         not interested in their values.

         Note that png_get_IHDR() returns 32-bit data into
         the application's width and height variables.
         This is an unsafe situation if these are 16-bit
         variables.  In such situations, the
         png_get_image_width() and png_get_image_height()
         functions described below are safer.

         width            = png_get_image_width(png_ptr,

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                              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);

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

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

     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)

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         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);

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         num_comments   - number of comments
         text_ptr       - array of png_text holding image
                          comments
         text_ptr[i].compression - type of compression used
                      on "text" PNG_TEXT_COMPRESSION_NONE
                                PNG_TEXT_COMPRESSION_zTXt
                                PNG_ITXT_COMPRESSION_NONE
                                PNG_ITXT_COMPRESSION_zTXt
         text_ptr[i].key   - keyword for comment.  Must contain
                              1-79 characters.
         text_ptr[i].text  - text comments for current
                              keyword.  Can be empty.
         text_ptr[i].text_length - length of text string,
                      after decompression, 0 for iTXt
         text_ptr[i].itxt_length - length of itxt string,
                      after decompression, 0 for tEXt/zTXt
         text_ptr[i].lang  - language of comment (empty
                              string for unknown).
         text_ptr[i].lang_key  - keyword in UTF-8
                              (empty string for unknown).
         Note that the itxt_length, lang, and lang_key
         members of the text_ptr structure only exist
         when the library is built with iTXt chunk support.

         num_text       - number of comments (same as
                          num_comments; you can put NULL here
                          to avoid the duplication)
         Note while png_set_text() will accept text, language,
         and translated keywords that can be NULL pointers, the
         structure returned by png_get_text will always contain
         regular zero-terminated C strings.  They might be
         empty strings but they will never be NULL pointers.

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

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

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

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         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;

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            res_x_and_y is 0 if res_x != res_y)

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

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

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

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

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

     Keywords should be limited to 79 Latin-1 characters without
     leading or trailing spaces, but non-consecutive spaces are
     allowed within the keyword.  It is possible to have the same
     keyword any number of times. The text_ptr is an array of
     png_text structures, each holding a pointer to a language
     string, a pointer to a keyword and a pointer to a text
     string.  The text string, language code, and translated key-
     word 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 dis-
     cussion 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

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     described in the order that they should occur.  This is
     important, as some of these change the color type and/or bit
     depth of the data, and some others only work on certain
     color types and bit depths.  Even though each transformation
     checks to see if it has data that it can do something with,
     you should make sure to only enable a transformation if it
     will be valid for the data.  For example, don't swap red and
     blue on grayscale data.

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

     Data will be decoded into the supplied row buffers packed
     into bytes unless the library has been told to transform it
     into another format. For example, 4 bit/pixel paletted or
     grayscale data will be returned 2 pixels/byte with the left-
     most 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);

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     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.

     As of libpng version 1.2.50, not all possible expansions are
     supported.

     In the following table, the 01 means grayscale with depth<8,
     31 means indexed with depth<8, other numerals represent the
     color type, "T" means the tRNS chunk is present, A means an
     alpha channel is present, and O means tRNS or alpha is
     present but all pixels in the image are opaque.

       FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O
     6A  6O
        TO
        01    -
        31        -
         0    1       -
        0T                -
        0O                    -
         2           GX           -
        2T                            -
        2O                                -
         3        1                           -
        3T                                        -
        3O                                            -
        4A                T                               -
        4O                                                    -
        6A               GX         TX           TX
     -
        6O                   GX                      TX
     -

     Within the matrix,
          "-" means the transformation is not supported.
          "X" means the transformation is obtained by
     png_set_expand().
          "1" means the transformation is obtained by
              png_set_expand_gray_1_2_4_to_8
          "G" means the transformation is obtained by
              png_set_gray_to_rgb().
          "P" means the transformation is obtained by
              png_set_expand_palette_to_rgb().
          "T" means the transformation is obtained by
              png_set_tRNS_to_alpha().

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

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

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

         if (color_type & PNG_COLOR_MASK_ALPHA)
             png_set_strip_alpha(png_ptr);

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

         png_set_invert_alpha(png_ptr);

     The PNG format only supports pixels with postmultiplied
     alpha. If you want to replace the pixels, after reading
     them, with pixels that have premultiplied color samples, you
     can do this with

         png_set_premultiply_alpha(png_ptr);

     If you do this, any input with a tRNS chunk will be expanded
     to have an alpha channel.

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

         if (bit_depth < 8)
             png_set_packing(png_ptr);

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

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         png_color_8p sig_bit;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

     With red_weight+green_weight<=100000, the normalized gray-
     level is computed:

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

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

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

     Libpng approximates this with

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

     which can be expressed with integers as

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

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

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

         png_color_16 my_background;
         png_color_16p image_background;

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

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

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

     Note that display_gamma is the overall gamma correction
     required to produce pleasing results, which depends on the
     lighting conditions in the surrounding environment.  In a
     dim or brightly lit room, no compensation other than the

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     physical gamma exponent of the monitor is needed, while in a
     dark room a slightly smaller exponent is better.

        double gamma, screen_gamma;

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

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

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

     If you need to reduce an RGB file to a paletted file, or if
     a paletted file has more entries then will fit on your
     screen, png_set_dither() will do that.  Note that this is a
     simple match dither that merely finds the closest color
     available.  This should work fairly well with optimized
     palettes, and fairly badly with linear color cubes.  If you
     pass a palette that is larger then maximum_colors, the file
     will reduce the number of colors in the palette so it will

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     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 sig-
     nificant 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

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     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 pro-
     cessed.

     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 allo-
     cating and freeing any memory required for the user struc-
     ture.

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

         voidp read_user_transform_ptr =
            png_get_user_transform_ptr(png_ptr);

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

         number_of_passes = png_set_interlace_handling(png_ptr);

     After setting the transformations, libpng can update your
     png_info structure to reflect any transformations you've
     requested with this call.  This is most useful to update the
     info structure's rowbytes field so you can use it to allo-
     cate 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.

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         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 allo-
     cation 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 neces-
     sary 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 pix-
     els.

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

         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);

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

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

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

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

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

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

     This will return the number of passes needed.  Currently,

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

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

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

         png_read_rows(png_ptr, row_pointers, NULL,
            number_of_rows);

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

         png_read_rows(png_ptr, NULL, row_pointers,
            number_of_rows);

     Finishing a sequential read

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

        png_read_end(png_ptr, end_info);

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

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

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

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     following function:

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

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

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

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

     This function only affects data that has already been allo-
     cated. 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.

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

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

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

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

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

     Reading PNG files progressively

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

     png_structp png_ptr; png_infop info_ptr;

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      /*  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)

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      {
         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

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            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.
          */
      }

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IV. Writing
     Much of this is very similar to reading.  However, every-
     thing of importance is repeated here, so you won't have to
     constantly look back up in the reading section to 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 ini-
     tialized. As these can be both relatively large, you may not
     want to store these on the stack, unless you have stack
     space to spare.  Of course, you will want to check if they
     return NULL.  If you are also reading, you won't want to
     name your read structure and your write structure both
     "png_ptr"; you can call them anything you like, such as
     "read_ptr" and "write_ptr".  Look at pngtest.c, for example.

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

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

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

         png_structp png_ptr = png_create_write_struct_2
            (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
             user_error_fn, user_warning_fn, (png_voidp)

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             user_mem_ptr, user_malloc_fn, user_free_fn);

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

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

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

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

         png_init_io(png_ptr, fp);

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

         png_set_sig_bytes(png_ptr, 8);

     to inform libpng that it should not write a signature.

     Write callbacks

     At this point, you can set up a callback function that will
     be called after each row has been written, which you can use

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     to control a progress meter or the like.  It's demonstrated
     in pngtest.c. You must supply a function

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

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

     To inform libpng about your function, use

         png_set_write_status_fn(png_ptr, write_row_callback);

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

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

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

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

     The png_set_compression_*() functions interface to the zlib
     compression library, and should mostly be ignored unless you
     really know what you are doing.  The only generally useful
     call is png_set_compression_level() which changes how much
     time zlib spends on trying to compress the image data.  See
     the Compression Library (zlib.h and algorithm.txt, distri-
     buted 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

     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 specif-
     ication.

     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

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                          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, because they might
     require access to some of the IHDR settings.  The remaining
     png_set_*() functions can be called in any order.

     If you wish, you can reset the compression_type,
     interlace_type, or filter_method later by calling
     png_set_IHDR() again; if you do this, the width, height,
     bit_depth, and color_type must be the same in each call.

         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)

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

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

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

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

         png_set_tRNS(png_ptr, info_ptr, trans, num_trans,
            trans_values);
         trans          - array of transparent
                          entries for palette (PNG_INFO_tRNS)
         trans_values   - graylevel or color sample values

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                          (in order red, green, blue) of the
                          single transparent color for
                          non-paletted images (PNG_INFO_tRNS)
         num_trans      - number of transparent entries
                          (PNG_INFO_tRNS)

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

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

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

         png_set_text(png_ptr, info_ptr, text_ptr, num_text);
         text_ptr       - array of png_text holding image
                          comments
         text_ptr[i].compression - type of compression used
                      on "text" PNG_TEXT_COMPRESSION_NONE
                                PNG_TEXT_COMPRESSION_zTXt
                                PNG_ITXT_COMPRESSION_NONE
                                PNG_ITXT_COMPRESSION_zTXt
         text_ptr[i].key   - keyword for comment.  Must contain
                      1-79 characters.
         text_ptr[i].text  - text comments for current
                              keyword.  Can be NULL or empty.
         text_ptr[i].text_length - length of text string,
                      after decompression, 0 for iTXt
         text_ptr[i].itxt_length - length of itxt string,
                      after decompression, 0 for tEXt/zTXt
         text_ptr[i].lang  - language of comment (NULL or
                              empty for unknown).
         text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
                              or empty for unknown).
         Note that the itxt_length, lang, and lang_key
         members of the text_ptr structure only exist
         when the library is built with iTXt chunk support.

         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.

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         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

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     png_text structures.  num_text is the number of valid struc-
     tures in the array. Each png_text structure holds a language
     code, a keyword, a text value, and a compression type.

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

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

     The keywords that are given in the PNG Specification are:

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

     The keyword-text pairs work like this.  Keywords should be
     short simple descriptions of what the comment is about.
     Some typical keywords are found in the PNG specification, as
     is some recommendations on keywords.  You can repeat key-
     words 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 con-
     nections don't have to wait for the disclaimer to go over
     the modem before they start seeing the image.  Finally, key-
     words should be full words, not abbreviations.  Keywords and
     text are in the ISO 8859-1 (Latin-1) character set (a super-
     set of regular ASCII) and can not contain NUL characters,
     and should not contain control or other unprintable

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     characters.  To make the comments widely readable, stick
     with basic ASCII, and avoid machine specific character set
     extensions like the IBM-PC character set.  The keyword must
     be present, but you can leave off the text string on non-
     compressed pairs. Compressed pairs must have a text string,
     as only the text string is compressed anyway, so the
     compression would be meaningless.

     PNG supports modification time via the png_time structure.
     Two conversion routines are provided,
     png_convert_from_time_t() for time_t and
     png_convert_from_struct_tm() for struct tm.  The time_t rou-
     tine 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 pos-
     sible 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 crea-
     tion, you should use a plain tEXt chunk with the "Creation
     Time" keyword.  This is necessary because the "creation
     time" of a PNG image is somewhat vague, depending on whether
     you mean the PNG file, the time the image was created in a
     non-PNG format, a still photo from which the image was
     scanned, or possibly the subject matter itself.  In order to
     facilitate machine-readable dates, it is recommended that
     the "Creation Time" tEXt chunk use RFC 1123 format dates
     (e.g. "22 May 1997 18:07:10 GMT"), although this isn't a
     requirement.  Unlike the tIME chunk, the "Creation Time"
     tEXt chunk is not expected to be automatically changed by
     the software.  To facilitate the use of RFC 1123 dates, a
     function png_convert_to_rfc1123(png_timep) is provided to
     convert from PNG time to an RFC 1123 format string.

     Writing unknown chunks

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

     The high-level write interface

     At this point there are two ways to proceed; through the
     high-level write interface, or through a sequence of low-

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     level write operations. You can use the high-level interface
     if your image data is present in the info structure.  All
     defined output transformations are permitted, enabled by the
     following masks.

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

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

         png_write_png(png_ptr, info_ptr, png_transforms, NULL)

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

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

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

     The low-level write interface

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

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         png_write_info(png_ptr, info_ptr);

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

         png_set_invert_alpha(png_ptr);

     This must appear before png_write_info() instead of later
     with the other transformations because in the case of palet-
     ted 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 sin-
     gle 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.

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     PNG files pack pixels of bit depths 1, 2, and 4 into bytes
     as small as they can, resulting in, for example, 8 pixels
     per byte for 1 bit files. If the data is supplied at 1 pixel
     per byte, use this code, which will correctly pack the pix-
     els 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 origi-
     nal 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);

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     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 sup-
     plied with this reversed (black being one and white being
     zero):

         png_set_invert_mono(png_ptr);

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

         png_set_write_user_transform_fn(png_ptr,
            write_transform_fn);

     You must supply the function

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

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

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

         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 sin-
     gle 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:

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         png_set_flush(png_ptr, nrows);

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

     Writing the image data

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

         png_write_image(png_ptr, row_pointers);

     where row_pointers is:

         png_byte *row_pointers[height];

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

     If you don't want to write the whole image at once, you can
     use png_write_rows() instead.  If the file is not inter-
     laced, 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;

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         png_write_row(png_ptr, row_pointer);

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

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

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

         number_of_passes =
            png_set_interlace_handling(png_ptr);

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

     Then write the complete image number_of_passes times.

         png_write_rows(png_ptr, row_pointers,
            number_of_rows);

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

     Finishing a sequential write

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

         png_write_end(png_ptr, info_ptr);

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

         png_destroy_write_struct(&png_ptr, &info_ptr);

     It is also possible to individually free the info_ptr

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     members that point to libpng-allocated storage with the fol-
     lowing function:

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

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

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

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

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

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

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

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            PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

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

     This function only affects data that has already been allo-
     cated. You can call this function before calling after the
     png_set_*() functions to control whether the user or
     png_destroy_*() is supposed to free the data. When the user
     assumes responsibility for libpng-allocated data, the appli-
     cation 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 han-
     dling in libpng goes through callbacks that are user-
     settable.  The default routines are in pngmem.c, pngrio.c,
     pngwio.c, and pngerror.c, respectively.  To change these
     functions, call the appropriate png_set_*_fn() function.

     Memory allocation is done through the functions
     png_malloc(), png_calloc(), and png_free().  These currently
     just call the standard C functions. png_calloc() calls
     png_malloc() and then png_memset() to clear the newly

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     allocated memory to zero.  If your pointers can't access
     more then 64K at a time, you will want to set MAXSEG_64K in
     zlib.h.  Since it is unlikely that the method of handling
     memory allocation on a platform will change between applica-
     tions, these functions must be modified in the library at
     compile time.  If you prefer to use a different method of
     allocating and freeing data, you can use
     png_create_read_struct_2() or png_create_write_struct_2() to
     register your own functions as described above.  These func-
     tions 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 fol-
     lows:

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

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

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

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

     On non-fatal errors, png_warning() is called to print a
     warning message, and then control returns to the calling
     code. By default png_error() and png_warning() print a mes-
     sage 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 call-
     backs.  These functions are normally supplied at the time
     that the png_struct is created. It is also possible to
     redirect errors and warnings to your own replacement func-
     tions after png_create_*_struct() has been called by cal-
     ling:

         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 replace-
     ment error functions should have parameters as follows:

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

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

     Custom chunks

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

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

     If you wish to write your own transformation for the data,
     look through the part of the code that does the transforma-
     tions, 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

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     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 pro-
     tected by the PNG_INTERNAL definition, which is only defined
     for those routines inside libpng itself.  The files in
     libpng proper only include png.h, which includes pngconf.h.

     Configuring zlib:

     There are special functions to configure the compression.
     Perhaps the most useful one changes the compression level,
     which currently uses input compression values in the range 0
     - 9.  The library normally uses the default compression

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     level (Z_DEFAULT_COMPRESSION = 6).  Tests have shown that
     for a large majority of images, compression values in the
     range 3-6 compress nearly as well as higher levels, and do
     so much faster.  For online applications it may be desirable
     to have maximum speed (Z_BEST_SPEED = 1).  With versions of
     zlib after v0.99, you can also specify no compression
     (Z_NO_COMPRESSION = 0), but this would create files larger
     than just storing the raw bitmap.  You can specify the
     compression level by calling:

         png_set_compression_level(png_ptr, level);

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

         png_set_compression_mem_level(png_ptr, level);

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

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

     Controlling row filtering

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

     The 'method' parameter sets the main filtering method, which
     is currently only '0' in the PNG 1.2 specification.  The
     'filters' parameter sets which filter(s), if any, should be

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     used for each scanline.  Possible values are PNG_ALL_FILTERS
     and PNG_NO_FILTERS to turn filtering on and off, respec-
     tively.

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

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

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

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

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

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

     The weights are multiplying factors that indicate to libpng
     that the row filter should be the same for successive rows
     unless another row filter is that many times better than the
     previous filter.  In the above example, if the previous 3
     filters were SUB, SUB, NONE, the SUB filter could have a
     "sum of absolute differences" 1.5 x 1.3 times higher than
     other filters and still be chosen, while the NONE filter
     could have a sum 1.1 times higher than other filters and

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     still be chosen.  Unspecified weights are taken to be 1.0,
     and the specified weights should probably be declining like
     those above in order to emphasize recent filters over older
     filters.

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

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

     Removing unwanted object code

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

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

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

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     for both reading and writing, and always need to be
     included. The progressive reader is in pngpread.c

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

     Requesting debug printout

     The macro definition PNG_DEBUG can be used to request debug-
     ging 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.

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     There aren't any such statements in this version of libpng,
     but if you insert some they will be printed.

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

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

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

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

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

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

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     initialization, which the old functions do not.  The func-
     tions png_read_destroy() and png_write_destroy() do not
     actually free the memory that libpng allocated for these
     structs, but just reset the data structures, so they can be
     used instead of png_destroy_read_struct() and
     png_destroy_write_struct() if you feel there is too much
     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 ini-
     tialized to zero.  It is still possible to set the error
     callbacks AFTER png_read_init(), or to change them with
     png_set_error_fn(), which is essentially the same function,
     but with a new name to force compilation errors with appli-
     cations that try to use the old method.

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

        png_uint_32 libpng_vn = png_access_version_number();

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

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

        png_uint_32 application_vn = PNG_LIBPNG_VER;

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

     Support for the iTXt chunk has been enabled by default as of
     version 1.2.41.

     Support for certain MNG features was enabled.

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

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

     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 selec-
     tion 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.

     These macros are deprecated:

         PNG_READ_TRANSFORMS_NOT_SUPPORTED

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         PNG_PROGRESSIVE_READ_NOT_SUPPORTED
         PNG_NO_SEQUENTIAL_READ_SUPPORTED
         PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
         PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
         PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED

     They have been replaced, respectively, by:

         PNG_NO_READ_TRANSFORMS
         PNG_NO_PROGRESSIVE_READ
         PNG_NO_SEQUENTIAL_READ
         PNG_NO_WRITE_TRANSFORMS
         PNG_NO_READ_ANCILLARY_CHUNKS
         PNG_NO_WRITE_ANCILLARY_CHUNKS

     PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.  It has been
     deprecated since libpng-1.0.16 and libpng-1.2.6.

     The function
         png_check_sig(sig, num) was replaced with
         !png_sig_cmp(sig, 0, num) It has been deprecated since
     libpng-0.90.

     The function
         png_set_gray_1_2_4_to_8() which also expands tRNS to
     alpha was replaced with
         png_set_expand_gray_1_2_4_to_8() which does not. It has
     been deprecated since libpng-1.0.18 and 1.2.9.

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 condi-
     tional compilation macros.  It is the best choice for use in
     configure scripts for detecting the presence of any libpng
     version since 0.88.  In an autoconf "configure.in" you could
     use

         AC_CHECK_LIB(png, png_get_io_ptr, ...

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

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

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

         http://libpng.sourceforge.net

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

         if (condition)
         {
            action;
         }

         else if (another condition)
         {
            another action;
         }

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

         if (condition)
            return (0);

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

     For macro definitions we use 2-space indentation, always
     leaving the "#" in the first column.

         #ifndef PNG_NO_FEATURE
         #  ifndef PNG_FEATURE_SUPPORTED
         #    define PNG_FEATURE_SUPPORTED
         #  endif
         #endif

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

         /* Single-line comment */
         statement;

         /* Multiple-line
          * comment
          */
         statement;

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     Very short comments can be placed at the end of the state-
     ment to which they pertain:

         statement;    /* comment */

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

     Functions and their curly braces 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,
     above the comment that says

         /* Maintainer: Put new public prototypes here ... */

     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 (except for those
     in pngtest) appear in the PNG_INTERNAL section of png.h
     above the comment that says

       /* Maintainer: Put new private prototypes here ^ and in
     libpngpf.3 */

     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:

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

     We prefer #ifdef and #ifndef to #if defined() and if
     !defined() when there is only one macro being tested.

     We do not use the TAB character for indentation in the C
     sources.

     Lines do not exceed 80 characters.

     Other rules can be inferred by inspecting the libpng source.

XIII. Y2K Compliance in libpng
     July 10, 2012

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

     This is your unofficial assurance that libpng from version
     0.71 and upward through 1.2.50 are Y2K compliant.  It is my
     belief that earlier versions were also Y2K compliant.

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

     The integer is
         "png_uint_16 year" in png_time_struct.

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

     There are seven time-related functions:

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

     All appear to handle dates properly in a Y2K environment.
     The png_convert_from_time_t() function calls gmtime() to
     convert from system clock time, which returns (year - 1900),

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     which we properly convert to the full 4-digit year.  There
     is a possibility that applications using libpng are not
     passing 4-digit years into the png_convert_to_rfc_1123()
     function, or that they are incorrectly passing only a 2-
     digit year instead of "year - 1900" into the
     png_convert_from_struct_tm() function, but this is not under
     our control.  The libpng documentation has always stated
     that it works with 4-digit years, and the APIs have been
     documented as such.

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

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

        Glenn Randers-Pehrson
        libpng maintainer
        PNG Development Group

SEE ALSO

     libpngpf(3), png(5)

     libpng:

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

     zlib:

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

     PNGspecification:RFC2083

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

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

AUTHORS

     This man page: Glenn Randers-Pehrson

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     <glennrp@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 documenta-
     tion.

     Libpng version 1.2.50 - July 10, 2012: Initially created in
     1995 by Guy Eric Schalnat, then of Group 42, Inc. Currently
     maintained by Glenn Randers-Pehrson
     (glennrp@users.sourceforge.net).

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

COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:
     Please refer to /usr/include/png.h for the full blurb.

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