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LIBPNG(3) LIBPNG(3)
NAME
libpng - Portable Network Graphics (PNG) Reference Library 1.2.27
SYNOPSIS
#include <png.h>
png_uint_32 png_access_version_number (void);
int png_check_sig (png_bytep sig, int num);
void png_chunk_error (png_structp png_ptr, png_const_charp error);
void png_chunk_warning (png_structp png_ptr, png_const_charp message);
void png_convert_from_struct_tm (png_timep ptime, struct tm FAR * ttime);
void png_convert_from_time_t (png_timep ptime, time_t ttime);
png_charp png_convert_to_rfc1123 (png_structp png_ptr, png_timep ptime);
png_infop png_create_info_struct (png_structp png_ptr);
png_structp png_create_read_struct (png_const_charp user_png_ver, png_voidp error_ptr,
png_error_ptr error_fn, png_error_ptr warn_fn);
png_structp png_create_read_struct_2(png_const_charp user_png_ver, png_voidp error_ptr,
png_error_ptr error_fn, png_error_ptr warn_fn, png_voidp mem_ptr, png_malloc_ptr mal-
loc_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 mal-
loc_fn, png_free_ptr free_fn);
int png_debug(int level, png_const_charp message);
int png_debug1(int level, png_const_charp message, p1);
int png_debug2(int level, png_const_charp message, p1, p2);
void png_destroy_info_struct (png_structp png_ptr, png_infopp info_ptr_ptr);
void png_destroy_read_struct (png_structpp png_ptr_ptr, png_infopp info_ptr_ptr,
png_infopp end_info_ptr_ptr);
void png_destroy_write_struct (png_structpp png_ptr_ptr, png_infopp info_ptr_ptr);
void png_error (png_structp png_ptr, png_const_charp error);
void png_free (png_structp png_ptr, png_voidp ptr);
void png_free_chunk_list (png_structp png_ptr);
void png_free_default(png_structp png_ptr, png_voidp ptr);
void png_free_data (png_structp png_ptr, png_infop info_ptr, int num);
png_byte png_get_bit_depth (png_structp png_ptr, png_infop info_ptr);
png_uint_32 png_get_bKGD (png_structp png_ptr, png_infop info_ptr, png_color_16p
*background);
png_byte png_get_channels (png_structp png_ptr, png_infop info_ptr);
png_uint_32 png_get_cHRM (png_structp png_ptr, png_infop info_ptr, double *white_x, double
*white_y, double *red_x, double *red_y, double *green_x, double *green_y, double *blue_x,
double *blue_y);
png_uint_32 png_get_cHRM_fixed (png_structp png_ptr, png_infop info_ptr, png_uint_32
*white_x, png_uint_32 *white_y, png_uint_32 *red_x, png_uint_32 *red_y, png_uint_32
*green_x, png_uint_32 *green_y, png_uint_32 *blue_x, png_uint_32 *blue_y);
png_byte png_get_color_type (png_structp png_ptr, png_infop info_ptr);
png_byte png_get_compression_type (png_structp png_ptr, png_infop info_ptr);
png_byte png_get_copyright (png_structp png_ptr);
png_voidp png_get_error_ptr (png_structp png_ptr);
png_byte png_get_filter_type (png_structp png_ptr, png_infop info_ptr);
png_uint_32 png_get_gAMA (png_structp png_ptr, png_infop info_ptr, double *file_gamma);
png_uint_32 png_get_gAMA_fixed (png_structp png_ptr, png_infop info_ptr, png_uint_32
*int_file_gamma);
png_byte png_get_header_ver (png_structp png_ptr);
png_byte png_get_header_version (png_structp png_ptr);
png_uint_32 png_get_hIST (png_structp png_ptr, png_infop info_ptr, png_uint_16p *hist);
png_uint_32 png_get_iCCP (png_structp png_ptr, png_infop info_ptr, png_charpp name, int
*compression_type, png_charpp profile, png_uint_32 *proflen);
png_uint_32 png_get_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32 *width,
png_uint_32 *height, int *bit_depth, int *color_type, int *interlace_type, int *compres-
sion_type, int *filter_type);
png_uint_32 png_get_image_height (png_structp png_ptr, png_infop info_ptr);
png_uint_32 png_get_image_width (png_structp png_ptr, png_infop info_ptr);
#if !defined(PNG_1_0_X) png_int_32 png_get_int_32 (png_bytep buf); #endif
png_byte png_get_interlace_type (png_structp png_ptr, png_infop info_ptr);
png_voidp png_get_io_ptr (png_structp png_ptr);
png_byte png_get_libpng_ver (png_structp png_ptr);
png_voidp png_get_mem_ptr(png_structp png_ptr);
png_uint_32 png_get_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32 *offset_x,
png_uint_32 *offset_y, int *unit_type);
png_uint_32 png_get_pCAL (png_structp png_ptr, png_infop info_ptr, png_charp *purpose,
png_int_32 *X0, png_int_32 *X1, int *type, int *nparams, png_charp *units, png_charpp
*params);
png_uint_32 png_get_pHYs (png_structp png_ptr, png_infop info_ptr, png_uint_32 *res_x,
png_uint_32 *res_y, int *unit_type);
float png_get_pixel_aspect_ratio (png_structp png_ptr, png_infop info_ptr);
png_uint_32 png_get_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);
png_voidp png_get_progressive_ptr (png_structp png_ptr);
png_uint_32 png_get_PLTE (png_structp png_ptr, png_infop info_ptr, png_colorp *palette,
int *num_palette);
png_byte png_get_rgb_to_gray_status (png_structp png_ptr) png_uint_32 png_get_rowbytes
(png_structp png_ptr, png_infop info_ptr);
png_bytepp png_get_rows (png_structp png_ptr, png_infop info_ptr);
png_uint_32 png_get_sBIT (png_structp png_ptr, png_infop info_ptr, png_color_8p *sig_bit);
png_bytep png_get_signature (png_structp png_ptr, png_infop info_ptr);
png_uint_32 png_get_sPLT (png_structp png_ptr, png_infop info_ptr, png_spalette_p
*splt_ptr);
png_uint_32 png_get_sRGB (png_structp png_ptr, png_infop info_ptr, int *intent);
png_uint_32 png_get_text (png_structp png_ptr, png_infop info_ptr, png_textp *text_ptr,
int *num_text);
png_uint_32 png_get_tIME (png_structp png_ptr, png_infop info_ptr, png_timep *mod_time);
png_uint_32 png_get_tRNS (png_structp png_ptr, png_infop info_ptr, png_bytep *trans, int
*num_trans, png_color_16p *trans_values);
#if !defined(PNG_1_0_X) png_uint_16 png_get_uint_16 (png_bytep buf);
png_uint_32 png_get_uint_31 (png_bytep buf);
png_uint_32 png_get_uint_32 (png_bytep buf); #endif
png_uint_32 png_get_unknown_chunks (png_structp png_ptr, png_infop info_ptr,
png_unknown_chunkpp unknowns);
png_voidp png_get_user_chunk_ptr (png_structp png_ptr);
png_uint_32 png_get_user_height_max( png_structp png_ptr);
png_voidp png_get_user_transform_ptr (png_structp png_ptr);
png_uint_32 png_get_user_width_max (png_structp png_ptr);
png_uint_32 png_get_valid (png_structp png_ptr, png_infop info_ptr, png_uint_32 flag);
png_int_32 png_get_x_offset_microns (png_structp png_ptr, png_infop info_ptr);
png_int_32 png_get_x_offset_pixels (png_structp png_ptr, png_infop info_ptr);
png_uint_32 png_get_x_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);
png_int_32 png_get_y_offset_microns (png_structp png_ptr, png_infop info_ptr);
png_int_32 png_get_y_offset_pixels (png_structp png_ptr, png_infop info_ptr);
png_uint_32 png_get_y_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);
png_uint_32 png_get_compression_buffer_size (png_structp png_ptr);
int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);
void png_init_io (png_structp png_ptr, FILE *fp);
DEPRECATED: void png_info_init (png_infop info_ptr);
DEPRECATED: void png_info_init_2 (png_infopp ptr_ptr, png_size_t png_info_struct_size);
png_voidp png_malloc (png_structp png_ptr, png_uint_32 size);
png_voidp png_malloc_default(png_structp png_ptr, png_uint_32 size);
voidp png_memcpy (png_voidp s1, png_voidp s2, png_size_t size);
png_voidp png_memcpy_check (png_structp png_ptr, png_voidp s1, png_voidp s2, png_uint_32
size);
voidp png_memset (png_voidp s1, int value, png_size_t size);
png_voidp png_memset_check (png_structp png_ptr, png_voidp s1, int value, png_uint_32
size);
DEPRECATED: void png_permit_empty_plte (png_structp png_ptr, int empty_plte_permitted);
void png_process_data (png_structp png_ptr, png_infop info_ptr, png_bytep buffer,
png_size_t buffer_size);
void png_progressive_combine_row (png_structp png_ptr, png_bytep old_row, png_bytep
new_row);
void png_read_destroy (png_structp png_ptr, png_infop info_ptr, png_infop end_info_ptr);
void png_read_end (png_structp png_ptr, png_infop info_ptr);
void png_read_image (png_structp png_ptr, png_bytepp image);
DEPRECATED: void png_read_init (png_structp png_ptr);
DEPRECATED: void png_read_init_2 (png_structpp ptr_ptr, png_const_charp user_png_ver,
png_size_t png_struct_size, png_size_t png_info_size);
void png_read_info (png_structp png_ptr, png_infop info_ptr);
void png_read_png (png_structp png_ptr, png_infop info_ptr, int transforms, png_voidp
params);
void png_read_row (png_structp png_ptr, png_bytep row, png_bytep display_row);
void png_read_rows (png_structp png_ptr, png_bytepp row, png_bytepp display_row,
png_uint_32 num_rows);
void png_read_update_info (png_structp png_ptr, png_infop info_ptr);
#if !defined(PNG_1_0_X) png_save_int_32 (png_bytep buf, png_int_32 i);
void png_save_uint_16 (png_bytep buf, unsigned int i);
void png_save_uint_32 (png_bytep buf, png_uint_32 i);
void png_set_add_alpha (png_structp png_ptr, png_uint_32 filler, int flags); #endif
void png_set_background (png_structp png_ptr, png_color_16p background_color, int back-
ground_gamma_code, int need_expand, double background_gamma);
void png_set_bgr (png_structp png_ptr);
void png_set_bKGD (png_structp png_ptr, png_infop info_ptr, png_color_16p background);
void png_set_cHRM (png_structp png_ptr, png_infop info_ptr, double white_x, double
white_y, double red_x, double red_y, double green_x, double green_y, double blue_x, double
blue_y);
void png_set_cHRM_fixed (png_structp png_ptr, png_infop info_ptr, png_uint_32 white_x,
png_uint_32 white_y, png_uint_32 red_x, png_uint_32 red_y, png_uint_32 green_x,
png_uint_32 green_y, png_uint_32 blue_x, png_uint_32 blue_y);
void png_set_compression_level (png_structp png_ptr, int level);
void png_set_compression_mem_level (png_structp png_ptr, int mem_level);
void png_set_compression_method (png_structp png_ptr, int method);
void png_set_compression_strategy (png_structp png_ptr, int strategy);
void png_set_compression_window_bits (png_structp png_ptr, int window_bits);
void png_set_crc_action (png_structp png_ptr, int crit_action, int ancil_action);
void png_set_dither (png_structp png_ptr, png_colorp palette, int num_palette, int maxi-
mum_colors, png_uint_16p histogram, int full_dither);
void png_set_error_fn (png_structp png_ptr, png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warning_fn);
void png_set_expand (png_structp png_ptr);
void png_set_expand_gray_1_2_4_to_8(png_structp png_ptr);
void png_set_filler (png_structp png_ptr, png_uint_32 filler, int flags);
void png_set_filter (png_structp png_ptr, int method, int filters);
void png_set_filter_heuristics (png_structp png_ptr, int heuristic_method, int
num_weights, png_doublep filter_weights, png_doublep filter_costs);
void png_set_flush (png_structp png_ptr, int nrows);
void png_set_gamma (png_structp png_ptr, double screen_gamma, double default_file_gamma);
void png_set_gAMA (png_structp png_ptr, png_infop info_ptr, double file_gamma);
void png_set_gAMA_fixed (png_structp png_ptr, png_infop info_ptr, png_uint_32 file_gamma);
void png_set_gray_1_2_4_to_8(png_structp png_ptr);
void png_set_gray_to_rgb (png_structp png_ptr);
void png_set_hIST (png_structp png_ptr, png_infop info_ptr, png_uint_16p hist);
void png_set_iCCP (png_structp png_ptr, png_infop info_ptr, png_charp name, int compres-
sion_type, png_charp profile, png_uint_32 proflen);
int png_set_interlace_handling (png_structp png_ptr);
void png_set_invalid (png_structp png_ptr, png_infop info_ptr, int mask);
void png_set_invert_alpha (png_structp png_ptr);
void png_set_invert_mono (png_structp png_ptr);
void png_set_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32 width, png_uint_32
height, int bit_depth, int color_type, int interlace_type, int compression_type, int fil-
ter_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_pro-
gressive_info_ptr info_fn, png_progressive_row_ptr row_fn, png_progressive_end_ptr
end_fn);
void png_set_PLTE (png_structp png_ptr, png_infop info_ptr, png_colorp palette, int
num_palette);
void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr read_data_fn);
void png_set_read_status_fn (png_structp png_ptr, png_read_status_ptr read_row_fn);
void png_set_read_user_transform_fn (png_structp png_ptr, png_user_transform_ptr
read_user_transform_fn);
void png_set_rgb_to_gray (png_structp png_ptr, int error_action, double red, double
green);
void png_set_rgb_to_gray_fixed (png_structp png_ptr, int error_action png_fixed_point red,
png_fixed_point green);
void png_set_rows (png_structp png_ptr, png_infop info_ptr, png_bytepp row_pointers);
void png_set_sBIT (png_structp png_ptr, png_infop info_ptr, png_color_8p sig_bit);
void png_set_sCAL (png_structp png_ptr, png_infop info_ptr, png_charp unit, double width,
double height);
void png_set_shift (png_structp png_ptr, png_color_8p true_bits);
void png_set_sig_bytes (png_structp png_ptr, int num_bytes);
void png_set_sPLT (png_structp png_ptr, png_infop info_ptr, png_spalette_p splt_ptr, int
num_spalettes);
void png_set_sRGB (png_structp png_ptr, png_infop info_ptr, int intent);
void png_set_sRGB_gAMA_and_cHRM (png_structp png_ptr, png_infop info_ptr, int intent);
void png_set_strip_16 (png_structp png_ptr);
void png_set_strip_alpha (png_structp png_ptr);
void png_set_swap (png_structp png_ptr);
void png_set_swap_alpha (png_structp png_ptr);
void png_set_text (png_structp png_ptr, png_infop info_ptr, png_textp text_ptr, int
num_text);
void png_set_tIME (png_structp png_ptr, png_infop info_ptr, png_timep mod_time);
void png_set_tRNS (png_structp png_ptr, png_infop info_ptr, png_bytep trans, int
num_trans, png_color_16p trans_values);
void png_set_tRNS_to_alpha(png_structp png_ptr);
png_uint_32 png_set_unknown_chunks (png_structp png_ptr, png_infop info_ptr,
png_unknown_chunkp unknowns, int num, int location);
void png_set_unknown_chunk_location(png_structp png_ptr, png_infop info_ptr, int chunk,
int location);
void png_set_read_user_chunk_fn (png_structp png_ptr, png_voidp user_chunk_ptr,
png_user_chunk_ptr read_user_chunk_fn);
void png_set_user_limits (png_structp png_ptr, png_uint_32 user_width_max, png_uint_32
user_height_max);
void png_set_user_transform_info (png_structp png_ptr, png_voidp user_transform_ptr, int
user_transform_depth, int user_transform_channels);
void png_set_write_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr write_data_fn,
png_flush_ptr output_flush_fn);
void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr write_row_fn);
void png_set_write_user_transform_fn (png_structp png_ptr, png_user_transform_ptr
write_user_transform_fn);
void png_set_compression_buffer_size(png_structp png_ptr, png_uint_32 size);
int png_sig_cmp (png_bytep sig, png_size_t start, png_size_t num_to_check);
void png_start_read_image (png_structp png_ptr);
void png_warning (png_structp png_ptr, png_const_charp message);
void png_write_chunk (png_structp png_ptr, png_bytep chunk_name, png_bytep data,
png_size_t length);
void png_write_chunk_data (png_structp png_ptr, png_bytep data, png_size_t length);
void png_write_chunk_end (png_structp png_ptr);
void png_write_chunk_start (png_structp png_ptr, png_bytep chunk_name, png_uint_32
length);
void png_write_destroy (png_structp png_ptr);
void png_write_end (png_structp png_ptr, png_infop info_ptr);
void png_write_flush (png_structp png_ptr);
void png_write_image (png_structp png_ptr, png_bytepp image);
DEPRECATED: void png_write_init (png_structp png_ptr);
DEPRECATED: void png_write_init_2 (png_structpp ptr_ptr, png_const_charp user_png_ver,
png_size_t png_struct_size, png_size_t png_info_size);
void png_write_info (png_structp png_ptr, png_infop info_ptr);
void png_write_info_before_PLTE (png_structp png_ptr, png_infop info_ptr);
void png_write_png (png_structp png_ptr, png_infop info_ptr, int transforms, png_voidp
params);
void png_write_row (png_structp png_ptr, png_bytep row);
void png_write_rows (png_structp png_ptr, png_bytepp row, png_uint_32 num_rows);
voidpf png_zalloc (voidpf png_ptr, uInt items, uInt size);
void png_zfree (voidpf png_ptr, voidpf ptr);
DESCRIPTION
The libpng library supports encoding, decoding, and various manipulations of the Portable
Network Graphics (PNG) format image files. It uses the zlib(3) compression library. 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 1.2.27 - April 29, 2008
Updated and distributed by Glenn Randers-Pehrson
<glennrp at users.sourceforge.net>
Copyright (c) 1998-2008 Glenn Randers-Pehrson
For conditions of distribution and use, see copyright
notice in png.h.
Based on:
libpng versions 0.97, January 1998, through 1.2.27 - April 29, 2008
Updated and distributed by Glenn Randers-Pehrson
Copyright (c) 1998-2008 Glenn Randers-Pehrson
libpng 1.0 beta 6 version 0.96 May 28, 1997
Updated and distributed by Andreas Dilger
Copyright (c) 1996, 1997 Andreas Dilger
libpng 1.0 beta 2 - version 0.88 January 26, 1996
For conditions of distribution and use, see copyright
notice in png.h. Copyright (c) 1995, 1996 Guy Eric
Schalnat, Group 42, Inc.
Updated/rewritten per request in the libpng FAQ
Copyright (c) 1995, 1996 Frank J. T. Wojcik
December 18, 1995 & January 20, 1996
I. Introduction
This file describes how to use and modify the PNG reference library (known as libpng) for
your own use. There are five sections to this file: introduction, structures, reading,
writing, and modification and configuration notes for various special platforms. In addi-
tion 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 Recommenda-
tion and as an ISO Standard (ISO/IEC 15948:2003 (E)) at <http://www.w3.org/TR/2003/REC-
PNG-20031110/ The W3C and ISO documents have identical technical content.
The PNG-1.2 specification is available at <http://www.libpng.org/pub/png/documents/>. It
is technically equivalent to the PNG specification (second edition) but has some addi-
tional material.
The PNG-1.0 specification is available as RFC 2083 <http://www.libpng.org/pub/png/docu-
ments/> and as a W3C Recommendation <http://www.w3.org/TR/REC.png.html>.
Some additional chunks are described in the special-purpose public chunks documents at
<http://www.libpng.org/pub/png/documents/>.
Other information about PNG, and the latest version of libpng, can be found at the PNG
home page, <http://www.libpng.org/pub/png/>.
Most users will not have to modify the library significantly; advanced users may want to
modify it more. All attempts were made to make it as complete as possible, while keeping
the code easy to understand. Currently, this library only supports C. Support for other
languages is being considered.
Libpng has been designed to handle multiple sessions at one time, to be easily modifiable,
to be portable to the vast majority of machines (ANSI, K&R, 16-, 32-, and 64-bit) avail-
able, and to be easy to use. The ultimate goal of libpng is to promote the acceptance of
the PNG file format in whatever way possible. While there is still work to be done (see
the TODO file), libpng should cover the majority of the needs of its users.
Libpng uses zlib for its compression and decompression of PNG files. Further information
about zlib, and the latest version of zlib, can be found at the zlib home page,
<http://www.info-zip.org/pub/infozip/zlib/>. The zlib compression utility is a general
purpose utility that is useful for more than PNG files, and can be used without libpng.
See the documentation delivered with zlib for more details. You can usually find the
source files for the zlib utility wherever you find the libpng source files.
Libpng is thread safe, provided the threads are using different instances of the struc-
tures. Each thread should have its own png_struct and png_info instances, and thus its
own image. Libpng does not protect itself against two threads using the same instance of
a structure.
II. Structures
There are two main structures that are important to libpng, png_struct and png_info. The
first, png_struct, is an internal structure that will not, for the most part, be used by a
user except as the first variable passed to every libpng function call.
The png_info structure is designed to provide information about the PNG file. At one
time, the fields of png_info were intended to be directly accessible to the user. 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 applica-
tions, but it is suggested that applications use the new interfaces if at all possible.
Applications that do make direct access to the members of png_struct (except for
png_ptr->jmpbuf) must be recompiled whenever the library is updated, and applications that
make direct access to the members of png_info must be recompiled if they were compiled or
loaded with libpng version 1.0.6, in which the members were in a different order. In ver-
sion 1.0.7, the members of the png_info structure reverted to the old order, as they were
in versions 0.97c through 1.0.5. Starting with version 2.0.0, both structures are going
to be hidden, and the contents of the structures will only be accessible through the
png_get/png_set functions.
The png.h header file is an invaluable reference for programming with libpng. And while
I'm on the topic, make sure you include the libpng header file:
#include <png.h>
III. Reading
We'll now walk you through the possible functions to call when reading in a PNG file
sequentially, briefly explaining the syntax and purpose of each one. See example.c and
png.h for more detail. While progressive reading is covered in the next section, you will
still need some of the functions discussed in this section to read a PNG file.
Setup
You will want to do the I/O initialization(*) before you get into libpng, so if it doesn't
work, you don't have much to undo. Of course, you will also want to insure that you are,
in fact, dealing with a PNG file. Libpng provides a simple check to see if a file is a
PNG file. To use it, pass in the first 1 to 8 bytes of the file to the function
png_sig_cmp(), and it will return 0 if the bytes match the corresponding bytes of the PNG
signature, or nonzero otherwise. Of course, the more bytes you pass in, the greater the
accuracy of the prediction.
If you are intending to keep the file pointer open for use in libpng, you must ensure you
don't read more than 8 bytes from the beginning of the file, and you also have to make a
call to png_set_sig_bytes_read() with the number of bytes you read from the beginning.
Libpng will then only check the bytes (if any) that your program didn't read.
(*): If you are not using the standard I/O functions, you will need to replace them with
custom functions. See the discussion under Customizing libpng.
FILE *fp = fopen(file_name, "rb");
if (!fp)
{
return (ERROR);
}
fread(header, 1, number, fp);
is_png = !png_sig_cmp(header, 0, number);
if (!is_png)
{
return (NOT_PNG);
}
Next, png_struct and png_info need to be allocated and initialized. In order to ensure
that the size of these structures is correct even with a dynamically linked libpng, there
are functions to initialize and allocate the structures. We also pass the library ver-
sion, optional pointers to error handling functions, and a pointer to a data struct for
use by the error functions, if necessary (the pointer and functions can be NULL if the
default error handlers are to be used). See the section on Changes to Libpng below
regarding the old initialization functions. The structure allocation functions quietly
return NULL if they fail to create the structure, so your application should check for
that.
png_structp png_ptr = png_create_read_struct
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
return (ERROR);
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_read_struct(&png_ptr,
(png_infopp)NULL, (png_infopp)NULL);
return (ERROR);
}
png_infop end_info = png_create_info_struct(png_ptr);
if (!end_info)
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
return (ERROR);
}
If you want to use your own memory allocation routines, define PNG_USER_MEM_SUPPORTED and
use png_create_read_struct_2() instead of png_create_read_struct():
png_structp png_ptr = png_create_read_struct_2
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn, (png_voidp)
user_mem_ptr, user_malloc_fn, user_free_fn);
The error handling routines passed to png_create_read_struct() and the memory alloc/free
routines passed to png_create_struct_2() are only necessary if you are not using the
libpng supplied error handling and memory alloc/free functions.
When libpng encounters an error, it expects to longjmp back to your routine. Therefore,
you will need to call setjmp and pass your png_jmpbuf(png_ptr). If you read the file from
different routines, you will need to update the jmpbuf field every time you enter a new
routine that will call a png_*() function.
See your documentation of setjmp/longjmp for your compiler for more information on
setjmp/longjmp. See the discussion on libpng error handling in the Customizing Libpng
section below for more information on the libpng error handling. If an error occurs, and
libpng longjmp's back to your setjmp, you will want to call png_destroy_read_struct() to
free any memory.
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_read_struct(&png_ptr, &info_ptr,
&end_info);
fclose(fp);
return (ERROR);
}
If you would rather avoid the complexity of setjmp/longjmp issues, you can compile libpng
with PNG_SETJMP_NOT_SUPPORTED, in which case errors will result in a call to PNG_ABORT()
which defaults to abort().
Now you need to set up the input code. The default for libpng is to use the C function
fread(). If you use this, you will need to pass a valid FILE * in the function
png_init_io(). Be sure that the file is opened in binary mode. If you wish to handle
reading data in another way, you need not call the png_init_io() function, but you must
then implement the libpng I/O methods discussed in the Customizing Libpng section below.
png_init_io(png_ptr, fp);
If you had previously opened the file and read any of the signature from the beginning in
order to see if this was a PNG file, you need to let libpng know that there are some bytes
missing from the start of the file.
png_set_sig_bytes(png_ptr, number);
Setting up callback code
You can set up a callback function to handle any unknown chunks in the input stream. You
must supply the function
read_chunk_callback(png_ptr ptr,
png_unknown_chunkp chunk);
{
/* The unknown chunk structure contains your
chunk data, along with similar data for any other
unknown chunks: */
png_byte name[5];
png_byte *data;
png_size_t size;
/* Note that libpng has already taken care of
the CRC handling */
/* put your code here. Search for your chunk in the
unknown chunk structure, process it, and return one
of the following: */
return (-n); /* chunk had an error */
return (0); /* did not recognize */
return (n); /* success */
}
(You can give your function another name that you like instead of "read_chunk_callback")
To inform libpng about your function, use
png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
read_chunk_callback);
This names not only the callback function, but also a user pointer that you can retrieve
with
png_get_user_chunk_ptr(png_ptr);
If you call the png_set_read_user_chunk_fn() function, then all unknown chunks will be
saved when read, in case your callback function will need one or more of them. This
behavior can be changed with the png_set_keep_unknown_chunks() function, described below.
At this point, you can set up a callback function that will be called after each row has
been read, which you can use to control a progress meter or the like. It's demonstrated
in pngtest.c. You must supply a function
void read_row_callback(png_ptr ptr, png_uint_32 row,
int pass);
{
/* put your code here */
}
(You can give it another name that you like instead of "read_row_callback")
To inform libpng about your function, use
png_set_read_status_fn(png_ptr, read_row_callback);
Width and height limits
The PNG specification allows the width and height of an image to be as large as 2^31-1
(0x7fffffff), or about 2.147 billion rows and columns. Since very few applications really
need to process such large images, we have imposed an arbitrary 1-million limit on rows
and columns. Larger images will be rejected immediately with a png_error() call. If you
wish to override this limit, you can use
png_set_user_limits(png_ptr, width_max, height_max);
to set your own limits, or use width_max = height_max = 0x7fffffffL to allow all valid
dimensions (libpng may reject some very large images anyway because of potential buffer
overflow conditions).
You should put this statement after you create the PNG structure and before calling
png_read_info(), png_read_png(), or png_process_data(). If you need to retrieve the lim-
its that are being applied, use
width_max = png_get_user_width_max(png_ptr);
height_max = png_get_user_height_max(png_ptr);
Unknown-chunk handling
Now you get to set the way the library processes unknown chunks in the input PNG stream.
Both known and unknown chunks will be read. Normal behavior is that known chunks will be
parsed into information in various info_ptr members while unknown chunks will be dis-
carded. To change this, you can call:
png_set_keep_unknown_chunks(png_ptr, keep,
chunk_list, num_chunks);
keep - 0: default unknown chunk handling
1: ignore; do not keep
2: keep only if safe-to-copy
3: keep even if unsafe-to-copy
You can use these definitions:
PNG_HANDLE_CHUNK_AS_DEFAULT 0
PNG_HANDLE_CHUNK_NEVER 1
PNG_HANDLE_CHUNK_IF_SAFE 2
PNG_HANDLE_CHUNK_ALWAYS 3
chunk_list - list of chunks affected (a byte string,
five bytes per chunk, NULL or ' ' if
num_chunks is 0)
num_chunks - number of chunks affected; if 0, all
unknown chunks are affected. If nonzero,
only the chunks in the list are affected
Unknown chunks declared in this way will be saved as raw data onto a list of
png_unknown_chunk structures. If a chunk that is normally known to libpng is named in the
list, it will be handled as unknown, according to the "keep" directive. If a chunk is
named in successive instances of png_set_keep_unknown_chunks(), the final instance will
take precedence. The IHDR and IEND chunks should not be named in chunk_list; if they are,
libpng will process them normally anyway.
Here is an example of the usage of png_set_keep_unknown_chunks(), where the private "vpAg"
chunk will later be processed by a user chunk callback function:
png_byte vpAg[5]={118, 112, 65, 103, (png_byte) ' '};
#if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
png_byte unused_chunks[]=
{
104, 73, 83, 84, (png_byte) ' ', /* hIST */
105, 84, 88, 116, (png_byte) ' ', /* iTXt */
112, 67, 65, 76, (png_byte) ' ', /* pCAL */
115, 67, 65, 76, (png_byte) ' ', /* sCAL */
115, 80, 76, 84, (png_byte) ' ', /* sPLT */
116, 73, 77, 69, (png_byte) ' ', /* tIME */
};
#endif
...
#if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
/* ignore all unknown chunks: */
png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0);
/* except for vpAg: */
png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
/* also ignore unused known chunks: */
png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
(int)sizeof(unused_chunks)/5);
#endif
The high-level read interface
At this point there are two ways to proceed; through the high-level read interface, or
through a sequence of low-level read operations. You can use the high-level interface if
(a) you are willing to read the entire image into memory, and (b) the input transforma-
tions you want to do are limited to the following set:
PNG_TRANSFORM_IDENTITY No transformation
PNG_TRANSFORM_STRIP_16 Strip 16-bit samples to
8 bits
PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel
PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit
samples to bytes
PNG_TRANSFORM_PACKSWAP Change order of packed
pixels to LSB first
PNG_TRANSFORM_EXPAND Perform set_expand()
PNG_TRANSFORM_INVERT_MONO Invert monochrome images
PNG_TRANSFORM_SHIFT Normalize pixels to the
sBIT depth
PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
to BGRA
PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
to AG
PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
to transparency
PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
(This excludes setting a background color, doing gamma transformation, dithering, and set-
ting filler.) If this is the case, simply do this:
png_read_png(png_ptr, info_ptr, png_transforms, NULL)
where png_transforms is an integer containing the bitwise OR of some set of transformation
flags. This call is equivalent to png_read_info(), followed the set of transformations
indicated by the transform mask, then png_read_image(), and finally png_read_end().
(The final parameter of this call is not yet used. Someday it might point to transforma-
tion parameters required by some future input transform.)
You must use png_transforms and not call any png_set_transform() functions when you use
png_read_png().
After you have called png_read_png(), you can retrieve the image data with
row_pointers = png_get_rows(png_ptr, info_ptr);
where row_pointers is an array of pointers to the pixel data for each row:
png_bytep row_pointers[height];
If you know your image size and pixel size ahead of time, you can allocate row_pointers
prior to calling png_read_png() with
if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
png_error (png_ptr,
"Image is too tall to process in memory");
if (width > PNG_UINT_32_MAX/pixel_size)
png_error (png_ptr,
"Image is too wide to process in memory");
row_pointers = png_malloc(png_ptr,
height*png_sizeof(png_bytep));
for (int i=0; i<height, i++)
row_pointers[i]=png_malloc(png_ptr,
width*pixel_size);
png_set_rows(png_ptr, info_ptr, &row_pointers);
Alternatively you could allocate your image in one big block and define row_pointers[i] to
point into the proper places in your block.
If you use png_set_rows(), the application is responsible for freeing row_pointers (and
row_pointers[i], if they were separately allocated).
If you don't allocate row_pointers ahead of time, png_read_png() will do it, and it'll be
free'ed when you call png_destroy_*().
The low-level read interface
If you are going the low-level route, you are now ready to read all the file information
up to the actual image data. You do this with a call to png_read_info().
png_read_info(png_ptr, info_ptr);
This will process all chunks up to but not including the image data.
Querying the info structure
Functions are used to get the information from the info_ptr once it has been read. Note
that these fields may not be completely filled in until png_read_end() has read the chunk
data following the image.
png_get_IHDR(png_ptr, info_ptr, &width, &height,
&bit_depth, &color_type, &interlace_type,
&compression_type, &filter_method);
width - holds the width of the image
in pixels (up to 2^31).
height - holds the height of the image
in pixels (up to 2^31).
bit_depth - holds the bit depth of one of the
image channels. (valid values are
1, 2, 4, 8, 16 and depend also on
the color_type. See also
significant bits (sBIT) below).
color_type - describes which color/alpha channels
are present.
PNG_COLOR_TYPE_GRAY
(bit depths 1, 2, 4, 8, 16)
PNG_COLOR_TYPE_GRAY_ALPHA
(bit depths 8, 16)
PNG_COLOR_TYPE_PALETTE
(bit depths 1, 2, 4, 8)
PNG_COLOR_TYPE_RGB
(bit_depths 8, 16)
PNG_COLOR_TYPE_RGB_ALPHA
(bit_depths 8, 16)
PNG_COLOR_MASK_PALETTE
PNG_COLOR_MASK_COLOR
PNG_COLOR_MASK_ALPHA
filter_method - (must be PNG_FILTER_TYPE_BASE
for PNG 1.0, and can also be
PNG_INTRAPIXEL_DIFFERENCING if
the PNG datastream is embedded in
a MNG-1.0 datastream)
compression_type - (must be PNG_COMPRESSION_TYPE_BASE
for PNG 1.0)
interlace_type - (PNG_INTERLACE_NONE or
PNG_INTERLACE_ADAM7)
Any or all of interlace_type, compression_type, of
filter_method can be NULL if you are
not interested in their values.
channels = png_get_channels(png_ptr, info_ptr);
channels - number of channels of info for the
color type (valid values are 1 (GRAY,
PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
4 (RGB_ALPHA or RGB + filler byte))
rowbytes = png_get_rowbytes(png_ptr, info_ptr);
rowbytes - number of bytes needed to hold a row
signature = png_get_signature(png_ptr, info_ptr);
signature - holds the signature read from the
file (if any). The data is kept in
the same offset it would be if the
whole signature were read (i.e. if an
application had already read in 4
bytes of signature before starting
libpng, the remaining 4 bytes would
be in signature[4] through signature[7]
(see png_set_sig_bytes())).
width = png_get_image_width(png_ptr,
info_ptr);
height = png_get_image_height(png_ptr,
info_ptr);
bit_depth = png_get_bit_depth(png_ptr,
info_ptr);
color_type = png_get_color_type(png_ptr,
info_ptr);
filter_method = png_get_filter_type(png_ptr,
info_ptr);
compression_type = png_get_compression_type(png_ptr,
info_ptr);
interlace_type = png_get_interlace_type(png_ptr,
info_ptr);
These are also important, but their validity depends on whether the chunk has been read.
The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr,
info_ptr, ...) functions return non-zero if the data has been read, or zero if it is miss-
ing. The parameters to the png_get_<chunk> are set directly if they are simple data
types, or a pointer into the info_ptr is returned for any complex types.
png_get_PLTE(png_ptr, info_ptr, &palette,
&num_palette);
palette - the palette for the file
(array of png_color)
num_palette - number of entries in the palette
png_get_gAMA(png_ptr, info_ptr, &gamma);
gamma - the gamma the file is written
at (PNG_INFO_gAMA)
png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
srgb_intent - the rendering intent (PNG_INFO_sRGB)
The presence of the sRGB chunk
means that the pixel data is in the
sRGB color space. This chunk also
implies specific values of gAMA and
cHRM.
png_get_iCCP(png_ptr, info_ptr, &name,
&compression_type, &profile, &proflen);
name - The profile name.
compression - The compression type; always
PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
You may give NULL to this argument to
ignore it.
profile - International Color Consortium color
profile data. May contain NULs.
proflen - length of profile data in bytes.
png_get_sBIT(png_ptr, info_ptr, &sig_bit);
sig_bit - the number of significant bits for
(PNG_INFO_sBIT) each of the gray,
red, green, and blue channels,
whichever are appropriate for the
given color type (png_color_16)
png_get_tRNS(png_ptr, info_ptr, &trans, &num_trans,
&trans_values);
trans - array of transparent entries for
palette (PNG_INFO_tRNS)
trans_values - graylevel or color sample values of
the single transparent color for
non-paletted images (PNG_INFO_tRNS)
num_trans - number of transparent entries
(PNG_INFO_tRNS)
png_get_hIST(png_ptr, info_ptr, &hist);
(PNG_INFO_hIST)
hist - histogram of palette (array of
png_uint_16)
png_get_tIME(png_ptr, info_ptr, &mod_time);
mod_time - time image was last modified
(PNG_VALID_tIME)
png_get_bKGD(png_ptr, info_ptr, &background);
background - background color (PNG_VALID_bKGD)
valid 16-bit red, green and blue
values, regardless of color_type
num_comments = png_get_text(png_ptr, info_ptr,
&text_ptr, &num_text);
num_comments - number of comments
text_ptr - array of png_text holding image
comments
text_ptr[i].compression - type of compression used
on "text" PNG_TEXT_COMPRESSION_NONE
PNG_TEXT_COMPRESSION_zTXt
PNG_ITXT_COMPRESSION_NONE
PNG_ITXT_COMPRESSION_zTXt
text_ptr[i].key - keyword for comment. Must contain
1-79 characters.
text_ptr[i].text - text comments for current
keyword. Can be empty.
text_ptr[i].text_length - length of text string,
after decompression, 0 for iTXt
text_ptr[i].itxt_length - length of itxt string,
after decompression, 0 for tEXt/zTXt
text_ptr[i].lang - language of comment (empty
string for unknown).
text_ptr[i].lang_key - keyword in UTF-8
(empty string for unknown).
num_text - number of comments (same as
num_comments; you can put NULL here
to avoid the duplication)
Note while png_set_text() will accept text, language,
and translated keywords that can be NULL pointers, the
structure returned by png_get_text will always contain
regular zero-terminated C strings. They might be
empty strings but they will never be NULL pointers.
num_spalettes = png_get_sPLT(png_ptr, info_ptr,
&palette_ptr);
palette_ptr - array of palette structures holding
contents of one or more sPLT chunks
read.
num_spalettes - number of sPLT chunks read.
png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
&unit_type);
offset_x - positive offset from the left edge
of the screen
offset_y - positive offset from the top edge
of the screen
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
&unit_type);
res_x - pixels/unit physical resolution in
x direction
res_y - pixels/unit physical resolution in
x direction
unit_type - PNG_RESOLUTION_UNKNOWN,
PNG_RESOLUTION_METER
png_get_sCAL(png_ptr, info_ptr, &unit, &width,
&height)
unit - physical scale units (an integer)
width - width of a pixel in physical scale units
height - height of a pixel in physical scale units
(width and height are doubles)
png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
&height)
unit - physical scale units (an integer)
width - width of a pixel in physical scale units
height - height of a pixel in physical scale units
(width and height are strings like "2.54")
num_unknown_chunks = png_get_unknown_chunks(png_ptr,
info_ptr, &unknowns)
unknowns - array of png_unknown_chunk
structures holding unknown chunks
unknowns[i].name - name of unknown chunk
unknowns[i].data - data of unknown chunk
unknowns[i].size - size of unknown chunk's data
unknowns[i].location - position of chunk in file
The value of "i" corresponds to the order in which the
chunks were read from the PNG file or inserted with the
png_set_unknown_chunks() function.
The data from the pHYs chunk can be retrieved in several convenient forms:
res_x = png_get_x_pixels_per_meter(png_ptr,
info_ptr)
res_y = png_get_y_pixels_per_meter(png_ptr,
info_ptr)
res_x_and_y = png_get_pixels_per_meter(png_ptr,
info_ptr)
res_x = png_get_x_pixels_per_inch(png_ptr,
info_ptr)
res_y = png_get_y_pixels_per_inch(png_ptr,
info_ptr)
res_x_and_y = png_get_pixels_per_inch(png_ptr,
info_ptr)
aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
info_ptr)
(Each of these returns 0 [signifying "unknown"] if
the data is not present or if res_x is 0;
res_x_and_y is 0 if res_x != res_y)
The data from the oFFs chunk can be retrieved in several convenient forms:
x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
y_offset = png_get_y_offset_inches(png_ptr, info_ptr);
(Each of these returns 0 [signifying "unknown" if both
x and y are 0] if the data is not present or if the
chunk is present but the unit is the pixel)
For more information, see the png_info definition in png.h and the PNG specification for
chunk contents. Be careful with trusting rowbytes, as some of the transformations could
increase the space needed to hold a row (expand, filler, gray_to_rgb, etc.). See
png_read_update_info(), below.
A quick word about text_ptr and num_text. PNG stores comments in keyword/text pairs, one
pair per chunk, with no limit on the number of text chunks, and a 2^31 byte limit on their
size. While there are suggested keywords, there is no requirement to restrict the use to
these strings. It is strongly suggested that keywords and text be sensible to humans
(that's the point), so don't use abbreviations. Non-printing symbols are not allowed.
See the PNG specification for more details. There is also no requirement to have text
after the keyword.
Keywords should be limited to 79 Latin-1 characters without leading or trailing spaces,
but non-consecutive spaces are allowed within the keyword. It is possible to have the
same keyword any number of times. The text_ptr is an array of png_text structures, each
holding a pointer to a language string, a pointer to a keyword and a pointer to a text
string. The text string, language code, and translated keyword may be empty or NULL
pointers. The keyword/text pairs are put into the array in the order that they are
received. However, some or all of the text chunks may be after the image, so, to make
sure you have read all the text chunks, don't mess with these until after you read the
stuff after the image. This will be mentioned again below in the discussion that goes
with png_read_end().
Input transformations
After you've read the header information, you can set up the library to handle any special
transformations of the image data. The various ways to transform the data will be
described in the order that they should occur. This is important, as some of these change
the color type and/or bit depth of the data, and some others only work on certain color
types and bit depths. Even though each transformation checks to see if it has data that
it can do something with, you should make sure to only enable a transformation if it will
be valid for the data. For example, don't swap red and blue on grayscale data.
The colors used for the background and transparency values should be supplied in the same
format/depth as the current image data. They are stored in the same format/depth as the
image data in a bKGD or tRNS chunk, so this is what libpng expects for this data. The
colors are transformed to keep in sync with the image data when an application calls the
png_read_update_info() routine (see below).
Data will be decoded into the supplied row buffers packed into bytes unless the library
has been told to transform it into another format. For example, 4 bit/pixel paletted or
grayscale data will be returned 2 pixels/byte with the leftmost pixel in the high-order
bits of the byte, unless png_set_packing() is called. 8-bit RGB data will be stored in
RGB RGB RGB format unless png_set_filler() or png_set_add_alpha() is called to insert
filler bytes, either before or after each RGB triplet. 16-bit RGB data will be returned
RRGGBB RRGGBB, with the most significant byte of the color value first, unless
png_set_strip_16() is called to transform it to regular RGB RGB triplets, or
png_set_filler() or png_set_add alpha() is called to insert filler bytes, either before or
after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can be modified with
png_set_filler(), png_set_add_alpha(), or png_set_strip_16().
The following code transforms grayscale images of less than 8 to 8 bits, changes paletted
images to RGB, and adds a full alpha channel if there is transparency information in a
tRNS chunk. This is most useful on grayscale images with bit depths of 2 or 4 or if there
is a multiple-image viewing application that wishes to treat all images in the same way.
if (color_type == PNG_COLOR_TYPE_PALETTE)
png_set_palette_to_rgb(png_ptr);
if (color_type == PNG_COLOR_TYPE_GRAY &&
bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);
if (png_get_valid(png_ptr, info_ptr,
PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
These three functions are actually aliases for png_set_expand(), added in libpng version
1.0.4, with the function names expanded to improve code readability. In some future ver-
sion they may actually do different things.
As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was added. It expands the
sample depth without changing tRNS to alpha. At the same time, png_set_gray_1_2_4_to_8()
was deprecated, and it will be removed from a future version.
PNG can have files with 16 bits per channel. If you only can handle 8 bits per channel,
this will strip the pixels down to 8 bit.
if (bit_depth == 16)
png_set_strip_16(png_ptr);
If, for some reason, you don't need the alpha channel on an image, and you want to remove
it rather than combining it with the background (but the image author certainly had in
mind that you *would* combine it with the background, so that's what you should probably
do):
if (color_type & PNG_COLOR_MASK_ALPHA)
png_set_strip_alpha(png_ptr);
In PNG files, the alpha channel in an image is the level of opacity. If you need the
alpha channel in an image to be the level of transparency instead of opacity, you can
invert the alpha channel (or the tRNS chunk data) after it's read, so that 0 is fully
opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit images) is fully trans-
parent, with
png_set_invert_alpha(png_ptr);
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as they can, resulting
in, for example, 8 pixels per byte for 1 bit files. This code expands to 1 pixel per byte
without changing the values of the pixels:
if (bit_depth < 8)
png_set_packing(png_ptr);
PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels stored in a PNG
image have been "scaled" or "shifted" up to the next higher possible bit depth (e.g. from
5 bits/sample in the range [0,31] to 8 bits/sample in the range [0, 255]). However, it is
also possible to convert the PNG pixel data back to the original bit depth of the image.
This call reduces the pixels back down to the original bit depth:
png_color_8p sig_bit;
if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
png_set_shift(png_ptr, sig_bit);
PNG files store 3-color pixels in red, green, blue order. This code changes the storage
of the pixels to blue, green, red:
if (color_type == PNG_COLOR_TYPE_RGB ||
color_type == PNG_COLOR_TYPE_RGB_ALPHA)
png_set_bgr(png_ptr);
PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them into 4 or 8
bytes for windowing systems that need them in this format:
if (color_type == PNG_COLOR_TYPE_RGB)
png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
where "filler" is the 8 or 16-bit number to fill with, and the location is either
PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether you want the filler before
the RGB or after. This transformation does not affect images that already have full alpha
channels. To add an opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER
which will generate RGBA pixels.
Note that png_set_filler() does not change the color type. If you want to do that, you
can add a true alpha channel with
if (color_type == PNG_COLOR_TYPE_RGB ||
color_type == PNG_COLOR_TYPE_GRAY)
png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
where "filler" contains the alpha value to assign to each pixel. This function was added
in libpng-1.2.7.
If you are reading an image with an alpha channel, and you need the data as ARGB instead
of the normal PNG format RGBA:
if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
png_set_swap_alpha(png_ptr);
For some uses, you may want a grayscale image to be represented as RGB. This code will do
that conversion:
if (color_type == PNG_COLOR_TYPE_GRAY ||
color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
png_set_gray_to_rgb(png_ptr);
Conversely, you can convert an RGB or RGBA image to grayscale or grayscale with alpha.
if (color_type == PNG_COLOR_TYPE_RGB ||
color_type == PNG_COLOR_TYPE_RGB_ALPHA)
png_set_rgb_to_gray_fixed(png_ptr, error_action,
int red_weight, int green_weight);
error_action = 1: silently do the conversion
error_action = 2: issue a warning if the original
image has any pixel where
red != green or red != blue
error_action = 3: issue an error and abort the
conversion if the original
image has any pixel where
red != green or red != blue
red_weight: weight of red component times 100000
green_weight: weight of green component times 100000
If either weight is negative, default
weights (21268, 71514) are used.
If you have set error_action = 1 or 2, you can later check whether the image really was
gray, after processing the image rows, with the png_get_rgb_to_gray_status(png_ptr) func-
tion. It will return a png_byte that is zero if the image was gray or 1 if there were any
non-gray pixels. bKGD and sBIT data will be silently converted to grayscale, using the
green channel data, regardless of the error_action setting.
With red_weight+green_weight<=100000, the normalized graylevel is computed:
int rw = red_weight * 65536;
int gw = green_weight * 65536;
int bw = 65536 - (rw + gw);
gray = (rw*red + gw*green + bw*blue)/65536;
The default values approximate those recommended in the Charles Poynton's Color FAQ,
<http://www.inforamp.net/~poynton/> Copyright (c) 1998-01-04 Charles Poynton <poynton at
inforamp.net>
Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
Libpng approximates this with
Y = 0.21268 * R + 0.7151 * G + 0.07217 * B
which can be expressed with integers as
Y = (6969 * R + 23434 * G + 2365 * B)/32768
The calculation is done in a linear colorspace, if the image gamma is known.
If you have a grayscale and you are using png_set_expand_depth(), png_set_expand(), or
png_set_gray_to_rgb to change to truecolor or to a higher bit-depth, you must either sup-
ply the background color as a gray value at the original file bit-depth (need_expand = 1)
or else supply the background color as an RGB triplet at the final, expanded bit depth
(need_expand = 0). Similarly, if you are reading a paletted image, you must either supply
the background color as a palette index (need_expand = 1) or as an RGB triplet that may or
may not be in the palette (need_expand = 0).
png_color_16 my_background;
png_color_16p image_background;
if (png_get_bKGD(png_ptr, info_ptr, &image_background))
png_set_background(png_ptr, image_background,
PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
else
png_set_background(png_ptr, &my_background,
PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
The png_set_background() function tells libpng to composite images with alpha or simple
transparency against the supplied background color. If the PNG file contains a bKGD chunk
(PNG_INFO_bKGD valid), you may use this color, or supply another color more suitable for
the current display (e.g., the background color from a web page). You need to tell libpng
whether the color is in the gamma space of the display (PNG_BACKGROUND_GAMMA_SCREEN for
colors you supply), the file (PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk),
or one that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't know why
anyone would use this, but it's here).
To properly display PNG images on any kind of system, the application needs to know what
the display gamma is. Ideally, the user will know this, and the application will allow
them to set it. One method of allowing the user to set the display gamma separately for
each system is to check for a SCREEN_GAMMA or DISPLAY_GAMMA environment variable, which
will hopefully be correctly set.
Note that display_gamma is the overall gamma correction required to produce pleasing
results, which depends on the lighting conditions in the surrounding environment. In a
dim or brightly lit room, no compensation other than the physical gamma exponent of the
monitor is needed, while in a dark room a slightly smaller exponent is better.
double gamma, screen_gamma;
if (/* We have a user-defined screen
gamma value */)
{
screen_gamma = user_defined_screen_gamma;
}
/* One way that applications can share the same
screen gamma value */
else if ((gamma_str = getenv("SCREEN_GAMMA"))
!= NULL)
{
screen_gamma = (double)atof(gamma_str);
}
/* If we don't have another value */
else
{
screen_gamma = 2.2; /* A good guess for a
PC monitor in a bright office or a dim room */
screen_gamma = 2.0; /* A good guess for a
PC monitor in a dark room */
screen_gamma = 1.7 or 1.0; /* A good
guess for Mac systems */
}
The png_set_gamma() function handles gamma transformations of the data. Pass both the
file gamma and the current screen_gamma. If the file does not have a gamma value, you can
pass one anyway if you have an idea what it is (usually 0.45455 is a good guess for GIF
images on PCs). Note that file gammas are inverted from screen gammas. See the discus-
sions 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 col-
ors in the palette so it will fit into maximum_colors. If there is a histogram, it will
use it to make more intelligent choices when reducing the palette. If there is no his-
togram, 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 signif-
icant bits first, the way PCs store them):
if (bit_depth == 16)
png_set_swap(png_ptr);
If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you need to change the
order the pixels are packed into bytes, you can use:
if (bit_depth < 8)
png_set_packswap(png_ptr);
Finally, you can write your own transformation function if none of the existing ones meets
your needs. This is done by setting a callback with
png_set_read_user_transform_fn(png_ptr,
read_transform_fn);
You must supply the function
void read_transform_fn(png_ptr ptr, row_info_ptr
row_info, png_bytep data)
See pngtest.c for a working example. Your function will be called after all of the other
transformations have been processed.
You can also set up a pointer to a user structure for use by your callback function, and
you can inform libpng that your transform function will change the number of channels or
bit depth with the function
png_set_user_transform_info(png_ptr, user_ptr,
user_depth, user_channels);
The user's application, not libpng, is responsible for allocating and freeing any memory
required for the user structure.
You can retrieve the pointer via the function png_get_user_transform_ptr(). For example:
voidp read_user_transform_ptr =
png_get_user_transform_ptr(png_ptr);
The last thing to handle is interlacing; this is covered in detail below, but you must
call the function here if you want libpng to handle expansion of the interlaced image.
number_of_passes = png_set_interlace_handling(png_ptr);
After setting the transformations, libpng can update your png_info structure to reflect
any transformations you've requested with this call. This is most useful to update the
info structure's rowbytes field so you can use it to allocate your image memory. This
function will also update your palette with the correct screen_gamma and background if
these have been given with the calls above.
png_read_update_info(png_ptr, info_ptr);
After you call png_read_update_info(), you can allocate any memory you need to hold the
image. The row data is simply raw byte data for all forms of images. As the actual 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_inter-
lace_handling() or call this function multiple times, or any of that other stuff necessary
with png_read_rows().
png_read_image(png_ptr, row_pointers);
where row_pointers is:
png_bytep row_pointers[height];
You can point to void or char or whatever you use for pixels.
If you don't want to read in the whole image at once, you can use png_read_rows() instead.
If there is no interlacing (check interlace_type == PNG_INTERLACE_NONE), this is simple:
png_read_rows(png_ptr, row_pointers, NULL,
number_of_rows);
where row_pointers is the same as in the png_read_image() call.
If you are doing this just one row at a time, you can do this with a single row_pointer
instead of an array of row_pointers:
png_bytep row_pointer = row;
png_read_row(png_ptr, row_pointer, NULL);
If the file is interlaced (interlace_type != 0 in the IHDR chunk), things get somewhat
harder. The only current (PNG Specification version 1.2) interlacing type for PNG is
(interlace_type == PNG_INTERLACE_ADAM7) is a somewhat complicated 2D interlace scheme,
known as Adam7, that breaks down an image into seven smaller images of varying size, based
on an 8x8 grid.
libpng can fill out those images or it can give them to you "as is". If you want them
filled out, there are two ways to do that. The one mentioned in the PNG specification is
to expand each pixel to cover those pixels that have not been read yet (the "rectangle"
method). This results in a blocky image for the first pass, which gradually smooths out
as more pixels are read. The other method is the "sparkle" method, where pixels are drawn
only in their final locations, with the rest of the image remaining whatever colors they
were initialized to before the start of the read. The first method usually looks better,
but tends to be slower, as there are more pixels to put in the rows.
If you don't want libpng to handle the interlacing details, just call png_read_rows()
seven times to read in all seven images. Each of the images is a valid image by itself,
or they can all be combined on an 8x8 grid to form a single image (although if you intend
to combine them you would be far better off using the libpng interlace handling).
The first pass will return an image 1/8 as wide as the entire image (every 8th column
starting in column 0) and 1/8 as high as the original (every 8th row starting in row 0),
the second will be 1/8 as wide (starting in column 4) and 1/8 as high (also starting in
row 0). The third pass will be 1/4 as wide (every 4th pixel starting in column 0) and 1/8
as high (every 8th row starting in row 4), and the fourth pass will be 1/4 as wide and 1/4
as high (every 4th column starting in column 2, and every 4th row starting in row 0). The
fifth pass will return an image 1/2 as wide, and 1/4 as high (starting at column 0 and row
2), while the sixth pass will be 1/2 as wide and 1/2 as high as the original (starting in
column 1 and row 0). The seventh and final pass will be as wide as the original, and 1/2
as high, containing all of the odd numbered scanlines. Phew!
If you want libpng to expand the images, call this before calling png_start_read_image()
or png_read_update_info():
if (interlace_type == PNG_INTERLACE_ADAM7)
number_of_passes
= png_set_interlace_handling(png_ptr);
This will return the number of passes needed. Currently, this is seven, but may change if
another interlace type is added. This function can be called even if the file is not
interlaced, where it will return one pass.
If you are not going to display the image after each pass, but are going to wait until the
entire image is read in, use the sparkle effect. This effect is faster and the end result
of either method is exactly the same. If you are planning on displaying the image after
each pass, the "rectangle" effect is generally considered the better looking one.
If you only want the "sparkle" effect, just call png_read_rows() as normal, with the third
parameter NULL. Make sure you make pass over the image number_of_passes times, and you
don't change the data in the rows between calls. You can change the locations of the
data, just not the data. Each pass only writes the pixels appropriate for that pass, and
assumes the data from previous passes is still valid.
png_read_rows(png_ptr, row_pointers, NULL,
number_of_rows);
If you only want the first effect (the rectangles), do the same as before except pass the
row buffer in the third parameter, and leave the second parameter NULL.
png_read_rows(png_ptr, NULL, row_pointers,
number_of_rows);
Finishing a sequential read
After you are finished reading the image through the low-level interface, you can finish
reading the file. If you are interested in comments or time, which may be stored either
before or after the image data, you should pass the separate png_info struct if you want
to keep the comments from before and after the image separate. If you are not interested,
you can pass NULL.
png_read_end(png_ptr, end_info);
When you are done, you can free all memory allocated by libpng like this:
png_destroy_read_struct(&png_ptr, &info_ptr,
&end_info);
It is also possible to individually free the info_ptr members that point to libpng-allo-
cated storage with the following function:
png_free_data(png_ptr, info_ptr, mask, seq)
mask - identifies data to be freed, a mask
containing the bitwise OR of one or
more of
PNG_FREE_PLTE, PNG_FREE_TRNS,
PNG_FREE_HIST, PNG_FREE_ICCP,
PNG_FREE_PCAL, PNG_FREE_ROWS,
PNG_FREE_SCAL, PNG_FREE_SPLT,
PNG_FREE_TEXT, PNG_FREE_UNKN,
or simply PNG_FREE_ALL
seq - sequence number of item to be freed
(-1 for all items)
This function may be safely called when the relevant storage has already been freed, or
has not yet been allocated, or was allocated by the user and not by libpng, and will in
those cases do nothing. The "seq" parameter is ignored if only one item of the selected
data type, such as PLTE, is allowed. If "seq" is not -1, and multiple items are allowed
for the data type identified in the mask, such as text or sPLT, only the n'th item in the
structure is freed, where n is "seq".
The default behavior is only to free data that was allocated internally by libpng. This
can be changed, so that libpng will not free the data, or so that it will free data that
was allocated by the user with png_malloc() or png_zalloc() and passed in via a
png_set_*() function, with
png_data_freer(png_ptr, info_ptr, freer, mask)
mask - which data elements are affected
same choices as in png_free_data()
freer - one of
PNG_DESTROY_WILL_FREE_DATA
PNG_SET_WILL_FREE_DATA
PNG_USER_WILL_FREE_DATA
This function only affects data that has already been allocated. You can call this func-
tion 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 responsibil-
ity for libpng-allocated data, the application must use png_free() to free it, and when
the user transfers responsibility to libpng for data that the user has allocated, the user
must have used png_malloc() or png_zalloc() to allocate it.
If you allocated your row_pointers in a single block, as suggested above in the descrip-
tion of the high level read interface, you must not transfer responsibility for freeing it
to the png_set_rows or png_read_destroy function, because they would also try to free the
individual row_pointers[i].
If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword separately,
do not transfer responsibility for freeing text_ptr to libpng, because when libpng fills a
png_text structure it combines these members with the key member, and png_free_data() will
free only text_ptr.key. Similarly, if you transfer responsibility for free'ing text_ptr
from libpng to your application, your application must not separately free those members.
The png_free_data() function will turn off the "valid" flag for anything it frees. If you
need to turn the flag off for a chunk that was freed by your application instead of by
libpng, you can use
png_set_invalid(png_ptr, info_ptr, mask);
mask - identifies the chunks to be made invalid,
containing the bitwise OR of one or
more of
PNG_INFO_gAMA, PNG_INFO_sBIT,
PNG_INFO_cHRM, PNG_INFO_PLTE,
PNG_INFO_tRNS, PNG_INFO_bKGD,
PNG_INFO_hIST, PNG_INFO_pHYs,
PNG_INFO_oFFs, PNG_INFO_tIME,
PNG_INFO_pCAL, PNG_INFO_sRGB,
PNG_INFO_iCCP, PNG_INFO_sPLT,
PNG_INFO_sCAL, PNG_INFO_IDAT
For a more compact example of reading a PNG image, see the file example.c.
Reading PNG files progressively
The progressive reader is slightly different then the non-progressive reader. Instead of
calling png_read_info(), png_read_rows(), and png_read_end(), you make one call to
png_process_data(), which calls callbacks when it has the info, a row, or the end of the
image. You set up these callbacks with png_set_progressive_read_fn(). You don't have to
worry about the input/output functions of libpng, as you are giving the library the data
directly in png_process_data(). I will assume that you have read the section on reading
PNG files above, so I will only highlight the differences (although I will show all of the
code).
png_structp png_ptr; png_infop info_ptr;
/* An example code fragment of how you would
initialize the progressive reader in your
application. */
int
initialize_png_reader()
{
png_ptr = png_create_read_struct
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
return (ERROR);
info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_read_struct(&png_ptr, (png_infopp)NULL,
(png_infopp)NULL);
return (ERROR);
}
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
return (ERROR);
}
/* This one's new. You can provide functions
to be called when the header info is valid,
when each row is completed, and when the image
is finished. If you aren't using all functions,
you can specify NULL parameters. Even when all
three functions are NULL, you need to call
png_set_progressive_read_fn(). You can use
any struct as the user_ptr (cast to a void pointer
for the function call), and retrieve the pointer
from inside the callbacks using the function
png_get_progressive_ptr(png_ptr);
which will return a void pointer, which you have
to cast appropriately.
*/
png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
info_callback, row_callback, end_callback);
return 0;
}
/* A code fragment that you call as you receive blocks
of data */
int
process_data(png_bytep buffer, png_uint_32 length)
{
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
return (ERROR);
}
/* This one's new also. Simply give it a chunk
of data from the file stream (in order, of
course). On machines with segmented memory
models machines, don't give it any more than
64K. The library seems to run fine with sizes
of 4K. Although you can give it much less if
necessary (I assume you can give it chunks of
1 byte, I haven't tried less then 256 bytes
yet). When this function returns, you may
want to display any rows that were generated
in the row callback if you don't already do
so there.
*/
png_process_data(png_ptr, info_ptr, buffer, length);
return 0;
}
/* This function is called (as set by
png_set_progressive_read_fn() above) when enough data
has been supplied so all of the header has been
read.
*/
void
info_callback(png_structp png_ptr, png_infop info)
{
/* Do any setup here, including setting any of
the transformations mentioned in the Reading
PNG files section. For now, you _must_ call
either png_start_read_image() or
png_read_update_info() after all the
transformations are set (even if you don't set
any). You may start getting rows before
png_process_data() returns, so this is your
last chance to prepare for that.
*/
}
/* This function is called when each row of image
data is complete */
void
row_callback(png_structp png_ptr, png_bytep new_row,
png_uint_32 row_num, int pass)
{
/* If the image is interlaced, and you turned
on the interlace handler, this function will
be called for every row in every pass. Some
of these rows will not be changed from the
previous pass. When the row is not changed,
the new_row variable will be NULL. The rows
and passes are called in order, so you don't
really need the row_num and pass, but I'm
supplying them because it may make your life
easier.
For the non-NULL rows of interlaced images,
you must call png_progressive_combine_row()
passing in the row and the old row. You can
call this function for NULL rows (it will just
return) and for non-interlaced images (it just
does the memcpy for you) if it will make the
code easier. Thus, you can just do this for
all cases:
*/
png_progressive_combine_row(png_ptr, old_row,
new_row);
/* where old_row is what was displayed for
previously for the row. Note that the first
pass (pass == 0, really) will completely cover
the old row, so the rows do not have to be
initialized. After the first pass (and only
for interlaced images), you will have to pass
the current row, and the function will combine
the old row and the new row.
*/
}
void
end_callback(png_structp png_ptr, png_infop info)
{
/* This function is called after the whole image
has been read, including any chunks after the
image (up to and including the IEND). You
will usually have the same info chunk as you
had in the header, although some data may have
been added to the comments and time fields.
Most people won't do much here, perhaps setting
a flag that marks the image as finished.
*/
}
IV. Writing
Much of this is very similar to reading. However, everything of importance is repeated
here, so you won't have to constantly look back up in the reading section to understand
writing.
Setup
You will want to do the I/O initialization before you get into libpng, so if it doesn't
work, you don't have anything to undo. If you are not using the standard I/O functions,
you will need to replace them with custom writing functions. See the discussion under
Customizing libpng.
FILE *fp = fopen(file_name, "wb");
if (!fp)
{
return (ERROR);
}
Next, png_struct and png_info need to be allocated and initialized. As these can be both
relatively large, you may not want to store these on the stack, unless you have stack
space to spare. Of course, you will want to check if they return NULL. If you are also
reading, you won't want to name your read structure and your write structure both
"png_ptr"; you can call them anything you like, such as "read_ptr" and "write_ptr". Look
at pngtest.c, for example.
png_structp png_ptr = png_create_write_struct
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
return (ERROR);
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_write_struct(&png_ptr,
(png_infopp)NULL);
return (ERROR);
}
If you want to use your own memory allocation routines, define PNG_USER_MEM_SUPPORTED and
use png_create_write_struct_2() instead of png_create_write_struct():
png_structp png_ptr = png_create_write_struct_2
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn, (png_voidp)
user_mem_ptr, user_malloc_fn, user_free_fn);
After you have these structures, you will need to set up the error handling. When libpng
encounters an error, it expects to longjmp() back to your routine. Therefore, you will
need to call setjmp() and pass the png_jmpbuf(png_ptr). If you write the file from dif-
ferent routines, you will need to update the png_jmpbuf(png_ptr) every time you enter a
new routine that will call a png_*() function. See your documentation of setjmp/longjmp
for your compiler for more information on setjmp/longjmp. See the discussion on libpng
error handling in the Customizing Libpng section below for more information on the libpng
error handling.
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(fp);
return (ERROR);
}
...
return;
If you would rather avoid the complexity of setjmp/longjmp issues, you can compile libpng
with PNG_SETJMP_NOT_SUPPORTED, in which case errors will result in a call to PNG_ABORT()
which defaults to abort().
Now you need to set up the output code. The default for libpng is to use the C function
fwrite(). If you use this, you will need to pass a valid FILE * in the function
png_init_io(). Be sure that the file is opened in binary mode. Again, if you wish to
handle writing data in another way, see the discussion on libpng I/O handling in the 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 applica-
tion, use
png_set_sig_bytes(png_ptr, 8);
to inform libpng that it should not write a signature.
Write callbacks
At this point, you can set up a callback function that will be called after each row has
been written, which you can use to control a progress meter or the like. It's demon-
strated 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 parame-
ter to png_set_filter() is the filter method, for which the only valid values are 0 (as of
the July 1999 PNG specification, version 1.2) or 64 (if you are writing a PNG datastream
that is to be embedded in a MNG datastream). The third parameter is a flag that indicates
which filter type(s) are to be tested for each scanline. See the PNG specification for
details on the specific filter types.
/* turn on or off filtering, and/or choose
specific filters. You can use either a single
PNG_FILTER_VALUE_NAME or the bitwise OR of one
or more PNG_FILTER_NAME masks. */
png_set_filter(png_ptr, 0,
PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE |
PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB |
PNG_FILTER_UP | PNG_FILTER_VALUE_UP |
PNG_FILTER_AVE | PNG_FILTER_VALUE_AVE |
PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
PNG_ALL_FILTERS);
If an application wants to start and stop using particular filters during compression, it
should start out with all of the filters (to ensure that the previous row of pixels will
be stored in case it's needed later), and then add and remove them after the start of com-
pression.
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 try-
ing to compress the image data. See the Compression Library (zlib.h and algorithm.txt,
distributed with zlib) for details on the compression levels.
/* set the zlib compression level */
png_set_compression_level(png_ptr,
Z_BEST_COMPRESSION);
/* set other zlib parameters */
png_set_compression_mem_level(png_ptr, 8);
png_set_compression_strategy(png_ptr,
Z_DEFAULT_STRATEGY);
png_set_compression_window_bits(png_ptr, 15);
png_set_compression_method(png_ptr, 8);
png_set_compression_buffer_size(png_ptr, 8192)
extern PNG_EXPORT(void,png_set_zbuf_size)
Setting the contents of info for output
You now need to fill in the png_info structure with all the data you wish to write before
the actual image. Note that the only thing you are allowed to write after the image is
the text chunks and the time chunk (as of PNG Specification 1.2, anyway). See
png_write_end() and the latest PNG specification for more information on that. If you
wish to write them before the image, fill them in now, and flag that data as being valid.
If you want to wait until after the data, don't fill them until png_write_end(). For all
the fields in png_info and their data types, see png.h. For explanations of what the
fields contain, see the PNG specification.
Some of the more important parts of the png_info are:
png_set_IHDR(png_ptr, info_ptr, width, height,
bit_depth, color_type, interlace_type,
compression_type, filter_method)
width - holds the width of the image
in pixels (up to 2^31).
height - holds the height of the image
in pixels (up to 2^31).
bit_depth - holds the bit depth of one of the
image channels.
(valid values are 1, 2, 4, 8, 16
and depend also on the
color_type. See also significant
bits (sBIT) below).
color_type - describes which color/alpha
channels are present.
PNG_COLOR_TYPE_GRAY
(bit depths 1, 2, 4, 8, 16)
PNG_COLOR_TYPE_GRAY_ALPHA
(bit depths 8, 16)
PNG_COLOR_TYPE_PALETTE
(bit depths 1, 2, 4, 8)
PNG_COLOR_TYPE_RGB
(bit_depths 8, 16)
PNG_COLOR_TYPE_RGB_ALPHA
(bit_depths 8, 16)
PNG_COLOR_MASK_PALETTE
PNG_COLOR_MASK_COLOR
PNG_COLOR_MASK_ALPHA
interlace_type - PNG_INTERLACE_NONE or
PNG_INTERLACE_ADAM7
compression_type - (must be
PNG_COMPRESSION_TYPE_DEFAULT)
filter_method - (must be PNG_FILTER_TYPE_DEFAULT
or, if you are writing a PNG to
be embedded in a MNG datastream,
can also be
PNG_INTRAPIXEL_DIFFERENCING)
If you call png_set_IHDR(), the call must appear before any of the other png_set_*() func-
tions, which might require access to some of the IHDR settings. The remaining png_set_*()
functions can be called in any order.
png_set_PLTE(png_ptr, info_ptr, palette,
num_palette);
palette - the palette for the file
(array of png_color)
num_palette - number of entries in the palette
png_set_gAMA(png_ptr, info_ptr, gamma);
gamma - the gamma the image was created
at (PNG_INFO_gAMA)
png_set_sRGB(png_ptr, info_ptr, srgb_intent);
srgb_intent - the rendering intent
(PNG_INFO_sRGB) The presence of
the sRGB chunk means that the pixel
data is in the sRGB color space.
This chunk also implies specific
values of gAMA and cHRM. Rendering
intent is the CSS-1 property that
has been defined by the International
Color Consortium
(http://www.color.org).
It can be one of
PNG_sRGB_INTENT_SATURATION,
PNG_sRGB_INTENT_PERCEPTUAL,
PNG_sRGB_INTENT_ABSOLUTE, or
PNG_sRGB_INTENT_RELATIVE.
png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
srgb_intent);
srgb_intent - the rendering intent
(PNG_INFO_sRGB) The presence of the
sRGB chunk means that the pixel
data is in the sRGB color space.
This function also causes gAMA and
cHRM chunks with the specific values
that are consistent with sRGB to be
written.
png_set_iCCP(png_ptr, info_ptr, name, compression_type,
profile, proflen);
name - The profile name.
compression - The compression type; always
PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
You may give NULL to this argument to
ignore it.
profile - International Color Consortium color
profile data. May contain NULs.
proflen - length of profile data in bytes.
png_set_sBIT(png_ptr, info_ptr, sig_bit);
sig_bit - the number of significant bits for
(PNG_INFO_sBIT) each of the gray, red,
green, and blue channels, whichever are
appropriate for the given color type
(png_color_16)
png_set_tRNS(png_ptr, info_ptr, trans, num_trans,
trans_values);
trans - array of transparent entries for
palette (PNG_INFO_tRNS)
trans_values - graylevel or color sample values of
the single transparent color for
non-paletted images (PNG_INFO_tRNS)
num_trans - number of transparent entries
(PNG_INFO_tRNS)
png_set_hIST(png_ptr, info_ptr, hist);
(PNG_INFO_hIST)
hist - histogram of palette (array of
png_uint_16)
png_set_tIME(png_ptr, info_ptr, mod_time);
mod_time - time image was last modified
(PNG_VALID_tIME)
png_set_bKGD(png_ptr, info_ptr, background);
background - background color (PNG_VALID_bKGD)
png_set_text(png_ptr, info_ptr, text_ptr, num_text);
text_ptr - array of png_text holding image
comments
text_ptr[i].compression - type of compression used
on "text" PNG_TEXT_COMPRESSION_NONE
PNG_TEXT_COMPRESSION_zTXt
PNG_ITXT_COMPRESSION_NONE
PNG_ITXT_COMPRESSION_zTXt
text_ptr[i].key - keyword for comment. Must contain
1-79 characters.
text_ptr[i].text - text comments for current
keyword. Can be NULL or empty.
text_ptr[i].text_length - length of text string,
after decompression, 0 for iTXt
text_ptr[i].itxt_length - length of itxt string,
after decompression, 0 for tEXt/zTXt
text_ptr[i].lang - language of comment (NULL or
empty for unknown).
text_ptr[i].translated_keyword - keyword in UTF-8 (NULL
or empty for unknown).
num_text - number of comments
png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
num_spalettes);
palette_ptr - array of png_sPLT_struct structures
to be added to the list of palettes
in the info structure.
num_spalettes - number of palette structures to be
added.
png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
unit_type);
offset_x - positive offset from the left
edge of the screen
offset_y - positive offset from the top
edge of the screen
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
unit_type);
res_x - pixels/unit physical resolution
in x direction
res_y - pixels/unit physical resolution
in y direction
unit_type - PNG_RESOLUTION_UNKNOWN,
PNG_RESOLUTION_METER
png_set_sCAL(png_ptr, info_ptr, unit, width, height)
unit - physical scale units (an integer)
width - width of a pixel in physical scale units
height - height of a pixel in physical scale units
(width and height are doubles)
png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
unit - physical scale units (an integer)
width - width of a pixel in physical scale units
height - height of a pixel in physical scale units
(width and height are strings like "2.54")
png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
num_unknowns)
unknowns - array of png_unknown_chunk
structures holding unknown chunks
unknowns[i].name - name of unknown chunk
unknowns[i].data - data of unknown chunk
unknowns[i].size - size of unknown chunk's data
unknowns[i].location - position to write chunk in file
0: do not write chunk
PNG_HAVE_IHDR: before PLTE
PNG_HAVE_PLTE: before IDAT
PNG_AFTER_IDAT: after IDAT
The "location" member is set automatically according to what part of the output file has
already been written. You can change its value after calling png_set_unknown_chunks() as
demonstrated in pngtest.c. Within each of the "locations", the chunks are sequenced
according to their position in the structure (that is, the value of "i", which is the
order in which the chunk was either read from the input file or defined with
png_set_unknown_chunks).
A quick word about text and num_text. text is an array of png_text structures. num_text
is the number of valid structures in the array. Each png_text structure holds a language
code, a keyword, a text value, and a compression type.
The compression types have the same valid numbers as the compression types of the image
data. Currently, the only valid number is zero. However, you can store text either com-
pressed 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_COMPRES-
SION_NONE or PNG_TEXT_COMPRESSION_zTXt any language code or translated keyword will not be
written out.
Until text gets around 1000 bytes, it is not worth compressing it. After the text has
been written out to the file, the compression type is set to PNG_TEXT_COMPRESSION_NONE_WR
or PNG_TEXT_COMPRESSION_zTXt_WR, so that it isn't written out again at the end (in case
you are calling png_write_end() with the same struct.
The keywords that are given in the PNG Specification are:
Title Short (one line) title or
caption for image
Author Name of image's creator
Description Description of image (possibly long)
Copyright Copyright notice
Creation Time Time of original image creation
(usually RFC 1123 format, see below)
Software Software used to create the image
Disclaimer Legal disclaimer
Warning Warning of nature of content
Source Device used to create the image
Comment Miscellaneous comment; conversion
from other image format
The keyword-text pairs work like this. Keywords should be short simple descriptions of
what the comment is about. Some typical keywords are found in the PNG specification, as
is some recommendations on keywords. You can repeat keywords in a file. You can even
write some text before the image and some after. For example, you may want to put a
description of the image before the image, but leave the disclaimer until after, so view-
ers working over modem connections don't have to wait for the disclaimer to go over the
modem before they start seeing the image. Finally, keywords should be full words, not
abbreviations. Keywords and text are in the ISO 8859-1 (Latin-1) character set (a super-
set of regular ASCII) and can not contain NUL characters, and should not contain control
or other unprintable characters. To make the comments widely readable, stick with basic
ASCII, and avoid machine specific character set extensions like the IBM-PC character set.
The keyword must be present, but you can leave off the text string on non-compressed
pairs. Compressed pairs must have a text string, as only the text string is compressed
anyway, so the compression would be meaningless.
PNG supports modification time via the png_time structure. Two conversion routines are
provided, png_convert_from_time_t() for time_t and png_convert_from_struct_tm() for struct
tm. The time_t routine uses gmtime(). You don't have to use either of these, but if you
wish to fill in the png_time structure directly, you should provide the time in universal
time (GMT) if possible instead of your local time. Note that the year number is the full
year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and that months start with
1.
If you want to store the time of the original image creation, you should use a plain tEXt
chunk with the "Creation Time" keyword. This is necessary because the "creation time" of
a PNG image is somewhat vague, depending on whether you mean the PNG file, the time the
image was created in a non-PNG format, a still photo from which the image was scanned, or
possibly the subject matter itself. In order to facilitate machine-readable dates, it is
recommended that the "Creation Time" tEXt chunk use RFC 1123 format dates (e.g. "22 May
1997 18:07:10 GMT"), although this isn't a requirement. Unlike the tIME chunk, the "Cre-
ation Time" tEXt chunk is not expected to be automatically changed by the software. To
facilitate the use of RFC 1123 dates, a function png_convert_to_rfc1123(png_timep) is pro-
vided to convert from PNG time to an RFC 1123 format string.
Writing unknown chunks
You can use the png_set_unknown_chunks function to queue up chunks for writing. You give
it a chunk name, raw data, and a size; that's all there is to it. The chunks will be
written by the next following png_write_info_before_PLTE, png_write_info, or png_write_end
function. Any chunks previously read into the info structure's unknown-chunk list will
also be written out in a sequence that satisfies the PNG specification's ordering rules.
The high-level write interface
At this point there are two ways to proceed; through the high-level write interface, or
through a sequence of low-level write operations. You can use the high-level interface if
your image data is present in the info structure. All defined output transformations are
permitted, enabled by the following masks.
PNG_TRANSFORM_IDENTITY No transformation
PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples
PNG_TRANSFORM_PACKSWAP Change order of packed
pixels to LSB first
PNG_TRANSFORM_INVERT_MONO Invert monochrome images
PNG_TRANSFORM_SHIFT Normalize pixels to the
sBIT depth
PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
to BGRA
PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
to AG
PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
to transparency
PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
PNG_TRANSFORM_STRIP_FILLER Strip out filler bytes.
If you have valid image data in the info structure (you can use png_set_rows() to put
image data in the info structure), simply do this:
png_write_png(png_ptr, info_ptr, png_transforms, NULL)
where png_transforms is an integer containing the bitwise OR of some set of transformation
flags. This call is equivalent to png_write_info(), followed the set of transformations
indicated by the transform mask, then png_write_image(), and finally png_write_end().
(The final parameter of this call is not yet used. Someday it might point to transforma-
tion parameters required by some future output transform.)
You must use png_transforms and not call any png_set_transform() functions when you use
png_write_png().
The low-level write interface
If you are going the low-level route instead, you are now ready to write all the file
information up to the actual image data. You do this with a call to png_write_info().
png_write_info(png_ptr, info_ptr);
Note that there is one transformation you may need to do before png_write_info(). In PNG
files, the alpha channel in an image is the level of opacity. If your data is supplied as
a level of transparency, you can invert the alpha channel before you write it, so that 0
is fully transparent and 255 (in 8-bit or paletted images) or 65535 (in 16-bit images) is
fully opaque, with
png_set_invert_alpha(png_ptr);
This must appear before png_write_info() instead of later with the other transformations
because in the case of paletted images the tRNS chunk data has to be inverted before the
tRNS chunk is written. If your image is not a paletted image, the tRNS data (which in
such cases represents a single color to be rendered as transparent) won't need to be
changed, and you can safely do this transformation after your png_write_info() call.
If you need to write a private chunk that you want to appear before the PLTE chunk when
PLTE is present, you can write the PNG info in two steps, and insert code to write your
own chunk between them:
png_write_info_before_PLTE(png_ptr, info_ptr);
png_set_unknown_chunks(png_ptr, info_ptr, ...);
png_write_info(png_ptr, info_ptr);
After you've written the file information, you can set up the library to handle any spe-
cial transformations of the image data. The various ways to transform the data will be
described in the order that they should occur. This is important, as some of these change
the color type and/or bit depth of the data, and some others only work on certain color
types and bit depths. Even though each transformation checks to see if it has data that
it can do something with, you should make sure to only enable a transformation if it will
be valid for the data. For example, don't swap red and blue on grayscale data.
PNG files store RGB pixels packed into 3 or 6 bytes. This code tells the library to strip
input data that has 4 or 8 bytes per pixel down to 3 or 6 bytes (or strip 2 or 4-byte
grayscale+filler data to 1 or 2 bytes per pixel).
png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
where the 0 is unused, and the location is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER,
depending upon whether the filler byte in the pixel is stored XRGB or RGBX.
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as they can, resulting
in, for example, 8 pixels per byte for 1 bit files. If the data is supplied at 1 pixel
per byte, use this code, which will correctly pack the pixels into a single byte:
png_set_packing(png_ptr);
PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your data is of another
bit depth, you can write an sBIT chunk into the file so that decoders can recover the
original data if desired.
/* Set the true bit depth of the image data */
if (color_type & PNG_COLOR_MASK_COLOR)
{
sig_bit.red = true_bit_depth;
sig_bit.green = true_bit_depth;
sig_bit.blue = true_bit_depth;
}
else
{
sig_bit.gray = true_bit_depth;
}
if (color_type & PNG_COLOR_MASK_ALPHA)
{
sig_bit.alpha = true_bit_depth;
}
png_set_sBIT(png_ptr, info_ptr, &sig_bit);
If the data is stored in the row buffer in a bit depth other than one supported by PNG
(e.g. 3 bit data in the range 0-7 for a 4-bit PNG), this will scale the values to appear
to be the correct bit depth as is required by PNG.
png_set_shift(png_ptr, &sig_bit);
PNG files store 16 bit pixels in network byte order (big-endian, ie. most significant bits
first). This code would be used if they are supplied the other way (little-endian, i.e.
least significant bits first, the way PCs store them):
if (bit_depth > 8)
png_set_swap(png_ptr);
If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you need to change the
order the pixels are packed into bytes, you can use:
if (bit_depth < 8)
png_set_packswap(png_ptr);
PNG files store 3 color pixels in red, green, blue order. This code would be used if they
are supplied as blue, green, red:
png_set_bgr(png_ptr);
PNG files describe monochrome as black being zero and white being one. This code would be
used if the pixels are supplied with this reversed (black being one and white being zero):
png_set_invert_mono(png_ptr);
Finally, you can write your own transformation function if none of the existing ones meets
your needs. This is done by setting a callback with
png_set_write_user_transform_fn(png_ptr,
write_transform_fn);
You must supply the function
void write_transform_fn(png_ptr ptr, row_info_ptr
row_info, png_bytep data)
See pngtest.c for a working example. Your function will be called before any of the other
transformations are processed.
You can also set up a pointer to a user structure for use by your callback function.
png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
The user_channels and user_depth parameters of this function are ignored when writing; you
can set them to zero as shown.
You can retrieve the pointer via the function png_get_user_transform_ptr(). For example:
voidp write_user_transform_ptr =
png_get_user_transform_ptr(png_ptr);
It is possible to have libpng flush any pending output, either manually, or automatically
after a certain number of lines have been written. To flush the output stream a single
time call:
png_write_flush(png_ptr);
and to have libpng flush the output stream periodically after a certain number of scan-
lines have been written, call:
png_set_flush(png_ptr, nrows);
Note that the distance between rows is from the last time png_write_flush() was called, or
the first row of the image if it has never been called. So if you write 50 lines, and
then png_set_flush 25, it will flush the output on the next scanline, and every 25 lines
thereafter, unless png_write_flush() is called before 25 more lines have been written. If
nrows is too small (less than about 10 lines for a 640 pixel wide RGB image) the image
compression may decrease noticeably (although this may be acceptable for real-time appli-
cations). Infrequent flushing will only degrade the compression performance by a few per-
cent over images that do not use flushing.
Writing the image data
That's it for the transformations. Now you can write the image data. The simplest way to
do this is in one function call. If you have the whole image in memory, you can just call
png_write_image() and libpng will write the image. You will need to pass in an array of
pointers to each row. This function automatically handles interlacing, so you don't need
to call png_set_interlace_handling() or call this function multiple times, or any of that
other stuff necessary with png_write_rows().
png_write_image(png_ptr, row_pointers);
where row_pointers is:
png_byte *row_pointers[height];
You can point to void or char or whatever you use for pixels.
If you don't want to write the whole image at once, you can use png_write_rows() instead.
If the file is not interlaced, this is simple:
png_write_rows(png_ptr, row_pointers,
number_of_rows);
row_pointers is the same as in the png_write_image() call.
If you are just writing one row at a time, you can do this with a single row_pointer
instead of an array of row_pointers:
png_bytep row_pointer = row;
png_write_row(png_ptr, row_pointer);
When the file is interlaced, things can get a good deal more complicated. The only cur-
rently (as of the PNG Specification version 1.2, dated July 1999) defined interlacing
scheme for PNG files is the "Adam7" interlace scheme, that breaks down an image into seven
smaller images of varying size. libpng will build these images for you, or you can do
them yourself. If you want to build them yourself, see the PNG specification for details
of which pixels to write when.
If you don't want libpng to handle the interlacing details, just use png_set_inter-
lace_handling() and call png_write_rows() the correct number of times to write all seven
sub-images.
If you want libpng to build the sub-images, call this before you start writing any rows:
number_of_passes =
png_set_interlace_handling(png_ptr);
This will return the number of passes needed. Currently, this is seven, but may change if
another interlace type is added.
Then write the complete image number_of_passes times.
png_write_rows(png_ptr, row_pointers,
number_of_rows);
As some of these rows are not used, and thus return immediately, you may want to read
about interlacing in the PNG specification, and only update the rows that are actually
used.
Finishing a sequential write
After you are finished writing the image, you should finish writing the file. If you are
interested in writing comments or time, you should pass an appropriately filled png_info
pointer. If you are not interested, you can pass NULL.
png_write_end(png_ptr, info_ptr);
When you are done, you can free all memory used by libpng like this:
png_destroy_write_struct(&png_ptr, &info_ptr);
It is also possible to individually free the info_ptr members that point to libpng-allo-
cated storage with the following function:
png_free_data(png_ptr, info_ptr, mask, seq)
mask - identifies data to be freed, a mask
containing the bitwise OR of one or
more of
PNG_FREE_PLTE, PNG_FREE_TRNS,
PNG_FREE_HIST, PNG_FREE_ICCP,
PNG_FREE_PCAL, PNG_FREE_ROWS,
PNG_FREE_SCAL, PNG_FREE_SPLT,
PNG_FREE_TEXT, PNG_FREE_UNKN,
or simply PNG_FREE_ALL
seq - sequence number of item to be freed
(-1 for all items)
This function may be safely called when the relevant storage has already been freed, or
has not yet been allocated, or was allocated by the user and not by libpng, and will in
those cases do nothing. The "seq" parameter is ignored if only one item of the selected
data type, such as PLTE, is allowed. If "seq" is not -1, and multiple items are allowed
for the data type identified in the mask, such as text or sPLT, only the n'th item in the
structure is freed, where n is "seq".
If you allocated data such as a palette that you passed in to libpng with png_set_*, you
must not free it until just before the call to png_destroy_write_struct().
The default behavior is only to free data that was allocated internally by libpng. This
can be changed, so that libpng will not free the data, or so that it will free data that
was allocated by the user with png_malloc() or png_zalloc() and passed in via a
png_set_*() function, with
png_data_freer(png_ptr, info_ptr, freer, mask)
mask - which data elements are affected
same choices as in png_free_data()
freer - one of
PNG_DESTROY_WILL_FREE_DATA
PNG_SET_WILL_FREE_DATA
PNG_USER_WILL_FREE_DATA
For example, to transfer responsibility for some data from a read structure to a write
structure, you could use
png_data_freer(read_ptr, read_info_ptr,
PNG_USER_WILL_FREE_DATA,
PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
png_data_freer(write_ptr, write_info_ptr,
PNG_DESTROY_WILL_FREE_DATA,
PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
thereby briefly reassigning responsibility for freeing to the user but immediately after-
wards reassigning it once more to the write_destroy function. Having done this, it would
then be safe to destroy the read structure and continue to use the PLTE, tRNS, and hIST
data in the write structure.
This function only affects data that has already been allocated. You can call this func-
tion before calling after the png_set_*() functions to control whether the user or
png_destroy_*() is supposed to free the data. When the user assumes responsibility for
libpng-allocated data, the application must use png_free() to free it, and when the user
transfers responsibility to libpng for data that the user has allocated, the user must
have used png_malloc() or png_zalloc() to allocate it.
If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword separately,
do not transfer responsibility for freeing text_ptr to libpng, because when libpng fills a
png_text structure it combines these members with the key member, and png_free_data() will
free only text_ptr.key. Similarly, if you transfer responsibility for free'ing text_ptr
from libpng to your application, your application must not separately free those members.
For a more compact example of writing a PNG image, see the file example.c.
V. Modifying/Customizing libpng:
There are two issues here. The first is changing how libpng does standard things like
memory allocation, input/output, and error handling. The second deals with more compli-
cated things like adding new chunks, adding new transformations, and generally changing
how libpng works. Both of those are compile-time issues; that is, they are generally
determined at the time the code is written, and there is rarely a need to provide the user
with a means of changing them.
Memory allocation, input/output, and error handling
All of the memory allocation, input/output, and error handling in libpng goes through
callbacks that are user-settable. The default routines are in pngmem.c, pngrio.c, png-
wio.c, and pngerror.c, respectively. To change these functions, call the appropriate
png_set_*_fn() function.
Memory allocation is done through the functions png_malloc() and png_free(). These cur-
rently just call the standard C functions. If your pointers can't access more then 64K at
a time, you will want to set MAXSEG_64K in zlib.h. Since it is unlikely that the method
of handling memory allocation on a platform will change between applications, these func-
tions must be modified in the library at compile time. If you prefer to use a different
method of allocating and freeing data, you can use png_create_read_struct_2() or png_cre-
ate_write_struct_2() to register your own functions as described above. These functions
also provide a void pointer that can be retrieved via
mem_ptr=png_get_mem_ptr(png_ptr);
Your replacement memory functions must have prototypes as follows:
png_voidp malloc_fn(png_structp png_ptr,
png_size_t size);
void free_fn(png_structp png_ptr, png_voidp ptr);
Your malloc_fn() must return NULL in case of failure. The png_malloc() function will nor-
mally call png_error() if it receives a NULL from the system memory allocator or from your
replacement malloc_fn().
Your free_fn() will never be called with a NULL ptr, since libpng's png_free() checks for
NULL before calling free_fn().
Input/Output in libpng is done through png_read() and png_write(), which currently just
call fread() and fwrite(). The FILE * is stored in png_struct and is initialized via
png_init_io(). If you wish to change the method of I/O, the library supplies callbacks
that you can set through the function png_set_read_fn() and png_set_write_fn() at run
time, instead of calling the png_init_io() function. These functions also provide a void
pointer that can be retrieved via the function png_get_io_ptr(). For example:
png_set_read_fn(png_structp read_ptr,
voidp read_io_ptr, png_rw_ptr read_data_fn)
png_set_write_fn(png_structp write_ptr,
voidp write_io_ptr, png_rw_ptr write_data_fn,
png_flush_ptr output_flush_fn);
voidp read_io_ptr = png_get_io_ptr(read_ptr);
voidp write_io_ptr = png_get_io_ptr(write_ptr);
The replacement I/O functions must have prototypes as follows:
void user_read_data(png_structp png_ptr,
png_bytep data, png_size_t length);
void user_write_data(png_structp png_ptr,
png_bytep data, png_size_t length);
void user_flush_data(png_structp png_ptr);
Supplying NULL for the read, write, or flush functions sets them back to using the default
C stream functions. It is an error to read from a write stream, and vice versa.
Error handling in libpng is done through png_error() and png_warning(). Errors handled
through png_error() are fatal, meaning that png_error() should never return to its caller.
Currently, this is handled via setjmp() and longjmp() (unless you have compiled libpng
with PNG_SETJMP_NOT_SUPPORTED, in which case it is handled via PNG_ABORT()), but you could
change this to do things like exit() if you should wish.
On non-fatal errors, png_warning() is called to print a warning message, and then control
returns to the calling code. By default png_error() and png_warning() print a message on
stderr via fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined
(because you don't want the messages) or PNG_NO_STDIO defined (because fprintf() isn't
available). If you wish to change the behavior of the error functions, you will need to
set up your own message callbacks. These functions are normally supplied at the time that
the png_struct is created. It is also possible to redirect errors and warnings to your
own replacement functions after png_create_*_struct() has been called by calling:
png_set_error_fn(png_structp png_ptr,
png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warning_fn);
png_voidp error_ptr = png_get_error_ptr(png_ptr);
If NULL is supplied for either error_fn or warning_fn, then the libpng default function
will be used, calling fprintf() and/or longjmp() if a problem is encountered. The
replacement error functions should have parameters as follows:
void user_error_fn(png_structp png_ptr,
png_const_charp error_msg);
void user_warning_fn(png_structp png_ptr,
png_const_charp warning_msg);
The motivation behind using setjmp() and longjmp() is the C++ throw and catch exception
handling methods. This makes the code much easier to write, as there is no need to check
every return code of every function call. However, there are some uncertainties about the
status of local variables after a longjmp, so the user may want to be careful about doing
anything after setjmp returns non-zero besides returning itself. Consult your compiler
documentation for more details. For an alternative approach, you may wish to use the
"cexcept" facility (see http://cexcept.sourceforge.net).
Custom chunks
If you need to read or write custom chunks, you may need to get deeper into the libpng
code. The library now has mechanisms for storing and writing chunks of unknown type; you
can even declare callbacks for custom chunks. However, this may not be good enough if the
library code itself needs to know about interactions between your chunk and existing
`intrinsic' chunks.
If you need to write a new intrinsic chunk, first read the PNG specification. Acquire a
first level of understanding of how it works. Pay particular attention to the sections
that describe chunk names, and look at how other chunks were designed, so you can do
things similarly. Second, check out the sections of libpng that read and write chunks.
Try to find a chunk that is similar to yours and use it as a template. More details can
be found in the comments inside the code. It is best to handle unknown chunks in a
generic method, via callback functions, instead of by modifying libpng functions.
If you wish to write your own transformation for the data, look through the part of the
code that does the transformations, and check out some of the simpler ones to get an idea
of how they work. Try to find a similar transformation to the one you want to add and
copy off of it. More details can be found in the comments inside the code itself.
Configuring for 16 bit platforms
You will want to look into zconf.h to tell zlib (and thus libpng) that it cannot allocate
more then 64K at a time. Even if you can, the memory won't be accessible. So limit zlib
and libpng to 64K by defining MAXSEG_64K.
Configuring for DOS
For DOS users who only have access to the lower 640K, you will have to limit zlib's memory
usage via a png_set_compression_mem_level() call. See zlib.h or zconf.h in the zlib
library for more information.
Configuring for Medium Model
Libpng's support for medium model has been tested on most of the popular compilers. Make
sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets defined, and FAR gets defined to far in
pngconf.h, and you should be all set. Everything in the library (except for zlib's struc-
ture) 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 ini-
tialization. They can be changed later via png_set_error_fn(). On some compilers, you
may also have to change the memory allocators (png_malloc, etc.).
Configuring for compiler xxx:
All includes for libpng are in pngconf.h. If you need to add/change/delete an include,
this is the place to do it. The includes that are not needed outside libpng are protected
by the PNG_INTERNAL definition, which is only defined for those routines inside libpng
itself. The files in libpng proper only include png.h, which includes pngconf.h.
Configuring zlib:
There are special functions to configure the compression. Perhaps the most useful one
changes the compression level, which currently uses input compression values in the range
0 - 9. The library normally uses the default compression level (Z_DEFAULT_COMPRESSION =
6). Tests have shown that for a large majority of images, compression values in the range
3-6 compress nearly as well as higher levels, and do so much faster. For online applica-
tions it may be desirable to have maximum speed (Z_BEST_SPEED = 1). With versions of zlib
after v0.99, you can also specify no compression (Z_NO_COMPRESSION = 0), but this would
create files larger than just storing the raw bitmap. You can specify the compression
level by calling:
png_set_compression_level(png_ptr, level);
Another useful one is to reduce the memory level used by the library. The memory level
defaults to 8, but it can be lowered if you are short on memory (running DOS, for example,
where you only have 640K). Note that the memory level does have an effect on compression;
among other things, lower levels will result in sections of incompressible data being
emitted in smaller stored blocks, with a correspondingly larger relative overhead of up to
15% in the worst case.
png_set_compression_mem_level(png_ptr, level);
The other functions are for configuring zlib. They are not recommended for normal use and
may result in writing an invalid PNG file. See zlib.h for more information on what these
mean.
png_set_compression_strategy(png_ptr,
strategy);
png_set_compression_window_bits(png_ptr,
window_bits);
png_set_compression_method(png_ptr, method);
png_set_compression_buffer_size(png_ptr, size);
Controlling row filtering
If you want to control whether libpng uses filtering or not, which filters are used, and
how it goes about picking row filters, you can call one of these functions. The selection
and configuration of row filters can have a significant impact on the size and encoding
speed and a somewhat lesser impact on the decoding speed of an image. Filtering is
enabled by default for RGB and grayscale images (with and without alpha), but not for
paletted images nor for any images with bit depths less than 8 bits/pixel.
The 'method' parameter sets the main filtering method, which is currently only '0' in the
PNG 1.2 specification. The 'filters' parameter sets which filter(s), if any, should be
used for each scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS to turn
filtering on and off, respectively.
Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB, PNG_FILTER_UP, PNG_FIL-
TER_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 initial-
ize its internal structures appropriately for all of the filter types. (Note that this
means the first row must always be adaptively filtered, because libpng currently does not
allocate the filter buffers until png_write_row() is called for the first time.)
filters = PNG_FILTER_NONE | PNG_FILTER_SUB
PNG_FILTER_UP | PNG_FILTER_AVE |
PNG_FILTER_PAETH | PNG_ALL_FILTERS;
png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
filters);
The second parameter can also be
PNG_INTRAPIXEL_DIFFERENCING if you are
writing a PNG to be embedded in a MNG
datastream. This parameter must be the
same as the value of filter_method used
in png_set_IHDR().
It is also possible to influence how libpng chooses from among the available filters.
This is done in one or both of two ways - by telling it how important it is to keep the
same filter for successive rows, and by telling it the relative computational costs of the
filters.
double weights[3] = {1.5, 1.3, 1.1},
costs[PNG_FILTER_VALUE_LAST] =
{1.0, 1.3, 1.3, 1.5, 1.7};
png_set_filter_heuristics(png_ptr,
PNG_FILTER_HEURISTIC_WEIGHTED, 3,
weights, costs);
The weights are multiplying factors that indicate to libpng that the row filter should be
the same for successive rows unless another row filter is that many times better than the
previous filter. In the above example, if the previous 3 filters were SUB, SUB, NONE, the
SUB filter could have a "sum of absolute differences" 1.5 x 1.3 times higher than other
filters and still be chosen, while the NONE filter could have a sum 1.1 times higher than
other filters and still be chosen. Unspecified weights are taken to be 1.0, and the spec-
ified 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 com-
piled. 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 capabil-
ities that you do want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the extra
transformations but still leave the library fully capable of reading and writing PNG files
with all known public chunks Use of the PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive
produces a library that is incapable of reading or writing ancillary chunks. If you are
not using the progressive reading capability, you can turn that off with
PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTERLACING capability, which you'll
still have).
All the reading and writing specific code are in separate files, so the linker should only
grab the files it needs. However, if you want to make sure, or if you are building a
stand alone library, all the reading files start with pngr and all the writing files start
with pngw. The files that don't match either (like png.c, pngtrans.c, etc.) are used for
both reading and writing, and always need to be included. The progressive reader is in
pngpread.c
If you are creating or distributing a dynamically linked library (a .so or DLL file), you
should not remove or disable any parts of the library, as this will cause applications
linked with different versions of the library to fail if they call functions not available
in your library. The size of the library itself should not be an issue, because only
those sections that are actually used will be loaded into memory.
Requesting debug printout
The macro definition PNG_DEBUG can be used to request debugging printout. Set it to an
integer value in the range 0 to 3. Higher numbers result in increasing amounts of debug-
ging 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, "mes-
sage" is the formatted string to be printed, and p1 and p2 are parameters that are to be
embedded in the string according to printf-style formatting directives. For example,
png_debug1(2, "foo=%d0, foo);
is expanded to
if(PNG_DEBUG > 2)
fprintf(PNG_DEBUG_FILE, "foo=%d0, foo);
When PNG_DEBUG is defined but is zero, the macros aren't defined, but you can still use
PNG_DEBUG to control your own debugging:
#ifdef PNG_DEBUG
fprintf(stderr, ...
#endif
When PNG_DEBUG = 1, the macros are defined, but only png_debug statements having level = 0
will be printed. There aren't any such statements in this version of libpng, but if you
insert some they will be printed.
VII. MNG support
The MNG specification (available at http://www.libpng.org/pub/mng) allows certain exten-
sions 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 mini-
mum, it must have the MNG 8-byte signature and the MHDR and MEND chunks. Libpng does not
provide support for these or any other MNG chunks; your application must provide its own
support for them. You may wish to consider using libmng (available at
http://www.libmng.com) instead.
VIII. Changes to Libpng from version 0.88
It should be noted that versions of libpng later than 0.96 are not distributed by the
original libpng author, Guy Schalnat, nor by Andreas Dilger, who had taken over from Guy
during 1996 and 1997, and distributed versions 0.89 through 0.96, but rather by another
member of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are still alive
and well, but they have moved on to other things.
The old libpng functions png_read_init(), png_write_init(), png_info_init(),
png_read_destroy(), and png_write_destroy() have been moved to PNG_INTERNAL in version
0.95 to discourage their use. These functions will be removed from libpng version 2.0.0.
The preferred method of creating and initializing the libpng structures is via the
png_create_read_struct(), png_create_write_struct(), and png_create_info_struct() because
they isolate the size of the structures from the application, allow version error check-
ing, and also allow the use of custom error handling routines during the initialization,
which the old functions do not. The functions png_read_destroy() and png_write_destroy()
do not actually free the memory that libpng allocated for these structs, but just reset
the data structures, so they can be used instead of png_destroy_read_struct() and
png_destroy_write_struct() if you feel there is too much system overhead allocating and
freeing the png_struct for each image read.
Setting the error callbacks via png_set_message_fn() before png_read_init() as was sug-
gested in libpng-0.88 is no longer supported because this caused applications that do not
use custom error functions to fail if the png_ptr was not initialized to zero. It is
still possible to set the error callbacks AFTER png_read_init(), or to change them with
png_set_error_fn(), which is essentially the same function, but with a new name to force
compilation errors with applications that try to use the old method.
Starting with version 1.0.7, you can find out which version of the library you are using
at run-time:
png_uint_32 libpng_vn = png_access_version_number();
The number libpng_vn is constructed from the major version, minor version with leading
zero, and release number with leading zero, (e.g., libpng_vn for version 1.0.7 is 10007).
You can also check which version of png.h you used when compiling your application:
png_uint_32 application_vn = PNG_LIBPNG_VER;
IX. Y2K Compliance in libpng
April 29, 2008
Since the PNG Development group is an ad-hoc body, we can't make an official declaration.
This is your unofficial assurance that libpng from version 0.71 and upward through 1.2.27
are Y2K compliant. It is my belief that earlier versions were also Y2K compliant.
Libpng only has three year fields. One is a 2-byte unsigned integer that will hold years
up to 65535. The other two hold the date in text format, and will hold years up to 9999.
The integer is
"png_uint_16 year" in png_time_struct.
The strings are
"png_charp time_buffer" in png_struct and
"near_time_buffer", which is a local character string in png.c.
There are seven time-related functions:
png_convert_to_rfc_1123() in png.c
(formerly png_convert_to_rfc_1152() in error)
png_convert_from_struct_tm() in pngwrite.c, called
in pngwrite.c
png_convert_from_time_t() in pngwrite.c
png_get_tIME() in pngget.c
png_handle_tIME() in pngrutil.c, called in pngread.c
png_set_tIME() in pngset.c
png_write_tIME() in pngwutil.c, called in pngwrite.c
All appear to handle dates properly in a Y2K environment. The png_convert_from_time_t()
function calls gmtime() to convert from system clock time, which returns (year - 1900),
which we properly convert to the full 4-digit year. There is a possibility that applica-
tions using libpng are not passing 4-digit years into the png_convert_to_rfc_1123() func-
tion, or that they are incorrectly passing only a 2-digit year instead of "year - 1900"
into the png_convert_from_struct_tm() function, but this is not under our control. The
libpng documentation has always stated that it works with 4-digit years, and the APIs have
been documented as such.
The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned integer to hold
the year, and can hold years as large as 65535.
zlib, upon which libpng depends, is also Y2K compliant. It contains no date-related code.
Glenn Randers-Pehrson
libpng maintainer
PNG Development Group
NOTE
Note about libpng version numbers:
Due to various miscommunications, unforeseen code incompatibilities and occasional factors
outside the authors' control, version numbering on the library has not always been consis-
tent and straightforward. The following table summarizes matters since version 0.89c,
which was the first widely used release:
source png.h png.h shared-lib
version string int version
------- ------ ----- ----------
0.89c ("beta 3") 0.89 89 1.0.89
0.90 ("beta 4") 0.90 90 0.90
0.95 ("beta 5") 0.95 95 0.95
0.96 ("beta 6") 0.96 96 0.96
0.97b ("beta 7") 1.00.97 97 1.0.1
0.97c 0.97 97 2.0.97
0.98 0.98 98 2.0.98
0.99 0.99 98 2.0.99
0.99a-m 0.99 99 2.0.99
1.00 1.00 100 2.1.0
1.0.0 1.0.0 100 2.1.0
1.0.0 (from here on, the 100 2.1.0
1.0.1 png.h string is 10001 2.1.0
1.0.1a-e identical to the 10002 from here on, the
1.0.2 source version) 10002 shared library is 2.V
1.0.2a-b 10003 where V is the source
1.0.1 10001 code version except as
1.0.1a-e 10002 2.1.0.1a-e noted.
1.0.2 10002 2.1.0.2
1.0.2a-b 10003 2.1.0.2a-b
1.0.3 10003 2.1.0.3
1.0.3a-d 10004 2.1.0.3a-d
1.0.4 10004 2.1.0.4
1.0.4a-f 10005 2.1.0.4a-f
1.0.5 (+ 2 patches) 10005 2.1.0.5
1.0.5a-d 10006 2.1.0.5a-d
1.0.5e-r 10100 2.1.0.5e-r
1.0.5s-v 10006 2.1.0.5s-v
1.0.6 (+ 3 patches) 10006 2.1.0.6
1.0.6d-g 10007 2.1.0.6d-g
1.0.6h 10007 10.6h
1.0.6i 10007 10.6i
1.0.6j 10007 2.1.0.6j
1.0.7beta11-14 DLLNUM 10007 2.1.0.7beta11-14
1.0.7beta15-18 1 10007 2.1.0.7beta15-18
1.0.7rc1-2 1 10007 2.1.0.7rc1-2
1.0.7 1 10007 2.1.0.7
1.0.8beta1-4 1 10008 2.1.0.8beta1-4
1.0.8rc1 1 10008 2.1.0.8rc1
1.0.8 1 10008 2.1.0.8
1.0.9beta1-6 1 10009 2.1.0.9beta1-6
1.0.9rc1 1 10009 2.1.0.9rc1
1.0.9beta7-10 1 10009 2.1.0.9beta7-10
1.0.9rc2 1 10009 2.1.0.9rc2
1.0.9 1 10009 2.1.0.9
1.0.10beta1 1 10010 2.1.0.10beta1
1.0.10rc1 1 10010 2.1.0.10rc1
1.0.10 1 10010 2.1.0.10
1.0.11beta1-3 1 10011 2.1.0.11beta1-3
1.0.11rc1 1 10011 2.1.0.11rc1
1.0.11 1 10011 2.1.0.11
1.0.12beta1-2 2 10012 2.1.0.12beta1-2
1.0.12rc1 2 10012 2.1.0.12rc1
1.0.12 2 10012 2.1.0.12
1.1.0a-f - 10100 2.1.1.0a-f abandoned
1.2.0beta1-2 2 10200 2.1.2.0beta1-2
1.2.0beta3-5 3 10200 3.1.2.0beta3-5
1.2.0rc1 3 10200 3.1.2.0rc1
1.2.0 3 10200 3.1.2.0
1.2.1beta-4 3 10201 3.1.2.1beta1-4
1.2.1rc1-2 3 10201 3.1.2.1rc1-2
1.2.1 3 10201 3.1.2.1
1.2.2beta1-6 12 10202 12.so.0.1.2.2beta1-6
1.0.13beta1 10 10013 10.so.0.1.0.13beta1
1.0.13rc1 10 10013 10.so.0.1.0.13rc1
1.2.2rc1 12 10202 12.so.0.1.2.2rc1
1.0.13 10 10013 10.so.0.1.0.13
1.2.2 12 10202 12.so.0.1.2.2
1.2.3rc1-6 12 10203 12.so.0.1.2.3rc1-6
1.2.3 12 10203 12.so.0.1.2.3
1.2.4beta1-3 13 10204 12.so.0.1.2.4beta1-3
1.2.4rc1 13 10204 12.so.0.1.2.4rc1
1.0.14 10 10014 10.so.0.1.0.14
1.2.4 13 10204 12.so.0.1.2.4
1.2.5beta1-2 13 10205 12.so.0.1.2.5beta1-2
1.0.15rc1 10 10015 10.so.0.1.0.15rc1
1.0.15 10 10015 10.so.0.1.0.15
1.2.5 13 10205 12.so.0.1.2.5
1.2.6beta1-4 13 10206 12.so.0.1.2.6beta1-4
1.2.6rc1-5 13 10206 12.so.0.1.2.6rc1-5
1.0.16 10 10016 10.so.0.1.0.16
1.2.6 13 10206 12.so.0.1.2.6
1.2.7beta1-2 13 10207 12.so.0.1.2.7beta1-2
1.0.17rc1 10 10017 10.so.0.1.0.17rc1
1.2.7rc1 13 10207 12.so.0.1.2.7rc1
1.0.17 10 10017 10.so.0.1.0.17
1.2.7 13 10207 12.so.0.1.2.7
1.2.8beta1-5 13 10208 12.so.0.1.2.8beta1-5
1.0.18rc1-5 10 10018 10.so.0.1.0.18rc1-5
1.2.8rc1-5 13 10208 12.so.0.1.2.8rc1-5
1.0.18 10 10018 10.so.0.1.0.18
1.2.8 13 10208 12.so.0.1.2.8
1.2.9beta1-3 13 10209 12.so.0.1.2.9beta1-3
1.2.9beta4-11 13 10209 12.so.0.9[.0]
1.2.9rc1 13 10209 12.so.0.9[.0]
1.2.9 13 10209 12.so.0.9[.0]
1.2.10beta1-8 13 10210 12.so.0.10[.0]
1.2.10rc1-3 13 10210 12.so.0.10[.0]
1.2.10 13 10210 12.so.0.10[.0]
1.2.11beta1-4 13 10211 12.so.0.11[.0]
1.0.19rc1-5 10 10019 10.so.0.19[.0]
1.2.11rc1-5 13 10211 12.so.0.11[.0]
1.0.19 10 10019 10.so.0.19[.0]
1.2.11 13 10211 12.so.0.11[.0]
1.0.20 10 10020 10.so.0.20[.0]
1.2.12 13 10212 12.so.0.12[.0]
1.2.13beta1 13 10213 12.so.0.13[.0]
1.0.21 10 10021 10.so.0.21[.0]
1.2.13 13 10213 12.so.0.13[.0]
1.2.14beta1-2 13 10214 12.so.0.14[.0]
1.0.22rc1 10 10022 10.so.0.22[.0]
1.2.14rc1 13 10214 12.so.0.14[.0]
1.2.15beta1-6 13 10215 12.so.0.15[.0]
1.0.23rc1-5 10 10023 10.so.0.23[.0]
1.2.15rc1-5 13 10215 12.so.0.15[.0]
1.0.23 10 10023 10.so.0.23[.0]
1.2.15 13 10215 12.so.0.15[.0]
1.2.16beta1-2 13 10216 12.so.0.16[.0]
1.2.16rc1 13 10216 12.so.0.16[.0]
1.0.24 10 10024 10.so.0.24[.0]
1.2.16 13 10216 12.so.0.16[.0]
1.2.17beta1-2 13 10217 12.so.0.17[.0]
1.0.25rc1 10 10025 10.so.0.25[.0]
1.2.17rc1-3 13 10217 12.so.0.17[.0]
1.0.25 10 10025 10.so.0.25[.0]
1.2.17 13 10217 12.so.0.17[.0]
1.0.26 10 10026 10.so.0.26[.0]
1.2.18 13 10218 12.so.0.18[.0]
1.2.19beta1-31 13 10219 12.so.0.19[.0]
1.0.27rc1-6 10 10027 10.so.0.27[.0]
1.2.19rc1-6 13 10219 12.so.0.19[.0]
1.0.27 10 10027 10.so.0.27[.0]
1.2.19 13 10219 12.so.0.19[.0]
1.2.20beta01-04 13 10220 12.so.0.20[.0]
1.0.28rc1-6 10 10028 10.so.0.28[.0]
1.2.20rc1-6 13 10220 12.so.0.20[.0]
1.0.28 10 10028 10.so.0.28[.0]
1.2.20 13 10220 12.so.0.20[.0]
1.2.21beta1-2 13 10221 12.so.0.21[.0]
1.2.21rc1-3 13 10221 12.so.0.21[.0]
1.0.29 10 10029 10.so.0.29[.0]
1.2.21 13 10221 12.so.0.21[.0]
1.2.22beta1-4 13 10222 12.so.0.22[.0]
1.0.30rc1 13 10030 10.so.0.30[.0]
1.2.22rc1 13 10222 12.so.0.22[.0]
1.0.30 10 10030 10.so.0.30[.0]
1.2.22 13 10222 12.so.0.22[.0]
1.2.23beta01-05 13 10223 12.so.0.23[.0]
1.2.23rc01 13 10223 12.so.0.23[.0]
1.2.23 13 10223 12.so.0.23[.0]
1.2.24beta01-02 13 10224 12.so.0.24[.0]
1.2.24rc01 13 10224 12.so.0.24[.0]
1.2.24 13 10224 12.so.0.24[.0]
1.2.25beta01-06 13 10225 12.so.0.25[.0]
1.2.25rc01-02 13 10225 12.so.0.25[.0]
1.0.31 10 10031 10.so.0.31[.0]
1.2.25 13 10225 12.so.0.25[.0]
1.2.26beta01-06 13 10226 12.so.0.26[.0]
1.2.26rc01 13 10226 12.so.0.26[.0]
1.2.26 13 10226 12.so.0.26[.0]
1.0.32 10 10032 10.so.0.32[.0]
1.2.27beta01-06 13 10227 12.so.0.27[.0]
Henceforth the source version will match the shared-library minor and patch numbers; the
shared-library major version number will be used for changes in backward compatibility, as
it is intended. The PNG_PNGLIB_VER macro, which is not used within libpng but is avail-
able for applications, is an unsigned integer of the form xyyzz corresponding to the
source version x.y.z (leading zeros in y and z). Beta versions were given the previous
public release number plus a letter, until version 1.0.6j; from then on they were given
the upcoming public release number plus "betaNN" or "rcN".
SEE ALSO
libpngpf(3), png(5)
libpng:
http://libpng.sourceforge.net (follow the [DOWNLOAD] link)
http://www.libpng.org/pub/png
zlib:
(generally) at the same location as libpng or at
ftp://ftp.info-zip.org/pub/infozip/zlib
PNGspecification:RFC2083
(generally) at the same location as libpng or at
ftp://ftp.rfc-editor.org:/in-notes/rfc2083.txt
or (as a W3C Recommendation) at
http://www.w3.org/TR/REC-png.html
In the case of any inconsistency between the PNG specification and this library, the spec-
ification takes precedence.
AUTHORS
This man page: Glenn Randers-Pehrson <glennrp at users.sourceforge.net>
The contributing authors would like to thank all those who helped with testing, bug fixes,
and patience. This wouldn't have been possible without all of you.
Thanks to Frank J. T. Wojcik for helping with the documentation.
Libpng version 1.2.27 - April 29, 2008: Initially created in 1995 by Guy Eric Schalnat,
then of Group 42, Inc. Currently maintained by Glenn Randers-Pehrson (glennrp at
users.sourceforge.net).
Supported by the PNG development group
png-mng-implement at lists.sf.net (subscription required; visit png-mng-implement at
lists.sourceforge.net (subscription required; visit https://lists.source-
forge.net/lists/listinfo/png-mng-implement to subscribe).
COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:
(This copy of the libpng notices is provided for your convenience. In case of any dis-
crepancy between this copy and the notices in the file png.h that is included in the
libpng distribution, the latter shall prevail.)
If you modify libpng you may insert additional notices immediately following this sen-
tence.
libpng versions 1.2.6, August 15, 2004, through 1.2.27, April 29, 2008, are Copyright (c)
2004,2006-2008 Glenn Randers-Pehrson, and are distributed according to the same disclaimer
and license as libpng-1.2.5 with the following individual added to the list of Contribut-
ing Authors
Cosmin Truta
libpng versions 1.0.7, July 1, 2000, through 1.2.5 - October 3, 2002, are Copyright (c)
2000-2002 Glenn Randers-Pehrson, and are distributed according to the same disclaimer and
license as libpng-1.0.6 with the following individuals added to the list of Contributing
Authors
Simon-Pierre Cadieux
Eric S. Raymond
Gilles Vollant
and with the following additions to the disclaimer:
There is no warranty against interference with your
enjoyment of the library or against infringement.
There is no warranty that our efforts or the library
will fulfill any of your particular purposes or needs.
This library is provided with all faults, and the entire
risk of satisfactory quality, performance, accuracy, and
effort is with the user.
libpng versions 0.97, January 1998, through 1.0.6, March 20, 2000, are Copyright (c) 1998,
1999 Glenn Randers-Pehrson Distributed according to the same disclaimer and license as
libpng-0.96, with the following individuals added to the list of Contributing Authors:
Tom Lane
Glenn Randers-Pehrson
Willem van Schaik
libpng versions 0.89, June 1996, through 0.96, May 1997, are Copyright (c) 1996, 1997
Andreas Dilger Distributed according to the same disclaimer and license as libpng-0.88,
with the following individuals added to the list of Contributing Authors:
John Bowler
Kevin Bracey
Sam Bushell
Magnus Holmgren
Greg Roelofs
Tom Tanner
libpng versions 0.5, May 1995, through 0.88, January 1996, are Copyright (c) 1995, 1996
Guy Eric Schalnat, Group 42, Inc.
For the purposes of this copyright and license, "Contributing Authors" is defined as the
following set of individuals:
Andreas Dilger
Dave Martindale
Guy Eric Schalnat
Paul Schmidt
Tim Wegner
The PNG Reference Library is supplied "AS IS". The Contributing Authors and Group 42,
Inc. disclaim all warranties, expressed or implied, including, without limitation, the
warranties of merchantability and of fitness for any purpose. The Contributing Authors
and Group 42, Inc. assume no liability for direct, indirect, incidental, special, exem-
plary, or consequential damages, which may result from the use of the PNG Reference
Library, even if advised of the possibility of such damage.
Permission is hereby granted to use, copy, modify, and distribute this source code, or
portions hereof, for any purpose, without fee, subject to the following restrictions:
1. The origin of this source code must not be misrepresented.
2. Altered versions must be plainly marked as such and
must not be misrepresented as being the original source.
3. This Copyright notice may not be removed or altered from
any source or altered source distribution.
The Contributing Authors and Group 42, Inc. specifically permit, without fee, and encour-
age the use of this source code as a component to supporting the PNG file format in com-
mercial products. If you use this source code in a product, acknowledgment is not
required but would be appreciated.
A "png_get_copyright" function is available, for convenient use in "about" boxes and the
like:
printf("%s",png_get_copyright(NULL));
Also, the PNG logo (in PNG format, of course) is supplied in the files "pngbar.png" and
"pngbar.jpg (88x31) and "pngnow.png" (98x31).
Libpng is OSI Certified Open Source Software. OSI Certified Open Source is a certifica-
tion mark of the Open Source Initiative.
Glenn Randers-Pehrson glennrp at users.sourceforge.net April 29, 2008
April 29, 2008 LIBPNG(3)
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