/usr/man/man.3/libpng.3
LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
NAME
libpng - Portable Network Graphics (PNG) Reference Library
1.2.12
SYNOPSIS
#include <png.h>
png_uint_32 png_access_version_number (void);
int png_check_sig (png_bytep sig, int num);
void png_chunk_error (png_structp png_ptr, png_const_charp
error);
void png_chunk_warning (png_structp png_ptr, png_const_charp
message);
void png_convert_from_struct_tm (png_timep ptime, struct tm
FAR * ttime);
void png_convert_from_time_t (png_timep ptime, time_t
ttime);
png_charp png_convert_to_rfc1123 (png_structp png_ptr,
png_timep ptime);
png_infop png_create_info_struct (png_structp png_ptr);
png_structp png_create_read_struct (png_const_charp
user_png_ver, png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warn_fn);
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png_structp png_create_read_struct_2(png_const_charp
user_png_ver, png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warn_fn, png_voidp mem_ptr, png_malloc_ptr
malloc_fn, png_free_ptr free_fn);
png_structp png_create_write_struct (png_const_charp
user_png_ver, png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warn_fn);
png_structp png_create_write_struct_2(png_const_charp
user_png_ver, png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warn_fn, png_voidp mem_ptr, png_malloc_ptr
malloc_fn, png_free_ptr free_fn);
int png_debug(int level, png_const_charp message);
int png_debug1(int level, png_const_charp message, p1);
int png_debug2(int level, png_const_charp message, p1, p2);
void png_destroy_info_struct (png_structp png_ptr,
png_infopp info_ptr_ptr);
void png_destroy_read_struct (png_structpp png_ptr_ptr,
png_infopp info_ptr_ptr, png_infopp end_info_ptr_ptr);
void png_destroy_write_struct (png_structpp png_ptr_ptr,
png_infopp info_ptr_ptr);
void png_error (png_structp png_ptr, png_const_charp error);
void png_free (png_structp png_ptr, png_voidp ptr);
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void png_free_chunk_list (png_structp png_ptr);
void png_free_default(png_structp png_ptr, png_voidp ptr);
void png_free_data (png_structp png_ptr, png_infop info_ptr,
int num);
png_byte png_get_bit_depth (png_structp png_ptr, png_infop
info_ptr);
png_uint_32 png_get_bKGD (png_structp png_ptr, png_infop
info_ptr, png_color_16p *background);
png_byte png_get_channels (png_structp png_ptr, png_infop
info_ptr);
png_uint_32 png_get_cHRM (png_structp png_ptr, png_infop
info_ptr, double *white_x, double *white_y, double *red_x,
double *red_y, double *green_x, double *green_y, double
*blue_x, double *blue_y);
png_uint_32 png_get_cHRM_fixed (png_structp png_ptr,
png_infop info_ptr, png_uint_32 *white_x, png_uint_32
*white_y, png_uint_32 *red_x, png_uint_32 *red_y,
png_uint_32 *green_x, png_uint_32 *green_y, png_uint_32
*blue_x, png_uint_32 *blue_y);
png_byte png_get_color_type (png_structp png_ptr, png_infop
info_ptr);
png_byte png_get_compression_type (png_structp png_ptr,
png_infop info_ptr);
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png_byte png_get_copyright (png_structp png_ptr);
png_voidp png_get_error_ptr (png_structp png_ptr);
png_byte png_get_filter_type (png_structp png_ptr, png_infop
info_ptr);
png_uint_32 png_get_gAMA (png_structp png_ptr, png_infop
info_ptr, double *file_gamma);
png_uint_32 png_get_gAMA_fixed (png_structp png_ptr,
png_infop info_ptr, png_uint_32 *int_file_gamma);
png_byte png_get_header_ver (png_structp png_ptr);
png_byte png_get_header_version (png_structp png_ptr);
png_uint_32 png_get_hIST (png_structp png_ptr, png_infop
info_ptr, png_uint_16p *hist);
png_uint_32 png_get_iCCP (png_structp png_ptr, png_infop
info_ptr, png_charpp name, int *compression_type, png_charpp
profile, png_uint_32 *proflen);
png_uint_32 png_get_IHDR (png_structp png_ptr, png_infop
info_ptr, png_uint_32 *width, png_uint_32 *height, int
*bit_depth, int *color_type, int *interlace_type, int
*compression_type, int *filter_type);
png_uint_32 png_get_image_height (png_structp png_ptr,
png_infop info_ptr);
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png_uint_32 png_get_image_width (png_structp png_ptr,
png_infop info_ptr);
#if !defined(PNG_1_0_X)
png_int_32 png_get_int_32 (png_bytep buf);
#endif
png_byte png_get_interlace_type (png_structp png_ptr,
png_infop info_ptr);
png_voidp png_get_io_ptr (png_structp png_ptr);
png_byte png_get_libpng_ver (png_structp png_ptr);
png_voidp png_get_mem_ptr(png_structp png_ptr);
png_uint_32 png_get_oFFs (png_structp png_ptr, png_infop
info_ptr, png_uint_32 *offset_x, png_uint_32 *offset_y, int
*unit_type);
png_uint_32 png_get_pCAL (png_structp png_ptr, png_infop
info_ptr, png_charp *purpose, png_int_32 *X0, png_int_32
*X1, int *type, int *nparams, png_charp *units, png_charpp
*params);
png_uint_32 png_get_pHYs (png_structp png_ptr, png_infop
info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int
*unit_type);
float png_get_pixel_aspect_ratio (png_structp png_ptr,
png_infop info_ptr);
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png_uint_32 png_get_pixels_per_meter (png_structp png_ptr,
png_infop info_ptr);
png_voidp png_get_progressive_ptr (png_structp png_ptr);
png_uint_32 png_get_PLTE (png_structp png_ptr, png_infop
info_ptr, png_colorp *palette, int *num_palette);
png_byte png_get_rgb_to_gray_status (png_structp png_ptr)
png_uint_32 png_get_rowbytes (png_structp png_ptr, png_infop
info_ptr);
png_bytepp png_get_rows (png_structp png_ptr, png_infop
info_ptr);
png_uint_32 png_get_sBIT (png_structp png_ptr, png_infop
info_ptr, png_color_8p *sig_bit);
png_bytep png_get_signature (png_structp png_ptr, png_infop
info_ptr);
png_uint_32 png_get_sPLT (png_structp png_ptr, png_infop
info_ptr, png_spalette_p *splt_ptr);
png_uint_32 png_get_sRGB (png_structp png_ptr, png_infop
info_ptr, int *intent);
png_uint_32 png_get_text (png_structp png_ptr, png_infop
info_ptr, png_textp *text_ptr, int *num_text);
png_uint_32 png_get_tIME (png_structp png_ptr, png_infop
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info_ptr, png_timep *mod_time);
png_uint_32 png_get_tRNS (png_structp png_ptr, png_infop
info_ptr, png_bytep *trans, int *num_trans, png_color_16p
*trans_values);
#if !defined(PNG_1_0_X)
png_uint_16 png_get_uint_16 (png_bytep buf);
png_uint_32 png_get_uint_31 (png_bytep buf);
png_uint_32 png_get_uint_32 (png_bytep buf);
#endif
png_uint_32 png_get_unknown_chunks (png_structp png_ptr,
png_infop info_ptr, png_unknown_chunkpp unknowns);
png_voidp png_get_user_chunk_ptr (png_structp png_ptr);
png_uint_32 png_get_user_height_max( png_structp png_ptr);
png_voidp png_get_user_transform_ptr (png_structp png_ptr);
png_uint_32 png_get_user_width_max (png_structp png_ptr);
png_uint_32 png_get_valid (png_structp png_ptr, png_infop
info_ptr, png_uint_32 flag);
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png_int_32 png_get_x_offset_microns (png_structp png_ptr,
png_infop info_ptr);
png_int_32 png_get_x_offset_pixels (png_structp png_ptr,
png_infop info_ptr);
png_uint_32 png_get_x_pixels_per_meter (png_structp png_ptr,
png_infop info_ptr);
png_int_32 png_get_y_offset_microns (png_structp png_ptr,
png_infop info_ptr);
png_int_32 png_get_y_offset_pixels (png_structp png_ptr,
png_infop info_ptr);
png_uint_32 png_get_y_pixels_per_meter (png_structp png_ptr,
png_infop info_ptr);
png_uint_32 png_get_compression_buffer_size (png_structp
png_ptr);
int png_handle_as_unknown (png_structp png_ptr, png_bytep
chunk_name);
void png_init_io (png_structp png_ptr, FILE *fp);
DEPRECATED: void png_info_init (png_infop info_ptr);
DEPRECATED: void png_info_init_2 (png_infopp ptr_ptr,
png_size_t png_info_struct_size);
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png_voidp png_malloc (png_structp png_ptr, png_uint_32
size);
png_voidp png_malloc_default(png_structp png_ptr,
png_uint_32 size);
voidp png_memcpy (png_voidp s1, png_voidp s2, png_size_t
size);
png_voidp png_memcpy_check (png_structp png_ptr, png_voidp
s1, png_voidp s2, png_uint_32 size);
voidp png_memset (png_voidp s1, int value, png_size_t size);
png_voidp png_memset_check (png_structp png_ptr, png_voidp
s1, int value, png_uint_32 size);
DEPRECATED: void png_permit_empty_plte (png_structp png_ptr,
int empty_plte_permitted);
void png_process_data (png_structp png_ptr, png_infop
info_ptr, png_bytep buffer, png_size_t buffer_size);
void png_progressive_combine_row (png_structp png_ptr,
png_bytep old_row, png_bytep new_row);
void png_read_destroy (png_structp png_ptr, png_infop
info_ptr, png_infop end_info_ptr);
void png_read_end (png_structp png_ptr, png_infop info_ptr);
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void png_read_image (png_structp png_ptr, png_bytepp image);
DEPRECATED: void png_read_init (png_structp png_ptr);
DEPRECATED: void png_read_init_2 (png_structpp ptr_ptr,
png_const_charp user_png_ver, png_size_t png_struct_size,
png_size_t png_info_size);
void png_read_info (png_structp png_ptr, png_infop
info_ptr);
void png_read_png (png_structp png_ptr, png_infop info_ptr,
int transforms, png_voidp params);
void png_read_row (png_structp png_ptr, png_bytep row,
png_bytep display_row);
void png_read_rows (png_structp png_ptr, png_bytepp row,
png_bytepp display_row, png_uint_32 num_rows);
void png_read_update_info (png_structp png_ptr, png_infop
info_ptr);
#if !defined(PNG_1_0_X)
png_save_int_32 (png_bytep buf, png_int_32 i);
void png_save_uint_16 (png_bytep buf, unsigned int i);
void png_save_uint_32 (png_bytep buf, png_uint_32 i);
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void png_set_add_alpha (png_structp png_ptr, png_uint_32
filler, int flags);
#endif
void png_set_background (png_structp png_ptr, png_color_16p
background_color, int background_gamma_code, int
need_expand, double background_gamma);
void png_set_bgr (png_structp png_ptr);
void png_set_bKGD (png_structp png_ptr, png_infop info_ptr,
png_color_16p background);
void png_set_cHRM (png_structp png_ptr, png_infop info_ptr,
double white_x, double white_y, double red_x, double red_y,
double green_x, double green_y, double blue_x, double
blue_y);
void png_set_cHRM_fixed (png_structp png_ptr, png_infop
info_ptr, png_uint_32 white_x, png_uint_32 white_y,
png_uint_32 red_x, png_uint_32 red_y, png_uint_32 green_x,
png_uint_32 green_y, png_uint_32 blue_x, png_uint_32
blue_y);
void png_set_compression_level (png_structp png_ptr, int
level);
void png_set_compression_mem_level (png_structp png_ptr, int
mem_level);
void png_set_compression_method (png_structp png_ptr, int
method);
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void png_set_compression_strategy (png_structp png_ptr, int
strategy);
void png_set_compression_window_bits (png_structp png_ptr,
int window_bits);
void png_set_crc_action (png_structp png_ptr, int
crit_action, int ancil_action);
void png_set_dither (png_structp png_ptr, png_colorp
palette, int num_palette, int maximum_colors, png_uint_16p
histogram, int full_dither);
void png_set_error_fn (png_structp png_ptr, png_voidp
error_ptr, png_error_ptr error_fn, png_error_ptr
warning_fn);
void png_set_expand (png_structp png_ptr);
void png_set_expand_gray_1_2_4_to_8(png_structp png_ptr);
void png_set_filler (png_structp png_ptr, png_uint_32
filler, int flags);
void png_set_filter (png_structp png_ptr, int method, int
filters);
void png_set_filter_heuristics (png_structp png_ptr, int
heuristic_method, int num_weights, png_doublep
filter_weights, png_doublep filter_costs);
void png_set_flush (png_structp png_ptr, int nrows);
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void png_set_gamma (png_structp png_ptr, double
screen_gamma, double default_file_gamma);
void png_set_gAMA (png_structp png_ptr, png_infop info_ptr,
double file_gamma);
void png_set_gAMA_fixed (png_structp png_ptr, png_infop
info_ptr, png_uint_32 file_gamma);
void png_set_gray_1_2_4_to_8(png_structp png_ptr);
void png_set_gray_to_rgb (png_structp png_ptr);
void png_set_hIST (png_structp png_ptr, png_infop info_ptr,
png_uint_16p hist);
void png_set_iCCP (png_structp png_ptr, png_infop info_ptr,
png_charp name, int compression_type, png_charp profile,
png_uint_32 proflen);
int png_set_interlace_handling (png_structp png_ptr);
void png_set_invalid (png_structp png_ptr, png_infop
info_ptr, int mask);
void png_set_invert_alpha (png_structp png_ptr);
void png_set_invert_mono (png_structp png_ptr);
void png_set_IHDR (png_structp png_ptr, png_infop info_ptr,
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png_uint_32 width, png_uint_32 height, int bit_depth, int
color_type, int interlace_type, int compression_type, int
filter_type);
void png_set_keep_unknown_chunks (png_structp png_ptr, int
keep, png_bytep chunk_list, int num_chunks);
void png_set_mem_fn(png_structp png_ptr, png_voidp mem_ptr,
png_malloc_ptr malloc_fn, png_free_ptr free_fn);
void png_set_oFFs (png_structp png_ptr, png_infop info_ptr,
png_uint_32 offset_x, png_uint_32 offset_y, int unit_type);
void png_set_packing (png_structp png_ptr);
void png_set_packswap (png_structp png_ptr);
void png_set_palette_to_rgb(png_structp png_ptr);
void png_set_pCAL (png_structp png_ptr, png_infop info_ptr,
png_charp purpose, png_int_32 X0, png_int_32 X1, int type,
int nparams, png_charp units, png_charpp params);
void png_set_pHYs (png_structp png_ptr, png_infop info_ptr,
png_uint_32 res_x, png_uint_32 res_y, int unit_type);
void png_set_progressive_read_fn (png_structp png_ptr,
png_voidp progressive_ptr, png_progressive_info_ptr info_fn,
png_progressive_row_ptr row_fn, png_progressive_end_ptr
end_fn);
void png_set_PLTE (png_structp png_ptr, png_infop info_ptr,
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png_colorp palette, int num_palette);
void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr,
png_rw_ptr read_data_fn);
void png_set_read_status_fn (png_structp png_ptr,
png_read_status_ptr read_row_fn);
void png_set_read_user_transform_fn (png_structp png_ptr,
png_user_transform_ptr read_user_transform_fn);
void png_set_rgb_to_gray (png_structp png_ptr, int
error_action, double red, double green);
void png_set_rgb_to_gray_fixed (png_structp png_ptr, int
error_action png_fixed_point red, png_fixed_point green);
void png_set_rows (png_structp png_ptr, png_infop info_ptr,
png_bytepp row_pointers);
void png_set_sBIT (png_structp png_ptr, png_infop info_ptr,
png_color_8p sig_bit);
void png_set_sCAL (png_structp png_ptr, png_infop info_ptr,
png_charp unit, double width, double height);
void png_set_shift (png_structp png_ptr, png_color_8p
true_bits);
void png_set_sig_bytes (png_structp png_ptr, int num_bytes);
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void png_set_sPLT (png_structp png_ptr, png_infop info_ptr,
png_spalette_p splt_ptr, int num_spalettes);
void png_set_sRGB (png_structp png_ptr, png_infop info_ptr,
int intent);
void png_set_sRGB_gAMA_and_cHRM (png_structp png_ptr,
png_infop info_ptr, int intent);
void png_set_strip_16 (png_structp png_ptr);
void png_set_strip_alpha (png_structp png_ptr);
void png_set_swap (png_structp png_ptr);
void png_set_swap_alpha (png_structp png_ptr);
void png_set_text (png_structp png_ptr, png_infop info_ptr,
png_textp text_ptr, int num_text);
void png_set_tIME (png_structp png_ptr, png_infop info_ptr,
png_timep mod_time);
void png_set_tRNS (png_structp png_ptr, png_infop info_ptr,
png_bytep trans, int num_trans, png_color_16p trans_values);
void png_set_tRNS_to_alpha(png_structp png_ptr);
png_uint_32 png_set_unknown_chunks (png_structp png_ptr,
png_infop info_ptr, png_unknown_chunkp unknowns, int num,
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int location);
void png_set_unknown_chunk_location(png_structp png_ptr,
png_infop info_ptr, int chunk, int location);
void png_set_read_user_chunk_fn (png_structp png_ptr,
png_voidp user_chunk_ptr, png_user_chunk_ptr
read_user_chunk_fn);
void png_set_user_limits (png_structp png_ptr, png_uint_32
user_width_max, png_uint_32 user_height_max);
void png_set_user_transform_info (png_structp png_ptr,
png_voidp user_transform_ptr, int user_transform_depth, int
user_transform_channels);
void png_set_write_fn (png_structp png_ptr, png_voidp
io_ptr, png_rw_ptr write_data_fn, png_flush_ptr
output_flush_fn);
void png_set_write_status_fn (png_structp png_ptr,
png_write_status_ptr write_row_fn);
void png_set_write_user_transform_fn (png_structp png_ptr,
png_user_transform_ptr write_user_transform_fn);
void png_set_compression_buffer_size(png_structp png_ptr,
png_uint_32 size);
int png_sig_cmp (png_bytep sig, png_size_t start, png_size_t
num_to_check);
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void png_start_read_image (png_structp png_ptr);
void png_warning (png_structp png_ptr, png_const_charp mes-
sage);
void png_write_chunk (png_structp png_ptr, png_bytep
chunk_name, png_bytep data, png_size_t length);
void png_write_chunk_data (png_structp png_ptr, png_bytep
data, png_size_t length);
void png_write_chunk_end (png_structp png_ptr);
void png_write_chunk_start (png_structp png_ptr, png_bytep
chunk_name, png_uint_32 length);
void png_write_destroy (png_structp png_ptr);
void png_write_end (png_structp png_ptr, png_infop
info_ptr);
void png_write_flush (png_structp png_ptr);
void png_write_image (png_structp png_ptr, png_bytepp
image);
DEPRECATED: void png_write_init (png_structp png_ptr);
DEPRECATED: void png_write_init_2 (png_structpp ptr_ptr,
png_const_charp user_png_ver, png_size_t png_struct_size,
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png_size_t png_info_size);
void png_write_info (png_structp png_ptr, png_infop
info_ptr);
void png_write_info_before_PLTE (png_structp png_ptr,
png_infop info_ptr);
void png_write_png (png_structp png_ptr, png_infop info_ptr,
int transforms, png_voidp params);
void png_write_row (png_structp png_ptr, png_bytep row);
void png_write_rows (png_structp png_ptr, png_bytepp row,
png_uint_32 num_rows);
voidpf png_zalloc (voidpf png_ptr, uInt items, uInt size);
void png_zfree (voidpf png_ptr, voidpf ptr);
DESCRIPTION
The libpng library supports encoding, decoding, and various
manipulations of the Portable Network Graphics (PNG) format
image files. It uses the zlib(3) compression library. Fol-
lowing is a copy of the libpng.txt file that accompanies
libpng.
LIBPNG.TXT
libpng.txt - A description on how to use and modify libpng
libpng version 1.2.12 - June 27, 2006
Updated and distributed by Glenn Randers-Pehrson
<glennrp at users.sourceforge.net>
Copyright (c) 1998-2005 Glenn Randers-Pehrson
For conditions of distribution and use, see copyright
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
notice in png.h.
based on:
libpng 1.0 beta 6 version 0.96 May 28, 1997
Updated and distributed by Andreas Dilger
Copyright (c) 1996, 1997 Andreas Dilger
libpng 1.0 beta 2 - version 0.88 January 26, 1996
For conditions of distribution and use, see copyright
notice in png.h. Copyright (c) 1995, 1996 Guy Eric
Schalnat, Group 42, Inc.
Updated/rewritten per request in the libpng FAQ
Copyright (c) 1995, 1996 Frank J. T. Wojcik
December 18, 1995 & January 20, 1996
I. Introduction
This file describes how to use and modify the PNG reference
library (known as libpng) for your own use. There are five
sections to this file: introduction, structures, reading,
writing, and modification and configuration notes for vari-
ous special platforms. In addition to this file, example.c
is a good starting point for using the library, as it is
heavily commented and should include everything most people
will need. We assume that libpng is already installed; see
the INSTALL file for instructions on how to install libpng.
Libpng was written as a companion to the PNG specification,
as a way of reducing the amount of time and effort it takes
to support the PNG file format in application programs.
The PNG specification (second edition), November 2003, is
available as a W3C Recommendation and as an ISO Standard
(ISO/IEC 15948:2003 (E)) at <http://www.w3.org/TR/2003/REC-
PNG-20031110/ The W3C and ISO documents have identical
technical content.
The PNG-1.2 specification is available at
<http://www.libpng.org/pub/png/documents/>
The PNG-1.0 specification is available as RFC 2083
<http://www.libpng.org/pub/png/documents/> and as a W3C
Recommendation <http://www.w3.org/TR/REC.png.html>. Some
additional chunks are described in the special-purpose pub-
lic 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/>.
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
Most users will not have to modify the library signifi-
cantly; advanced users may want to modify it more. All
attempts were made to make it as complete as possible, while
keeping the code easy to understand. Currently, this
library only supports C. Support for other languages is
being considered.
Libpng has been designed to handle multiple sessions at one
time, to be easily modifiable, to be portable to the vast
majority of machines (ANSI, K&R, 16-, 32-, and 64-bit)
available, and to be easy to use. The ultimate goal of
libpng is to promote the acceptance of the PNG file format
in whatever way possible. While there is still work to be
done (see the TODO file), libpng should cover the majority
of the needs of its users.
Libpng uses zlib for its compression and decompression of
PNG files. Further information about zlib, and the latest
version of zlib, can be found at the zlib home page,
<http://www.info-zip.org/pub/infozip/zlib/>. The zlib
compression utility is a general purpose utility that is
useful for more than PNG files, and can be used without
libpng. See the documentation delivered with zlib for more
details. You can usually find the source files for the zlib
utility wherever you find the libpng source files.
Libpng is thread safe, provided the threads are using dif-
ferent instances of the structures. Each thread should have
its own png_struct and png_info instances, and thus its own
image. Libpng does not protect itself against two threads
using the same instance of a structure. Note: thread safety
may be defeated by use of some of the MMX assembler code in
pnggccrd.c, which is only compiled when the user defines
PNG_THREAD_UNSAFE_OK.
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 applications, but it
is suggested that applications use the new interfaces if at
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
all possible.
Applications that do make direct access to the members of
png_struct (except for png_ptr->jmpbuf) must be recompiled
whenever the library is updated, and applications that make
direct access to the members of png_info must be recompiled
if they were compiled or loaded with libpng version 1.0.6,
in which the members were in a different order. In version
1.0.7, the members of the png_info structure reverted to the
old order, as they were in versions 0.97c through 1.0.5.
Starting with version 2.0.0, both structures are going to be
hidden, and the contents of the structures will only be
accessible through the png_get/png_set functions.
The png.h header file is an invaluable reference for pro-
gramming with libpng. And while I'm on the topic, make sure
you include the libpng header file:
#include <png.h>
III. Reading
We'll now walk you through the possible functions to call
when reading in a PNG file sequentially, briefly explaining
the syntax and purpose of each one. See example.c and png.h
for more detail. While progressive reading is covered in
the next section, you will still need some of the functions
discussed in this section to read a PNG file.
Setup
You will want to do the I/O initialization(*) before you get
into libpng, so if it doesn't work, you don't have much to
undo. Of course, you will also want to insure that you are,
in fact, dealing with a PNG file. Libpng provides a simple
check to see if a file is a PNG file. To use it, pass in
the first 1 to 8 bytes of the file to the function
png_sig_cmp(), and it will return 0 if the bytes match the
corresponding bytes of the PNG signature, or nonzero other-
wise. 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.
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
FILE *fp = fopen(file_name, "rb");
if (!fp)
{
return (ERROR);
}
fread(header, 1, number, fp);
is_png = !png_sig_cmp(header, 0, number);
if (!is_png)
{
return (NOT_PNG);
}
Next, png_struct and png_info need to be allocated and ini-
tialized. In order to ensure that the size of these struc-
tures is correct even with a dynamically linked libpng,
there are functions to initialize and allocate the struc-
tures. We also pass the library version, optional pointers
to error handling functions, and a pointer to a data struct
for use by the error functions, if necessary (the pointer
and functions can be NULL if the default error handlers are
to be used). See the section on Changes to Libpng below
regarding the old initialization functions. The structure
allocation functions quietly return NULL if they fail to
create the structure, so your application should check for
that.
png_structp png_ptr = png_create_read_struct
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
return (ERROR);
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_read_struct(&png_ptr,
(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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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 Cus-
tomizing Libpng section below.
png_init_io(png_ptr, fp);
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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: */
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. 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);
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);
{
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
/* put your code here */
}
(You can give it another name that you like instead of
"read_row_callback")
To inform libpng about your function, use
png_set_read_status_fn(png_ptr, read_row_callback);
Width and height limits
The PNG specification allows the width and height of an
image to be as large as 2^31-1 (0x7fffffff), or about 2.147
billion rows and columns. Since very few applications
really need to process such large images, we have imposed an
arbitrary 1-million limit on rows and columns. Larger
images will be rejected immediately with a png_error() call.
If you wish to override this limit, you can use
png_set_user_limits(png_ptr, width_max, height_max);
to set your own limits, or use width_max = height_max =
0x7fffffffL to allow all valid dimensions (libpng may reject
some very large images anyway because of potential buffer
overflow conditions).
You should put this statement after you create the PNG
structure and before calling png_read_info(),
png_read_png(), or png_process_data(). If you need to
retrieve the limits that are being applied, use
width_max = png_get_user_width_max(png_ptr);
height_max = png_get_user_height_max(png_ptr);
Unknown-chunk handling
Now you get to set the way the library processes unknown
chunks in the input PNG stream. Both known and unknown
chunks will be read. Normal behavior is that known chunks
will be parsed into information in various info_ptr members;
unknown chunks will be discarded. To change this, you can
call:
png_set_keep_unknown_chunks(png_ptr, keep,
chunk_list, num_chunks);
keep - 0: do not handle as unknown
1: 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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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.
The high-level read interface
At this point there are two ways to proceed; through the
high-level read interface, or through a sequence of low-
level read operations. You can use the high-level interface
if (a) you are willing to read the entire image into memory,
and (b) the input transformations you want to do are limited
to the following set:
PNG_TRANSFORM_IDENTITY No transformation
PNG_TRANSFORM_STRIP_16 Strip 16-bit samples to
8 bits
PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel
PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit
samples to bytes
PNG_TRANSFORM_PACKSWAP Change order of packed
pixels to LSB first
PNG_TRANSFORM_EXPAND Perform set_expand()
PNG_TRANSFORM_INVERT_MONO Invert monochrome images
PNG_TRANSFORM_SHIFT Normalize pixels to the
sBIT depth
PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
to BGRA
PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
to AG
PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
to transparency
PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
(This excludes setting a background color, doing gamma
transformation, dithering, and setting filler.) If this is
the case, simply do this:
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
png_read_png(png_ptr, info_ptr, png_transforms, NULL)
where png_transforms is an integer containing the logical OR
of some set of transformation flags. This call is
equivalent to png_read_info(), followed the set of transfor-
mations indicated by the transform mask, then
png_read_image(), and finally png_read_end().
(The final parameter of this call is not yet used. Someday
it might point to transformation parameters required by some
future input transform.)
You must use png_transforms and not call any
png_set_transform() functions when you use png_read_png().
After you have called png_read_png(), you can retrieve the
image data with
row_pointers = png_get_rows(png_ptr, info_ptr);
where row_pointers is an array of pointers to the pixel data
for each row:
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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
PNG_INTRAPIXEL_DIFFERENCING if
the PNG datastream is embedded in
a MNG-1.0 datastream)
compression_type - (must be PNG_COMPRESSION_TYPE_BASE
for PNG 1.0)
interlace_type - (PNG_INTERLACE_NONE or
PNG_INTERLACE_ADAM7)
Any or all of interlace_type, compression_type, of
filter_method can be NULL if you are
not interested in their values.
channels = png_get_channels(png_ptr, info_ptr);
channels - number of channels of info for the
color type (valid values are 1 (GRAY,
PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
4 (RGB_ALPHA or RGB + filler byte))
rowbytes = png_get_rowbytes(png_ptr, info_ptr);
rowbytes - number of bytes needed to hold a row
signature = png_get_signature(png_ptr, info_ptr);
signature - holds the signature read from the
file (if any). The data is kept in
the same offset it would be if the
whole signature were read (i.e. if an
application had already read in 4
bytes of signature before starting
libpng, the remaining 4 bytes would
be in signature[4] through signature[7]
(see png_set_sig_bytes())).
width = png_get_image_width(png_ptr,
info_ptr);
height = png_get_image_height(png_ptr,
info_ptr);
bit_depth = png_get_bit_depth(png_ptr,
info_ptr);
color_type = png_get_color_type(png_ptr,
info_ptr);
filter_method = png_get_filter_type(png_ptr,
info_ptr);
compression_type = png_get_compression_type(png_ptr,
info_ptr);
interlace_type = png_get_interlace_type(png_ptr,
info_ptr);
These are also important, but their validity depends on
whether the chunk has been read. The png_get_valid(png_ptr,
info_ptr, PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr,
info_ptr, ...) functions return non-zero if the data has
been read, or zero if it is missing. The parameters to the
Last change: June 27, 2006 30
LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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)
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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);
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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,
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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 com-
ments in keyword/text pairs, one pair per chunk, with no
limit on the number of text chunks, and a 2^31 byte limit on
their size. While there are suggested keywords, there is no
requirement to restrict the use to these strings. It is
strongly suggested that keywords and text be sensible to
humans (that's the point), so don't use abbreviations.
Non-printing symbols are not allowed. See the PNG specifi-
cation for more details. There is also no requirement to
have text after the keyword.
Keywords should be limited to 79 Latin-1 characters without
leading or trailing spaces, but non-consecutive spaces are
allowed within the keyword. It is possible to have the same
keyword any number of times. The text_ptr is an array of
png_text structures, each holding a pointer to a language
string, a pointer to a keyword and a pointer to a text
string. The text string, language code, and translated key-
word may be empty or NULL pointers. The keyword/text pairs
are put into the array in the order that they are received.
However, some or all of the text chunks may be after the
image, so, to make sure you have read all the text chunks,
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
don't mess with these until after you read the stuff after
the image. This will be mentioned again below in the dis-
cussion that goes with png_read_end().
Input transformations
After you've read the header information, you can set up the
library to handle any special transformations of the image
data. The various ways to transform the data will be
described in the order that they should occur. This is
important, as some of these change the color type and/or bit
depth of the data, and some others only work on certain
color types and bit depths. Even though each transformation
checks to see if it has data that it can do something with,
you should make sure to only enable a transformation if it
will be valid for the data. For example, don't swap red and
blue on grayscale data.
The colors used for the background and transparency values
should be supplied in the same format/depth as the current
image data. They are stored in the same format/depth as the
image data in a bKGD or tRNS chunk, so this is what libpng
expects for this data. The colors are transformed to keep
in sync with the image data when an application calls the
png_read_update_info() routine (see below).
Data will be decoded into the supplied row buffers packed
into bytes unless the library has been told to transform it
into another format. For example, 4 bit/pixel paletted or
grayscale data will be returned 2 pixels/byte with the left-
most pixel in the high-order bits of the byte, unless
png_set_packing() is called. 8-bit RGB data will be stored
in RGB RGB RGB format unless png_set_filler() or
png_set_add_alpha() is called to insert filler bytes, either
before or after each RGB triplet. 16-bit RGB data will be
returned RRGGBB RRGGBB, with the most significant byte of
the color value first, unless png_set_strip_16() is called
to transform it to regular RGB RGB triplets, or
png_set_filler() or png_set_add alpha() is called to insert
filler bytes, either before or after each RRGGBB triplet.
Similarly, 8-bit or 16-bit grayscale data can be modified
with png_set_filler(), png_set_add_alpha(), or
png_set_strip_16().
The following code transforms grayscale images of less than
8 to 8 bits, changes paletted images to RGB, and adds a full
alpha channel if there is transparency information in a tRNS
chunk. This is most useful on grayscale images with bit
depths of 2 or 4 or if there is a multiple-image viewing
application that wishes to treat all images in the same way.
if (color_type == PNG_COLOR_TYPE_PALETTE)
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
png_set_palette_to_rgb(png_ptr);
if (color_type == PNG_COLOR_TYPE_GRAY &&
bit_depth < 8)
png_set_expand_gray_1_2_4_to_8(png_ptr);
if (png_get_valid(png_ptr, info_ptr,
PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
These three functions are actually aliases for
png_set_expand(), added in libpng version 1.0.4, with the
function names expanded to improve code readability. In
some future version they may actually do different things.
As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8()
was added. It expands the sample depth without changing
tRNS to alpha. At the same time, png_set_gray_1_2_4_to_8()
was deprecated, and it will be removed from a future ver-
sion.
PNG can have files with 16 bits per channel. If you only
can handle 8 bits per channel, this will strip the pixels
down to 8 bit.
if (bit_depth == 16)
png_set_strip_16(png_ptr);
If, for some reason, you don't need the alpha channel on an
image, and you want to remove it rather than combining it
with the background (but the image author certainly had in
mind that you *would* combine it with the background, so
that's what you should probably do):
if (color_type & PNG_COLOR_MASK_ALPHA)
png_set_strip_alpha(png_ptr);
In PNG files, the alpha channel in an image is the level of
opacity. If you need the alpha channel in an image to be
the level of transparency instead of opacity, you can invert
the alpha channel (or the tRNS chunk data) after it's read,
so that 0 is fully opaque and 255 (in 8-bit or paletted
images) or 65535 (in 16-bit images) is fully transparent,
with
png_set_invert_alpha(png_ptr);
PNG files pack pixels of bit depths 1, 2, and 4 into bytes
as small as they can, resulting in, for example, 8 pixels
per byte for 1 bit files. This code expands to 1 pixel per
byte without changing the values of the pixels:
if (bit_depth < 8)
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
png_set_packing(png_ptr);
PNG files have possible bit depths of 1, 2, 4, 8, and 16.
All pixels stored in a PNG image have been "scaled" or
"shifted" up to the next higher possible bit depth (e.g.
from 5 bits/sample in the range [0,31] to 8 bits/sample in
the range [0, 255]). However, it is also possible to con-
vert the PNG pixel data back to the original bit depth of
the image. This call reduces the pixels back down to the
original bit depth:
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:
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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 grays-
cale or grayscale with alpha.
if (color_type == PNG_COLOR_TYPE_RGB ||
color_type == PNG_COLOR_TYPE_RGB_ALPHA)
png_set_rgb_to_gray_fixed(png_ptr, error_action,
int red_weight, int green_weight);
error_action = 1: silently do the conversion
error_action = 2: issue a warning if the original
image has any pixel where
red != green or red != blue
error_action = 3: issue an error and abort the
conversion if the original
image has any pixel where
red != green or red != blue
red_weight: weight of red component times 100000
green_weight: weight of green component times 100000
If either weight is negative, default
weights (21268, 71514) are used.
If you have set error_action = 1 or 2, you can later check
whether the image really was gray, after processing the
image rows, with the png_get_rgb_to_gray_status(png_ptr)
function. It will return a png_byte that is zero if the
image was gray or 1 if there were any non-gray pixels. bKGD
and sBIT data will be silently converted to grayscale, using
the green channel data, regardless of the error_action set-
ting.
With red_weight+green_weight<=100000, the normalized gray-
level is computed:
int rw = red_weight * 65536;
int gw = green_weight * 65536;
int bw = 65536 - (rw + gw);
gray = (rw*red + gw*green + bw*blue)/65536;
The default values approximate those recommended in the
Charles Poynton's Color FAQ,
<http://www.inforamp.net/~poynton/> Copyright (c) 1998-01-04
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
Charles Poynton <poynton at inforamp.net>
Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
Libpng approximates this with
Y = 0.21268 * R + 0.7151 * G + 0.07217 * B
which can be expressed with integers as
Y = (6969 * R + 23434 * G + 2365 * B)/32768
The calculation is done in a linear colorspace, if the image
gamma is known.
If you have a grayscale and you are using
png_set_expand_depth(), png_set_expand(), or
png_set_gray_to_rgb to change to truecolor or to a higher
bit-depth, you must either supply the background color as a
gray value at the original file bit-depth (need_expand = 1)
or else supply the background color as an RGB triplet at the
final, expanded bit depth (need_expand = 0). Similarly, if
you are reading a paletted image, you must either supply the
background color as a palette index (need_expand = 1) or as
an RGB triplet that may or may not be in the palette
(need_expand = 0).
png_color_16 my_background;
png_color_16p image_background;
if (png_get_bKGD(png_ptr, info_ptr, &image_background))
png_set_background(png_ptr, image_background,
PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
else
png_set_background(png_ptr, &my_background,
PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
The png_set_background() function tells libpng to composite
images with alpha or simple transparency against the sup-
plied background color. If the PNG file contains a bKGD
chunk (PNG_INFO_bKGD valid), you may use this color, or sup-
ply another color more suitable for the current display
(e.g., the background color from a web page). You need to
tell libpng whether the color is in the gamma space of the
display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply),
the file (PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD
chunk), or one that is neither of these gammas
(PNG_BACKGROUND_GAMMA_UNIQUE - I don't know why anyone would
use this, but it's here).
To properly display PNG images on any kind of system, the
application needs to know what the display gamma is.
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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 dis-
cussions on gamma in the PNG specification for an excellent
description of what gamma is, and why all applications
should support it. It is strongly recommended that PNG
viewers support gamma correction.
if (png_get_gAMA(png_ptr, info_ptr, &gamma))
png_set_gamma(png_ptr, screen_gamma, gamma);
else
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
png_set_gamma(png_ptr, screen_gamma, 0.45455);
If you need to reduce an RGB file to a paletted file, or if
a paletted file has more entries then will fit on your
screen, png_set_dither() will do that. Note that this is a
simple match dither that merely finds the closest color
available. This should work fairly well with optimized
palettes, and fairly badly with linear color cubes. If you
pass a palette that is larger then maximum_colors, the file
will reduce the number of colors in the palette so it will
fit into maximum_colors. If there is a histogram, it will
use it to make more intelligent choices when reducing the
palette. If there is no histogram, it may not do as good a
job.
if (color_type & PNG_COLOR_MASK_COLOR)
{
if (png_get_valid(png_ptr, info_ptr,
PNG_INFO_PLTE))
{
png_uint_16p histogram = NULL;
png_get_hIST(png_ptr, info_ptr,
&histogram);
png_set_dither(png_ptr, palette, num_palette,
max_screen_colors, histogram, 1);
}
else
{
png_color std_color_cube[MAX_SCREEN_COLORS] =
{ ... colors ... };
png_set_dither(png_ptr, std_color_cube,
MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
NULL,0);
}
}
PNG files describe monochrome as black being zero and white
being one. The following code will reverse this (make black
be one and white be zero):
if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
png_set_invert_mono(png_ptr);
This function can also be used to invert grayscale and
gray-alpha images:
if (color_type == PNG_COLOR_TYPE_GRAY ||
color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
png_set_invert_mono(png_ptr);
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
PNG files store 16 bit pixels in network byte order (big-
endian, ie. most significant bits first). This code changes
the storage to the other way (little-endian, i.e. least sig-
nificant bits first, the way PCs store them):
if (bit_depth == 16)
png_set_swap(png_ptr);
If you are using packed-pixel images (1, 2, or 4
bits/pixel), and you need to change the order the pixels are
packed into bytes, you can use:
if (bit_depth < 8)
png_set_packswap(png_ptr);
Finally, you can write your own transformation function if
none of the existing ones meets your needs. This is done by
setting a callback with
png_set_read_user_transform_fn(png_ptr,
read_transform_fn);
You must supply the function
void read_transform_fn(png_ptr ptr, row_info_ptr
row_info, png_bytep data)
See pngtest.c for a working example. Your function will be
called after all of the other transformations have been pro-
cessed.
You can also set up a pointer to a user structure for use by
your callback function, and you can inform libpng that your
transform function will change the number of channels or bit
depth with the function
png_set_user_transform_info(png_ptr, user_ptr,
user_depth, user_channels);
The user's application, not libpng, is responsible for allo-
cating and freeing any memory required for the user struc-
ture.
You can retrieve the pointer via the function
png_get_user_transform_ptr(). For example:
voidp read_user_transform_ptr =
png_get_user_transform_ptr(png_ptr);
The last thing to handle is interlacing; this is covered in
detail below, but you must call the function here if you
want libpng to handle expansion of the interlaced image.
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
number_of_passes = png_set_interlace_handling(png_ptr);
After setting the transformations, libpng can update your
png_info structure to reflect any transformations you've
requested with this call. This is most useful to update the
info structure's rowbytes field so you can use it to allo-
cate your image memory. This function will also update your
palette with the correct screen_gamma and background if
these have been given with the calls above.
png_read_update_info(png_ptr, info_ptr);
After you call png_read_update_info(), you can allocate any
memory you need to hold the image. The row data is simply
raw byte data for all forms of images. As the actual allo-
cation varies among applications, no example will be given.
If you are allocating one large chunk, you will need to
build an array of pointers to each row, as it will be needed
for some of the functions below.
Reading image data
After you've allocated memory, you can read the image data.
The simplest way to do this is in one function call. If you
are allocating enough memory to hold the whole image, you
can just call png_read_image() and libpng will read in all
the image data and put it in the memory area supplied. You
will need to pass in an array of pointers to each row.
This function automatically handles interlacing, so you
don't need to call png_set_interlace_handling() or call this
function multiple times, or any of that other stuff neces-
sary with png_read_rows().
png_read_image(png_ptr, row_pointers);
where row_pointers is:
png_bytep row_pointers[height];
You can point to void or char or whatever you use for pix-
els.
If you don't want to read in the whole image at once, you
can use png_read_rows() instead. If there is no interlacing
(check interlace_type == PNG_INTERLACE_NONE), this is sim-
ple:
png_read_rows(png_ptr, row_pointers, NULL,
number_of_rows);
where row_pointers is the same as in the png_read_image()
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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 com-
plicated 2D interlace scheme, known as Adam7, that breaks
down an image into seven smaller images of varying size,
based on an 8x8 grid.
libpng can fill out those images or it can give them to you
"as is". If you want them filled out, there are two ways to
do that. The one mentioned in the PNG specification is to
expand each pixel to cover those pixels that have not been
read yet (the "rectangle" method). This results in a blocky
image for the first pass, which gradually smooths out as
more pixels are read. The other method is the "sparkle"
method, where pixels are drawn only in their final loca-
tions, with the rest of the image remaining whatever colors
they were initialized to before the start of the read. The
first method usually looks better, but tends to be slower,
as there are more pixels to put in the rows.
If you don't want libpng to handle the interlacing details,
just call png_read_rows() seven times to read in all seven
images. Each of the images is a valid image by itself, or
they can all be combined on an 8x8 grid to form a single
image (although if you intend to combine them you would be
far better off using the libpng interlace handling).
The first pass will return an image 1/8 as wide as the
entire image (every 8th column starting in column 0) and 1/8
as high as the original (every 8th row starting in row 0),
the second will be 1/8 as wide (starting in column 4) and
1/8 as high (also starting in row 0). The third pass will
be 1/4 as wide (every 4th pixel starting in column 0) and
1/8 as high (every 8th row starting in row 4), and the
fourth pass will be 1/4 as wide and 1/4 as high (every 4th
column starting in column 2, and every 4th row starting in
row 0). The fifth pass will return an image 1/2 as wide,
and 1/4 as high (starting at column 0 and row 2), while the
sixth pass will be 1/2 as wide and 1/2 as high as the origi-
nal (starting in column 1 and row 0). The seventh and final
pass will be as wide as the original, and 1/2 as high, con-
taining all of the odd numbered scanlines. Phew!
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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 "rec-
tangle" effect is generally considered the better looking
one.
If you only want the "sparkle" effect, just call
png_read_rows() as normal, with the third parameter NULL.
Make sure you make pass over the image number_of_passes
times, and you don't change the data in the rows between
calls. You can change the locations of the data, just not
the data. Each pass only writes the pixels appropriate for
that pass, and assumes the data from previous passes is
still valid.
png_read_rows(png_ptr, row_pointers, NULL,
number_of_rows);
If you only want the first effect (the rectangles), do the
same as before except pass the row buffer in the third
parameter, and leave the second parameter NULL.
png_read_rows(png_ptr, NULL, row_pointers,
number_of_rows);
Finishing a sequential read
After you are finished reading the image through the low-
level interface, you can finish reading the file. If you
are interested in comments or time, which may be stored
either before or after the image data, you should pass the
separate png_info struct if you want to keep the comments
from before and after the image separate. If you are not
interested, you can pass NULL.
png_read_end(png_ptr, end_info);
When you are done, you can free all memory allocated by
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
libpng like this:
png_destroy_read_struct(&png_ptr, &info_ptr,
&end_info);
It is also possible to individually free the info_ptr
members that point to libpng-allocated storage with the fol-
lowing function:
png_free_data(png_ptr, info_ptr, mask, seq)
mask - identifies data to be freed, a mask
containing the logical OR of one or
more of
PNG_FREE_PLTE, PNG_FREE_TRNS,
PNG_FREE_HIST, PNG_FREE_ICCP,
PNG_FREE_PCAL, PNG_FREE_ROWS,
PNG_FREE_SCAL, PNG_FREE_SPLT,
PNG_FREE_TEXT, PNG_FREE_UNKN,
or simply PNG_FREE_ALL
seq - sequence number of item to be freed
(-1 for all items)
This function may be safely called when the relevant storage
has already been freed, or has not yet been allocated, or
was allocated by the user and not by libpng, and will in
those cases do nothing. The "seq" parameter is ignored if
only one item of the selected data type, such as PLTE, is
allowed. If "seq" is not -1, and multiple items are allowed
for the data type identified in the mask, such as text or
sPLT, only the n'th item in the structure is freed, where n
is "seq".
The default behavior is only to free data that was allocated
internally by libpng. This can be changed, so that libpng
will not free the data, or so that it will free data that
was allocated by the user with png_malloc() or png_zalloc()
and passed in via a png_set_*() function, with
png_data_freer(png_ptr, info_ptr, freer, mask)
mask - which data elements are affected
same choices as in png_free_data()
freer - one of
PNG_DESTROY_WILL_FREE_DATA
PNG_SET_WILL_FREE_DATA
PNG_USER_WILL_FREE_DATA
This function only affects data that has already been allo-
cated. You can call this function after reading the PNG
data but before calling any png_set_*() functions, to con-
trol whether the user or the png_set_*() function is respon-
sible for freeing any existing data that might be present,
and again after the png_set_*() functions to control whether
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
the user or png_destroy_*() is supposed to free the data.
When the user assumes responsibility for libpng-allocated
data, the application must use png_free() to free it, and
when the user transfers responsibility to libpng for data
that the user has allocated, the user must have used
png_malloc() or png_zalloc() to allocate it.
If you allocated your row_pointers in a single block, as
suggested above in the description of the high level read
interface, you must not transfer responsibility for freeing
it to the png_set_rows or png_read_destroy function, because
they would also try to free the individual row_pointers[i].
If you allocated text_ptr.text, text_ptr.lang, and
text_ptr.translated_keyword separately, do not transfer
responsibility for freeing text_ptr to libpng, because when
libpng fills a png_text structure it combines these members
with the key member, and png_free_data() will free only
text_ptr.key. Similarly, if you transfer responsibility for
free'ing text_ptr from libpng to your application, your
application must not separately free those members.
The png_free_data() function will turn off the "valid" flag
for anything it frees. If you need to turn the flag off for
a chunk that was freed by your application instead of by
libpng, you can use
png_set_invalid(png_ptr, info_ptr, mask);
mask - identifies the chunks to be made invalid,
containing the logical 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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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);
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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.
*/
}
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
/* 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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
a flag that marks the image as finished.
*/
}
IV. Writing
Much of this is very similar to reading. However, every-
thing of importance is repeated here, so you won't have to
constantly look back up in the reading section to understand
writing.
Setup
You will want to do the I/O initialization before you get
into libpng, so if it doesn't work, you don't have anything
to undo. If you are not using the standard I/O functions,
you will need to replace them with custom writing functions.
See the discussion under Customizing libpng.
FILE *fp = fopen(file_name, "wb");
if (!fp)
{
return (ERROR);
}
Next, png_struct and png_info need to be allocated and ini-
tialized. As these can be both relatively large, you may
not want to store these on the stack, unless you have stack
space to spare. Of course, you will want to check if they
return NULL. If you are also reading, you won't want to
name your read structure and your write structure both
"png_ptr"; you can call them anything you like, such as
"read_ptr" and "write_ptr". Look at pngtest.c, for example.
png_structp png_ptr = png_create_write_struct
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
return (ERROR);
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_write_struct(&png_ptr,
(png_infopp)NULL);
return (ERROR);
}
If you want to use your own memory allocation routines,
define PNG_USER_MEM_SUPPORTED and use
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
png_create_write_struct_2() instead of
png_create_write_struct():
png_structp png_ptr = png_create_write_struct_2
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn, (png_voidp)
user_mem_ptr, user_malloc_fn, user_free_fn);
After you have these structures, you will need to set up the
error handling. When libpng encounters an error, it expects
to longjmp() back to your routine. Therefore, you will need
to call setjmp() and pass the png_jmpbuf(png_ptr). If you
write the file from different routines, you will need to
update the png_jmpbuf(png_ptr) every time you enter a new
routine that will call a png_*() function. See your docu-
mentation of setjmp/longjmp for your compiler for more
information on setjmp/longjmp. See the discussion on libpng
error handling in the Customizing Libpng section below for
more information on the libpng error handling.
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(fp);
return (ERROR);
}
...
return;
If you would rather avoid the complexity of setjmp/longjmp
issues, you can compile libpng with
PNG_SETJMP_NOT_SUPPORTED, in which case errors will result
in a call to PNG_ABORT() which defaults to abort().
Now you need to set up the output code. The default for
libpng is to use the C function fwrite(). If you use this,
you will need to pass a valid FILE * in the function
png_init_io(). Be sure that the file is opened in binary
mode. Again, if you wish to handle writing data in another
way, see the discussion on libpng I/O handling in the Cus-
tomizing Libpng section below.
png_init_io(png_ptr, fp);
If you are embedding your PNG into a datastream such as MNG,
and don't want libpng to write the 8-byte signature, or if
you have already written the signature in your application,
use
png_set_sig_bytes(png_ptr, 8);
to inform libpng that it should not write a signature.
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
Write callbacks
At this point, you can set up a callback function that will
be called after each row has been written, which you can use
to control a progress meter or the like. It's demonstrated
in pngtest.c. You must supply a function
void write_row_callback(png_ptr, png_uint_32 row,
int pass);
{
/* put your code here */
}
(You can give it another name that you like instead of
"write_row_callback")
To inform libpng about your function, use
png_set_write_status_fn(png_ptr, write_row_callback);
You now have the option of modifying how the compression
library will run. The following functions are mainly for
testing, but may be useful in some cases, like if you need
to write PNG files extremely fast and are willing to give up
some compression, or if you want to get the maximum possible
compression at the expense of slower writing. If you have
no special needs in this area, let the library do what it
wants by not calling this function at all, as it has been
tuned to deliver a good speed/compression ratio. The second
parameter to png_set_filter() is the filter method, for
which the only valid values are 0 (as of the July 1999 PNG
specification, version 1.2) or 64 (if you are writing a PNG
datastream that is to be embedded in a MNG datastream). The
third parameter is a flag that indicates which filter
type(s) are to be tested for each scanline. See the PNG
specification for details on the specific filter types.
/* turn on or off filtering, and/or choose
specific filters. You can use either a single
PNG_FILTER_VALUE_NAME or the logical 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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
be stored in case it's needed later), and then add and
remove them after the start of compression.
If you are writing a PNG datastream that is to be embedded
in a MNG datastream, the second parameter can be either 0 or
64.
The png_set_compression_*() functions interface to the zlib
compression library, and should mostly be ignored unless you
really know what you are doing. The only generally useful
call is png_set_compression_level() which changes how much
time zlib spends on trying to compress the image data. See
the Compression Library (zlib.h and algorithm.txt, distri-
buted with zlib) for details on the compression levels.
/* set the zlib compression level */
png_set_compression_level(png_ptr,
Z_BEST_COMPRESSION);
/* set other zlib parameters */
png_set_compression_mem_level(png_ptr, 8);
png_set_compression_strategy(png_ptr,
Z_DEFAULT_STRATEGY);
png_set_compression_window_bits(png_ptr, 15);
png_set_compression_method(png_ptr, 8);
png_set_compression_buffer_size(png_ptr, 8192)
extern PNG_EXPORT(void,png_set_zbuf_size)
Setting the contents of info for
You now need to fill in the png_info structure with all the
data you wish to write before the actual image. Note that
the only thing you are allowed to write after the image is
the text chunks and the time chunk (as of PNG Specification
1.2, anyway). See png_write_end() and the latest PNG
specification for more information on that. If you wish to
write them before the image, fill them in now, and flag that
data as being valid. If you want to wait until after the
data, don't fill them until png_write_end(). For all the
fields in png_info and their data types, see png.h. For
explanations of what the fields contain, see the PNG specif-
ication.
Some of the more important parts of the png_info are:
png_set_IHDR(png_ptr, info_ptr, width, height,
bit_depth, color_type, interlace_type,
compression_type, filter_method)
width - holds the width of the image
in pixels (up to 2^31).
height - holds the height of the image
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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)
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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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 struc-
tures in the array. Each png_text structure holds a
language code, a keyword, a text value, and a compression
type.
The compression types have the same valid numbers as the
compression types of the image data. Currently, the only
valid number is zero. However, you can store text either
compressed or uncompressed, unlike images, which always have
to be compressed. So if you don't want the text compressed,
set the compression type to PNG_TEXT_COMPRESSION_NONE.
Because tEXt and zTXt chunks don't have a language field, if
you specify PNG_TEXT_COMPRESSION_NONE or
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PNG_TEXT_COMPRESSION_zTXt any language code or translated
keyword will not be written out.
Until text gets around 1000 bytes, it is not worth compress-
ing it. After the text has been written out to the file,
the compression type is set to PNG_TEXT_COMPRESSION_NONE_WR
or PNG_TEXT_COMPRESSION_zTXt_WR, so that it isn't written
out again at the end (in case you are calling
png_write_end() with the same struct.
The keywords that are given in the PNG Specification are:
Title Short (one line) title or
caption for image
Author Name of image's creator
Description Description of image (possibly long)
Copyright Copyright notice
Creation Time Time of original image creation
(usually RFC 1123 format, see below)
Software Software used to create the image
Disclaimer Legal disclaimer
Warning Warning of nature of content
Source Device used to create the image
Comment Miscellaneous comment; conversion
from other image format
The keyword-text pairs work like this. Keywords should be
short simple descriptions of what the comment is about.
Some typical keywords are found in the PNG specification, as
is some recommendations on keywords. You can repeat key-
words in a file. You can even write some text before the
image and some after. For example, you may want to put a
description of the image before the image, but leave the
disclaimer until after, so viewers working over modem con-
nections don't have to wait for the disclaimer to go over
the modem before they start seeing the image. Finally, key-
words should be full words, not abbreviations. Keywords and
text are in the ISO 8859-1 (Latin-1) character set (a super-
set of regular ASCII) and can not contain NUL characters,
and should not contain control or other unprintable charac-
ters. To make the comments widely readable, stick with
basic ASCII, and avoid machine specific character set exten-
sions 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
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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 crea-
tion, you should use a plain tEXt chunk with the "Creation
Time" keyword. This is necessary because the "creation
time" of a PNG image is somewhat vague, depending on whether
you mean the PNG file, the time the image was created in a
non-PNG format, a still photo from which the image was
scanned, or possibly the subject matter itself. In order to
facilitate machine-readable dates, it is recommended that
the "Creation Time" tEXt chunk use RFC 1123 format dates
(e.g. "22 May 1997 18:07:10 GMT"), although this isn't a
requirement. Unlike the tIME chunk, the "Creation Time"
tEXt chunk is not expected to be automatically changed by
the software. To facilitate the use of RFC 1123 dates, a
function png_convert_to_rfc1123(png_timep) is provided to
convert from PNG time to an RFC 1123 format string.
Writing unknown chunks
You can use the png_set_unknown_chunks function to queue up
chunks for writing. You give it a chunk name, raw data, and
a size; that's all there is to it. The chunks will be writ-
ten by the next following png_write_info_before_PLTE,
png_write_info, or png_write_end function. Any chunks pre-
viously 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 inter-
face 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
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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 logical OR
of some set of transformation flags. This call is
equivalent to png_write_info(), followed the set of
transformations indicated by the transform mask, then
png_write_image(), and finally png_write_end().
(The final parameter of this call is not yet used. Someday
it might point to transformation parameters required by some
future output transform.)
You must use png_transforms and not call any
png_set_transform() functions when you use png_write_png().
The low-level write interface
If you are going the low-level route instead, you are now
ready to write all the file information up to the actual
image data. You do this with a call to png_write_info().
png_write_info(png_ptr, info_ptr);
Note that there is one transformation you may need to do
before png_write_info(). In PNG files, the alpha channel in
an image is the level of opacity. If your data is supplied
as a level of transparency, you can invert the alpha channel
before you write it, so that 0 is fully transparent and 255
(in 8-bit or paletted images) or 65535 (in 16-bit images) is
fully opaque, with
png_set_invert_alpha(png_ptr);
This must appear before png_write_info() instead of later
with the other transformations because in the case of palet-
ted images the tRNS chunk data has to be inverted before the
tRNS chunk is written. If your image is not a paletted
image, the tRNS data (which in such cases represents a sin-
gle color to be rendered as transparent) won't need to be
changed, and you can safely do this transformation after
your png_write_info() call.
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If you need to write a private chunk that you want to appear
before the PLTE chunk when PLTE is present, you can write
the PNG info in two steps, and insert code to write your own
chunk between them:
png_write_info_before_PLTE(png_ptr, info_ptr);
png_set_unknown_chunks(png_ptr, info_ptr, ...);
png_write_info(png_ptr, info_ptr);
After you've written the file information, you can set up
the library to handle any special transformations of the
image data. The various ways to transform the data will be
described in the order that they should occur. This is
important, as some of these change the color type and/or bit
depth of the data, and some others only work on certain
color types and bit depths. Even though each transformation
checks to see if it has data that it can do something with,
you should make sure to only enable a transformation if it
will be valid for the data. For example, don't swap red and
blue on grayscale data.
PNG files store RGB pixels packed into 3 or 6 bytes. This
code tells the library to strip input data that has 4 or 8
bytes per pixel down to 3 or 6 bytes (or strip 2 or 4-byte
grayscale+filler data to 1 or 2 bytes per pixel).
png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
where the 0 is unused, and the location is either
PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon
whether the filler byte in the pixel is stored XRGB or RGBX.
PNG files pack pixels of bit depths 1, 2, and 4 into bytes
as small as they can, resulting in, for example, 8 pixels
per byte for 1 bit files. If the data is supplied at 1
pixel per byte, use this code, which will correctly pack the
pixels into a single byte:
png_set_packing(png_ptr);
PNG files reduce possible bit depths to 1, 2, 4, 8, and 16.
If your data is of another bit depth, you can write an sBIT
chunk into the file so that decoders can recover the origi-
nal data if desired.
/* Set the true bit depth of the image data */
if (color_type & PNG_COLOR_MASK_COLOR)
{
sig_bit.red = true_bit_depth;
sig_bit.green = true_bit_depth;
sig_bit.blue = true_bit_depth;
}
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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 sup-
plied with this reversed (black being one and white being
zero):
png_set_invert_mono(png_ptr);
Finally, you can write your own transformation function if
none of the existing ones meets your needs. This is done by
setting a callback with
png_set_write_user_transform_fn(png_ptr,
write_transform_fn);
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You must supply the function
void write_transform_fn(png_ptr ptr, row_info_ptr
row_info, png_bytep data)
See pngtest.c for a working example. Your function will be
called before any of the other transformations are pro-
cessed.
You can also set up a pointer to a user structure for use by
your callback function.
png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
The user_channels and user_depth parameters of this function
are ignored when writing; you can set them to zero as shown.
You can retrieve the pointer via the function
png_get_user_transform_ptr(). For example:
voidp write_user_transform_ptr =
png_get_user_transform_ptr(png_ptr);
It is possible to have libpng flush any pending output,
either manually, or automatically after a certain number of
lines have been written. To flush the output stream a sin-
gle time call:
png_write_flush(png_ptr);
and to have libpng flush the output stream periodically
after a certain number of scanlines have been written, call:
png_set_flush(png_ptr, nrows);
Note that the distance between rows is from the last time
png_write_flush() was called, or the first row of the image
if it has never been called. So if you write 50 lines, and
then png_set_flush 25, it will flush the output on the next
scanline, and every 25 lines thereafter, unless
png_write_flush() is called before 25 more lines have been
written. If nrows is too small (less than about 10 lines
for a 640 pixel wide RGB image) the image compression may
decrease noticeably (although this may be acceptable for
real-time applications). Infrequent flushing will only
degrade the compression performance by a few percent over
images that do not use flushing.
Writing the image data
That's it for the transformations. Now you can write the
image data. The simplest way to do this is in one function
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call. If you have the whole image in memory, you can just
call png_write_image() and libpng will write the image. You
will need to pass in an array of pointers to each row. This
function automatically handles interlacing, so you don't
need to call png_set_interlace_handling() or call this func-
tion multiple times, or any of that other stuff necessary
with png_write_rows().
png_write_image(png_ptr, row_pointers);
where row_pointers is:
png_byte *row_pointers[height];
You can point to void or char or whatever you use for pix-
els.
If you don't want to write the whole image at once, you can
use png_write_rows() instead. If the file is not inter-
laced, this is simple:
png_write_rows(png_ptr, row_pointers,
number_of_rows);
row_pointers is the same as in the png_write_image() call.
If you are just writing one row at a time, you can do this
with a single row_pointer instead of an array of
row_pointers:
png_bytep row_pointer = row;
png_write_row(png_ptr, row_pointer);
When the file is interlaced, things can get a good deal more
complicated. The only currently (as of the PNG Specifica-
tion version 1.2, dated July 1999) defined interlacing
scheme for PNG files is the "Adam7" interlace scheme, that
breaks down an image into seven smaller images of varying
size. libpng will build these images for you, or you can do
them yourself. If you want to build them yourself, see the
PNG specification for details of which pixels to write when.
If you don't want libpng to handle the interlacing details,
just use png_set_interlace_handling() and call
png_write_rows() the correct number of times to write all
seven sub-images.
If you want libpng to build the sub-images, call this before
you start writing any rows:
number_of_passes =
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png_set_interlace_handling(png_ptr);
This will return the number of passes needed. Currently,
this is seven, but may change if another interlace type is
added.
Then write the complete image number_of_passes times.
png_write_rows(png_ptr, row_pointers,
number_of_rows);
As some of these rows are not used, and thus return immedi-
ately, you may want to read about interlacing in the PNG
specification, and only update the rows that are actually
used.
Finishing a sequential write
After you are finished writing the image, you should finish
writing the file. If you are interested in writing comments
or time, you should pass an appropriately filled png_info
pointer. If you are not interested, you can pass NULL.
png_write_end(png_ptr, info_ptr);
When you are done, you can free all memory used by libpng
like this:
png_destroy_write_struct(&png_ptr, &info_ptr);
It is also possible to individually free the info_ptr
members that point to libpng-allocated storage with the fol-
lowing function:
png_free_data(png_ptr, info_ptr, mask, seq)
mask - identifies data to be freed, a mask
containing the logical 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
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allowed. If "seq" is not -1, and multiple items are allowed
for the data type identified in the mask, such as text or
sPLT, only the n'th item in the structure is freed, where n
is "seq".
If you allocated data such as a palette that you passed in
to libpng with png_set_*, you must not free it until just
before the call to png_destroy_write_struct().
The default behavior is only to free data that was allocated
internally by libpng. This can be changed, so that libpng
will not free the data, or so that it will free data that
was allocated by the user with png_malloc() or png_zalloc()
and passed in via a png_set_*() function, with
png_data_freer(png_ptr, info_ptr, freer, mask)
mask - which data elements are affected
same choices as in png_free_data()
freer - one of
PNG_DESTROY_WILL_FREE_DATA
PNG_SET_WILL_FREE_DATA
PNG_USER_WILL_FREE_DATA
For example, to transfer responsibility for some data from a
read structure to a write structure, you could use
png_data_freer(read_ptr, read_info_ptr,
PNG_USER_WILL_FREE_DATA,
PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
png_data_freer(write_ptr, write_info_ptr,
PNG_DESTROY_WILL_FREE_DATA,
PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
thereby briefly reassigning responsibility for freeing to
the user but immediately afterwards reassigning it once more
to the write_destroy function. Having done this, it would
then be safe to destroy the read structure and continue to
use the PLTE, tRNS, and hIST data in the write structure.
This function only affects data that has already been allo-
cated. You can call this function before calling after the
png_set_*() functions to control whether the user or
png_destroy_*() is supposed to free the data. When the user
assumes responsibility for libpng-allocated data, the appli-
cation must use png_free() to free it, and when the user
transfers responsibility to libpng for data that the user
has allocated, the user must have used png_malloc() or
png_zalloc() to allocate it.
If you allocated text_ptr.text, text_ptr.lang, and
text_ptr.translated_keyword separately, do not transfer
responsibility for freeing text_ptr to libpng, because when
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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 three issues here. The first is changing how
libpng does standard things like memory allocation,
input/output, and error handling. The second deals with
more complicated things like adding new chunks, adding new
transformations, and generally changing how libpng works.
Both of those are compile-time issues; that is, they are
generally determined at the time the code is written, and
there is rarely a need to provide the user with a means of
changing them. The third is a run-time issue: choosing
between and/or tuning one or more alternate versions of com-
putationally intensive routines; specifically, optimized
assembly-language (and therefore compiler- and platform-
dependent) versions.
Memory allocation, input/output, and error handling
All of the memory allocation, input/output, and error han-
dling in libpng goes through callbacks that are user-
settable. The default routines are in pngmem.c, pngrio.c,
pngwio.c, and pngerror.c, respectively. To change these
functions, call the appropriate png_set_*_fn() function.
Memory allocation is done through the functions png_malloc()
and png_free(). These currently just call the standard C
functions. If your pointers can't access more then 64K at a
time, you will want to set MAXSEG_64K in zlib.h. Since it
is unlikely that the method of handling memory allocation on
a platform will change between applications, these functions
must be modified in the library at compile time. If you
prefer to use a different method of allocating and freeing
data, you can use png_create_read_struct_2() or
png_create_write_struct_2() to register your own functions
as described above. These functions also provide a void
pointer that can be retrieved via
mem_ptr=png_get_mem_ptr(png_ptr);
Your replacement memory functions must have prototypes as
follows:
png_voidp malloc_fn(png_structp png_ptr,
png_size_t size);
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void free_fn(png_structp png_ptr, png_voidp ptr);
Your malloc_fn() must return NULL in case of failure. The
png_malloc() function will normally call png_error() if it
receives a NULL from the system memory allocator or from
your replacement malloc_fn().
Input/Output in libpng is done through png_read() and
png_write(), which currently just call fread() and fwrite().
The FILE * is stored in png_struct and is initialized via
png_init_io(). If you wish to change the method of I/O, the
library supplies callbacks that you can set through the
function png_set_read_fn() and png_set_write_fn() at run
time, instead of calling the png_init_io() function. These
functions also provide a void pointer that can be retrieved
via the function png_get_io_ptr(). For example:
png_set_read_fn(png_structp read_ptr,
voidp read_io_ptr, png_rw_ptr read_data_fn)
png_set_write_fn(png_structp write_ptr,
voidp write_io_ptr, png_rw_ptr write_data_fn,
png_flush_ptr output_flush_fn);
voidp read_io_ptr = png_get_io_ptr(read_ptr);
voidp write_io_ptr = png_get_io_ptr(write_ptr);
The replacement I/O functions must have prototypes as fol-
lows:
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
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message on stderr via fprintf() unless the library is com-
piled 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 func-
tions after png_create_*_struct() has been called by cal-
ling:
png_set_error_fn(png_structp png_ptr,
png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warning_fn);
png_voidp error_ptr = png_get_error_ptr(png_ptr);
If NULL is supplied for either error_fn or warning_fn, then
the libpng default function will be used, calling fprintf()
and/or longjmp() if a problem is encountered. The replace-
ment error functions should have parameters as follows:
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 return-
ing 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
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designed, so you can do things similarly. Second, check out
the sections of libpng that read and write chunks. Try to
find a chunk that is similar to yours and use it as a tem-
plate. More details can be found in the comments inside the
code. It is best to handle unknown chunks in a generic
method, via callback functions, instead of by modifying
libpng functions.
If you wish to write your own transformation for the data,
look through the part of the code that does the transforma-
tions, and check out some of the simpler ones to get an idea
of how they work. Try to find a similar transformation to
the one you want to add and copy off of it. More details
can be found in the comments inside the code itself.
Configuring for 16 bit platforms
You will want to look into zconf.h to tell zlib (and thus
libpng) that it cannot allocate more then 64K at a time.
Even if you can, the memory won't be accessible. So limit
zlib and libpng to 64K by defining MAXSEG_64K.
Configuring for DOS
For DOS users who only have access to the lower 640K, you
will have to limit zlib's memory usage via a
png_set_compression_mem_level() call. See zlib.h or zconf.h
in the zlib library for more information.
Configuring for Medium Model
Libpng's support for medium model has been tested on most of
the popular compilers. Make sure MAXSEG_64K gets defined,
USE_FAR_KEYWORD gets defined, and FAR gets defined to far in
pngconf.h, and you should be all set. Everything in the
library (except for zlib's structure) is expecting far data.
You must use the typedefs with the p or pp on the end for
pointers (or at least look at them and be careful). Make
note that the rows of data are defined as png_bytepp, which
is an unsigned char far * far *.
Configuring for gui/windowing platforms:
You will need to write new error and warning functions that
use the GUI interface, as described previously, and set them
to be the error and warning functions at the time that
png_create_*_struct() is called, in order to have them
available during the structure initialization. They can be
changed later via png_set_error_fn(). On some compilers,
you may also have to change the memory allocators
(png_malloc, etc.).
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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 pro-
tected by the PNG_INTERNAL definition, which is only defined
for those routines inside libpng itself. The files in
libpng proper only include png.h, which includes pngconf.h.
Configuring zlib:
There are special functions to configure the compression.
Perhaps the most useful one changes the compression level,
which currently uses input compression values in the range 0
- 9. The library normally uses the default compression
level (Z_DEFAULT_COMPRESSION = 6). Tests have shown that
for a large majority of images, compression values in the
range 3-6 compress nearly as well as higher levels, and do
so much faster. For online applications it may be desirable
to have maximum speed (Z_BEST_SPEED = 1). With versions of
zlib after v0.99, you can also specify no compression
(Z_NO_COMPRESSION = 0), but this would create files larger
than just storing the raw bitmap. You can specify the
compression level by calling:
png_set_compression_level(png_ptr, level);
Another useful one is to reduce the memory level used by the
library. The memory level defaults to 8, but it can be
lowered if you are short on memory (running DOS, for exam-
ple, where you only have 640K). Note that the memory level
does have an effect on compression; among other things,
lower levels will result in sections of incompressible data
being emitted in smaller stored blocks, with a correspond-
ingly larger relative overhead of up to 15% in the worst
case.
png_set_compression_mem_level(png_ptr, level);
The other functions are for configuring zlib. They are not
recommended for normal use and may result in writing an
invalid PNG file. See zlib.h for more information on what
these mean.
png_set_compression_strategy(png_ptr,
strategy);
png_set_compression_window_bits(png_ptr,
window_bits);
png_set_compression_method(png_ptr, method);
png_set_compression_buffer_size(png_ptr, size);
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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, respec-
tively.
Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can
be bitwise ORed together with '|' to specify one or more
filters to use. These filters are described in more detail
in the PNG specification. If you intend to change the
filter type during the course of writing the image, you
should start with flags set for all of the filters you
intend to use so that libpng can initialize its internal
structures appropriately for all of the filter types. (Note
that this means the first row must always be adaptively fil-
tered, 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 rela-
tive computational costs of the filters.
double weights[3] = {1.5, 1.3, 1.1},
costs[PNG_FILTER_VALUE_LAST] =
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
{1.0, 1.3, 1.3, 1.5, 1.7};
png_set_filter_heuristics(png_ptr,
PNG_FILTER_HEURISTIC_WEIGHTED, 3,
weights, costs);
The weights are multiplying factors that indicate to libpng
that the row filter should be the same for successive rows
unless another row filter is that many times better than the
previous filter. In the above example, if the previous 3
filters were SUB, SUB, NONE, the SUB filter could have a
"sum of absolute differences" 1.5 x 1.3 times higher than
other filters and still be chosen, while the NONE filter
could have a sum 1.1 times higher than other filters and
still be chosen. Unspecified weights are taken to be 1.0,
and the specified weights should probably be declining like
those above in order to emphasize recent filters over older
filters.
The filter costs specify for each filter type a relative
decoding cost to be considered when selecting row filters.
This means that filters with higher costs are less likely to
be chosen over filters with lower costs, unless their "sum
of absolute differences" is that much smaller. The costs do
not necessarily reflect the exact computational speeds of
the various filters, since this would unduly influence the
final image size.
Note that the numbers above were invented purely for this
example and are given only to help explain the function
usage. Little testing has been done to find optimum values
for either the costs or the weights.
Removing unwanted object code
There are a bunch of #define's in pngconf.h that control
what parts of libpng are compiled. All the defines end in
_SUPPORTED. If you are never going to use a capability, you
can change the #define to #undef before recompiling libpng
and save yourself code and data space, or you can turn off
individual capabilities with defines that begin with
PNG_NO_.
You can also turn all of the transforms and ancillary chunk
capabilities off en masse with compiler directives that
define PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or
WRITE]_ANCILLARY_CHUNKS, or all four, along with directives
to turn on any of the capabilities that you do want. The
PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the
extra transformations but still leave the library fully
capable of reading and writing PNG files with all known pub-
lic chunks Use of the PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
directive produces a library that is incapable of reading or
writing ancillary chunks. If you are not using the progres-
sive reading capability, you can turn that off with
PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTER-
LACING 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 dis-
able any parts of the library, as this will cause applica-
tions linked with different versions of the library to fail
if they call functions not available in your library. The
size of the library itself should not be an issue, because
only those sections that are actually used will be loaded
into memory.
Requesting debug printout
The macro definition PNG_DEBUG can be used to request debug-
ging printout. Set it to an integer value in the range 0 to
3. Higher numbers result in increasing amounts of debugging
information. The information is printed to the "stderr"
file, unless another file name is specified in the
PNG_DEBUG_FILE macro definition.
When PNG_DEBUG > 0, the following functions (macros) become
available:
png_debug(level, message)
png_debug1(level, message, p1)
png_debug2(level, message, p1, p2)
in which "level" is compared to PNG_DEBUG to decide whether
to print the message, "message" is the formatted string to
be printed, and p1 and p2 are parameters that are to be
embedded in the string according to printf-style formatting
directives. For example,
png_debug1(2, "foo=%d0, foo);
is expanded to
if(PNG_DEBUG > 2)
fprintf(PNG_DEBUG_FILE, "foo=%d0, foo);
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
When PNG_DEBUG is defined but is zero, the macros aren't
defined, but you can still use PNG_DEBUG to control your own
debugging:
#ifdef PNG_DEBUG
fprintf(stderr, ...
#endif
When PNG_DEBUG = 1, the macros are defined, but only
png_debug statements having level = 0 will be printed.
There aren't any such statements in this version of libpng,
but if you insert some they will be printed.
VI. Runtime optimization
A new feature in libpng 1.2.0 is the ability to dynamically
switch between standard and optimized versions of some rou-
tines. Currently these are limited to three computationally
intensive tasks when reading PNG files: decoding row
filters, expanding interlacing, and combining interlaced or
transparent row data with previous row data. Currently the
optimized versions are available only for x86 (Intel, AMD,
etc.) platforms with MMX support, though this may change in
future versions. (For example, the non-MMX assembler optim-
izations for zlib might become similarly runtime-selectable
in future releases, in which case libpng could be extended
to support them. Alternatively, the compile-time choice of
floating-point versus integer routines for gamma correction
might become runtime-selectable.)
Because such optimizations tend to be very platform- and
compiler-dependent, both in how they are written and in how
they perform, the new runtime code in libpng has been writ-
ten to allow programs to query, enable, and disable either
specific optimizations or all such optimizations. For exam-
ple, to enable all possible optimizations (bearing in mind
that some "optimizations" may actually run more slowly in
rare cases):
#if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
png_uint_32 mask, flags;
flags = png_get_asm_flags(png_ptr);
mask = png_get_asm_flagmask(PNG_SELECT_READ |
PNG_SELECT_WRITE);
png_set_asm_flags(png_ptr, flags | mask);
#endif
To enable only optimizations relevant to reading PNGs, use
PNG_SELECT_READ by itself when calling
png_get_asm_flagmask(); similarly for optimizing only writ-
ing. To disable all optimizations:
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
#if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
flags = png_get_asm_flags(png_ptr);
mask = png_get_asm_flagmask(PNG_SELECT_READ |
PNG_SELECT_WRITE);
png_set_asm_flags(png_ptr, flags & ~mask);
#endif
To enable or disable only MMX-related features, use
png_get_mmx_flagmask() in place of png_get_asm_flagmask().
The mmx version takes one additional parameter:
#if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
int selection = PNG_SELECT_READ | PNG_SELECT_WRITE;
int compilerID;
mask = png_get_mmx_flagmask(selection, &compilerID);
#endif
On return, compilerID will indicate which version of the MMX
assembler optimizations was compiled. Currently two flavors
exist: Microsoft Visual C++ (compilerID == 1) and GNU C
(a.k.a. gcc/gas, compilerID == 2). On non-x86 platforms or
on systems compiled without MMX optimizations, a value of -1
is used.
Note that both png_get_asm_flagmask() and
png_get_mmx_flagmask() return all valid, settable optimiza-
tion bits for the version of the library that's currently in
use. In the case of shared (dynamically linked) libraries,
this may include optimizations that did not exist at the
time the code was written and compiled. It is also possi-
ble, of course, to enable only known, specific optimiza-
tions; for example:
#if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
flags = PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
| PNG_ASM_FLAG_MMX_READ_INTERLACE |
PNG_ASM_FLAG_MMX_READ_FILTER_SUB |
PNG_ASM_FLAG_MMX_READ_FILTER_UP |
PNG_ASM_FLAG_MMX_READ_FILTER_AVG |
PNG_ASM_FLAG_MMX_READ_FILTER_PAETH ;
png_set_asm_flags(png_ptr, flags);
#endif
This method would enable only the MMX read-optimizations
available at the time of libpng 1.2.0's release, regardless
of whether a later version of the DLL were actually being
used. (Also note that these functions did not exist in ver-
sions older than 1.2.0, so any attempt to run a dynamically
linked app on such an older version would fail.)
To determine whether the processor supports MMX instructions
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
at all, use the png_mmx_support() function:
#if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
mmxsupport = png_mmx_support();
#endif
It returns -1 if MMX support is not compiled into libpng, 0
if MMX code is compiled but MMX is not supported by the pro-
cessor, or 1 if MMX support is fully available. Note that
png_mmx_support(), png_get_mmx_flagmask(), and
png_get_asm_flagmask() all may be called without allocating
and ini- tializing any PNG structures (for example, as part
of a usage screen or "about" box).
The following code can be used to prevent an application
from using the thread_unsafe features, even if libpng was
built with PNG_THREAD_UNSAFE_OK defined:
#if defined(PNG_USE_PNGGCCRD) &&
defined(PNG_ASSEMBLER_CODE_SUPPORTED) &&
defined(PNG_THREAD_UNSAFE_OK)
/* Disable thread-unsafe features of pnggccrd */
if (png_access_version_number() >= 10200)
{
png_uint_32 mmx_disable_mask = 0;
png_uint_32 asm_flags;
mmx_disable_mask |= (
PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
| PNG_ASM_FLAG_MMX_READ_FILTER_SUB
| PNG_ASM_FLAG_MMX_READ_FILTER_AVG
| PNG_ASM_FLAG_MMX_READ_FILTER_PAETH );
asm_flags = png_get_asm_flags(png_ptr);
png_set_asm_flags(png_ptr, asm_flags &
~mmx_disable_mask);
} #endif
For more extensive examples of runtime querying, enabling
and disabling of optimized features, see
contrib/gregbook/readpng2.c in the libpng source-code dis-
tribution.
VII. MNG support
The MNG specification (available at
http://www.libpng.org/pub/mng) allows certain extensions to
PNG for PNG images that are embedded in MNG datastreams.
Libpng can support some of these extensions. To enable
them, use the png_permit_mng_features() function:
feature_set = png_permit_mng_features(png_ptr, mask)
mask is a png_uint_32 containing the logical OR of the
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
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 logical AND of
your mask with the set of MNG features that is
supported by the version of libpng that you are using.
It is an error to use this function when reading or writing
a standalone PNG file with the PNG 8-byte signature. The
PNG datastream must be wrapped in a MNG datastream. As a
minimum, it must have the MNG 8-byte signature and the MHDR
and MEND chunks. Libpng does not provide support for these
or any other MNG chunks; your application must provide its
own support for them. You may wish to consider using libmng
(available at http://www.libmng.com) instead.
VIII. Changes to Libpng from version
It should be noted that versions of libpng later than 0.96
are not distributed by the original libpng author, Guy
Schalnat, nor by Andreas Dilger, who had taken over from Guy
during 1996 and 1997, and distributed versions 0.89 through
0.96, but rather by another member of the original PNG
Group, Glenn Randers-Pehrson. Guy and Andreas are still
alive and well, but they have moved on to other things.
The old libpng functions png_read_init(), png_write_init(),
png_info_init(), png_read_destroy(), and png_write_destroy()
have been moved to PNG_INTERNAL in version 0.95 to
discourage their use. These functions will be removed from
libpng version 2.0.0.
The preferred method of creating and initializing the libpng
structures is via the png_create_read_struct(),
png_create_write_struct(), and png_create_info_struct()
because they isolate the size of the structures from the
application, allow version error checking, and also allow
the use of custom error handling routines during the ini-
tialization, which the old functions do not. The functions
png_read_destroy() and png_write_destroy() do not actually
free the memory that libpng allocated for these structs, but
just reset the data structures, so they can be used instead
of png_destroy_read_struct() and png_destroy_write_struct()
if you feel there is too much system overhead allocating and
freeing the png_struct for each image read.
Setting the error callbacks via png_set_message_fn() before
png_read_init() as was suggested in libpng-0.88 is no longer
supported because this caused applications that do not use
custom error functions to fail if the png_ptr was not ini-
tialized to zero. It is still possible to set the error
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
callbacks AFTER png_read_init(), or to change them with
png_set_error_fn(), which is essentially the same function,
but with a new name to force compilation errors with appli-
cations that try to use the old method.
Starting with version 1.0.7, you can find out which version
of the library you are using at run-time:
png_uint_32 libpng_vn = png_access_version_number();
The number libpng_vn is constructed from the major version,
minor version with leading zero, and release number with
leading zero, (e.g., libpng_vn for version 1.0.7 is 10007).
You can also check which version of png.h you used when com-
piling your application:
png_uint_32 application_vn = PNG_LIBPNG_VER;
IX. Y2K Compliance in libpng
June 27, 2006
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 1.2.12 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
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
png_set_tIME() in pngset.c
png_write_tIME() in pngwutil.c, called in pngwrite.c
All appear to handle dates properly in a Y2K environment.
The png_convert_from_time_t() function calls gmtime() to
convert from system clock time, which returns (year - 1900),
which we properly convert to the full 4-digit year. There
is a possibility that applications using libpng are not
passing 4-digit years into the png_convert_to_rfc_1123()
function, or that they are incorrectly passing only a 2-
digit year instead of "year - 1900" into the
png_convert_from_struct_tm() function, but this is not under
our control. The libpng documentation has always stated
that it works with 4-digit years, and the APIs have been
documented as such.
The tIME chunk itself is also Y2K compliant. It uses a 2-
byte unsigned integer to hold the year, and can hold years
as large as 65535.
zlib, upon which libpng depends, is also Y2K compliant. It
contains no date-related code.
Glenn Randers-Pehrson
libpng maintainer
PNG Development Group
NOTE
Note about libpng version numbers:
Due to various miscommunications, unforeseen code incompati-
bilities and occasional factors outside the authors' con-
trol, version numbering on the library has not always been
consistent and straightforward. The following table summar-
izes 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
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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
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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]
Henceforth the source version will match the shared-library
minor and patch numbers; the shared-library major version
number will be used for changes in backward compatibility,
as it is intended. The PNG_PNGLIB_VER macro, which is not
used within libpng but is available for applications, is an
unsigned integer of the form xyyzz corresponding to the
source version x.y.z (leading zeros in y and z). Beta
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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://ds.internic.net/rfc/rfc2083.txt
or (as a W3C Recommendation) at
http://www.w3.org/TR/REC-png.html
In the case of any inconsistency between the PNG specifica-
tion and this library, the specification takes precedence.
AUTHORS
This man page: Glenn Randers-Pehrson <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 documenta-
tion.
Libpng version 1.2.12 - June 27, 2006: 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 (subscrip-
tion required; visit
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
https://lists.sourceforge.net/lists/listinfo/png-mng-
implement to subscribe).
COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:
(This copy of the libpng notices is provided for your con-
venience. In case of any discrepancy between this copy and
the notices in the file png.h that is included in the libpng
distribution, the latter shall prevail.)
If you modify libpng you may insert additional notices
immediately following this sentence.
libpng versions 1.2.6, August 15, 2004, through 1.2.12, June
27, 2006, are Copyright (c) 2004-2006 Glenn Randers-Pehrson,
and are distributed according to the same disclaimer and
license as libpng-1.2.5 with the following individual added
to the list of Contributing Authors
Cosmin Truta
libpng versions 1.0.7, July 1, 2000, through 1.2.5 - October
3, 2002, are Copyright (c) 2000-2002 Glenn Randers-Pehrson,
and are distributed according to the same disclaimer and
license as libpng-1.0.6 with the following individuals added
to the list of Contributing Authors
Simon-Pierre Cadieux
Eric S. Raymond
Gilles Vollant
and with the following additions to the disclaimer:
There is no warranty against interference with your
enjoyment of the library or against infringement.
There is no warranty that our efforts or the library
will fulfill any of your particular purposes or needs.
This library is provided with all faults, and the entire
risk of satisfactory quality, performance, accuracy, and
effort is with the user.
libpng versions 0.97, January 1998, through 1.0.6, March 20,
2000, are Copyright (c) 1998, 1999 Glenn Randers-Pehrson
Distributed according to the same disclaimer and license as
libpng-0.96, with the following individuals added to the
list of Contributing Authors:
Tom Lane
Glenn Randers-Pehrson
Willem van Schaik
libpng versions 0.89, June 1996, through 0.96, May 1997, are
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
Copyright (c) 1996, 1997 Andreas Dilger Distributed accord-
ing 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, "Contribut-
ing 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 Contri-
buting Authors and Group 42, Inc. disclaim all warranties,
expressed or implied, including, without limitation, the
warranties of merchantability and of fitness for any pur-
pose. The Contributing Authors and Group 42, Inc. assume
no liability for direct, indirect, incidental, special,
exemplary, or consequential damages, which may result from
the use of the PNG Reference Library, even if advised of the
possibility of such damage.
Permission is hereby granted to use, copy, modify, and dis-
tribute this source code, or portions hereof, for any pur-
pose, without fee, subject to the following restrictions:
1. The origin of this source code must not be
misrepresented.
2. Altered versions must be plainly marked as such and
must not be misrepresented as being the original source.
3. This Copyright notice may not be removed or altered from
any source or altered source distribution.
The Contributing Authors and Group 42, Inc. specifically
permit, without fee, and encourage the use of this source
code as a component to supporting the PNG file format in
commercial products. If you use this source code in a
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LIBPNG(3) C LIBRARY FUNCTIONS LIBPNG(3)
product, acknowledgment is not required but would be appre-
ciated.
A "png_get_copyright" function is available, for convenient
use in "about" boxes and the like:
printf("%s",png_get_copyright(NULL));
Also, the PNG logo (in PNG format, of course) is supplied in
the files "pngbar.png" and "pngbar.jpg (88x31) and
"pngnow.png" (98x31).
Libpng is OSI Certified Open Source Software. OSI Certified
Open Source is a certification mark of the Open Source Ini-
tiative.
Glenn Randers-Pehrson glennrp at users.sourceforge.net June
27, 2006
Last change: June 27, 2006 87
See also libpngpf(3)
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