libpng - Portable Network Graphics (PNG) Reference Library
1.0.8
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, voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warn_fn);
png_structp png_create_read_struct_2(png_const_charp
malloc_fn, png_free_ptr free_fn);
png_structp png_create_write_struct (png_const_charp
user_png_ver, voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warn_fn);
png_structp png_create_write_struct_2(png_const_charp
user_png_ver, png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warn_fn, png_voidp mem_ptr, png_malloc_ptr
malloc_fn, png_free_ptr free_fn);
int png_debug(int level, png_const_charp message);
int png_debug1(int level, png_const_charp message, p1);
int png_debug2(int level, png_const_charp message, p1,
p2);
void png_destroy_info_struct (png_structp png_ptr,
png_infopp info_ptr_ptr);
void png_destroy_read_struct (png_structpp png_ptr_ptr,
png_infopp info_ptr_ptr, png_infopp end_info_ptr_ptr);
void png_destroy_write_struct (png_structpp png_ptr_ptr,
png_infopp info_ptr_ptr);
void png_error (png_structp png_ptr, png_const_charp
error);
void png_free (png_structp png_ptr, png_voidp ptr);
void png_free_default(png_structp png_ptr, png_voidp ptr);
void png_free_data (png_structp png_ptr, png_infop
info_ptr, int num);
png_byte png_get_bit_depth (png_structp png_ptr, png_infop
info_ptr);
png_uint_32 png_get_bKGD (png_structp png_ptr, png_infop
info_ptr, png_color_16p *background);
png_byte png_get_channels (png_structp png_ptr, png_infop
info_ptr);
png_uint_32 png_get_cHRM (png_structp png_ptr, png_infop
info_ptr, double *white_x, double *white_y, double *red_x,
double *red_y, double *green_x, double *green_y, double
*blue_x, double *blue_y);
png_uint_32 png_get_cHRM_fixed (png_structp png_ptr,
png_infop info_ptr, png_uint_32 *white_x, png_uint_32
*white_y, png_uint_32 *red_x, png_uint_32 *red_y,
png_uint_32 *green_x, png_uint_32 *green_y, png_uint_32
*blue_x, png_uint_32 *blue_y);
png_byte png_get_color_type (png_structp png_ptr,
png_infop info_ptr);
png_byte png_get_compression_type (png_structp png_ptr,
png_infop info_ptr);
png_byte png_get_filter_type (png_structp png_ptr,
png_infop info_ptr);
png_uint_32 png_get_gAMA (png_structp png_ptr, png_infop
info_ptr, double *file_gamma);
png_uint_32 png_get_gAMA_fixed (png_structp png_ptr,
png_infop info_ptr, png_uint_32 *int_file_gamma);
png_byte png_get_header_ver (png_structp png_ptr);
png_byte png_get_header_version (png_structp png_ptr);
png_uint_32 png_get_hIST (png_structp png_ptr, png_infop
info_ptr, png_uint_16p *hist);
png_uint_32 png_get_iCCP (png_structp png_ptr, png_infop
info_ptr, png_charpp name, int *compression_type,
png_charpp profile, png_uint_32 *proflen);
png_uint_32 png_get_IHDR (png_structp png_ptr, png_infop
info_ptr, png_uint_32 *width, png_uint_32 *height, int
*bit_depth, int *color_type, int *interlace_type, int
*compression_type, int *filter_type);
png_uint_32 png_get_image_height (png_structp png_ptr,
png_infop info_ptr);
png_uint_32 png_get_image_width (png_structp png_ptr,
png_infop info_ptr);
png_infop info_ptr);
png_voidp png_get_io_ptr (png_structp png_ptr);
png_byte png_get_libpng_ver (png_structp png_ptr);
png_voidp png_get_mem_ptr(png_structp png_ptr);
png_uint_32 png_get_oFFs (png_structp png_ptr, png_infop
info_ptr, png_uint_32 *offset_x, png_uint_32 *offset_y,
int *unit_type);
png_uint_32 png_get_pCAL (png_structp png_ptr, png_infop
info_ptr, png_charp *purpose, png_int_32 *X0, png_int_32
*X1, int *type, int *nparams, png_charp *units, png_charpp
*params);
png_uint_32 png_get_pHYs (png_structp png_ptr, png_infop
info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int
*unit_type);
float png_get_pixel_aspect_ratio (png_structp png_ptr,
png_infop info_ptr);
png_uint_32 png_get_pixels_per_meter (png_structp png_ptr,
png_infop info_ptr);
png_voidp png_get_progressive_ptr (png_structp png_ptr);
png_uint_32 png_get_PLTE (png_structp png_ptr, png_infop
info_ptr, png_colorp *palette, int *num_palette);
png_uint_32 png_get_rowbytes (png_structp png_ptr,
png_infop info_ptr);
png_bytepp png_get_rows (png_structp png_ptr, png_infop
info_ptr);
png_uint_32 png_get_sBIT (png_structp png_ptr, png_infop
info_ptr, png_color_8p *sig_bit);
png_bytep png_get_signature (png_structp png_ptr,
png_infop info_ptr);
png_uint_32 png_get_sPLT (png_structp png_ptr, png_infop
info_ptr, png_spalette_p *splt_ptr);
png_uint_32 png_get_sRGB (png_structp png_ptr, png_infop
info_ptr, int *intent);
png_uint_32 png_get_text (png_structp png_ptr, png_infop
info_ptr, png_textp *text_ptr, int *num_text);
png_uint_32 png_get_tIME (png_structp png_ptr, png_infop
info_ptr, png_timep *mod_time);
png_uint_32 png_get_tRNS (png_structp png_ptr, png_infop
info_ptr, png_bytep *trans, int *num_trans, png_color_16p
*trans_values);
png_uint_32 png_get_unknown_chunks (png_structp png_ptr,
png_infop info_ptr, png_unknown_chunkpp unknowns);
png_ptr);
png_uint_32 png_get_valid (png_structp png_ptr, png_infop
info_ptr, png_uint_32 flag);
png_int_32 png_get_x_offset_microns (png_structp png_ptr,
png_infop info_ptr);
png_int_32 png_get_x_offset_pixels (png_structp png_ptr,
png_infop info_ptr);
png_uint_32 png_get_x_pixels_per_meter (png_structp
png_ptr, png_infop info_ptr);
png_int_32 png_get_y_offset_microns (png_structp png_ptr,
png_infop info_ptr);
png_int_32 png_get_y_offset_pixels (png_structp png_ptr,
png_infop info_ptr);
png_uint_32 png_get_y_pixels_per_meter (png_structp
png_ptr, png_infop info_ptr);
png_uint_32 png_get_compression_buffer_size (png_structp
png_ptr);
void png_info_init (png_infop info_ptr);
void png_init_io (png_structp png_ptr, FILE *fp);
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);
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);
image);
DEPRECATED: void png_read_init (png_structp png_ptr);
DEPRECATED: void png_read_init_2 (png_structp png_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, 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);
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);
double red_y, double green_x, double green_y, double
blue_x, double blue_y);
void png_set_cHRM_fixed (png_structp png_ptr, png_infop
info_ptr, png_uint_32 white_x, png_uint_32 white_y,
png_uint_32 red_x, png_uint_32 red_y, png_uint_32 green_x,
png_uint_32 green_y, png_uint_32 blue_x, png_uint_32
blue_y);
void png_set_compression_level (png_structp png_ptr, int
level);
void png_set_compression_mem_level (png_structp png_ptr,
int mem_level);
void png_set_compression_method (png_structp png_ptr, int
method);
void png_set_compression_strategy (png_structp png_ptr,
int strategy);
void png_set_compression_window_bits (png_structp png_ptr,
int window_bits);
void png_set_crc_action (png_structp png_ptr, int
crit_action, int ancil_action);
void png_set_dither (png_structp png_ptr, png_colorp
palette, int num_palette, int maximum_colors, png_uint_16p
histogram, int full_dither);
void png_set_error_fn (png_structp png_ptr, png_voidp
error_ptr, png_error_ptr error_fn, png_error_ptr warn
ing_fn);
void png_set_filler (png_structp png_ptr, png_uint_32
filler, int flags);
void png_set_filter (png_structp png_ptr, int method, int
filters);
void png_set_filter_heuristics (png_structp png_ptr, int
heuristic_method, int num_weights, png_doublep fil
ter_weights, png_doublep filter_costs);
void png_set_flush (png_structp png_ptr, int nrows);
void png_set_gamma (png_structp png_ptr, double
screen_gamma, double default_file_gamma);
void png_set_gAMA (png_structp png_ptr, png_infop
info_ptr, double file_gamma);
void png_set_gAMA_fixed (png_structp png_ptr, png_infop
info_ptr, png_uint_32 file_gamma);
void png_set_gray_1_2_4_to_8(png_structp png_ptr);
void png_set_gray_to_rgb (png_structp png_ptr);
void png_set_hIST (png_structp png_ptr, png_infop
info_ptr, png_uint_16p hist);
void png_set_iCCP (png_structp png_ptr, png_infop
int png_set_interlace_handling (png_structp png_ptr);
void png_set_invalid (png_structp png_ptr, png_infop
info_ptr, int mask);
void png_set_invert_alpha (png_structp png_ptr);
void png_set_invert_mono (png_structp png_ptr);
void png_set_IHDR (png_structp png_ptr, png_infop
info_ptr, png_uint_32 width, png_uint_32 height, int
bit_depth, int color_type, int interlace_type, int com
pression_type, int filter_type);
void png_set_keep_unknown_chunks (png_structp png_ptr, int
keep, png_bytep chunk_list, int num_chunks);
void png_set_mem_fn(png_structp png_ptr, png_voidp
mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);
void png_set_oFFs (png_structp png_ptr, png_infop
info_ptr, png_uint_32 offset_x, png_uint_32 offset_y, int
unit_type);
void png_set_packing (png_structp png_ptr);
void png_set_packswap (png_structp png_ptr);
void png_set_palette_to_rgb(png_structp png_ptr);
info_ptr, png_charp purpose, png_int_32 X0, png_int_32 X1,
int type, int nparams, png_charp units, png_charpp
params);
void png_set_pHYs (png_structp png_ptr, png_infop
info_ptr, png_uint_32 res_x, png_uint_32 res_y, int
unit_type);
void png_set_progressive_read_fn (png_structp png_ptr,
png_voidp progressive_ptr, png_progressive_info_ptr
info_fn, png_progressive_row_ptr row_fn, png_progres
sive_end_ptr end_fn);
void png_set_PLTE (png_structp png_ptr, png_infop
info_ptr, png_colorp palette, int num_palette);
void png_set_read_fn (png_structp png_ptr, png_voidp
io_ptr, png_rw_ptr read_data_fn);
void png_set_read_status_fn (png_structp png_ptr,
png_read_status_ptr read_row_fn);
void png_set_read_user_transform_fn (png_structp png_ptr,
png_user_transform_ptr read_user_transform_fn);
void png_set_rgb_to_gray (png_structp png_ptr, int
error_action, double red, double green);
void png_set_rgb_to_gray_fixed (png_structp png_ptr, int
error_action png_fixed_point red, png_fixed_point green);
void png_set_rows (png_structp png_ptr, png_infop
info_ptr, png_bytepp row_pointers);
info_ptr, png_color_8p sig_bit);
void png_set_sCAL (png_structp png_ptr, png_infop
info_ptr, png_charp unit, double width, double height);
void png_set_shift (png_structp png_ptr, png_color_8p
true_bits);
void png_set_sig_bytes (png_structp png_ptr, int
num_bytes);
void png_set_sPLT (png_structp png_ptr, png_infop
info_ptr, png_spalette_p splt_ptr, int num_spalettes);
void png_set_sRGB (png_structp png_ptr, png_infop
info_ptr, int intent);
void png_set_sRGB_gAMA_and_cHRM (png_structp png_ptr,
png_infop info_ptr, int intent);
void png_set_strip_16 (png_structp png_ptr);
void png_set_strip_alpha (png_structp png_ptr);
void png_set_swap (png_structp png_ptr);
void png_set_swap_alpha (png_structp png_ptr);
void png_set_text (png_structp png_ptr, png_infop
info_ptr, png_textp text_ptr, int num_text);
info_ptr, png_timep mod_time);
void png_set_tRNS (png_structp png_ptr, png_infop
info_ptr, png_bytep trans, int num_trans, png_color_16p
trans_values);
void png_set_tRNS_to_alpha(png_structp png_ptr);
png_uint_32 png_set_unknown_chunks (png_structp png_ptr,
png_infop info_ptr, png_unknown_chunkp unknowns, int num,
int location);
void png_set_unknown_chunk_location(png_structp png_ptr,
png_infop info_ptr, int chunk, int location);
void png_set_read_user_chunk_fn (png_structp png_ptr,
png_voidp user_chunk_ptr, png_user_chunk_ptr
read_user_chunk_fn);
void png_set_user_transform_info (png_structp png_ptr,
png_voidp user_transform_ptr, int user_transform_depth,
int user_transform_channels);
void png_set_write_fn (png_structp png_ptr, png_voidp
io_ptr, png_rw_ptr write_data_fn, png_flush_ptr out
put_flush_fn);
void png_set_write_status_fn (png_structp png_ptr,
png_write_status_ptr write_row_fn);
void png_set_write_user_transform_fn (png_structp png_ptr,
png_user_transform_ptr write_user_transform_fn);
png_uint_32 size);
int png_sig_cmp (png_bytep sig, png_size_t start,
png_size_t num_to_check);
void png_start_read_image (png_structp png_ptr);
void png_warning (png_structp png_ptr, png_const_charp
message);
void png_write_chunk (png_structp png_ptr, png_bytep
chunk_name, png_bytep data, png_size_t length);
void png_write_chunk_data (png_structp png_ptr, png_bytep
data, png_size_t length);
void png_write_chunk_end (png_structp png_ptr);
void png_write_chunk_start (png_structp png_ptr, png_bytep
chunk_name, png_uint_32 length);
void png_write_destroy (png_structp png_ptr);
void png_write_destroy_info (png_infop info_ptr);
void png_write_end (png_structp png_ptr, png_infop
info_ptr);
void png_write_flush (png_structp png_ptr);
image);
DEPRECATED: void png_write_init (png_structp png_ptr);
DEPRECATED: void png_write_init_2 (png_structp png_ptr,
png_const_charp user_png_ver, png_size_t png_struct_size,
png_size_t png_info_size);
void png_write_info (png_structp png_ptr, png_infop
info_ptr);
void png_write_info_before_PLTE (png_structp png_ptr,
png_infop info_ptr);
void png_write_png (png_structp png_ptr, png_infop
info_ptr, int transforms, 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);
DESCRIPTION
The libpng library supports encoding, decoding, and vari
ous manipulations of the Portable Network Graphics (PNG)
format image files. It uses the zlib(3) compression
library. Following is a copy of the libpng.txt file that
accompanies libpng.
LIBPNG.TXT
libpng.txt - A description on how to use and modify libpng
libpng version 1.0.8 - July 24, 2000
Updated and distributed by Glenn Randers-Pehrson
<randeg@alum.rpi.edu>
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 refer
ence library (known as libpng) for your own use. There
are five sections to this file: introduction, structures,
reading, writing, and modification and configuration notes
for various special platforms. In addition to this file,
example.c is a good starting point for using the library,
as it is heavily commented and should include everything
most people will need. We assume that libpng is already
installed; see the INSTALL file for instructions on how to
install libpng.
Libpng was written as a companion to the PNG specifica
tion, as a way of reducing the amount of time and effort
it takes to support the PNG file format in application
programs.
The PNG-1.2 specification is available at
<http://www.cdrom.com/pub/png> (will be moving to
<http://www.libpng.org>) and at <ftp://ftp.uu.net/graph
ics/png/documents/>.
The PNG-1.0 specification is available as RFC 2083
<ftp://ftp.uu.net/graphics/png/documents/> and as a W3C
Recommendation <http://www.w3.org/TR/REC.png.html>. Some
additional chunks are described in the special-purpose
public chunks documents at <ftp://ftp.uu.net/graph
ics/png/documents/>.
Other information about PNG, and the latest version of
libpng, can be found at the PNG home page,
<http://www.cdrom.com/pub/png/> (will be moving to
<http://www.libpng.org>) and at <ftp://ftp.uu.net/graph
ics/png/>.
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,
<ftp://ftp.freesoftware.com/pub/infozip/zlib/>. The zlib
compression utility is a general purpose utility that is
useful for more than PNG files, and can be used without
libpng. See the documentation delivered with zlib for
more details. You can usually find the source files for
the zlib utility wherever you find the libpng source
files.
Libpng is thread safe, provided the threads are using dif
ferent instances of the structures. Each thread should
have its own png_struct and png_info instances, and thus
its own image. Libpng does not protect itself against two
threads using the same instance of a structure.
II. Structures
There are two main structures that are important to
libpng, png_struct and png_info. The first, png_struct,
is an internal structure that will not, for the most part,
be used by a user except as the first variable passed to
every libpng function call.
The png_info structure is designed to provide information
about the PNG file. At one time, the fields of png_info
were intended to be directly accessible to the user. How
ever, this tended to cause problems with applications
using dynamically loaded libraries, and as a result a set
of interface functions for png_info (the png_get_*() and
png_set_*() functions) was developed. The fields of
png_info are still available for older applications, but
it is suggested that applications use the new interfaces
if at all possible.
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 ver
sions 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 explain
ing the syntax and purpose of each one. See example.c and
png.h for more detail. While progressive reading is cov
ered in the next section, you will still need some of the
functions discussed in this section to read a PNG file.
Setup
You will want to do the I/O initialization(*) before you
get into libpng, so if it doesn't work, you don't have
much to undo. Of course, you will also want to insure
that you are, in fact, dealing with a PNG file. Libpng
provides a simple check to see if a file is a PNG file.
To use it, pass in the first 1 to 8 bytes of the file to
the function png_sig_cmp(), and it will return 0 if the
bytes match the corresponding bytes of the PNG signature,
or nonzero otherwise. Of course, the more bytes you pass
in, the greater the accuracy of the prediction.
If you are intending to keep the file pointer open for use
in libpng, you must ensure you don't read more than 8
bytes from the beginning of the file, and you also have to
make a call to png_set_sig_bytes_read() with the number of
bytes you read from the beginning. Libpng will then only
check the bytes (if any) that your program didn't read.
(*): If you are not using the standard I/O functions, you
will need to replace them with custom functions. See the
discussion under Customizing libpng.
FILE *fp = fopen(file_name, "rb");
return (ERROR);
}
fread(header, 1, number, fp);
is_png = !png_sig_cmp(header, 0, number);
if (!is_png)
{
return (NOT_PNG);
}
Next, png_struct and png_info need to be allocated and
initialized. In order to ensure that the size of these
structures is correct even with a dynamically linked
libpng, there are functions to initialize and allocate the
structures. We also pass the library version, optional
pointers to error handling functions, and a pointer to a
data struct for use by the error functions, if necessary
(the pointer and functions can be NULL if the default
error handlers are to be used). See the section on
Changes to Libpng below regarding the old initialization
functions. The structure allocation functions quietly
return NULL if they fail to create the structure, so your
application should check for that.
png_structp png_ptr = png_create_read_struct
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
return (ERROR);
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_read_struct(&png_ptr,
(png_infopp)NULL, (png_infopp)NULL);
return (ERROR);
}
png_infop end_info = png_create_info_struct(png_ptr);
if (!end_info)
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
return (ERROR);
}
If you want to use your own memory allocation routines,
define PNG_USER_MEM_SUPPORTED and use png_cre
ate_read_struct_2() instead of png_create_read_struct():
png_structp png_ptr = png_create_read_struct_2
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
The error handling routines passed to png_cre
ate_read_struct() and the memory alloc/free routines
passed to png_create_struct_2() are only necessary if you
are not using the libpng supplied error handling and mem
ory 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 discus
sion 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_SUP
PORTED, in which case errors will result in a call to
PNG_ABORT() which defaults to abort().
Now you need to set up the input code. The default for
libpng is to use the C function fread(). If you use this,
you will need to pass a valid FILE * in the function
png_init_io(). Be sure that the file is opened in binary
mode. If you wish to handle reading data in another way,
you need not call the png_init_io() function, but you must
then implement the libpng I/O methods discussed in the
Customizing Libpng section below.
png_init_io(png_ptr, fp);
If you had previously opened the file and read any of the
signature from the beginning in order to see if this was a
PNG file, you need to let libpng know that there are some
bytes missing from the start of the file.
png_set_sig_bytes(png_ptr, number);
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 follow
ing: */
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 demon
strated in pngtest.c. You must supply a function
void read_row_callback(png_ptr ptr, png_uint_32 row,
int pass);
{
/* put your code here */
}
(You can give it another name that you like instead of
"read_row_callback")
To inform libpng about your function, use
png_set_read_status_fn(png_ptr, read_row_callback);
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 mem
bers; unknown chunks will be discarded. To change this,
you can call:
png_set_keep_unknown_chunks(png_ptr, info_ptr, keep,
chunk_list, num_chunks);
keep - 0: do not keep
1: keep only if safe-to-copy
2: keep even if unsafe-to-copy
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
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 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 inter
face 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 sam
ples 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
(This excludes setting a background color, doing gamma
transformation, dithering, and setting filler.) If this
is the case, simply do this:
png_read_png(png_ptr, info_ptr, png_transforms, NULL)
where png_transforms is an integer containing the logical
OR of some set of transformation flags. This call is
equivalent to png_read_info(), followed the set of trans
formations indicated by the transform mask, then
png_read_image(), and finally png_read_end().
(The final parameter of this call is not yet used. Some
day it might point to transformation parameters required
by some future input transform.)
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
row_pointers = png_malloc(png_ptr,
height*sizeof(png_bytep));
for (int i=0; i<height, i++)
row_pointers[i]=png_malloc(png_ptr,
width*pixel_size);
png_set_rows(png_ptr, info_ptr, &row_pointers);
Alternatively you could allocate your image in one big
block and define row_pointers[i] to point into the proper
places in your block.
If you use png_set_rows(), the application is responsible
for freeing row_pointers (and row_pointers[i], if they
were separately allocated).
If you don't allocate row_pointers ahead of time,
png_read_png() will do it, and it'll be free'ed when you
call png_destroy_*().
The low-level read interface
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_type);
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_type - (must be PNG_FILTER_TYPE_BASE
for PNG 1.0)
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_type can be NULL if you are
channels = png_get_channels(png_ptr, info_ptr);
channels - number of channels of info for the
color type (valid values are 1 (GRAY,
PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
4 (RGB_ALPHA or RGB + filler byte))
rowbytes = png_get_rowbytes(png_ptr, info_ptr);
rowbytes - number of bytes needed to hold a row
signature = png_get_signature(png_ptr, info_ptr);
signature - holds the signature read from the
file (if any). The data is kept in
the same offset it would be if the
whole signature were read (i.e. if an
application had already read in 4
bytes of signature before starting
libpng, the remaining 4 bytes would
be in signature[4] through signa
ture[7]
(see png_set_sig_bytes())).
width = png_get_image_width(png_ptr,
info_ptr);
height = png_get_image_height(png_ptr,
info_ptr);
bit_depth = png_get_bit_depth(png_ptr,
info_ptr);
color_type = png_get_color_type(png_ptr,
info_ptr);
filter_type = png_get_filter_type(png_ptr,
info_ptr);
compression_type = png_get_compression_type(png_ptr,
info_ptr);
interlace_type = png_get_interlace_type(png_ptr,
info_ptr);
These are also important, but their validity depends on
whether the chunk has been read. The
png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
png_get_<chunk>(png_ptr, info_ptr, ...) functions return
non-zero if the data has been read, or zero if it is miss
ing. The parameters to the png_get_<chunk> are set
directly if they are simple data types, or a pointer into
the info_ptr is returned for any complex types.
png_get_PLTE(png_ptr, info_ptr, &palette,
&num_palette);
palette - the palette for the file
(array of png_color)
num_palette - number of entries in the palette
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, &compres
sion_type,
&profile, &proflen);
name - The profile name.
compression - The compression type; always
PNG_COMPRESSION_TYPE_BASE
for PNG 1.0. You may give NULL to
this argument
to ignore it.
profile - International Color Consortium color
profile
data. May contain NULs.
proflen - length of profile data in bytes.
png_get_sBIT(png_ptr, info_ptr, &sig_bit);
sig_bit - the number of significant bits for
(PNG_INFO_sBIT) each of the gray,
red, green, and blue channels,
whichever are appropriate for the
given color type (png_color_16)
png_get_tRNS(png_ptr, info_ptr, &trans, &num_trans,
&trans_values);
trans - array of transparent entries for
palette (PNG_INFO_tRNS)
trans_values - graylevel or color sample values of
the single transparent color for
non-paletted images (PNG_INFO_tRNS)
num_trans - number of transparent entries
(PNG_INFO_tRNS)
png_get_hIST(png_ptr, info_ptr, &hist);
(PNG_INFO_hIST)
hist - histogram of palette (array of
png_uint_16)
png_get_tIME(png_ptr, info_ptr, &mod_time);
mod_time - time image was last modified
(PNG_VALID_tIME)
png_get_bKGD(png_ptr, info_ptr, &background);
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].translated_keyword - keyword in UTF-8
(empty string for unknown).
num_text - number of comments (same as num_com
ments;
you can put NULL here to avoid the
duplication)
Note while png_set_text() will accept text, language,
and
translated keywords that can be NULL pointers, the
structure
returned by png_get_text will always contain regular
zero-terminated C strings. They might be empty
strings but
they will never be NULL pointers.
num_spalettes = png_get_sPLT(png_ptr, info_ptr,
&palette_ptr);
palette_ptr - array of palette structures holding
contents of one or more sPLT chunks
read.
num_spalettes - number of sPLT chunks read.
png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
&unit_type);
offset_x - positive offset from the left edge
of the screen
offset_y - positive offset from the top edge
of the screen
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROME
TER
&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 struc
tures holding
unknown chunks
unknowns[i].name - name of unknown chunk
unknowns[i].data - data of unknown chunk
unknowns[i].size - size of unknown chunk's data
unknowns[i].location - position of chunk in file
The value of "i" corresponds to the order in which the
chunks were read
from the PNG file or inserted with the
png_set_unknown_chunks() function.
The data from the pHYs chunk can be retrieved in several
convenient forms:
res_x = png_get_x_pixels_per_meter(png_ptr,
info_ptr)
res_y = png_get_y_pixels_per_meter(png_ptr,
info_ptr)
res_x_and_y = png_get_pixels_per_meter(png_ptr,
info_ptr)
res_x = png_get_x_pixels_per_inch(png_ptr,
info_ptr)
res_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 sensi
ble to humans (that's the point), so don't use abbrevia
tions. Non-printing symbols are not allowed. See the PNG
specification for more details. There is also no require
ment to have text after the keyword.
Keywords should be limited to 79 Latin-1 characters with
out 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 trans
lated keyword may be empty or NULL pointers. The key
word/text pairs are put into the array in the order that
they are received. However, some or all of the text
after you read the stuff after the image. This will be
mentioned again below in the discussion that goes with
png_read_end().
Input transformations
After you've read the header information, you can set up
the library to handle any special transformations of the
image data. The various ways to transform the data will
be described in the order that they should occur. This is
important, as some of these change the color type and/or
bit depth of the data, and some others only work on cer
tain color types and bit depths. Even though each trans
formation checks to see if it has data that it can do
something with, you should make sure to only enable a
transformation if it will be valid for the data. For
example, don't swap red and blue on grayscale data.
The colors used for the background and transparency values
should be supplied in the same format/depth as the current
image data. They are stored in the same format/depth as
the image data in a bKGD or tRNS chunk, so this is what
libpng expects for this data. The colors are transformed
to keep in sync with the image data when an application
calls the png_read_update_info() routine (see below).
Data will be decoded into the supplied row buffers packed
into bytes unless the library has been told to transform
it into another format. For example, 4 bit/pixel paletted
or grayscale data will be returned 2 pixels/byte with the
leftmost pixel in the high-order bits of the byte, unless
png_set_packing() is called. 8-bit RGB data will be
stored in RGB RGB RGB format unless png_set_filler() 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() 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() or png_set_strip_16().
The following code transforms grayscale images of less
than 8 to 8 bits, changes paletted images to RGB, and adds
a full alpha channel if there is transparency information
in a tRNS chunk. This is most useful on grayscale images
with bit depths of 2 or 4 or if there is a multiple-image
viewing application that wishes to treat all images in the
same way.
if (color_type == PNG_COLOR_TYPE_PALETTE)
png_set_palette_to_rgb(png_ptr);
bit_depth < 8) png_set_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.
PNG can have files with 16 bits per channel. If you only
can handle 8 bits per channel, this will strip the pixels
down to 8 bit.
if (bit_depth == 16)
png_set_strip_16(png_ptr);
If, for some reason, you don't need the alpha channel on
an image, and you want to remove it rather than combining
it with the background (but the image author certainly had
in mind that you *would* combine it with the background,
so that's what you should probably do):
if (color_type & PNG_COLOR_MASK_ALPHA)
png_set_strip_alpha(png_ptr);
In PNG files, the alpha channel in an image is the level
of opacity. If you need the alpha channel in an image to
be the level of transparency instead of opacity, you can
invert the alpha channel (or the tRNS chunk data) after
it's read, so that 0 is fully opaque and 255 (in 8-bit or
paletted images) or 65535 (in 16-bit images) is fully
transparent, with
png_set_invert_alpha(png_ptr);
PNG files pack pixels of bit depths 1, 2, and 4 into bytes
as small as they can, resulting in, for example, 8 pixels
per byte for 1 bit files. This code expands to 1 pixel
per byte without changing the values of the pixels:
if (bit_depth < 8)
png_set_packing(png_ptr);
PNG files have possible bit depths of 1, 2, 4, 8, and 16.
All pixels stored in a PNG image have been "scaled" or
"shifted" up to the next higher possible bit depth (e.g.
from 5 bits/sample in the range [0,31] to 8 bits/sample in
the range [0, 255]). However, it is also possible to 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:
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 bytes. This code
expands them into 4 bytes for windowing systems that need
them in this format:
if (bit_depth == 8 && 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.
If you are reading an image with an alpha channel, and you
need the data as ARGB instead of the normal PNG format
RGBA:
if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
png_set_swap_alpha(png_ptr);
For some uses, you may want a grayscale image to be repre
sented as RGB. This code will do that conversion:
if (color_type == PNG_COLOR_TYPE_GRAY ||
color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
png_set_gray_to_rgb(png_ptr);
Conversely, you can convert an RGB or RGBA image to
grayscale or grayscale with alpha.
if (color_type == PNG_COLOR_TYPE_RGB ||
color_type == PNG_COLOR_TYPE_RGB_ALPHA)
png_set_rgb_to_gray_fixed(png_ptr, error_action,
int red_weight, int green_weight);
error_action = 1: silently do the conversion
error_action = 2: issue a warning if the original
image has any pixel where
red != green or red != blue
image has any pixel where
red != green or red != blue
red_weight: weight of red component times 100000
green_weight: weight of green component times
100000
If either weight is negative,
default
weights (21268, 71514) are used.
If you have set error_action = 1 or 2, you can later check
whether the image really was gray, after processing the
image rows, with the png_get_rgb_to_gray_status(png_ptr)
function. It will return a png_byte that is zero if the
image was gray or 1 if there were any non-gray pixels.
bKGD and sBIT data will be silently converted to
grayscale, using the green channel data, regardless of the
error_action setting.
With red_weight+green_weight<=100000, the normalized
graylevel is computed:
int rw = red_weight * 65536;
int gw = green_weight * 65536;
int bw = 65536 - (rw + gw);
gray = (rw*red + gw*green + bw*blue)/65536;
The default values approximate those recommended in the
Charles Poynton's Color FAQ,
<http://www.inforamp.net/~poynton/> Copyright (c)
1998-01-04 Charles Poynton poynton@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() or png_set_expand() to change 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 palet
that may or may not be in the palette (need_expand = 0).
png_color_16 my_background;
png_color_16p image_background;
if (png_get_bKGD(png_ptr, info_ptr, &image_back
ground))
png_set_background(png_ptr, image_background,
PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
else
png_set_background(png_ptr, &my_background,
PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
The png_set_background() function tells libpng to compos
ite images with alpha or simple transparency against the
supplied background color. If the PNG file contains a
bKGD chunk (PNG_INFO_bKGD valid), you may use this color,
or supply another color more suitable for the current dis
play (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 col
ors 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. Ide
ally, 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)
}
/* If we don't have another value */
else
{
screen_gamma = 2.2; /* A good guess for a
PC monitor in a bright office or a dim room */
screen_gamma = 2.0; /* A good guess for a
PC monitor in a dark room */
screen_gamma = 1.7 or 1.0; /* A good
guess for Mac systems */
}
The png_set_gamma() function handles gamma transformations
of the data. Pass both the file gamma and the current
screen_gamma. If the file does not have a gamma value,
you can pass one anyway if you have an idea what it is
(usually 0.45455 is a good guess for GIF images on PCs).
Note that file gammas are inverted from screen gammas.
See the discussions on gamma in the PNG specification for
an excellent description of what gamma is, and why all
applications should support it. It is strongly recom
mended that PNG viewers support gamma correction.
if (png_get_gAMA(png_ptr, info_ptr, &gamma))
png_set_gamma(png_ptr, screen_gamma, gamma);
else
png_set_gamma(png_ptr, screen_gamma, 0.45455);
If you need to reduce an RGB file to a paletted file, or
if a paletted file has more entries then will fit on your
screen, png_set_dither() will do that. Note that this is
a simple match dither that merely finds the closest color
available. This should work fairly well with optimized
palettes, and fairly badly with linear color cubes. If
you pass a palette that is larger then maximum_colors, the
file will reduce the number of colors in the palette so it
will fit into maximum_colors. If there is a histogram, it
will use it to make more intelligent choices when reducing
the palette. If there is no histogram, it may not do as
good a job.
if (color_type & PNG_COLOR_MASK_COLOR)
{
if (png_get_valid(png_ptr, info_ptr,
PNG_INFO_PLTE))
{
png_color_16p histogram;
png_get_hIST(png_ptr, info_ptr,
&histogram);
png_set_dither(png_ptr, palette, num_palette,
max_screen_colors, histogram, 1);
{
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_GRAY)
png_set_invert_mono(png_ptr);
PNG files store 16 bit pixels in network byte order (big-
endian, ie. most significant bits first). This code
changes the storage to the other way (little-endian, i.e.
least significant bits first, the way PCs store them):
if (bit_depth == 16)
png_set_swap(png_ptr);
If you are using packed-pixel images (1, 2, or 4
bits/pixel), and you need to change the order the pixels
are packed into bytes, you can use:
if (bit_depth < 8)
png_set_packswap(png_ptr);
Finally, you can write your own transformation function if
none of the existing ones meets your needs. This is done
by setting a callback with
png_set_read_user_transform_fn(png_ptr,
read_transform_fn);
You must supply the function
void read_transform_fn(png_ptr ptr, row_info_ptr
row_info, png_bytep data)
See pngtest.c for a working example. Your function will
be called after all of the other transformations have been
processed.
You can also set up a pointer to a user structure for use
by your callback function, and you can inform libpng that
your transform function will change the number of channels
or bit depth with the function
user_depth, user_channels);
The user's application, not libpng, is responsible for
allocating and freeing any memory required for the user
structure.
You can retrieve the pointer via the function
png_get_user_transform_ptr(). For example:
voidp read_user_transform_ptr =
png_get_user_transform_ptr(png_ptr);
The last thing to handle is interlacing; this is covered
in detail below, but you must call the function here if
you want libpng to handle expansion of the interlaced
image.
number_of_passes = png_set_interlace_han
dling(png_ptr);
After setting the transformations, libpng can update your
png_info structure to reflect any transformations you've
requested with this call. This is most useful to update
the info structure's rowbytes field so you can use it to
allocate your image memory. This function will also
update your palette with the correct screen_gamma and
background if these have been given with the calls above.
png_read_update_info(png_ptr, info_ptr);
After you call png_read_update_info(), you can allocate
any memory you need to hold the image. The row data is
simply raw byte data for all forms of images. As the
actual allocation varies among applications, no example
will be given. If you are allocating one large chunk, you
will need to build an array of pointers to each row, as it
will be needed for some of the functions below.
Reading image data
After you've allocated memory, you can read the image
data. The simplest way to do this is in one function
call. If you are allocating enough memory to hold the
whole image, you can just call png_read_image() and libpng
will read in all the image data and put it in the memory
area supplied. You will need to pass in an array of
pointers to each row.
This function automatically handles interlacing, so you
don't need to call png_set_interlace_handling() or call
this function multiple times, or any of that other stuff
necessary with png_read_rows().
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 interlac
ing (check interlace_type == PNG_INTERLACE_NONE), this is
simple:
png_read_rows(png_ptr, row_pointers, NULL,
number_of_rows);
where row_pointers is the same as in the png_read_image()
call.
If you are doing this just one row at a time, you can do
this with a single row_pointer instead of an array of
row_pointers:
png_bytep row_pointer = row;
png_read_row(png_ptr, row_pointer, NULL);
If the file is interlaced (interlace_type != 0 in the IHDR
chunk), things get somewhat harder. The only current (PNG
Specification version 1.2) interlacing type for PNG is
(interlace_type == PNG_INTERLACE_ADAM7) is a somewhat 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 specifica
tion 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 gradu
ally smooths out as more pixels are read. The other
method is the "sparkle" method, where pixels are drawn
only in their final locations, with the rest of the image
remaining whatever colors they were initialized to before
the start of the read. The first method usually looks
better, but tends to be slower, as there are more pixels
to put in the rows.
If you don't want libpng to handle the interlacing
details, just call png_read_rows() seven times to read in
all seven images. Each of the images is a valid image by
itself, or they can all be combined on an 8x8 grid to form
dling).
The first pass will return an image 1/8 as wide as the
entire image (every 8th column starting in column 0) and
1/8 as high as the original (every 8th row starting in row
0), the second will be 1/8 as wide (starting in column 4)
and 1/8 as high (also starting in row 0). The third pass
will be 1/4 as wide (every 4th pixel starting in column 0)
and 1/8 as high (every 8th row starting in row 4), and the
fourth pass will be 1/4 as wide and 1/4 as high (every 4th
column starting in column 2, and every 4th row starting in
row 0). The fifth pass will return an image 1/2 as wide,
and 1/4 as high (starting at column 0 and row 2), while
the sixth pass will be 1/2 as wide and 1/2 as high as the
original (starting in column 1 and row 0). The seventh
and final pass will be as wide as the original, and 1/2 as
high, containing all of the odd numbered scanlines. Phew!
If you want libpng to expand the images, call this before
calling png_start_read_image() or png_read_update_info():
if (interlace_type == PNG_INTERLACE_ADAM7)
number_of_passes
= png_set_interlace_handling(png_ptr);
This will return the number of passes needed. Currently,
this is seven, but may change if another interlace type is
added. This function can be called even if the file is
not interlaced, where it will return one pass.
If you are not going to display the image after each pass,
but are going to wait until the entire image is read in,
use the sparkle effect. This effect is faster and the end
result of either method is exactly the same. If you are
planning on displaying the image after each pass, the
"rectangle" effect is generally considered the better
looking one.
If you only want the "sparkle" effect, just call
png_read_rows() as normal, with the third parameter NULL.
Make sure you make pass over the image number_of_passes
times, and you don't change the data in the rows between
calls. You can change the locations of the data, just not
the data. Each pass only writes the pixels appropriate
for that pass, and assumes the data from previous passes
is still valid.
png_read_rows(png_ptr, row_pointers, NULL,
number_of_rows);
If you only want the first effect (the rectangles), do the
same as before except pass the row buffer in the third
number_of_rows);
Finishing a sequential read
After you are finished reading the image through either
the high- or low-level interfaces, 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 sepa
rate. If you are not interested, you can pass NULL.
png_read_end(png_ptr, end_info);
When you are done, you can free all memory allocated by
libpng like this:
png_destroy_read_struct(&png_ptr, &info_ptr,
&end_info);
It is also possible to individually free the info_ptr mem
bers that point to libpng-allocated storage with the fol
lowing function:
png_free_data(png_ptr, info_ptr, mask, n)
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
n - sequence number of item to be freed
(-1 for all items)
This function may be safely called when the relevant stor
age 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 "n" parameter is ignored
if only one item of the selected data type, such as PLTE,
is allowed. If "n" 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 is freed.
The default behavior is only to free data that was allo
cated internally by libpng. This can be changed, so that
libpng will not free the data, or so that it will free
data that was allocated by the user with png_malloc() or
png_zalloc() and passed in via a png_set_*() function,
with
mask - which data elements are affected
same choices as in png_free_data()
freer - one of
PNG_DESTROY_WILL_FREE_DATA
PNG_SET_WILL_FREE_DATA
PNG_USER_WILL_FREE_DATA
This function only affects data that has already been
allocated. You can call this function after reading the
PNG data but before calling any png_set_*() functions, to
control whether the user or the png_set_*() function is
responsible for freeing any existing data that might be
present, and again after the png_set_*() functions to con
trol whether the user or png_destroy_*() is supposed to
free the data. When the user assumes responsibility for
libpng-allocated data, the application must use png_free()
to free it, and when the user transfers responsibility to
libpng for data that the user has allocated, the user must
have used png_malloc() or png_zalloc() to allocate it (the
png_zalloc() function is the same as png_malloc() except
that it also zeroes the newly-allocated memory).
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 free
ing 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 responsi
bility for free'ing text_ptr from libpng to your applica
tion, your application must not separately free those mem
bers.
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_iCCP, PNG_INFO_sPLT,
PNG_INFO_sCAL, PNG_INFO_IDAT
For a more compact example of reading a PNG image, see the
file example.c.
Reading PNG files progressively
The progressive reader is slightly different then the non-
progressive reader. Instead of calling png_read_info(),
png_read_rows(), and png_read_end(), you make one call to
png_process_data(), which calls callbacks when it has the
info, a row, or the end of the image. You set up these
callbacks with png_set_progressive_read_fn(). You don't
have to worry about the input/output functions of libpng,
as you are giving the library the data directly in
png_process_data(). I will assume that you have read the
section on reading PNG files above, so I will only high
light 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,
you can specify NULL parameters. Even when all
three functions are NULL, you need to call
png_set_progressive_read_fn(). You can use
any struct as the user_ptr (cast to a void pointer
for the function call), and retrieve the pointer
from inside the callbacks using the function
png_get_progressive_ptr(png_ptr);
which will return a void pointer, which you have
to cast appropriately.
*/
png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
info_callback, row_callback, end_callback);
return 0;
}
/* A code fragment that you call as you receive blocks
of data */
int
process_data(png_bytep buffer, png_uint_32 length)
{
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
return (ERROR);
}
/* This one's new also. Simply give it a chunk
of data from the file stream (in order, of
course). On machines with segmented memory
models machines, don't give it any more than
64K. The library seems to run fine with sizes
of 4K. Although you can give it much less if
necessary (I assume you can give it chunks of
1 byte, I haven't tried less then 256 bytes
yet). When this function returns, you may
want to display any rows that were generated
in the row callback if you don't already do
so there.
*/
png_process_data(png_ptr, info_ptr, buffer, length);
return 0;
}
/* This function is called (as set by
png_set_progressive_read_fn() above) when enough data
has been supplied so all of the header has been
read.
*/
{
/* Do any setup here, including setting any of
the transformations mentioned in the Reading
PNG files section. For now, you _must_ call
either png_start_read_image() or
png_read_update_info() after all the
transformations are set (even if you don't set
any). You may start getting rows before
png_process_data() returns, so this is your
last chance to prepare for that.
*/
}
/* This function is called when each row of image
data is complete */
void
row_callback(png_structp png_ptr, png_bytep new_row,
png_uint_32 row_num, int pass)
{
/* If the image is interlaced, and you turned
on the interlace handler, this function will
be called for every row in every pass. Some
of these rows will not be changed from the
previous pass. When the row is not changed,
the new_row variable will be NULL. The rows
and passes are called in order, so you don't
really need the row_num and pass, but I'm
supplying them because it may make your life
easier.
For the non-NULL rows of interlaced images,
you must call png_progressive_combine_row()
passing in the row and the old row. You can
call this function for NULL rows (it will just
return) and for non-interlaced images (it just
does the memcpy for you) if it will make the
code easier. Thus, you can just do this for
all cases:
*/
png_progressive_combine_row(png_ptr, old_row,
new_row);
/* where old_row is what was displayed for
previously for the row. Note that the first
pass (pass == 0, really) will completely cover
the old row, so the rows do not have to be
initialized. After the first pass (and only
for interlaced images), you will have to pass
the current row, and the function will combine
the old row and the new row.
*/
end_callback(png_structp png_ptr, png_infop info)
{
/* This function is called after the whole image
has been read, including any chunks after the
image (up to and including the IEND). You
will usually have the same info chunk as you
had in the header, although some data may have
been added to the comments and time fields.
Most people won't do much here, perhaps setting
a flag that marks the image as finished.
*/
}
IV. Writing
Much of this is very similar to reading. However, every
thing of importance is repeated here, so you won't have to
constantly look back up in the reading section to under
stand 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 any
thing to undo. If you are not using the standard I/O func
tions, you will need to replace them with custom writing
functions. See the discussion under Customizing libpng.
FILE *fp = fopen(file_name, "wb");
if (!fp)
{
return (ERROR);
}
Next, png_struct and png_info need to be allocated and
initialized. As these can be both relatively large, you
may not want to store these on the stack, unless you have
stack space to spare. Of course, you will want to check
if they return NULL. If you are also reading, you won't
want to name your read structure and your write structure
both "png_ptr"; you can call them anything you like, such
as "read_ptr" and "write_ptr". Look at pngtest.c, for
example.
png_structp png_ptr = png_create_write_struct
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
return (ERROR);
if (!info_ptr)
{
png_destroy_write_struct(&png_ptr,
(png_infopp)NULL);
return (ERROR);
}
If you want to use your own memory allocation routines,
define PNG_USER_MEM_SUPPORTED and use png_cre
ate_write_struct_2() instead of png_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_jmp
buf(png_ptr). If you write the file from different rou
tines, you will need to update the png_jmpbuf(png_ptr)
every time you enter a new routine that will call a
png_*() function. See your documentation of
setjmp/longjmp for your compiler for more information on
setjmp/longjmp. See the discussion on libpng error han
dling in the Customizing Libpng section below for more
information on the libpng error handling.
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(fp);
return (ERROR);
}
...
return;
If you would rather avoid the complexity of setjmp/longjmp
issues, you can compile libpng with PNG_SETJMP_NOT_SUP
PORTED, in which case errors will result in a call to
PNG_ABORT() which defaults to abort().
Now you need to set up the output code. The default for
libpng is to use the C function fwrite(). If you use
this, you will need to pass a valid FILE * in the function
png_init_io(). Be sure that the file is opened in binary
mode. Again, if you wish to handle writing data in
another way, see the discussion on libpng I/O handling in
the Customizing Libpng section below.
png_init_io(png_ptr, fp);
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 value is '0' (as of the
July 1999 PNG specification, version 1.2). The third
parameter is a flag that indicates which filter type(s)
are to be tested for each scanline. See the PNG specifi
cation 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 be stored in case it's needed later), and then add
zlib compression library, and should mostly be ignored
unless you really know what you are doing. The only gen
erally useful call is png_set_compression_level() which
changes how much time zlib spends on trying to compress
the image data. See the Compression Library (zlib.h and
algorithm.txt, distributed with zlib) for details on the
compression levels.
/* set the zlib compression level */
png_set_compression_level(png_ptr,
Z_BEST_COMPRESSION);
/* set other zlib parameters */
png_set_compression_mem_level(png_ptr, 8);
png_set_compression_strategy(png_ptr,
Z_DEFAULT_STRATEGY);
png_set_compression_window_bits(png_ptr, 15);
png_set_compression_method(png_ptr, 8);
png_set_compression_buffer_size(png_ptr, 8192)
extern PNG_EXPORT(void,png_set_zbuf_size)
Setting the contents of info for output
You now need to fill in the png_info structure with all
the data you wish to write before the actual image. Note
that the only thing you are allowed to write after the
image is the text chunks and the time chunk (as of PNG
Specification 1.2, anyway). See png_write_end() and the
latest PNG specification for more information on that. If
you wish to write them before the image, fill them in now,
and flag that data as being valid. If you want to wait
until after the data, don't fill them until
png_write_end(). For all the fields in png_info and their
data types, see png.h. For explanations of what the
fields contain, see the PNG specification.
Some of the more important parts of the png_info are:
png_set_IHDR(png_ptr, info_ptr, width, height,
bit_depth, color_type, interlace_type,
compression_type, filter_type)
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).
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_type - (must be PNG_FILTER_TYPE_DEFAULT)
png_set_PLTE(png_ptr, info_ptr, palette,
num_palette);
palette - the palette for the file
(array of png_color)
num_palette - number of entries in the palette
png_set_gAMA(png_ptr, info_ptr, gamma);
gamma - the gamma the image was created
at (PNG_INFO_gAMA)
png_set_sRGB(png_ptr, info_ptr, srgb_intent);
srgb_intent - the rendering intent
(PNG_INFO_sRGB) The presence of
the sRGB chunk means that the pixel
data is in the sRGB color space.
This chunk also implies specific
values of gAMA and cHRM. Rendering
intent is the CSS-1 property that
has been defined by the International
Color Consortium
(http://www.color.org).
It can be one of
PNG_sRGB_INTENT_SATURATION,
PNG_sRGB_INTENT_PERCEPTUAL,
PNG_sRGB_INTENT_ABSOLUTE, or
PNG_sRGB_INTENT_RELATIVE.
png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
srgb_intent);
srgb_intent - the rendering intent
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, compres
sion_type,
profile, proflen);
name - The profile name.
compression - The compression type; always
PNG_COMPRESSION_TYPE_BASE
for PNG 1.0. You may give NULL to
this argument
to ignore it.
profile - International Color Consortium color
profile
data. May contain NULs.
proflen - length of profile data in bytes.
png_set_sBIT(png_ptr, info_ptr, sig_bit);
sig_bit - the number of significant bits for
(PNG_INFO_sBIT) each of the gray,
red,
green, and blue channels, whichever
are
appropriate for the given color type
(png_color_16)
png_set_tRNS(png_ptr, info_ptr, trans, num_trans,
trans_values);
trans - array of transparent entries for
palette (PNG_INFO_tRNS)
trans_values - graylevel or color sample values of
the single transparent color for
non-paletted images (PNG_INFO_tRNS)
num_trans - number of transparent entries
(PNG_INFO_tRNS)
png_set_hIST(png_ptr, info_ptr, hist);
(PNG_INFO_hIST)
hist - histogram of palette (array of
png_uint_16)
png_set_tIME(png_ptr, info_ptr, mod_time);
mod_time - time image was last modified
(PNG_VALID_tIME)
png_set_bKGD(png_ptr, info_ptr, background);
background - background color (PNG_VALID_bKGD)
png_set_text(png_ptr, info_ptr, text_ptr, num_text);
text_ptr[i].compression - type of compression used
on "text" PNG_TEXT_COMPRESSION_NONE
PNG_TEXT_COMPRESSION_zTXt
PNG_ITXT_COMPRESSION_NONE
PNG_ITXT_COMPRESSION_zTXt
text_ptr[i].key - keyword for comment. Must contain
1-79 characters.
text_ptr[i].text - text comments for current
keyword. Can be NULL or empty.
text_ptr[i].text_length - length of text string,
after decompression, 0 for iTXt
text_ptr[i].itxt_length - length of itxt string,
after decompression, 0 for tEXt/zTXt
text_ptr[i].lang - language of comment (NULL or
empty for unknown).
text_ptr[i].translated_keyword - keyword in UTF-8
(NULL
or empty for unknown).
num_text - number of comments
png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
num_spalettes);
palette_ptr - array of png_sPLT_struct structures
to be
added to the list of palettes in the
info
structure.
num_spalettes - number of palette structures to be
added.
png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
unit_type);
offset_x - positive offset from the left
edge of the screen
offset_y - positive offset from the top
edge of the screen
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
unit_type);
res_x - pixels/unit physical resolution
in x direction
res_y - pixels/unit physical resolution
in y direction
unit_type - PNG_RESOLUTION_UNKNOWN,
PNG_RESOLUTION_METER
png_set_sCAL(png_ptr, info_ptr, unit, width, height)
unit - physical scale units (an integer)
width - width of a pixel in physical scale units
height - height of a pixel in physical scale
units
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 struc
tures holding
unknown chunks
unknowns[i].name - name of unknown chunk
unknowns[i].data - data of unknown chunk
unknowns[i].size - size of unknown chunk's data
unknowns[i].location - position to write chunk in file
0: do not write chunk
PNG_HAVE_IHDR: before PLTE
PNG_HAVE_PLTE: before IDAT
PNG_AFTER_IDAT: after IDAT
The "location" member is set automatically according
to
what part of the output file has already been written.
You can change its value after calling
png_set_unknown_chunks()
as demonstrated in pngtest.c. Within each of the
"locations",
the chunks are sequenced according to their position
in the
structure (that is, the value of "i", which is the
order in which
the chunk was either read from the input file or
defined with
png_set_unknown_chunks).
A quick word about text and num_text. text is an array of
png_text structures. num_text is the number of valid
structures in the array. Each png_text structure holds a
language code, a keyword, a text value, and a compression
type.
The compression types have the same valid numbers as the
compression types of the image data. Currently, the only
valid number is zero. However, you can store text either
compressed or uncompressed, unlike images, which always
have to be compressed. So if you don't want the text com
pressed, set the compression type to PNG_TEXT_COMPRES
SION_NONE. Because tEXt and zTXt chunks don't have a lan
guage field, if you specify PNG_TEXT_COMPRESSION_NONE or
PNG_TEXT_COMPRESSION_zTXt any language code or translated
keyword will not be written out.
the file, the compression type is set to PNG_TEXT_COMPRES
SION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR, so that it
isn't written out again at the end (in case you are call
ing png_write_end() with the same struct.
The keywords that are given in the PNG Specification are:
Title Short (one line) title or
caption for image
Author Name of image's creator
Description Description of image (possibly long)
Copyright Copyright notice
Creation Time Time of original image creation
(usually RFC 1123 format, see below)
Software Software used to create the image
Disclaimer Legal disclaimer
Warning Warning of nature of content
Source Device used to create the image
Comment Miscellaneous comment; conversion
from other image format
The keyword-text pairs work like this. Keywords should be
short simple descriptions of what the comment is about.
Some typical keywords are found in the PNG specification,
as is some recommendations on keywords. You can repeat
keywords in a file. You can even write some text before
the image and some after. For example, you may want to
put a description of the image before the image, but leave
the disclaimer until after, so viewers working over modem
connections don't have to wait for the disclaimer to go
over the modem before they start seeing the image.
Finally, keywords should be full words, not abbreviations.
Keywords and text are in the ISO 8859-1 (Latin-1) charac
ter set (a superset of regular ASCII) and can not contain
NUL characters, and should not contain control or other
unprintable characters. To make the comments widely read
able, stick with basic ASCII, and avoid machine specific
character set extensions like the IBM-PC character set.
The keyword must be present, but you can leave off the
text string on non-compressed pairs. Compressed pairs
must have a text string, as only the text string is com
pressed anyway, so the compression would be meaningless.
PNG supports modification time via the png_time structure.
Two conversion routines are provided, png_con
vert_from_time_t() for time_t and png_con
vert_from_struct_tm() for struct tm. The time_t routine
uses gmtime(). You don't have to use either of these, but
if you wish to fill in the png_time structure directly,
you should provide the time in universal time (GMT) if
possible instead of your local time. Note that the year
number is the full year (e.g. 1998, rather than 98 - PNG
ation, you should use a plain tEXt chunk with the "Cre
ation Time" keyword. This is necessary because the "cre
ation time" of a PNG image is somewhat vague, depending on
whether you mean the PNG file, the time the image was cre
ated 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 rec
ommended 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 automati
cally changed by the software. To facilitate the use of
RFC 1123 dates, a function png_con
vert_to_rfc1123(png_timep) is provided to convert from PNG
time to an RFC 1123 format string.
Writing unknown chunks
You can use the png_set_unknown_chunks function to queue
up chunks for writing. You give it a chunk name, raw
data, and a size; that's all there is to it. The chunks
will be written by the next following
png_write_info_before_PLTE, png_write_info, or
png_write_end function. Any chunks previously read into
the info structure's unknown-chunk list will also be writ
ten out in a sequence that satisfies the PNG specifica
tion'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 sam
ples
PNG_TRANSFORM_PACKSWAP Change order of packed
pixels to LSB first
PNG_TRANSFORM_INVERT_MONO Invert monochrome images
PNG_TRANSFORM_SHIFT Normalize pixels to the
sBIT depth
PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA to
BGRA
PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA to
AG
PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
to transparency
PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
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 trans
formations indicated by the transform mask, then
png_write_image(), and finally png_write_end().
(The final parameter of this call is not yet used. Some
day it might point to transformation parameters required
by some future output transform.)
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 sup
plied as a level of transparency, you can invert the alpha
channel before you write it, so that 0 is fully transpar
ent and 255 (in 8-bit or paletted images) or 65535 (in
16-bit images) is fully opaque, with
png_set_invert_alpha(png_ptr);
This must appear before png_write_info() instead of later
with the other transformations because in the case of
paletted images the tRNS chunk data has to be inverted
before the tRNS chunk is written. If your image is not a
paletted image, the tRNS data (which in such cases repre
sents a single color to be rendered as transparent) won't
need to be changed, and you can safely do this transforma
tion after your png_write_info() call.
If you need to write a private chunk that you want to
appear before the PLTE chunk when PLTE is present, you can
write the PNG info in two steps, and insert code to write
your own chunk between them:
png_write_info_before_PLTE(png_ptr, info_ptr);
png_set_unknown_chunks(png_ptr, info_ptr, ...);
png_write_info(png_ptr, info_ptr);
After you've written the file information, you can set up
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 cer
tain color types and bit depths. Even though each trans
formation checks to see if it has data that it can do
something with, you should make sure to only enable a
transformation if it will be valid for the data. For
example, don't swap red and blue on grayscale data.
PNG files store RGB pixels packed into 3 or 6 bytes. This
code tells the library to strip input data that has 4 or 8
bytes per pixel down to 3 or 6 bytes (or strip 2 or 4-byte
grayscale+filler data to 1 or 2 bytes per pixel).
png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
where the 0 is unused, and the location is either
PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon
whether the filler byte in the pixel is stored XRGB or
RGBX.
PNG files pack pixels of bit depths 1, 2, and 4 into bytes
as small as they can, resulting in, for example, 8 pixels
per byte for 1 bit files. If the data is supplied at 1
pixel per byte, use this code, which will correctly pack
the pixels into a single byte:
png_set_packing(png_ptr);
PNG files reduce possible bit depths to 1, 2, 4, 8, and
16. If your data is of another bit depth, you can write
an sBIT chunk into the file so that decoders can recover
the original data if desired.
/* Set the true bit depth of the image data */
if (color_type & PNG_COLOR_MASK_COLOR)
{
sig_bit.red = true_bit_depth;
sig_bit.green = true_bit_depth;
sig_bit.blue = true_bit_depth;
}
else
{
sig_bit.gray = true_bit_depth;
}
if (color_type & PNG_COLOR_MASK_ALPHA)
{
sig_bit.alpha = true_bit_depth;
}
png_set_sBIT(png_ptr, info_ptr, &sig_bit);
range 0-7 for a 4-bit PNG), this will scale the values to
appear to be the correct bit depth as is required by PNG.
png_set_shift(png_ptr, &sig_bit);
PNG files store 16 bit pixels in network byte order (big-
endian, ie. most significant bits first). This code would
be used if they are supplied the other way (little-endian,
i.e. least significant bits first, the way PCs store
them):
if (bit_depth > 8)
png_set_swap(png_ptr);
If you are using packed-pixel images (1, 2, or 4
bits/pixel), and you need to change the order the pixels
are packed into bytes, you can use:
if (bit_depth < 8)
png_set_packswap(png_ptr);
PNG files store 3 color pixels in red, green, blue order.
This code would be used if they are supplied as blue,
green, red:
png_set_bgr(png_ptr);
PNG files describe monochrome as black being zero and
white being one. This code would be used if the pixels are
supplied with this reversed (black being one and white
being zero):
png_set_invert_mono(png_ptr);
Finally, you can write your own transformation function if
none of the existing ones meets your needs. This is done
by setting a callback with
png_set_write_user_transform_fn(png_ptr,
write_transform_fn);
You must supply the function
void write_transform_fn(png_ptr ptr, row_info_ptr
row_info, png_bytep data)
See pngtest.c for a working example. Your function will
be called before any of the other transformations are pro
cessed.
You can also set up a pointer to a user structure for use
by your callback function.
The user_channels and user_depth parameters of this func
tion are ignored when writing; you can set them to zero as
shown.
You can retrieve the pointer via the function
png_get_user_transform_ptr(). For example:
voidp write_user_transform_ptr =
png_get_user_transform_ptr(png_ptr);
It is possible to have libpng flush any pending output,
either manually, or automatically after a certain number
of lines have been written. To flush the output stream a
single time call:
png_write_flush(png_ptr);
and to have libpng flush the output stream periodically
after a certain number of scanlines have been written,
call:
png_set_flush(png_ptr, nrows);
Note that the distance between rows is from the last time
png_write_flush() was called, or the first row of the
image if it has never been called. So if you write 50
lines, and then png_set_flush 25, it will flush the output
on the next scanline, and every 25 lines thereafter,
unless png_write_flush() is called before 25 more lines
have been written. If nrows is too small (less than about
10 lines for a 640 pixel wide RGB image) the image com
pression may decrease noticeably (although this may be
acceptable for real-time applications). Infrequent flush
ing 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 func
tion 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 interlac
ing, so you don't need to call png_set_interlace_han
dling() or call this function multiple times, or any of
that other stuff necessary with png_write_rows().
png_write_image(png_ptr, row_pointers);
where row_pointers is:
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
interlaced, this is simple:
png_write_rows(png_ptr, row_pointers,
number_of_rows);
row_pointers is the same as in the png_write_image() call.
If you are just writing one row at a time, you can do this
with a single row_pointer instead of an array of
row_pointers:
png_bytep row_pointer = row;
png_write_row(png_ptr, row_pointer);
When the file is interlaced, things can get a good deal
more complicated. The only currently (as of the PNG Spec
ification version 1.2, dated July 1999) defined interlac
ing 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 your
self, see the PNG specification for details of which pix
els to write when.
If you don't want libpng to handle the interlacing
details, just use png_set_interlace_handling() and call
png_write_rows() the correct number of times to write all
seven sub-images.
If you want libpng to build the sub-images, call this
before you start writing any rows:
number_of_passes =
png_set_interlace_handling(png_ptr);
This will return the number of passes needed. Currently,
this is seven, but may change if another interlace type is
added.
Then write the complete image number_of_passes times.
png_write_rows(png_ptr, row_pointers,
number_of_rows);
As some of these rows are not used, and thus return imme
used.
Finishing a sequential write
After you are finished writing the image, you should fin
ish 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 mem
bers that point to libpng-allocated storage with the fol
lowing function:
png_free_data(png_ptr, info_ptr, mask, n)
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
n - sequence number of item to be freed
(-1 for all items)
This function may be safely called when the relevant stor
age 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 "n" parameter is ignored
if only one item of the selected data type, such as PLTE,
is allowed. If "n" 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 is freed.
If you allocated data such as a palette that you passed in
to libpng with png_set_*, you must not free it until just
before the call to png_destroy_write_struct().
The default behavior is only to free data that was allo
cated internally by libpng. This can be changed, so that
libpng will not free the data, or so that it will free
data that was allocated by the user with png_malloc() or
png_zalloc() and passed in via a png_set_*() function,
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 con
tinue to use the PLTE, tRNS, and hIST data in the write
structure.
This function only affects data that has already been
allocated. You can call this function before calling
after the png_set_*() functions to control whether the
user or png_destroy_*() is supposed to free the data.
When the user assumes responsibility for libpng-allocated
data, the application must use png_free() to free it, and
when the user transfers responsibility to libpng for data
that the user has allocated, the user must have used
png_malloc() or png_zalloc() to allocate it.
If you allocated text_ptr.text, text_ptr.lang, and
text_ptr.translated_keyword separately, do not transfer
responsibility for freeing text_ptr to libpng, because
when libpng fills a png_text structure it combines these
members with the key member, and png_free_data() will free
only text_ptr.key. Similarly, if you transfer responsi
bility for free'ing text_ptr from libpng to your applica
tion, your application must not separately free those mem
bers. For a more compact example of writing a PNG image,
see the file example.c.
V. Modifying/Customizing libpng:
There are two issues here. The first is changing how
libpng does standard things like memory allocation,
input/output, and error handling. The second deals with
more complicated things like adding new chunks, adding new
transformations, and generally changing how libpng works.
dling in libpng goes through callbacks that are user set
table. The default routines are in pngmem.c, pngrio.c,
pngwio.c, and pngerror.c, respectively. To change these
functions, call the appropriate png_set_*_fn() function.
Memory allocation is done through the functions png_mal
loc(), png_zalloc(), 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 modi
fied in the library at compile time. If you prefer to use
a different method of allocating and freeing data, you can
use
png_set_mem_fn(png_structp png_ptr, png_voidp mem_ptr,
png_malloc_ptr
malloc_fn, png_free_ptr free_fn)
This function also provides 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_uint_32
size);
void free_fn(png_structp png_ptr, png_voidp ptr);
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 ini
tialized 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);
lows:
void user_read_data(png_structp png_ptr,
png_bytep data, png_uint_32 length);
void user_write_data(png_structp png_ptr,
png_bytep data, png_uint_32 length);
void user_flush_data(png_structp png_ptr);
Supplying NULL for the read, write, or flush functions
sets them back to using the default C stream functions.
It is an error to read from a write stream, and vice
versa.
Error handling in libpng is done through png_error() and
png_warning(). Errors handled through png_error() are
fatal, meaning that png_error() should never return to its
caller. Currently, this is handled via setjmp() and
longjmp() (unless you have compiled libpng with
PNG_SETJMP_NOT_SUPPORTED, in which case it is handled via
PNG_ABORT()), but you could change this to do things like
exit() if you should wish.
On non-fatal errors, png_warning() is called to print a
warning message, and then control returns to the calling
code. By default png_error() and png_warning() print a
message on stderr via fprintf() unless the library is 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 functions after png_create_*_struct() has
been called by calling:
png_set_error_fn(png_structp png_ptr,
png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warning_fn);
png_voidp error_ptr = png_get_error_ptr(png_ptr);
If NULL is supplied for either error_fn or warning_fn,
then the libpng default function will be used, calling
fprintf() and/or longjmp() if a problem is encountered.
The replacement error functions should have parameters as
follows:
void user_error_fn(png_structp png_ptr,
png_const_charp error_msg);
void user_warning_fn(png_structp png_ptr,
png_const_charp warning_msg);
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. How
ever, there are some uncertainties about the status of
local variables after a longjmp, so the user may want to
be careful about doing anything after setjmp returns non-
zero besides returning itself. Consult your compiler doc
umentation 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.
Hoewver, this may not be good enough if the library code
itself needs to know about interactions between your chunk
and existing `intrinsic' chunks.
If you need to write a new intrinsic chunk, first read the
PNG specification. Acquire a first level of understanding
of how it works. Pay particular attention to the sections
that describe chunk names, and look at how other chunks
were designed, so you can do things similarly. Second,
check out the sections of libpng that read and write
chunks. Try to find a chunk that is similar to yours and
use it as a template. More details can be found in the
comments inside the code. It is best to handle unknown
chunks in a generic method, via callback functions,
instead of by modifying libpng functions.
If you wish to write your own transformation for the data,
look through the part of the code that does the transfor
mations, and check out some of the simpler ones to get an
idea of how they work. Try to find a similar transforma
tion 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_com
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 com
pilers, you may also have to change the memory allocators
(png_malloc, etc.).
Configuring for compiler xxx:
All includes for libpng are in pngconf.h. If you need to
add/change/delete an include, this is the place to do it.
The includes that are not needed outside libpng are 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 desir
able to have maximum speed (Z_BEST_SPEED = 1). With ver
sions of zlib after v0.99, you can also specify no com
pression (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);
the library. The memory level defaults to 8, but it can
be lowered if you are short on memory (running DOS, for
example, where you only have 640K).
png_set_compression_mem_level(png_ptr, level);
The other functions are for configuring zlib. They are
not recommended for normal use and may result in writing
an invalid PNG file. See zlib.h for more information on
what these mean.
png_set_compression_strategy(png_ptr,
strategy);
png_set_compression_window_bits(png_ptr,
window_bits);
png_set_compression_method(png_ptr, method);
png_set_compression_buffer_size(png_ptr, size);
Controlling row filtering
If you want to control whether libpng uses filtering or
not, which filters are used, and how it goes about picking
row filters, you can call one of these functions. The
selection and configuration of row filters can have a sig
nificant impact on the size and encoding speed and a some
what lesser impact on the decoding speed of an image.
Filtering is enabled by default for RGB and grayscale
images (with and without alpha), but not for paletted
images nor for any images with bit depths less than 8
bits/pixel.
The 'method' parameter sets the main filtering method,
which is currently only '0' in the PNG 1.2 specification.
The 'filters' parameter sets which filter(s), if any,
should be used for each scanline. Possible values are
PNG_ALL_FILTERS and PNG_NO_FILTERS to turn filtering on
and off, respectively.
Individual filter types are PNG_FILTER_NONE, PNG_FIL
TER_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 fil
ters you intend to use so that libpng can initialize its
internal structures appropriately for all of the filter
types.
filters = PNG_FILTER_NONE | PNG_FILTER_SUB
PNG_FILTER_UP | PNG_FILTER_AVE |
PNG_FILTER_PAETH | PNG_ALL_FILTERS;
PNG_FILTER_VALUE_SUB, PNG_FILTER_VALUE_UP,
PNG_FILTER_VALUE_AVE, PNG_FILTER_VALUE_PAETH
png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
filters);
It is also possible to influence how libpng chooses from
among the available filters. This is done in two ways -
by telling it how important it is to keep the same filter
for successive rows, and by telling it the relative compu
tational costs of the filters.
double weights[3] = {1.5, 1.3, 1.1},
costs[PNG_FILTER_VALUE_LAST] =
{1.0, 1.3, 1.3, 1.5, 1.7};
png_set_filter_selection(png_ptr,
PNG_FILTER_SELECTION_WEIGHTED, 3,
weights, costs);
The weights are multiplying factors that indicate to
libpng that the row filter should be the same for succes
sive 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 val
ues 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,
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 direc
tives disable the extra transformations but still leave
the library fully capable of reading and writing PNG files
with all known public chunks Use of the PNG_NO_READ[or
WRITE]_ANCILLARY_CHUNKS directive produces a library that
is incapable of reading or writing ancillary chunks. If
you are not using the progressive reading capability, you
can turn that off with PNG_NO_PROGRESSIVE_READ (don't con
fuse this with the INTERLACING capability, which you'll
still have).
All the reading and writing specific code are in separate
files, so the linker should only grab the files it needs.
However, if you want to make sure, or if you are building
a stand alone library, all the reading files start with
pngr and all the writing files start with pngw. The files
that don't match either (like png.c, pngtrans.c, etc.)
are used for both reading and writing, and always need to
be included. The progressive reader is in pngpread.c
If you are creating or distributing a dynamically linked
library (a .so or DLL file), you should not remove or 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
debugging printout. Set it to an integer value in the
range 0 to 3. Higher numbers result in increasing amounts
of debugging information. The information is printed to
the "stderr" file, unless another file name is specified
in the PNG_DEBUG_FILE macro definition.
When PNG_DEBUG > 0, the following functions (macros)
become available:
png_debug(level, message)
png_debug1(level, message, p1)
png_debug2(level, message, p1, p2)
whether to print the message, "message" is the formatted
string to be printed, and p1 and p2 are parameters that
are to be embedded in the string according to printf-style
formatting directives. For example,
png_debug1(2, "foo=%d0, foo);
is expanded to
if(PNG_DEBUG > 2)
fprintf(PNG_DEBUG_FILE, "foo=%d0, foo);
When PNG_DEBUG is defined but is zero, the macros aren't
defined, but you can still use PNG_DEBUG to control your
own debugging:
#ifdef PNG_DEBUG
fprintf(stderr, ...
#endif
When PNG_DEBUG = 1, the macros are defined, but only
png_debug statements having level = 0 will be printed.
There aren't any such statements in this version of
libpng, but if you insert some they will be printed.
VI. Changes to Libpng from version 0.88
It should be noted that versions of libpng later than 0.96
are not distributed by the original libpng author, Guy
Schalnat, nor by Andreas Dilger, who had taken over from
Guy during 1996 and 1997, and distributed versions 0.89
through 0.96, but rather by another member of the original
PNG Group, Glenn Randers-Pehrson. Guy and Andreas are
still alive and well, but they have moved on to other
things.
The old libpng functions png_read_init(),
png_write_init(), png_info_init(), png_read_destroy(), and
png_write_destory() 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 func
tions png_read_destroy() and png_write_destroy() do not
actually free the memory that libpng allocated for these
structs, but just reset the data structures, so they can
system overhead allocating and freeing the png_struct for
each image read.
Setting the error callbacks via png_set_message_fn()
before png_read_init() as was suggested in libpng-0.88 is
no longer supported because this caused applications that
do not use custom error functions to fail if the png_ptr
was not initialized to zero. It is still possible to set
the error callbacks AFTER png_read_init(), or to change
them with png_set_error_fn(), which is essentially the
same function, but with a new name to force compilation
errors with applications that try to use the old method.
Starting with version 1.0.7, you can find out which ver
sion of the library you are using at run-time:
png_uint_32 libpng_vn = png_access_version_number();
The number libpng_vn is constructed from the major ver
sion, minor version with leading zero, and release number
with leading zero, (e.g., libpng_vn for version 1.0.7 is
10007).
You can also check which version of png.h you used when
compiling your application:
png_uint_32 application_vn = PNG_LIBPNG_VER;
VII. Y2K Compliance in libpng
July 24, 2000
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 1.0.8 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.
(formerly png_convert_to_rfc_1152() in error)
png_convert_from_struct_tm() in pngwrite.c, called in
pngwrite.c
png_convert_from_time_t() in pngwrite.c
png_get_tIME() in pngget.c
png_handle_tIME() in pngrutil.c, called in pngread.c
png_set_tIME() in pngset.c
png_write_tIME() in pngwutil.c, called in pngwrite.c
All appear to handle dates properly in a Y2K environment.
The png_convert_from_time_t() function calls gmtime() to
convert from system clock time, which returns (year -
1900), which we properly convert to the full 4-digit year.
There is a possibility that applications using libpng are
not passing 4-digit years into the png_con
vert_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 incom
patibilities and occasional factors outside the authors'
control, version numbering on the library has not always
been consistent and straightforward. The following table
summarizes matters since version 0.89c, which was the
first widely used release:
source png.h png.h shared-lib
version string int version
------- ------ ----- ----------
0.89c ("1.0 beta 3") 0.89 89 1.0.89
0.90 ("1.0 beta 4") 0.90 90 0.90 [should
have been 2.0.90]
0.95 ("1.0 beta 5") 0.95 95 0.95 [should
have been 2.0.96]
0.97b ("1.00.97 beta 7") 1.00.97 97 1.0.1 [should
have been 2.0.97]
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 [100
should be 10000]
1.0.0 1.0.0 100 2.1.0 [100
should be 10000]
1.0.1 1.0.1 10001 2.1.0
1.0.1a-e 1.0.1a-e 10002 2.1.0.1a-e
1.0.2 1.0.2 10002 2.1.0.2
1.0.2a-b 1.0.2a-b 10003 2.1.0.2a-b
1.0.3 1.0.3 10003 2.1.0.3
1.0.3a-d 1.0.3a-d 10004 2.1.0.3a-d
1.0.4 1.0.4 10004 2.1.0.4
1.0.4a-f 1.0.4a-f 10005 2.1.0.4a-f
1.0.5 (+ 2 patches) 1.0.5 10005 2.1.0.5
1.0.5a-d 1.0.5a-d 10006 2.1.0.5a-d
1.0.5e-r 1.0.5e-r 10100 2.1.0.5e-r
(not compatible)
1.0.5s-v 1.0.5s-v 10006 2.1.0.5s-v
(compatible)
1.0.6 (+ 3 patches) 1.0.6 10006 2.1.0.6
1.0.6d 1.0.6d 10007 2.1.0.6d
1.0.7 1.0.7 10007 2.1.0.7
(still compatible)
Henceforth the source version will match the shared-
library minor
and patch numbers; the shared-library major version
number will be
used for changes in backward compatibility, as it is
intended. The
PNG_PNGLIB_VER macro, which is not used within libpng
but is available
for applications, is an unsigned integer of the form
xyyzz corresponding
to the source version x.y.z (leading zeros in y and z).
Beta versions
are given the previous public release number plus a
letter or two.
SEE ALSO
libpngpf(3), png(5)
libpng:
ftp://ftp.uu.net/graphics/png
(generally) at the same location as libpng or at
ftp://ftp.uu.net/pub/archiving/zip/zlib
ftp://ftp.freesoftware.com/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 specifi
cation and this library, the specification takes prece
dence.
AUTHORS
This man page: Glenn Randers-Pehrson <randeg@alum.rpi.edu>
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 documen
tation.
Libpng version 1.0.8 - July 24, 2000: Initially created in
1995 by Guy Eric Schalnat, then of Group 42, Inc. Cur
rently maintained by Glenn Randers-Pehrson (ran
deg@alum.rpi.edu).
Supported by the PNG development group
(png-implement@ccrc.wustl.edu).
COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:
If you modify libpng you may insert additional notices
immediately following this sentence.
libpng versions 1.0.7, July 1, 2000, through 1.0.8, July
24, 2000, are Copyright (c) 2000 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
enjoyment of the
library or against infringement. There is no warranty
that our
efforts or the library will fulfill any of your partic
ular purposes
or needs. This library is provided with all faults,
and the entire
risk of satisfactory quality, performance, accuracy,
and effort is with
the user.
libpng versions 0.97, January 1998, through 1.0.6, March
20, 2000, are Copyright (c) 1998, 1999 Glenn Randers-
Pehrson Distributed according to the same disclaimer and
license as libpng-0.96, with the following individuals
added to the list of Contributing Authors:
Tom Lane
Glenn Randers-Pehrson
Willem van Schaik
libpng versions 0.89, June 1996, through 0.96, May 1997,
are Copyright (c) 1996, 1997 Andreas Dilger Distributed
according to the same disclaimer and license as
libpng-0.88, with the following individuals added to the
list of Contributing Authors:
John Bowler
Kevin Bracey
Sam Bushell
Magnus Holmgren
Greg Roelofs
Tom Tanner
libpng versions 0.5, May 1995, through 0.88, January 1996,
are Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42,
Inc.
For the purposes of this copyright and license, "Con
tributing Authors" is defined as the following set of
individuals:
Andreas Dilger
Dave Martindale
Guy Eric Schalnat
Paul Schmidt
Tim Wegner
The PNG Reference Library is supplied "AS IS". The Con
tributing Authors and Group 42, Inc. disclaim all war
ranties, expressed or implied, including, without limita
tion, the warranties of merchantability and of fitness for
cial, exemplary, or consequential damages, which may
result from the use of the PNG Reference Library, even if
advised of the possibility of such damage.
Permission is hereby granted to use, copy, modify, and
distribute this source code, or portions hereof, for any
purpose, without fee, subject to the following restric
tions:
1. The origin of this source code must not be misrepre
sented.
2. Altered versions must be plainly marked as such and
must not
be misrepresented as being the original source.
3. This Copyright notice may not be removed or altered
from any
source or altered source distribution.
The Contributing Authors and Group 42, Inc. specifically
permit, without fee, and encourage the use of this source
code as a component to supporting the PNG file format in
commercial products. If you use this source code in a
product, acknowledgment is not required but would be
appreciated.
A "png_get_copyright" function is available, for conve
nient use in "about" boxes and the like:
printf("%s",png_get_copyright(NULL));
Also, the PNG logo (in PNG format, of course) is supplied
in the files "pngbar.png" and "pngbar.jpg (88x31) and
"pngnow.png" (98x31).
Libpng is OSI Certified Open Source Software. OSI Certi
fied Open Source is a certification mark of the Open
Source Initiative.
Glenn Randers-Pehrson randeg@alum.rpi.edu July 24, 2000
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