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libpng-manual.txt - A description on how to use and modify libpng
libpng version 1.6.26 - October 20, 2016
Updated and distributed by Glenn Randers-Pehrson
<glennrp at users.sourceforge.net>
Copyright (c) 1998-2016 Glenn Randers-Pehrson
This document is released under the libpng license.
For conditions of distribution and use, see the disclaimer
and license in png.h
Based on:
libpng versions 0.97, January 1998, through 1.6.26 - October 20, 2016
Updated and distributed by Glenn Randers-Pehrson
Copyright (c) 1998-2016 Glenn Randers-Pehrson
libpng 1.0 beta 6 - version 0.96 - May 28, 1997
Updated and distributed by Andreas Dilger
Copyright (c) 1996, 1997 Andreas Dilger
libpng 1.0 beta 2 - version 0.88 - January 26, 1996
For conditions of distribution and use, see copyright
notice in png.h. Copyright (c) 1995, 1996 Guy Eric
Schalnat, Group 42, Inc.
Updated/rewritten per request in the libpng FAQ
Copyright (c) 1995, 1996 Frank J. T. Wojcik
December 18, 1995 & January 20, 1996
I. Introduction
II. Structures
III. Reading
IV. Writing
V. Simplified API
VI. Modifying/Customizing libpng
VII. MNG support
VIII. Changes to Libpng from version 0.88
IX. Changes to Libpng from version 1.0.x to 1.2.x
X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
XI. Changes to Libpng from version 1.4.x to 1.5.x
XII. Changes to Libpng from version 1.5.x to 1.6.x
XIII. Detecting libpng
XIV. Source code repository
XV. Coding style
XVI. Y2K Compliance in libpng
I. Introduction
This file describes how to use and modify the PNG reference library
(known as libpng) for your own use. 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 configure and install libpng.
For examples of libpng usage, see the files "example.c", "pngtest.c",
and the files in the "contrib" directory, all of which are included in
the libpng distribution.
Libpng was written as a companion to the PNG specification, as a way
of reducing the amount of time and effort it takes to support the PNG
file format in application programs.
The PNG specification (second edition), November 2003, is available as
a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at
The W3C and ISO documents have identical technical content.
The PNG-1.2 specification is available at
It is technically equivalent
to the PNG specification (second edition) but has some additional material.
The PNG-1.0 specification is available as RFC 2083
<http://png-mng.sourceforge.net/pub/png/spec/1.0/> and as a
W3C Recommendation <http://www.w3.org/TR/REC-png-961001>.
Some additional chunks are described in the special-purpose public chunks
documents at <http://www.libpng.org/pub/png/spec/register/>
Other information
about PNG, and the latest version of libpng, can be found at the PNG home
page, <http://www.libpng.org/pub/png/>.
Most users will not have to modify the library significantly; advanced
users may want to modify it more. All attempts were made to make it as
complete as possible, while keeping the code easy to understand.
Currently, this library only supports C. Support for other languages
is being considered.
Libpng has been designed to handle multiple sessions at one time,
to be easily modifiable, to be portable to the vast majority of
machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
to use. The ultimate goal of libpng is to promote the acceptance of
the PNG file format in whatever way possible. While there is still
work to be done (see the TODO file), libpng should cover the
majority of the needs of its users.
Libpng uses zlib for its compression and decompression of PNG files.
Further information about zlib, and the latest version of zlib, can
be found at the zlib home page, <http://zlib.net/>.
The zlib compression utility is a general purpose utility that is
useful for more than PNG files, and can be used without libpng.
See the documentation delivered with zlib for more details.
You can usually find the source files for the zlib utility wherever you
find the libpng source files.
Libpng is thread safe, provided the threads are using different
instances of the structures. Each thread should have its own
png_struct and png_info instances, and thus its own image.
Libpng does not protect itself against two threads using the
same instance of a structure.
II. Structures
There are two main structures that are important to libpng, png_struct
and png_info. Both are internal structures that are no longer exposed
in the libpng interface (as of libpng 1.5.0).
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. However, this tended to cause problems
with applications using dynamically loaded libraries, and as a result
a set of interface functions for png_info (the png_get_*() and png_set_*()
functions) was developed, and direct access to the png_info fields was
The png_struct structure is the object used by the library to decode a
single image. As of 1.5.0 this structure is also not exposed.
Almost all libpng APIs require a pointer to a png_struct as the first argument.
Many (in particular the png_set and png_get APIs) also require a pointer
to png_info as the second argument. Some application visible macros
defined in png.h designed for basic data access (reading and writing
integers in the PNG format) don't take a png_info pointer, but it's almost
always safe to assume that a (png_struct*) has to be passed to call an API
You can have more than one png_info structure associated with an image,
as illustrated in pngtest.c, one for information valid prior to the
IDAT chunks and another (called "end_info" below) for things after them.
The png.h header file is an invaluable reference for programming with libpng.
And while I'm on the topic, make sure you include the libpng header file:
#include <png.h>
and also (as of libpng-1.5.0) the zlib header file, if you need it:
#include <zlib.h>
The png.h header file defines a number of integral types used by the
APIs. Most of these are fairly obvious; for example types corresponding
to integers of particular sizes and types for passing color values.
One exception is how non-integral numbers are handled. For application
convenience most APIs that take such numbers have C (double) arguments;
however, internally PNG, and libpng, use 32 bit signed integers and encode
the value by multiplying by 100,000. As of libpng 1.5.0 a convenience
macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point)
which is simply (png_int_32).
All APIs that take (double) arguments also have a matching API that
takes the corresponding fixed point integer arguments. The fixed point
API has the same name as the floating point one with "_fixed" appended.
The actual range of values permitted in the APIs is frequently less than
the full range of (png_fixed_point) (-21474 to +21474). When APIs require
a non-negative argument the type is recorded as png_uint_32 above. Consult
the header file and the text below for more information.
Special care must be take with sCAL chunk handling because the chunk itself
uses non-integral values encoded as strings containing decimal floating point
numbers. See the comments in the header file.
The main header file function declarations are frequently protected by C
preprocessing directives of the form:
#ifdef PNG_feature_SUPPORTED
#ifdef PNG_feature_SUPPORTED
The library can be built without support for these APIs, although a
standard build will have all implemented APIs. Application programs
should check the feature macros before using an API for maximum
portability. From libpng 1.5.0 the feature macros set during the build
of libpng are recorded in the header file "pnglibconf.h" and this file
is always included by png.h.
If you don't need to change the library configuration from the default, skip to
the next section ("Reading").
Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all
of the build project files in the 'projects' directory simply copy
scripts/pnglibconf.h.prebuilt to pnglibconf.h. This means that these build
systems do not permit easy auto-configuration of the library - they only
support the default configuration.
The easiest way to make minor changes to the libpng configuration when
auto-configuration is supported is to add definitions to the command line
using (typically) CPPFLAGS. For example:
will change the internal libpng math implementation for gamma correction and
other arithmetic calculations to fixed point, avoiding the need for fast
floating point support. The result can be seen in the generated pnglibconf.h -
make sure it contains the changed feature macro setting.
If you need to make more extensive configuration changes - more than one or two
feature macro settings - you can either add -DPNG_USER_CONFIG to the build
command line and put a list of feature macro settings in pngusr.h or you can set
DFA_XTRA (a makefile variable) to a file containing the same information in the
form of 'option' settings.
A. Changing pnglibconf.h
A variety of methods exist to build libpng. Not all of these support
reconfiguration of pnglibconf.h. To reconfigure pnglibconf.h it must either be
rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand.
Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to
pnglibconf.h and changing the lines defining the supported features, paying
very close attention to the 'option' information in scripts/pnglibconf.dfa
that describes those features and their requirements. This is easy to get
B. Configuration using DFA_XTRA
Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later
variant such as 'nawk' or 'gawk', is available. The configure build will
automatically find an appropriate awk and build pnglibconf.h.
The scripts/pnglibconf.mak file contains a set of make rules for doing the
same thing if configure is not used, and many of the makefiles in the scripts
directory use this approach.
When rebuilding simply write a new file containing changed options and set
DFA_XTRA to the name of this file. This causes the build to append the new file
to the end of scripts/pnglibconf.dfa. The pngusr.dfa file should contain lines
of the following forms:
everything = off
This turns all optional features off. Include it at the start of pngusr.dfa to
make it easier to build a minimal configuration. You will need to turn at least
some features on afterward to enable either reading or writing code, or both.
option feature on
option feature off
Enable or disable a single feature. This will automatically enable other
features required by a feature that is turned on or disable other features that
require a feature which is turned off. Conflicting settings will cause an error
message to be emitted by awk.
setting feature default value
Changes the default value of setting 'feature' to 'value'. There are a small
number of settings listed at the top of pnglibconf.h, they are documented in the
source code. Most of these values have performance implications for the library
but most of them have no visible effect on the API. Some can also be overridden
from the API.
This method of building a customized pnglibconf.h is illustrated in
contrib/pngminim/*. See the "$(PNGCONF):" target in the makefile and
pngusr.dfa in these directories.
C. Configuration using PNG_USER_CONFIG
If -DPNG_USER_CONFIG is added to the CPPFLAGS when pnglibconf.h is built,
the file pngusr.h will automatically be included before the options in
scripts/pnglibconf.dfa are processed. Your pngusr.h file should contain only
macro definitions turning features on or off or setting settings.
Apart from the global setting "everything = off" all the options listed above
can be set using macros in pngusr.h:
#define PNG_feature_SUPPORTED
is equivalent to:
option feature on
#define PNG_NO_feature
is equivalent to:
option feature off
#define PNG_feature value
is equivalent to:
setting feature default value
Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
pngusr file you supply override the contents of scripts/pnglibconf.dfa
If confusing or incomprehensible behavior results it is possible to
examine the intermediate file pnglibconf.dfn to find the full set of
dependency information for each setting and option. Simply locate the
feature in the file and read the C comments that precede it.
This method is also illustrated in the contrib/pngminim/* makefiles and
III. Reading
We'll now walk you through the possible functions to call when reading
in a PNG file sequentially, briefly explaining the syntax and purpose
of each one. See example.c and png.h for more detail. While
progressive reading is covered in the next section, you will still
need some of the functions discussed in this section to read a PNG
You will want to do the I/O initialization(*) before you get into libpng,
so if it doesn't work, you don't have much to undo. Of course, you
will also want to insure that you are, in fact, dealing with a PNG
file. Libpng provides a simple check to see if a file is a PNG file.
To use it, pass in the first 1 to 8 bytes of the file to the function
png_sig_cmp(), and it will return 0 (false) if the bytes match the
corresponding bytes of the PNG signature, or nonzero (true) otherwise.
Of course, the more bytes you pass in, the greater the accuracy of the
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()
with the number of bytes you read from the beginning. Libpng will
then only check the bytes (if any) that your program didn't read.
(*): If you are not using the standard I/O functions, you will need
to replace them with custom functions. See the discussion under
Customizing libpng.
FILE *fp = fopen(file_name, "rb");
if (!fp)
return (ERROR);
if (fread(header, 1, number, fp) != number)
return (ERROR);
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_infopp)NULL, (png_infopp)NULL);
return (ERROR);
If you want to use your own memory allocation routines,
use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use
png_create_read_struct_2() instead of png_create_read_struct():
png_structp png_ptr = png_create_read_struct_2
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn, (png_voidp)
user_mem_ptr, user_malloc_fn, user_free_fn);
The error handling routines passed to png_create_read_struct()
and the memory alloc/free routines passed to png_create_struct_2()
are only necessary if you are not using the libpng supplied error
handling and memory alloc/free functions.
When libpng encounters an error, it expects to longjmp back
to your routine. Therefore, you will need to call setjmp and pass
your png_jmpbuf(png_ptr). If you read the file from different
routines, you will need to update the longjmp buffer every time you enter
a new routine that will call a png_*() function.
See your documentation of setjmp/longjmp for your compiler for more
information on setjmp/longjmp. See the discussion on libpng error
handling in the Customizing Libpng section below for more information
on the libpng error handling. If an error occurs, and libpng longjmp's
back to your setjmp, you will want to call png_destroy_read_struct() to
free any memory.
if (setjmp(png_jmpbuf(png_ptr)))
png_destroy_read_struct(&png_ptr, &info_ptr,
return (ERROR);
Pass (png_infopp)NULL instead of &end_info if you didn't create
an end_info structure.
If you would rather avoid the complexity of setjmp/longjmp issues,
you can compile libpng with PNG_NO_SETJMP, in which case
errors will result in a call to PNG_ABORT() which defaults to abort().
You can #define PNG_ABORT() to a function that does something
more useful than abort(), as long as your function does not
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 change the zlib compression buffer size to be used while
reading compressed data with
png_set_compression_buffer_size(png_ptr, buffer_size);
where the default size is 8192 bytes. Note that the buffer size
is changed immediately and the buffer is reallocated immediately,
instead of setting a flag to be acted upon later.
If you want CRC errors to be handled in a different manner than
the default, use
png_set_crc_action(png_ptr, crit_action, ancil_action);
The values for png_set_crc_action() say how libpng is to handle CRC errors in
ancillary and critical chunks, and whether to use the data contained
therein. Starting with libpng-1.6.26, this also governs how an ADLER32 error
is handled while reading the IDAT chunk. Note that it is impossible to
"discard" data in a critical chunk.
Choices for (int) crit_action are
PNG_CRC_DEFAULT 0 error/quit
PNG_CRC_ERROR_QUIT 1 error/quit
PNG_CRC_WARN_USE 3 warn/use data
PNG_CRC_QUIET_USE 4 quiet/use data
PNG_CRC_NO_CHANGE 5 use the current value
Choices for (int) ancil_action are
PNG_CRC_DEFAULT 0 error/quit
PNG_CRC_ERROR_QUIT 1 error/quit
PNG_CRC_WARN_DISCARD 2 warn/discard data
PNG_CRC_WARN_USE 3 warn/use data
PNG_CRC_QUIET_USE 4 quiet/use data
PNG_CRC_NO_CHANGE 5 use the current value
When the setting for crit_action is PNG_CRC_QUIET_USE, the CRC and ADLER32
checksums are not only ignored, but they are not evaluated.
Setting up callback code
You can set up a callback function to handle any unknown chunks in the
input stream. You must supply the function
read_chunk_callback(png_structp png_ptr,
png_unknown_chunkp chunk);
/* The unknown chunk structure contains your
chunk data, along with similar data for any other
unknown chunks: */
png_byte name[5];
png_byte *data;
png_size_t size;
/* Note that libpng has already taken care of
the CRC handling */
/* put your code here. Search for your chunk in the
unknown chunk structure, process it, and return one
of the following: */
return (-n); /* chunk had an error */
return (0); /* did not recognize */
return (n); /* success */
(You can give your function another name that you like instead of
To inform libpng about your function, use
png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
This names not only the callback function, but also a user pointer that
you can retrieve with
If you call the png_set_read_user_chunk_fn() function, then all unknown
chunks which the callback does not handle will be saved when read. You can
cause them to be discarded by returning '1' ("handled") instead of '0'. This
behavior will change in libpng 1.7 and the default handling set by the
png_set_keep_unknown_chunks() function, described below, will be used when the
callback returns 0. If you want the existing behavior you should set the global
default to PNG_HANDLE_CHUNK_IF_SAFE now; this is compatible with all current
versions of libpng and with 1.7. Libpng 1.6 issues a warning if you keep the
default, or PNG_HANDLE_CHUNK_NEVER, and the callback returns 0.
At this point, you can set up a callback function that will be
called after each row has been read, which you can use to control
a progress meter or the like. It's demonstrated in pngtest.c.
You must supply a function
void read_row_callback(png_structp png_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);
When this function is called the row has already been completely processed and
the 'row' and 'pass' refer to the next row to be handled. For the
non-interlaced case the row that was just handled is simply one less than the
passed in row number, and pass will always be 0. For the interlaced case the
same applies unless the row value is 0, in which case the row just handled was
the last one from one of the preceding passes. Because interlacing may skip a
pass you cannot be sure that the preceding pass is just 'pass-1'; if you really
need to know what the last pass is record (row,pass) from the callback and use
the last recorded value each time.
As with the user transform you can find the output row using the
Unknown-chunk handling
Now you get to set the way the library processes unknown chunks in the
input PNG stream. Both known and unknown chunks will be read. Normal
behavior is that known chunks will be parsed into information in
various info_ptr members while unknown chunks will be discarded. This
behavior can be wasteful if your application will never use some known
chunk types. To change this, you can call:
png_set_keep_unknown_chunks(png_ptr, keep,
chunk_list, num_chunks);
keep - 0: default unknown chunk handling
1: ignore; do not keep
2: keep only if safe-to-copy
3: keep even if unsafe-to-copy
You can use these definitions:
chunk_list - list of chunks affected (a byte string,
five bytes per chunk, NULL or '\0' if
num_chunks is positive; ignored if
numchunks <= 0).
num_chunks - number of chunks affected; if 0, all
unknown chunks are affected. If positive,
only the chunks in the list are affected,
and if negative all unknown chunks and
all known chunks except for the IHDR,
PLTE, tRNS, IDAT, and IEND chunks are
Unknown chunks declared in this way will be saved as raw data onto a
list of png_unknown_chunk structures. If a chunk that is normally
known to libpng is named in the list, it will be handled as unknown,
according to the "keep" directive. If a chunk is named in successive
instances of png_set_keep_unknown_chunks(), the final instance will
take precedence. The IHDR and IEND chunks should not be named in
chunk_list; if they are, libpng will process them normally anyway.
If you know that your application will never make use of some particular
chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.
Here is an example of the usage of png_set_keep_unknown_chunks(),
where the private "vpAg" chunk will later be processed by a user chunk
callback function:
png_byte vpAg[5]={118, 112, 65, 103, (png_byte) '\0'};
png_byte unused_chunks[]=
104, 73, 83, 84, (png_byte) '\0', /* hIST */
105, 84, 88, 116, (png_byte) '\0', /* iTXt */
112, 67, 65, 76, (png_byte) '\0', /* pCAL */
115, 67, 65, 76, (png_byte) '\0', /* sCAL */
115, 80, 76, 84, (png_byte) '\0', /* sPLT */
116, 73, 77, 69, (png_byte) '\0', /* tIME */
/* ignore all unknown chunks
* (use global setting "2" for libpng16 and earlier):
png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);
/* except for vpAg: */
png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
/* also ignore unused known chunks: */
png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
(int)(sizeof unused_chunks)/5);
User limits
The PNG specification allows the width and height of an image to be as
large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
For safety, libpng imposes a default limit of 1 million rows and columns.
Larger images will be rejected immediately with a png_error() call. If
you wish to change these limits, you can use
png_set_user_limits(png_ptr, width_max, height_max);
to set your own limits (libpng may reject some very wide images
anyway because of potential buffer overflow conditions).
You should put this statement after you create the PNG structure and
before calling png_read_info(), png_read_png(), or png_process_data().
When writing a PNG datastream, put this statement before calling
png_write_info() or png_write_png().
If you need to retrieve the limits that are being applied, use
width_max = png_get_user_width_max(png_ptr);
height_max = png_get_user_height_max(png_ptr);
The PNG specification sets no limit on the number of ancillary chunks
allowed in a PNG datastream. By default, libpng imposes a limit of
a total of 1000 sPLT, tEXt, iTXt, zTXt, and unknown chunks to be stored.
If you have set up both info_ptr and end_info_ptr, the limit applies
separately to each. You can change the limit on the total number of such
chunks that will be stored, with
png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);
where 0x7fffffffL means unlimited. You can retrieve this limit with
chunk_cache_max = png_get_chunk_cache_max(png_ptr);
Libpng imposes a limit of 8 Megabytes (8,000,000 bytes) on the amount of
memory that a compressed chunk other than IDAT can occupy, when decompressed.
You can change this limit with
png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
and you can retrieve the limit with
chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);
Any chunks that would cause either of these limits to be exceeded will
be ignored.
Information about your system
If you intend to display the PNG or to incorporate it in other image data you
need to tell libpng information about your display or drawing surface so that
libpng can convert the values in the image to match the display.
From libpng-1.5.4 this information can be set before reading the PNG file
header. In earlier versions png_set_gamma() existed but behaved incorrectly if
called before the PNG file header had been read and png_set_alpha_mode() did not
If you need to support versions prior to libpng-1.5.4 test the version number
as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures
described in the appropriate manual page.
You give libpng the encoding expected by your system expressed as a 'gamma'
value. You can also specify a default encoding for the PNG file in
case the required information is missing from the file. By default libpng
assumes that the PNG data matches your system, to keep this default call:
png_set_gamma(png_ptr, screen_gamma, output_gamma);
or you can use the fixed point equivalent:
png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
If you don't know the gamma for your system it is probably 2.2 - a good
approximation to the IEC standard for display systems (sRGB). If images are
too contrasty or washed out you got the value wrong - check your system
Many systems permit the system gamma to be changed via a lookup table in the
display driver, a few systems, including older Macs, change the response by
default. As of 1.5.4 three special values are available to handle common
PNG_DEFAULT_sRGB: Indicates that the system conforms to the
IEC 61966-2-1 standard. This matches almost
all systems.
PNG_GAMMA_MAC_18: Indicates that the system is an older
(pre Mac OS 10.6) Apple Macintosh system with
the default settings.
PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
that the system expects data with no gamma
You would use the linear (unencoded) value if you need to process the pixel
values further because this avoids the need to decode and re-encode each
component value whenever arithmetic is performed. A lot of graphics software
uses linear values for this reason, often with higher precision component values
to preserve overall accuracy.
The output_gamma value expresses how to decode the output values, not how
they are encoded. The values used correspond to the normal numbers used to
describe the overall gamma of a computer display system; for example 2.2 for
an sRGB conformant system. The values are scaled by 100000 in the _fixed
version of the API (so 220000 for sRGB.)
The inverse of the value is always used to provide a default for the PNG file
encoding if it has no gAMA chunk and if png_set_gamma() has not been called
to override the PNG gamma information.
When the ALPHA_OPTIMIZED mode is selected the output gamma is used to encode
opaque pixels however pixels with lower alpha values are not encoded,
regardless of the output gamma setting.
When the standard Porter Duff handling is requested with mode 1 the output
encoding is set to be linear and the output_gamma value is only relevant
as a default for input data that has no gamma information. The linear output
encoding will be overridden if png_set_gamma() is called - the results may be
highly unexpected!
The following numbers are derived from the sRGB standard and the research
behind it. sRGB is defined to be approximated by a PNG gAMA chunk value of
0.45455 (1/2.2) for PNG. The value implicitly includes any viewing
correction required to take account of any differences in the color
environment of the original scene and the intended display environment; the
value expresses how to *decode* the image for display, not how the original
data was *encoded*.
sRGB provides a peg for the PNG standard by defining a viewing environment.
sRGB itself, and earlier TV standards, actually use a more complex transform
(a linear portion then a gamma 2.4 power law) than PNG can express. (PNG is
limited to simple power laws.) By saying that an image for direct display on
an sRGB conformant system should be stored with a gAMA chunk value of 45455
( and of the ISO PNG specification) the PNG specification
makes it possible to derive values for other display systems and
The Mac value is deduced from the sRGB based on an assumption that the actual
extra viewing correction used in early Mac display systems was implemented as
a power 1.45 lookup table.
Any system where a programmable lookup table is used or where the behavior of
the final display device characteristics can be changed requires system
specific code to obtain the current characteristic. However this can be
difficult and most PNG gamma correction only requires an approximate value.
By default, if png_set_alpha_mode() is not called, libpng assumes that all
values are unencoded, linear, values and that the output device also has a
linear characteristic. This is only very rarely correct - it is invariably
better to call png_set_alpha_mode() with PNG_DEFAULT_sRGB than rely on the
default if you don't know what the right answer is!
The special value PNG_GAMMA_MAC_18 indicates an older Mac system (pre Mac OS
10.6) which used a correction table to implement a somewhat lower gamma on an
otherwise sRGB system.
Both these values are reserved (not simple gamma values) in order to allow
more precise correction internally in the future.
NOTE: the values can be passed to either the fixed or floating
point APIs, but the floating point API will also accept floating point
The second thing you may need to tell libpng about is how your system handles
alpha channel information. Some, but not all, PNG files contain an alpha
channel. To display these files correctly you need to compose the data onto a
suitable background, as described in the PNG specification.
Libpng only supports composing onto a single color (using png_set_background;
see below). Otherwise you must do the composition yourself and, in this case,
you may need to call png_set_alpha_mode:
#if PNG_LIBPNG_VER >= 10504
png_set_alpha_mode(png_ptr, mode, screen_gamma);
png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
The screen_gamma value is the same as the argument to png_set_gamma; however,
how it affects the output depends on the mode. png_set_alpha_mode() sets the
file gamma default to 1/screen_gamma, so normally you don't need to call
png_set_gamma. If you need different defaults call png_set_gamma() before
png_set_alpha_mode() - if you call it after it will override the settings made
by png_set_alpha_mode().
The mode is as follows:
PNG_ALPHA_PNG: The data is encoded according to the PNG
specification. Red, green and blue, or gray, components are
gamma encoded color values and are not premultiplied by the
alpha value. The alpha value is a linear measure of the
contribution of the pixel to the corresponding final output pixel.
You should normally use this format if you intend to perform
color correction on the color values; most, maybe all, color
correction software has no handling for the alpha channel and,
anyway, the math to handle pre-multiplied component values is
unnecessarily complex.
Before you do any arithmetic on the component values you need
to remove the gamma encoding and multiply out the alpha
channel. See the PNG specification for more detail. It is
important to note that when an image with an alpha channel is
scaled, linear encoded, pre-multiplied component values must
be used!
The remaining modes assume you don't need to do any further color correction or
that if you do, your color correction software knows all about alpha (it
probably doesn't!). They 'associate' the alpha with the color information by
storing color channel values that have been scaled by the alpha. The
advantage is that the color channels can be resampled (the image can be
scaled) in this form. The disadvantage is that normal practice is to store
linear, not (gamma) encoded, values and this requires 16-bit channels for
still images rather than the 8-bit channels that are just about sufficient if
gamma encoding is used. In addition all non-transparent pixel values,
including completely opaque ones, must be gamma encoded to produce the final
image. These are the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' modes
described below (the latter being the two common names for associated alpha
color channels). Note that PNG files always contain non-associated color
channels; png_set_alpha_mode() with one of the modes causes the decoder to
convert the pixels to an associated form before returning them to your
Since it is not necessary to perform arithmetic on opaque color values so
long as they are not to be resampled and are in the final color space it is
possible to optimize the handling of alpha by storing the opaque pixels in
the PNG format (adjusted for the output color space) while storing partially
opaque pixels in the standard, linear, format. The accuracy required for
standard alpha composition is relatively low, because the pixels are
isolated, therefore typically the accuracy loss in storing 8-bit linear
values is acceptable. (This is not true if the alpha channel is used to
simulate transparency over large areas - use 16 bits or the PNG mode in
this case!) This is the 'OPTIMIZED' mode. For this mode a pixel is
treated as opaque only if the alpha value is equal to the maximum value.
PNG_ALPHA_STANDARD: The data libpng produces is encoded in the
standard way assumed by most correctly written graphics software.
The gamma encoding will be removed by libpng and the
linear component values will be pre-multiplied by the
alpha channel.
With this format the final image must be re-encoded to
match the display gamma before the image is displayed.
If your system doesn't do that, yet still seems to
perform arithmetic on the pixels without decoding them,
it is broken - check out the modes below.
With PNG_ALPHA_STANDARD libpng always produces linear
component values, whatever screen_gamma you supply. The
screen_gamma value is, however, used as a default for
the file gamma if the PNG file has no gamma information.
If you call png_set_gamma() after png_set_alpha_mode() you
will override the linear encoding. Instead the
pre-multiplied pixel values will be gamma encoded but
the alpha channel will still be linear. This may
actually match the requirements of some broken software,
but it is unlikely.
While linear 8-bit data is often used it has
insufficient precision for any image with a reasonable
dynamic range. To avoid problems, and if your software
supports it, use png_set_expand_16() to force all
components to 16 bits.
except that completely opaque pixels are gamma encoded according to
the screen_gamma value. Pixels with alpha less than 1.0
will still have linear components.
Use this format if you have control over your
compositing software and so don't do other arithmetic
(such as scaling) on the data you get from libpng. Your
compositing software can simply copy opaque pixels to
the output but still has linear values for the
non-opaque pixels.
In normal compositing, where the alpha channel encodes
partial pixel coverage (as opposed to broad area
translucency), the inaccuracies of the 8-bit
representation of non-opaque pixels are irrelevant.
You can also try this format if your software is broken;
it might look better.
PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however, all component
values, including the alpha channel are gamma encoded. This is
broken because, in practice, no implementation that uses this choice
correctly undoes the encoding before handling alpha composition. Use this
choice only if other serious errors in the software or hardware you use
mandate it. In most cases of broken software or hardware the bug in the
final display manifests as a subtle halo around composited parts of the
image. You may not even perceive this as a halo; the composited part of
the image may simply appear separate from the background, as though it had
been cut out of paper and pasted on afterward.
If you don't have to deal with bugs in software or hardware, or if you can fix
them, there are three recommended ways of using png_set_alpha_mode():
png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
You can do color correction on the result (libpng does not currently
support color correction internally). When you handle the alpha channel
you need to undo the gamma encoding and multiply out the alpha.
png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
If you are using the high level interface, don't call png_set_expand_16();
instead pass PNG_TRANSFORM_EXPAND_16 to the interface.
With this mode you can't do color correction, but you can do arithmetic,
including composition and scaling, on the data without further processing.
png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
You can avoid the expansion to 16-bit components with this mode, but you
lose the ability to scale the image or perform other linear arithmetic.
All you can do is compose the result onto a matching output. Since this
mode is libpng-specific you also need to write your own composition
The following are examples of calls to png_set_alpha_mode to achieve the
required overall gamma correction and, where necessary, alpha
png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
This is the default libpng handling of the alpha channel - it is not
pre-multiplied into the color components. In addition the call states
that the output is for a sRGB system and causes all PNG files without gAMA
chunks to be assumed to be encoded using sRGB.
png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
In this case the output is assumed to be something like an sRGB conformant
display preceeded by a power-law lookup table of power 1.45. This is how
early Mac systems behaved.
png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);
This is the classic Jim Blinn approach and will work in academic
environments where everything is done by the book. It has the shortcoming
of assuming that input PNG data with no gamma information is linear - this
is unlikely to be correct unless the PNG files where generated locally.
Most of the time the output precision will be so low as to show
significant banding in dark areas of the image.
png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);
This is a somewhat more realistic Jim Blinn inspired approach. PNG files
are assumed to have the sRGB encoding if not marked with a gamma value and
the output is always 16 bits per component. This permits accurate scaling
and processing of the data. If you know that your input PNG files were
generated locally you might need to replace PNG_DEFAULT_sRGB with the
correct value for your system.
png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);
If you just need to composite the PNG image onto an existing background
and if you control the code that does this you can use the optimization
setting. In this case you just copy completely opaque pixels to the
output. For pixels that are not completely transparent (you just skip
those) you do the composition math using png_composite or png_composite_16
below then encode the resultant 8-bit or 16-bit values to match the output
Other cases
If neither the PNG nor the standard linear encoding work for you because
of the software or hardware you use then you have a big problem. The PNG
case will probably result in halos around the image. The linear encoding
will probably result in a washed out, too bright, image (it's actually too
contrasty.) Try the ALPHA_OPTIMIZED mode above - this will probably
substantially reduce the halos. Alternatively try:
png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);
This option will also reduce the halos, but there will be slight dark
halos round the opaque parts of the image where the background is light.
In the OPTIMIZED mode the halos will be light halos where the background
is dark. Take your pick - the halos are unavoidable unless you can get
your hardware/software fixed! (The OPTIMIZED approach is slightly
When the default gamma of PNG files doesn't match the output gamma.
If you have PNG files with no gamma information png_set_alpha_mode allows
you to provide a default gamma, but it also sets the ouput gamma to the
matching value. If you know your PNG files have a gamma that doesn't
match the output you can take advantage of the fact that
png_set_alpha_mode always sets the output gamma but only sets the PNG
default if it is not already set:
png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
The first call sets both the default and the output gamma values, the
second call overrides the output gamma without changing the default. This
is easier than achieving the same effect with png_set_gamma. You must use
PNG_ALPHA_PNG for the first call - internal checking in png_set_alpha will
fire if more than one call to png_set_alpha_mode and png_set_background is
made in the same read operation, however multiple calls with PNG_ALPHA_PNG
are ignored.
If you don't need, or can't handle, the alpha channel you can call
png_set_background() to remove it by compositing against a fixed color. Don't
call png_set_strip_alpha() to do this - it will leave spurious pixel values in
transparent parts of this image.
png_set_background(png_ptr, &background_color,
The background_color is an RGB or grayscale value according to the data format
libpng will produce for you. Because you don't yet know the format of the PNG
file, if you call png_set_background at this point you must arrange for the
format produced by libpng to always have 8-bit or 16-bit components and then
store the color as an 8-bit or 16-bit color as appropriate. The color contains
separate gray and RGB component values, so you can let libpng produce gray or
RGB output according to the input format, but low bit depth grayscale images
must always be converted to at least 8-bit format. (Even though low bit depth
grayscale images can't have an alpha channel they can have a transparent
You set the transforms you need later, either as flags to the high level
interface or libpng API calls for the low level interface. For reference the
settings and API calls required are:
8-bit values:
png_set_expand(png_ptr); png_set_scale_16(png_ptr);
If you must get exactly the same inaccurate results
produced by default in versions prior to libpng-1.5.4,
use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
16-bit values:
In either case palette image data will be expanded to RGB. If you just want
color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr)
to the list.
Calling png_set_background before the PNG file header is read will not work
prior to libpng-1.5.4. Because the failure may result in unexpected warnings or
errors it is therefore much safer to call png_set_background after the head has
been read. Unfortunately this means that prior to libpng-1.5.4 it cannot be
used with the high level interface.
The high-level read interface
At this point there are two ways to proceed; through the high-level
read interface, or through a sequence of low-level read operations.
You can use the high-level interface if (a) you are willing to read
the entire image into memory, and (b) the input transformations
you want to do are limited to the following set:
PNG_TRANSFORM_SCALE_16 Strip 16-bit samples to
8-bit accurately
PNG_TRANSFORM_STRIP_16 Chop 16-bit samples to
8-bit less accurately
PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel
PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit
samples to bytes
PNG_TRANSFORM_PACKSWAP Change order of packed
pixels to LSB first
PNG_TRANSFORM_EXPAND Perform set_expand()
PNG_TRANSFORM_INVERT_MONO Invert monochrome images
PNG_TRANSFORM_SHIFT Normalize pixels to the
sBIT depth
to AG
PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
to transparency
PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples
to RGB (or GA to RGBA)
PNG_TRANSFORM_EXPAND_16 Expand samples to 16 bits
(This excludes setting a background color, doing gamma transformation,
quantizing, and setting filler.) If this is the case, simply do this:
png_read_png(png_ptr, info_ptr, png_transforms, NULL)
where png_transforms is an integer containing the bitwise OR of some
set of transformation flags. This call is equivalent to png_read_info(),
followed the set of transformations indicated by the transform mask,
then png_read_image(), and finally png_read_end().
(The final parameter of this call is not yet used. Someday it might point
to transformation parameters required by some future input transform.)
You must use png_transforms and not call any png_set_transform() functions
when you use png_read_png().
After you have called png_read_png(), you can retrieve the image data
row_pointers = png_get_rows(png_ptr, info_ptr);
where row_pointers is an array of pointers to the pixel data for each row:
png_bytep row_pointers[height];
If you know your image size and pixel size ahead of time, you can allocate
row_pointers prior to calling png_read_png() with
if (height > PNG_UINT_32_MAX/(sizeof (png_byte)))
png_error (png_ptr,
"Image is too tall to process in memory");
if (width > PNG_UINT_32_MAX/pixel_size)
png_error (png_ptr,
"Image is too wide to process in memory");
row_pointers = png_malloc(png_ptr,
height*(sizeof (png_bytep)));
for (int i=0; i<height, i++)
row_pointers[i]=NULL; /* security precaution */
for (int i=0; i<height, i++)
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 by libpng when you call png_destroy_*().
The low-level read interface
If you are going the low-level route, you are now ready to read all
the file information up to the actual image data. You do this with a
call to png_read_info().
png_read_info(png_ptr, info_ptr);
This will process all chunks up to but not including the image data.
This also copies some of the data from the PNG file into the decode structure
for use in later transformations. Important information copied in is:
1) The PNG file gamma from the gAMA chunk. This overwrites the default value
provided by an earlier call to png_set_gamma or png_set_alpha_mode.
2) Prior to libpng-1.5.4 the background color from a bKGd chunk. This
damages the information provided by an earlier call to png_set_background
resulting in unexpected behavior. Libpng-1.5.4 no longer does this.
3) The number of significant bits in each component value. Libpng uses this to
optimize gamma handling by reducing the internal lookup table sizes.
4) The transparent color information from a tRNS chunk. This can be modified by
a later call to png_set_tRNS.
Querying the info structure
Functions are used to get the information from the info_ptr once it
has been read. Note that these fields may not be completely filled
in until png_read_end() has read the chunk data following the image.
png_get_IHDR(png_ptr, info_ptr, &width, &height,
&bit_depth, &color_type, &interlace_type,
&compression_type, &filter_method);
width - holds the width of the image
in pixels (up to 2^31).
height - holds the height of the image
in pixels (up to 2^31).
bit_depth - holds the bit depth of one of the
image channels. (valid values are
1, 2, 4, 8, 16 and depend also on
the color_type. See also
significant bits (sBIT) below).
color_type - describes which color/alpha channels
are present.
(bit depths 1, 2, 4, 8, 16)
(bit depths 8, 16)
(bit depths 1, 2, 4, 8)
(bit_depths 8, 16)
(bit_depths 8, 16)
interlace_type - (PNG_INTERLACE_NONE or
compression_type - (must be PNG_COMPRESSION_TYPE_BASE
for PNG 1.0)