Libpng-manual.txt - A description on how to use and modify libpng

libpng version 1.5.13 - September 27, 2012

Updated and distributed by Glenn Randers-Pehrson

<glennrp at users.sourceforge.net>

Copyright (c) 1998-2012 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 1.5.13 - September 27, 2012

Updated and distributed by Glenn Randers-Pehrson

Copyright (c) 1998-2012 Glenn Randers-Pehrson

libpng 1.0 beta 6 version 0.96 May 28, 1997

Updated and distributed by Andreas Dilger

Copyright (c) 1996, 1997 Andreas Dilger

libpng 1.0 beta 2 - version 0.88 January 26, 1996

For conditions of distribution and use, see copyright

notice in png.h. Copyright (c) 1995, 1996 Guy Eric

Schalnat, Group 42, Inc.

Updated/rewritten per request in the libpng FAQ

Copyright (c) 1995, 1996 Frank J. T. Wojcik

December 18, 1995 & January 20, 1996

I. Introduction

This file describes how to use and modify the PNG reference library

(known as libpng) for your own use. There are five sections to this

file: introduction, structures, reading, writing, and modification and

configuration notes for various special platforms. In addition to this

file, example.c is a good starting point for using the library, as

it is heavily commented and should include everything most people

will need. We assume that libpng is already installed; see the

INSTALL file for instructions on how to install libpng.

For examples of libpng usage, see the files "example.c", "pngtest.c",

and the files in the "contrib" directory, all of which are included in

the libpng distribution.

Libpng was written as a companion to the PNG specification, as a way

of reducing the amount of time and effort it takes to support the PNG

file format in application programs.

The PNG specification (second edition), November 2003, is available as

a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2003 (E)) at

<http://www.w3.org/TR/2003/REC-PNG-20031110/

The W3C and ISO documents have identical technical content.

The PNG-1.2 specification is available at

<http://www.libpng.org/pub/png/documents/>. It is technically equivalent

to the PNG specification (second edition) but has some additional material.

The PNG-1.0 specification is available

as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a

W3C Recommendation <http://www.w3.org/TR/REC.png.html>.

Some additional chunks are described in the special-purpose public chunks

documents at <http://www.libpng.org/pub/png/documents/>.

Other information

about PNG, and the latest version of libpng, can be found at the PNG home

page, <http://www.libpng.org/pub/png/>.

Most users will not have to modify the library 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://www.info-zip.org/pub/infozip/zlib/>.

The zlib compression utility is a general purpose utility that is

useful for more than PNG files, and can be used without libpng.

See the documentation delivered with zlib for more details.

You can usually find the source files for the zlib utility wherever you

find the libpng source files.

Libpng is thread safe, provided the threads are using different

instances of the 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

deprecated..

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

function.

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>

Types

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.

Configuration

The main header file function declarations are frequently protected by C

preprocessing directives of the form:

#ifdef PNG_feature_SUPPORTED

declare-function

#endif

...

#ifdef PNG_feature_SUPPORTED

use-function

#endif

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:

CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC

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

wrong.

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_USR_CONFIG

If -DPNG_USR_CONFIG is added to the CFLAGS 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

pngusr.h.

III. Reading

We'll now walk you through the possible functions to call when reading

in a PNG file sequentially, briefly explaining the syntax and purpose

of each one. See example.c and png.h for more detail. While

progressive reading is covered in the next section, you will still

need some of the functions discussed in this section to read a PNG

file.

Setup

You will want to do the I/O initialization(*) before you get into libpng,

so if it doesn't work, you don't have much to undo. Of course, you

will also want to insure that you are, in fact, dealing with a PNG

file. Libpng provides a simple check to see if a file is a PNG file.

To use it, pass in the first 1 to 8 bytes of the file to the function

png_sig_cmp(), and it will return 0 (false) if the bytes match the

corresponding bytes of the PNG signature, or nonzero (true) otherwise.

Of course, the more bytes you pass in, the greater the accuracy of the

prediction.

If you are intending to keep the file pointer open for use in libpng,

you must ensure you don't read more than 8 bytes from the beginning

of the file, and you also have to make a call to png_set_sig_bytes_read()

with the number of bytes you read from the beginning. Libpng will

then only check the bytes (if any) that your program didn't read.

(*): If you are not using the standard I/O functions, you will need

to replace them with custom functions. See the discussion under

Customizing libpng.

FILE *fp = fopen(file_name, "rb");

if (!fp)

{

return (ERROR);

}

fread(header, 1, number, fp);

is_png = !png_sig_cmp(header, 0, number);

if (!is_png)

{

return (NOT_PNG);

}

Next, png_struct and png_info need to be allocated and 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);

}

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,

&end_info);

fclose(fp);

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

return.

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. 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

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

"read_chunk_callback")

To inform libpng about your function, use

png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,

read_chunk_callback);

This names not only the callback function, but also a user pointer that

you can retrieve with

png_get_user_chunk_ptr(png_ptr);

If you call the png_set_read_user_chunk_fn() function, then all unknown

chunks will be saved when read, in case your callback function will need

one or more of them. This behavior can be changed with the

png_set_keep_unknown_chunks() function, described below.

At this point, you can set up a callback function that will be

called after each row has been read, which you can use to control

a progress meter or the like. It's demonstrated in pngtest.c.

You must supply a function

void read_row_callback(png_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

PNG_ROW_FROM_PASS_ROW macro.

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:

PNG_HANDLE_CHUNK_AS_DEFAULT 0

PNG_HANDLE_CHUNK_NEVER 1

PNG_HANDLE_CHUNK_IF_SAFE 2

PNG_HANDLE_CHUNK_ALWAYS 3

chunk_list - list of chunks affected (a byte string,

five bytes per chunk, NULL or '\0' if

num_chunks is 0)

num_chunks - number of chunks affected; if 0, all

unknown chunks are affected. If nonzero,

only the chunks in the list are affected

Unknown chunks declared in this way will be saved as raw data onto a

list of png_unknown_chunk structures. If a chunk that is normally

known to libpng is named in the list, it will be handled as unknown,

according to the "keep" directive. If a chunk is named in successive

instances of png_set_keep_unknown_chunks(), the final instance will

take precedence. The IHDR and IEND chunks should not be named in

chunk_list; if they are, libpng will process them normally anyway.

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'};

#if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)

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 */

};

#endif

...

#if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)

/* ignore all unknown chunks: */

png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0);

/* except for vpAg: */

png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);

/* also ignore unused known chunks: */

png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,

(int)sizeof(unused_chunks)/5);

#endif

User limits

The PNG specification allows the width and height of an image to be as

large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.

Since very few applications really need to process such large images,

we have imposed an arbitrary 1-million limit on rows and columns.

Larger images will be rejected immediately with a png_error() call. If

you wish to change this limit, you can use

png_set_user_limits(png_ptr, width_max, height_max);

to set your own limits, or use width_max = height_max = 0x7fffffffL

to allow all valid dimensions (libpng may reject some very large images

anyway because of potential buffer overflow conditions).

You should put this statement after you create the PNG structure and

before calling png_read_info(), png_read_png(), or png_process_data().

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. You can impose a limit on the total number

of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with

png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

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

chunk_cache_max = png_get_chunk_cache_max(png_ptr);

This limit also applies to the number of buffers that can be allocated

by png_decompress_chunk() while decompressing iTXt, zTXt, and iCCP chunks.

You can also set a limit on the amount of memory that a compressed chunk

other than IDAT can occupy, 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

exist.

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, 1/screen_gamma/*file gamma*/);

or you can use the fixed point equivalent:

png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma, 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

documentation!

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

situations:

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 encoding.

You would use the linear (unencoded) value if you need to process the pixel

values further because this avoids the need to decode and reencode 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 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);

#else

png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);

#endif

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!)

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.

PNG_ALPHA_OPTIMIZED: This mode is the same

as PNG_ALPHA_STANDARD 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

an appropriate format to try if your software, or more

likely hardware, is totally broken, i.e., if it performs

linear arithmetic directly on gamma encoded values.

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,

screen_gamma);

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,

screen_gamma);

png_set_expand_16(png_ptr);

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,

screen_gamma);

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

software.

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,

PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);

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

color!)

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_TRANSFORM_SCALE_16 | PNG_EXPAND

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)

instead.

16-bit values:

PNG_TRANSFORM_EXPAND_16

png_set_expand_16(png_ptr);

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_IDENTITY No transformation

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

PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA

to BGRA

PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA

to AG

PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity

to transparency

PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples

PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples

to RGB (or GA to RGBA)

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

with

row_pointers = png_get_rows(png_ptr, info_ptr);

where row_pointers is an array of pointers to the pixel data for each row:

png_bytep row_pointers[height];

If you know your image size and pixel size ahead of time, you can allocate

row_pointers prior to calling png_read_png() with

if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))

png_error (png_ptr,

"Image is too tall to process in memory");

if (width > PNG_UINT_32_MAX/pixel_size)

png_error (png_ptr,

"Image is too wide to process in memory");

row_pointers = png_malloc(png_ptr,

height*png_sizeof(png_bytep));

for (int i=0; i<height, i++)

row_pointers[i]=NULL; /* security precaution */

for (int i=0; i<height, i++)

row_pointers[i]=png_malloc(png_ptr,

width*pixel_size);

png_set_rows(png_ptr, info_ptr, &row_pointers);

Alternatively you could allocate your image in one big block and define

row_pointers[i] to point into the proper places in your block.

If you use png_set_rows(), the application is responsible for freeing

row_pointers (and row_pointers[i], if they were separately allocated).

If you don't allocate row_pointers ahead of time, png_read_png() will

do it, and it'll be free'ed 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);