libpng version 1.6.21 - January 15, 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 1.6.21 - January 15, 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

TABLE OF CONTENTS

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 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://png-mng.sourcefor ge.net/pub/png/spec/1.2/. 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.sourcefor ge.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/pn g/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 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_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 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() 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_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 0error/quit
PNG_CRC_ERROR_QUIT1error/quit
PNG_CRC_WARN_USE 3warn/use data
PNG_CRC_QUIET_USE 4quiet/use data
PNG_CRC_NO_CHANGE 5use the current value

Choices for (int) ancil_action are

PNG_CRC_DEFAULT 0error/quit
PNG_CRC_ERROR_QUIT 1error/quit
PNG_CRC_WARN_DISCARD2warn/discard data
PNG_CRC_WARN_USE 3warn/use data
PNG_CRC_QUIET_USE 4quiet/use data
PNG_CRC_NO_CHANGE 5use 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 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 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
PNG_HANDLE_CHUNK_AS_DEFAULT0default unknown chunk handling
PNG_HANDLE_CHUNK_NEVER 1ignore; do not keep
PNG_HANDLE_CHUNK_IF_SAFE 2keep only if safe-to-copy
PNG_HANDLE_CHUNK_ALWAYS 3keep even if unsafe-to-copy
chunk_list List of chunks affected (a byte string, five bytes per chunk, NULL or '\0' if num_chunks is positive; ignored if num_chunks <= 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 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 * (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); #endif

User limits

The PNG specification allows the width and height of an image to be as large as 231-1 (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 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, output_gamma);

or you can use the fixed point equivalent:

png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma, PNG_FP_1*output_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_sRGBIndicates that the system conforms to the IEC 61966-2-1 standard. This matches almost all systems.
PNG_GAMMA_MAC_18Indicates that the system is an older (pre Mac OS 10.6) Apple Macintosh system with the default settings.
PNG_GAMMA_LINEARJust 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 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 (11.3.3.2 and 11.3.3.5 of the ISO PNG specification) the PNG specification makes it possible to derive values for other display systems and environments.

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 values.

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!). 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 application.

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.

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 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, 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.

The following are examples of calls to png_set_alpha_mode to achieve the required overall gamma correction and, where necessary, alpha premultiplication.

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_expand_16(pp); 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 encoding.

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 faster.)

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, 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_IDENTITYNo transformation
PNG_TRANSFORM_SCALE_16Strip 16-bit samples to 8-bit accurately
PNG_TRANSFORM_STRIP_16Chop 16-bit samples to 8-bit less accurately
PNG_TRANSFORM_STRIP_ALPHADiscard the alpha channel
PNG_TRANSFORM_PACKINGExpand 1, 2 and 4-bit samples to bytes
PNG_TRANSFORM_PACKSWAPChange order of packed pixels to LSB first
PNG_TRANSFORM_EXPANDPerform set_expand()
PNG_TRANSFORM_INVERT_MONOInvert monochrome images
PNG_TRANSFORM_SHIFTNormalize pixels to the sBIT depth
PNG_TRANSFORM_BGRFlip RGB to BGR, RGBA to BGRA
PNG_TRANSFORM_SWAP_ALPHAFlip RGBA to ARGB or GA to AG
PNG_TRANSFORM_INVERT_ALPHAChange alpha from opacity to transparency
PNG_TRANSFORM_SWAP_ENDIANByte-swap 16-bit samples
PNG_TRANSFORM_GRAY_TO_RGBExpand grayscale samples to RGB (or GA to RGBA)
PNG_TRANSFORM_EXPAND_16Expand 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/(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++) 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);

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 231).
height Holds the height of the image in pixels (up to 231).
bit_depth Holds the bit depth of one of the image channels. (Valid values are 1, 2, 4, 8, 16 and depend also on the color_type. See also significant bits (sBIT) below).
color_type Describes which color/alpha channels are present.
PNG_COLOR_TYPE_GRAY bit depths 1, 2, 4, 8, 16
PNG_COLOR_TYPE_GRAY_ALPHA bit depths 8, 16
PNG_COLOR_TYPE_PALETTE bit depths 1, 2, 4, 8
PNG_COLOR_TYPE_RGB bit_depths 8, 16
PNG_COLOR_TYPE_RGB_ALPHA bit_depths 8, 16
PNG_COLOR_MASK_PALETTE
PNG_COLOR_MASK_COLOR
PNG_COLOR_MASK_ALPHA
interlace_type PNG_INTERLACE_NONE or PNG_INTERLACE_ADAM7
compression_type Must be PNG_COMPRESSION_TYPE_BASE for PNG 1.0
filter_method Must be PNG_FILTER_TYPE_BASE for PNG 1.0, and can also be PNG_INTRAPIXEL_DIFFERENCING if the PNG datastream is embedded in a MNG-1.0 datastream.

Any of width, height, color_type, bit_depth, interlace_type, compression_type, or filter_method can be NULL if you are not interested in their values.

Note that png_get_IHDR() returns 32-bit data into the application's width and height variables. This is an unsafe situation if these are not png_uint_32 variables. In such situations, the png_get_image_width() and png_get_image_height() functions described below are safer.

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);
interlace_type = png_get_interlace_type(png_ptr, info_ptr);
compression_type = png_get_compression_type(png_ptr, info_ptr);
filter_method = png_get_filter_type(png_ptr, info_ptr);
channels = png_get_channels(png_ptr, info_ptr);

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);

Number of bytes needed to hold a row.

signature = png_get_signature(png_ptr, info_ptr);

Holds the signature read from the file (if any). The data is kept in the same offset it would be if the whole signature were read (i.e. if an application had already read in 4 bytes of signature before starting libpng, the remaining 4 bytes would be in signature[4] through signature[7] (see png_set_sig_bytes()).

These are also important, but their validity depends on whether the chunk has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the data has been read, or zero if it is missing. The parameters to the 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.

The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks is simply returned to give the application information about how the image was encoded. Libpng itself only does transformations using the file gamma when combining semitransparent pixels with the background color, and, since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit linear pixels within the simplified API. Libpng also uses the file gamma when converting RGB to gray, beginning with libpng-1.0.5, if the application calls png_set_rgb_to_gray().

png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette);
palette The palette for the file (array of png_color)
num_palette Number of entries in the palette
png_get_gAMA(png_ptr, info_ptr, &file_gamma); png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);
file_gamma The gamma at which the file is written (PNG_INFO_gAMA)
int_file_gamma 100,000 times the gamma at which the file is written
png_get_cHRM(png_ptr, info_ptr, &white_x, &white_y, &red_x, &red_y, &green_x, &green_y, &blue_x, &blue_y) png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z, &green_X, &green_Y, &green_Z, &blue_X, &blue_Y, &blue_Z) png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x, &int_white_y, &int_red_x, &int_red_y, &int_green_x, &int_green_y, &int_blue_x, &int_blue_y) png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y, &int_red_Z, &int_green_X, &int_green_Y, &int_green_Z, &int_blue_X, &int_blue_Y, &int_blue_Z)
{white,red,green,blue}_{x,y} A color space encoding specified using the chromaticities of the end points and the white point. (PNG_INFO_cHRM)
{red,green,blue}_{X,Y,Z} A color space encoding specified using the encoding end points - the CIE tristimulus specification of the intended color of the red, green and blue channels in the PNG RGB data. The white point is simply the sum of the three end points. (PNG_INFO_cHRM)
png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
srgb_intent The rendering intent (PNG_INFO_sRGB). The presence of the sRGB chunk means that the pixel data is in the sRGB color space. This chunk also implies specific values of gAMA and cHRM.
png_get_iCCP(png_ptr, info_ptr, &name, &compression_type, &profile, &proflen);
name The profile name.
compression_type 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_alpha, &num_trans, &trans_color);
trans_alpha Array of alpha (transparency) entries for palette (PNG_INFO_tRNS).
num_trans Number of transparent entries (PNG_INFO_tRNS).
trans_color Graylevel or color sample values of the single transparent color for non-paletted images (PNG_INFO_tRNS).
(PNG_INFO_hIST) png_get_hIST(png_ptr, info_ptr, &hist);
hist Histogram of palette (array of png_uint_16).
png_get_tIME(png_ptr, info_ptr, &mod_time);
mod_time Time image was last modified (PNG_VALID_tIME).
png_get_bKGD(png_ptr, info_ptr, &background);
background Background color (of type png_color_16p) (PNG_VALID_bKGD). Valid 16-bit red, green and blue values, regardless of color_type.
num_comments = png_get_text(png_ptr, info_ptr, &text_ptr, &num_text);
num_comments Number of comments.
text_ptr Array of png_text holding image comments.
text_ptr[i].compression type of compression used on text: PNG_TEXT_COMPRESSION_NONE, PNG_TEXT_COMPRESSION_zTXt, PNG_ITXT_COMPRESSION_NONE, or PNG_ITXT_COMPRESSION_zTXt.
text_ptr[i].key Keyword for comment. Must contain 1-79 characters.
text_ptr[i].text Text comments for current keyword. Can be empty.
text_ptr[i].text_length Length of text string, after decompression, 0 for iTXt.
text_ptr[i].itxt_length Length of itxt string, after decompression, 0 for tEXt/zTXt.
text_ptr[i].lang Language of comment (empty string for unknown).
text_ptr[i].lang_key Keyword in UTF-8 (empty string for unknown).

Note that the itxt_length, lang, and lang_key members of the text_ptr structure only exist when the library is built with iTXt chunk support. Prior to libpng-1.4.0 the library was built by default without iTXt support. Also note that when iTXt is supported, they contain NULL pointers when the compression field contains PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt.

num_text Number of comments (same as num_comments; you can put NULL here to avoid the duplication).

Note while png_set_text() will accept text, language, and translated keywords that can be NULL pointers, the structure returned by png_get_text will always contain regular zero-terminated C strings. They might be empty strings but they will never be NULL pointers.

num_spalettes = png_get_sPLT(png_ptr, info_ptr, &palette_ptr);
num_spalettes Number of sPLT chunks read.
palette_ptr Array of palette structures holding contents of one or more 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 (can be negative).
offset_y Positive offset from the top edge of the screen (can be negative).
unit_type PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, &unit_type);
res_x Pixels/unit physical resolution in x direction.
res_y Pixels/unit physical resolution in x direction.
unit_type PNG_RESOLUTION_UNKNOWN, PNG_RESOLUTION_METER
png_get_sCAL(png_ptr, info_ptr, &unit, &width, &height)
unit Physical scale units (an integer).
width Width of a pixel in physical scale units.
height Height of a pixel in physical scale units (width and height are doubles).
png_get_sCAL_s(png_ptr, info_ptr, &unit, &width, &height)
unit Physical scale units (an integer).
width Width of a pixel in physical scale units (expressed as a string).
height Height of a pixel in physical scale units (width and height are strings like "2.54").
num_unknown_chunks = png_get_unknown_chunks(png_ptr, info_ptr, &unknowns)
unknowns Array of png_unknown_chunk structures holding unknown chunks.
unknowns[i].name Name of unknown chunk.
unknowns[i].data Data of unknown chunk.
unknowns[i].size Size of unknown chunk's data.
unknowns[i].location Position of chunk in file.

The value of i corresponds to the order in which the chunks were read from the PNG file or inserted with the png_set_unknown_chunks() function.

The value of location is a bitwise OR of

PNG_HAVE_IHDR (0x01)
PNG_HAVE_PLTE (0x02)
PNG_AFTER_IDAT (0x08)

The data from the pHYs chunk can be retrieved in several convenient forms:

res_x = png_get_x_pixels_per_meter(png_ptr, info_ptr); res_y = png_get_y_pixels_per_meter(png_ptr, info_ptr); res_x_and_y = png_get_pixels_per_meter(png_ptr, info_ptr); res_x = png_get_x_pixels_per_inch(png_ptr, info_ptr); res_y = png_get_y_pixels_per_inch(png_ptr, info_ptr); res_x_and_y = png_get_pixels_per_inch(png_ptr, info_ptr); aspect_ratio = png_get_pixel_aspect_ratio(png_ptr, info_ptr);

Each of these returns 0 [signifying "unknown"] if the data is not present or if res_x is 0; res_x_and_y is 0 if res_x != res_y.

Note that because of the way the resolutions are stored internally, the inch conversions won't come out to exactly even number. For example, 72 dpi is stored as 0.28346 pixels/meter, and when this is retrieved it is 71.9988 dpi, so be sure to round the returned value appropriately if you want to display a reasonable-looking result.

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. The remark about inexact inch conversions applies here as well, because a value in inches can't always be converted to microns and back without some loss of precision.

For more information, see the PNG specification for chunk contents. Be careful with trusting rowbytes, as some of the transformations could increase the space needed to hold a row (expand, filler, gray_to_rgb, etc.). See png_read_update_info(), below.

A quick word about text_ptr and num_text. PNG stores comments in keyword/text pairs, one pair per chunk, with no limit on the number of text chunks, and a 231 byte limit on their size. While there are suggested keywords, there is no requirement to restrict the use to these strings. It is strongly suggested that keywords and text be sensible to humans (that's the point), so don't use abbreviations. Non-printing symbols are not allowed. See the PNG specification for more details. There is also no requirement to have text after the keyword.

Keywords should be limited to 79 Latin-1 characters without leading or trailing spaces, but non-consecutive spaces are allowed within the keyword. It is possible to have the same keyword any number of times. The text_ptr is an array of png_text structures, each holding a pointer to a language string, a pointer to a keyword and a pointer to a text string. The text string, language code, and translated keyword may be empty or NULL pointers. The keyword/text pairs are put into the array in the order that they are received. However, some or all of the text chunks may be after the image, so, to make sure you have read all the text chunks, don't mess with these until after you read the stuff after the image. This will be mentioned again below in the discussion that goes with png_read_end().

Input transformations

After you've read the header information, you can set up the library to handle any special transformations of the image data. The various ways to transform the data will be described in the order that they should occur. This is important, as some of these change the color type and/or bit depth of the data, and some others only work on certain color types and bit depths.

Transformations you request are ignored if they don't have any meaning for a particular input data format. However some transformations can have an effect as a result of a previous transformation. If you specify a contradictory set of transformations, for example both adding and removing the alpha channel, you cannot predict the final result.

The color used for the transparency values should be supplied in the same format/depth as the current image data. It is stored in the same format/depth as the image data in a tRNS chunk, so this is what libpng expects for this data.

The color used for the background value depends on the need_expand argument as described below.

Data will be decoded into the supplied row buffers packed into bytes unless the library has been told to transform it into another format. For example, 4 bit/pixel paletted or grayscale data will be returned 2 pixels/byte with the leftmost pixel in the high-order bits of the byte, unless png_set_packing() is called. 8-bit RGB data will be stored in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha() is called to insert filler bytes, either before or after each RGB triplet.

16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant byte of the color value first, unless png_set_scale_16() is called to transform it to regular RGB RGB triplets, or png_set_filler() or png_set_add alpha() is called to insert two filler bytes, either before or after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(), or png_set_scale_16().

The following code transforms grayscale images of less than 8 to 8 bits, changes paletted images to RGB, and adds a full alpha channel if there is transparency information in a tRNS chunk. This is most useful on grayscale images with bit depths of 2 or 4 or if there is a multiple-image viewing application that wishes to treat all images in the same way.

if (color_type == PNG_COLOR_TYPE_PALETTE) png_set_palette_to_rgb(png_ptr); if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr); if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);

The first two functions are actually aliases for png_set_expand(), added in libpng version 1.0.4, with the function names expanded to improve code readability. In some future version they may actually do different things.

As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was added. It expands the sample depth without changing tRNS to alpha.

As of libpng version 1.5.2, png_set_expand_16() was added. It behaves as png_set_expand(); however, the resultant channels have 16 bits rather than 8. Use this when the output color or gray channels are made linear to avoid fairly severe accuracy loss.

if (bit_depth < 16) png_set_expand_16(png_ptr);

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) #if PNG_LIBPNG_VER >= 10504 png_set_scale_16(png_ptr); #else png_set_strip_16(png_ptr); #endif

(The more accurate png_set_scale_16() API became available in libpng version 1.5.4).

If you need to process the alpha channel on the image separately from the image data (for example if you convert it to a bitmap mask) it is possible to have libpng strip the channel leaving just RGB or gray data:

if (color_type & PNG_COLOR_MASK_ALPHA) png_set_strip_alpha(png_ptr);

If you strip the alpha channel you need to find some other way of dealing with the information. If, instead, you want to convert the image to an opaque version with no alpha channel use png_set_background; see below.

As of libpng version 1.5.2, almost all useful expansions are supported, the major ommissions are conversion of grayscale to indexed images (which can be done trivially in the application) and conversion of indexed to grayscale (which can be done by a trivial manipulation of the palette.)

In the following table, the 01 means grayscale with depth<8, 31 means indexed with depth<8, other numerals represent the color type, "T" means the tRNS chunk is present, A means an alpha channel is present, and O means tRNS or alpha is present but all pixels in the image are opaque.

FROM
TO
013100T0O22T2O33T3O4A4O6A6O
01 - [G] - - - - - - - - - - - - -
31 [Q] Q [Q] [Q] [Q] Q Q Q Q Q Q [Q] [Q] Q Q
0 1 G + . . G G G G G G B B GB GB
0T lt Gt t + . Gt G G Gt G G Bt Bt GBt GBt
0O lt Gt t . + Gt Gt G Gt Gt G Bt Bt GBt GBt
2 C P C C C + . . C - - CB CB B B
2T Ct - Ct C C t + t - - - CBt CBt Bt Bt
2O Ct - Ct C C t t + - - - CBt CBt Bt Bt
3 [Q] p [Q] [Q] [Q] Q Q Q + . . [Q] [Q] Q Q
3T [Qt] p [Qt] [Q] [Q] Qt Qt Qt t + t [Qt] [Qt] Qt Qt
3O [Qt] p [Qt] [Q] [Q] Qt Qt Qt t t + [Qt] [Qt] Qt Qt
4A lA G A T T GA GT GT GA GT GT + BA G GBA
4O lA GBA A T T GA GT GT GA GT GT BA + GBA G
6A CA PA CA C C A T tT PA P P C CBA + BA
6O CA PBA CA C C A tT T PA P P CBA C BA +

Within the matrix,

+identifies entries where from and to are the same.
-means the transformation is not supported.
.means nothing is necessary (a tRNS chunk can just be ignored).
tmeans the transformation is obtained by png_set_tRNS.
Ameans the transformation is obtained by png_set_add_alpha().
Xmeans the transformation is obtained by png_set_expand().
1means the transformation is obtained by png_set_expand_gray_1_2_4_to_8() (and by png_set_expand() if there is no transparency in the original or the final format).
Cmeans the transformation is obtained by png_set_gray_to_rgb().
Gmeans the transformation is obtained by png_set_rgb_to_gray().
Pmeans the transformation is obtained by png_set_expand_palette_to_rgb().
pmeans the transformation is obtained by png_set_packing().
Qmeans the transformation is obtained by png_set_quantize().
Tmeans the transformation is obtained by png_set_tRNS_to_alpha().
Bmeans the transformation is obtained by png_set_background() or png_strip_alpha().

When an entry has multiple transforms listed all are required to cause the right overall transformation. When two transforms are separated by a comma either will do the job. When transforms are enclosed in [] the transform should do the job but this is currently unimplemented - a different format will result if the suggested transformations are used.

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 convert the PNG pixel data back to the original bit depth of the image. This call reduces the pixels back down to the original bit depth:

png_color_8p sig_bit; if (png_get_sBIT(png_ptr, info_ptr, &sig_bit)) png_set_shift(png_ptr, sig_bit);

PNG files store 3-color pixels in red, green, blue order. This code changes the storage of the pixels to blue, green, red:

if (color_type == PNG_COLOR_TYPE_RGB || color_type == PNG_COLOR_TYPE_RGB_ALPHA) png_set_bgr(png_ptr);

PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them into 4 or 8 bytes for windowing systems that need them in this format:

if (color_type == PNG_COLOR_TYPE_RGB) png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);

where filler is the 8-bit 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. When filling an 8-bit pixel, the least significant 8 bits of the number are used, if a 16-bit number is supplied. This transformation does not affect images that already have full alpha channels. To add an opaque alpha channel, use filler=0xffff and PNG_FILLER_AFTER which will generate RGBA pixels.

Note that png_set_filler() does not change the color type. If you want to do that, you can add a true alpha channel with

if (color_type == PNG_COLOR_TYPE_RGB || color_type == PNG_COLOR_TYPE_GRAY) png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);

where filler contains the alpha value to assign to each pixel. The png_set_add_alpha() function was added in libpng-1.2.7.

If you are reading an image with an alpha channel, and you need the data as ARGB instead of the normal PNG format RGBA:

if (color_type == PNG_COLOR_TYPE_RGB_ALPHA) png_set_swap_alpha(png_ptr);

For some uses, you may want a grayscale image to be represented as RGB. This code will do that conversion:

if (color_type == PNG_COLOR_TYPE_GRAY || color_type == PNG_COLOR_TYPE_GRAY_ALPHA) png_set_gray_to_rgb(png_ptr);

Conversely, you can convert an RGB or RGBA image to grayscale or grayscale with alpha.

if (color_type == PNG_COLOR_TYPE_RGB || color_type == PNG_COLOR_TYPE_RGB_ALPHA) png_set_rgb_to_gray(png_ptr, error_action, double red_weight, double green_weight);
error_action 1: silently do the conversion
2: issue a warning if the original image has any pixel where red != green or red != blue
3: issue an error and abort the conversion if the original image has any pixel where red != green or red != blue
red_weight Weight of red component.
green_weight Weight of green component. If either weight is negative, default weights are used.

In the corresponding fixed point API the red_weight and green_weight values are simply scaled by 100,000:

png_set_rgb_to_gray(png_ptr, error_action, png_fixed_point red_weight, png_fixed_point green_weight);

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. Background and sBIT data will be silently converted to grayscale, using the green channel data for sBIT, regardless of the error_action setting.

The default values come from the PNG file cHRM chunk if present; otherwise, the defaults correspond to the ITU-R recommendation 709, and also the sRGB color space, as recommended in the Charles Poynton's Colour FAQ, Copyright (c) 2006-11-28 Charles Poynton, in section 9:

http: //www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9

Y = 0.2126 * R + 0.7152 * G + 0.0722 * B

Previous versions of this document, 1998 through 2002, recommended a slightly different formula:

Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

Libpng uses an integer approximation:

Y = (6968 * R + 23434 * G + 2366 * B)/32768

The calculation is done in a linear colorspace, if the image gamma can be determined.

The png_set_background() function has been described already; it tells libpng to composite images with alpha or simple transparency against the supplied background color. For compatibility with versions of libpng earlier than libpng-1.5.4 it is recommended that you call the function after reading the file header, even if you don't want to use the color in a bKGD chunk, if one exists.

If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid), you may use this color, or supply another color more suitable for the current display (e.g., the background color from a web page). You need to tell libpng how the color is represented, both the format of the component values in the color (the number of bits) and the gamma encoding of the color. The function takes two arguments, background_gamma_mode and need_expand to convey this information; however, only two combinations are likely to be useful:

png_color_16 my_background; png_color_16p image_background; if (png_get_bKGD(png_ptr, info_ptr, &image_background)) png_set_background(png_ptr, image_background, PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1); else png_set_background(png_ptr, &my_background, PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);

The second call was described above - my_background is in the format of the final display output produced by libpng. Because you now know the format of the PNG it is possible to avoid the need to choose either 8-bit or 16-bit output and to retain palette images (the palette colors will be modified appropriately and the tRNS chunk removed.) However, if you are doing this, take great care not to ask for transformations without checking first that they apply!

In the first call the background color has the original bit depth and color type of the PNG file. So, for palette images the color is supplied as a palette index and for low bit greyscale images the color is a reduced bit value in image_background->gray.

If you didn't call png_set_gamma() before reading the file header, for example if you need your code to remain compatible with older versions of libpng prior to libpng-1.5.4, this is the place to call it.

Do not call it if you called png_set_alpha_mode(); doing so will damage the settings put in place by png_set_alpha_mode(). (If png_set_alpha_mode() is supported then you can certainly do png_set_gamma() before reading the PNG header.)

This API unconditionally sets the screen and file gamma values, so it will override the value in the PNG file unless it is called before the PNG file reading starts. For this reason you must always call it with the PNG file value when you call it in this position:

if (png_get_gAMA(png_ptr, info_ptr, &file_gamma)) png_set_gamma(png_ptr, screen_gamma, file_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 than will fit on your screen, png_set_quantize() will do that. Note that this is a simple match quantization that merely finds the closest color available. This should work fairly well with optimized palettes, but fairly badly with linear color cubes. If you pass a palette that is larger than 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, libpng will use it to make more intelligent choices when reducing the palette. If there is no histogram, it may not do as good a job.

if (color_type & PNG_COLOR_MASK_COLOR) { if (png_get_valid(png_ptr, info_ptr, PNG_INFO_PLTE)) { png_uint_16p histogram = NULL; png_get_hIST(png_ptr, info_ptr, &histogram); png_set_quantize(png_ptr, palette, num_palette, max_screen_colors, histogram, 1); } else { png_color std_color_cube[MAX_SCREEN_COLORS] = { ... colors ... }; png_set_quantize(png_ptr, std_color_cube, MAX_SCREEN_COLORS, MAX_SCREEN_COLORS, NULL,0); } }

PNG files describe monochrome as black being zero and white being one. The following code will reverse this (make black be one and white be zero):

if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY) png_set_invert_mono(png_ptr);

This function can also be used to invert grayscale and gray-alpha images:

if (color_type == PNG_COLOR_TYPE_GRAY || color_type == PNG_COLOR_TYPE_GRAY_ALPHA) png_set_invert_mono(png_ptr);

PNG files store 16-bit pixels in network byte order (big-endian, ie. most significant bits first). This code changes the storage to the other way (little-endian, i.e. least 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_structp png_ptr, png_row_infop 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. Take care with interlaced images if you do the interlace yourself - the width of the row is the width in row_info, not the overall image width.

If supported, libpng provides two information routines that you can use to find where you are in processing the image:

png_get_current_pass_number(png_structp png_ptr); png_get_current_row_number(png_structp png_ptr);

Don't try using these outside a transform callback - firstly they are only supported if user transforms are supported, secondly they may well return unexpected results unless the row is actually being processed at the moment they are called.

With interlaced images the value returned is the row in the input sub-image image. Use PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).

The discussion of interlace handling above contains more information on how to use these values.

You can also set up a pointer to a user structure for use by your callback function, and you can inform libpng that your transform function will change the number of channels or bit depth with the function

png_set_user_transform_info(png_ptr, user_ptr, user_depth, user_channels);

The user's application, not libpng, is responsible for allocating and freeing any memory required for the user structure.

You can retrieve the pointer via the function png_get_user_transform_ptr(). For example:

voidp read_user_transform_ptr = png_get_user_transform_ptr(png_ptr);

The last thing to handle is interlacing; this is covered in detail below, but you must call the function here if you want libpng to handle expansion of the interlaced image.

number_of_passes = png_set_interlace_handling(png_ptr);

After setting the transformations, libpng can update your png_info structure to reflect any transformations you've requested with this call.

png_read_update_info(png_ptr, info_ptr);

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. You may only call png_read_update_info() once with a particular 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.

Remember: Before you call png_read_update_info(), the png_get_*() functions return the values corresponding to the original PNG image. After you call png_read_update_info the values refer to the image that libpng will output. Consequently you must call all the png_set_ functions before you call png_read_update_info(). This is particularly important for png_set_interlace_handling() - if you are going to call png_read_update_info() you must call png_set_interlace_handling() before it unless you want to receive interlaced output.

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() (unless you call png_read_update_info()) or call this function multiple times, or any of that other stuff necessary with png_read_rows().

png_read_image(png_ptr, row_pointers);

where row_pointers is:

png_bytep row_pointers[height];

You can point to void or char or whatever you use for pixels.

If you don't want to read in the whole image at once, you can use png_read_rows() instead. If there is no interlacing (check interlace_type == PNG_INTERLACE_NONE), this is simple:

png_read_rows(png_ptr, row_pointers, NULL, number_of_rows);

where row_pointers is the same as in the png_read_image() call.

If you are doing this just one row at a time, you can do this with a single row_pointer instead of an array of row_pointers:

png_bytep row_pointer = row; png_read_row(png_ptr, row_pointer, NULL);

If the file is interlaced (interlace_type != 0 in the IHDR chunk), things get somewhat harder. The only current (PNG Specification version 1.2) interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7); a somewhat complicated 2D interlace scheme, known as Adam7, that breaks down an image into seven smaller images of varying size, based on an 8x8 grid. This number is defined (from libpng 1.5) as PNG_INTERLACE_ADAM7_PASSES in png.h

libpng can fill out those images or it can give them to you "as is". It is almost always better to have libpng handle the interlacing for you. If you want the images filled out, there are two ways to do that. The one mentioned in the PNG specification is to expand each pixel to cover those pixels that have not been read yet (the "rectangle" method). This results in a blocky image for the first pass, which gradually smooths out as more pixels are read. The other method is the "sparkle" method, where pixels are drawn only in their final locations, with the rest of the image remaining whatever colors they were initialized to before the start of the read. The first method usually looks better, but tends to be slower, as there are more pixels to put in the rows.

If, as is likely, 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. You then need to read the whole image number_of_passes times. Each time will distribute the pixels from the current pass to the correct place in the output image, so you need to supply the same rows to png_read_rows in each pass.

If you are not going to display the image after each pass, but are going to wait until the entire image is read in, use the sparkle effect. This effect is faster and the end result of either method is exactly the same. If you are planning on displaying the image after each pass, the "rectangle" effect is generally considered the better looking one.

If you only want the "sparkle" effect, just call png_read_rows() as normal, with the third parameter NULL. Make sure you make pass over the image number_of_passes times, and you don't change the data in the rows between calls. You can change the locations of the data, just not the data. Each pass only writes the pixels appropriate for that pass, and assumes the data from previous passes is still valid.

png_read_rows(png_ptr, row_pointers, NULL, number_of_rows);

If you only want the first effect (the rectangles), do the same as before except pass the row buffer in the third parameter, and leave the second parameter NULL.

png_read_rows(png_ptr, NULL, row_pointers, number_of_rows);

If you don't want libpng to handle the interlacing details, just call png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images. Each of the images is a valid image by itself; however, you will almost certainly need to distribute the pixels from each sub-image to the correct place. This is where everything gets very tricky.

If you want to retrieve the separate images you must pass the correct number of rows to each successive call of png_read_rows(). The calculation gets pretty complicated for small images, where some sub-images may not even exist because either their width or height ends up zero. libpng provides two macros to help you in 1.5 and later versions:

png_uint_32 width = PNG_PASS_COLS(image_width, pass_number); png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);

Respectively, these tell you the width and height of the sub-image corresponding to the numbered pass. pass is in in the range 0 to 6 - this can be confusing because the specification refers to the same passes as 1 to 7! Be careful, you must check both the width and height before calling png_read_rows() and not call it for that pass if either is zero.

You can, of course, read each sub-image row by row. If you want to produce optimal code to make a pixel-by-pixel transformation of an interlaced image this is the best approach; read each row of each pass, transform it, and write it out to a new interlaced image.

If you want to de-interlace the image yourself libpng provides further macros to help that tell you where to place the pixels in the output image. Because the interlacing scheme is rectangular - sub-image pixels are always arranged on a rectangular grid - all you need to know for each pass is the starting column and row in the output image of the first pixel plus the spacing between each pixel. As of libpng 1.5 there are four macros to retrieve this information:

png_uint_32 x = PNG_PASS_START_COL(pass); png_uint_32 y = PNG_PASS_START_ROW(pass); png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass); png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);

These allow you to write the obvious loop:

png_uint_32 input_y = 0; png_uint_32 output_y = PNG_PASS_START_ROW(pass); while (output_y < output_image_height) { png_uint_32 input_x = 0; png_uint_32 output_x = PNG_PASS_START_COL(pass); while (output_x < output_image_width) { image[output_y][output_x] = subimage[pass][input_y][input_x++]; output_x += xStep; } ++input_y; output_y += yStep; }

Notice that the steps between successive output rows and columns are returned as shifts. This is possible because the pixels in the subimages are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original image. In practice you may need to directly calculate the output coordinate given an input coordinate. libpng provides two further macros for this purpose:

png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass); png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);

Finally a pair of macros are provided to tell you if a particular image row or column appears in a given pass:

int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass); int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);

Bear in mind that you will probably also need to check the width and height of the pass in addition to the above to be sure the pass even exists!

With any luck you are convinced by now that you don't want to do your own interlace handling. In reality normally the only good reason for doing this is if you are processing PNG files on a pixel-by-pixel basis and don't want to load the whole file into memory when it is interlaced.

libpng includes a test program, pngvalid, that illustrates reading and writing of interlaced images. If you can't get interlacing to work in your code and don't want to leave it to libpng (the recommended approach), see how pngvalid.c does it.

Finishing a sequential read

After you are finished reading the image through the low-level interface, you can finish reading the file.

If you want to use a different CRC action for handling CRC errors in chunks after the image data, you can call png_set_crc_action() again at this point.

If you are interested in comments or time, which may be stored either before or after the image data, you should pass the separate png_info struct if you want to keep the comments from before and after the image separate.

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); } png_read_end(png_ptr, end_info);

If you are not interested, you should still call png_read_end() but you can pass NULL, avoiding the need to create an end_info structure. If you do this, libpng will not process any chunks after IDAT other than skipping over them and perhaps (depending on whether you have called png_set_crc_action) checking their CRCs while looking for the IEND chunk.

png_read_end(png_ptr, (png_infop)NULL);

If you don't call png_read_end(), then your file pointer will be left pointing to the first chunk after the last IDAT, which is probably not what you want if you expect to read something beyond the end of the PNG datastream.

When you are done, you can free all memory allocated by libpng like this:

png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);

or, if you didn't create an end_info structure,

png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);

It is also possible to individually free the info_ptr members that point to libpng-allocated storage with the following function:

png_free_data(png_ptr, info_ptr, mask, seq)
mask Identifies data to be freed, a mask containing the bitwise OR of one or more of PNG_FREE_PLTE, PNG_FREE_TRNS, PNG_FREE_HIST, PNG_FREE_ICCP, PNG_FREE_PCAL, PNG_FREE_ROWS, PNG_FREE_SCAL, PNG_FREE_SPLT, PNG_FREE_TEXT, PNG_FREE_UNKN, or simply PNG_FREE_ALL.
seq Sequence number of item to be freed (-1 for all items).

This function may be safely called when the relevant storage has already been freed, or has not yet been allocated, or was allocated by the user and not by libpng, and will in those cases do nothing. The seq parameter is ignored if only one item of the selected data type, such as PLTE, is allowed. If seq is not -1, and multiple items are allowed for the data type identified in the mask, such as text or sPLT, only the nth item in the structure is freed, where n is seq.

The default behavior is only to free data that was allocated internally by libpng. This can be changed, so that libpng will not free the data, or so that it will free data that was allocated by the user with png_malloc() or png_calloc() and passed in via a png_set_*() function, with

png_data_freer(png_ptr, info_ptr, freer, mask)
freer One of PNG_DESTROY_WILL_FREE_DATA, PNG_SET_WILL_FREE_DATA, or PNG_USER_WILL_FREE_DATA.
mask Which data elements are affected same choices as in png_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 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_calloc() to allocate it.

If you allocated your row_pointers in a single block, as suggested above in the description of the high level read interface, you must not transfer responsibility for freeing it to the png_set_rows or png_read_destroy function, because they would also try to free the individual row_pointers[i].

If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword separately, do not transfer responsibility for freeing text_ptr to libpng, because when libpng fills a png_text structure it combines these members with the key member, and png_free_data() will free only text_ptr.key. Similarly, if you transfer responsibility for free'ing text_ptr from libpng to your application, your application must not separately free those members.

The png_free_data() function will turn off the "valid" flag for anything it frees. If you need to turn the flag off for a chunk that was freed by your application instead of by libpng, you can use

png_set_invalid(png_ptr, info_ptr, mask);
mask Identifies the chunks to be made invalid, containing the bitwise OR of one or more of PNG_INFO_gAMA, PNG_INFO_sBIT, PNG_INFO_cHRM, PNG_INFO_PLTE, PNG_INFO_tRNS, PNG_INFO_bKGD, PNG_INFO_hIST, PNG_INFO_pHYs, PNG_INFO_oFFs, PNG_INFO_tIME, PNG_INFO_pCAL, PNG_INFO_sRGB, PNG_INFO_iCCP, PNG_INFO_sPLT, PNG_INFO_sCAL, PNG_INFO_IDAT.

For a more compact example of reading a PNG image, see the file example.c.

Reading PNG files progressively

The progressive reader is slightly different from the non-progressive reader. Instead of calling png_read_info(), png_read_rows(), and png_read_end(), you make one call to png_process_data(), which calls callbacks when it has the info, a row, or the end of the image. You set up these callbacks with png_set_progressive_read_fn(). You don't have to worry about the input/output functions of libpng, as you are giving the library the data directly in png_process_data(). I will assume that you have read the section on reading PNG files above, so I will only highlight the differences (although I will show all of the code).

png_structp png_ptr; png_infop info_ptr; /* An example code fragment of how you would initialize the progressive reader in your application. */ int initialize_png_reader() { png_ptr = png_create_read_struct (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, user_error_fn, user_warning_fn); if (!png_ptr) return (ERROR); info_ptr = png_create_info_struct(png_ptr); if (!info_ptr) { png_destroy_read_struct(&png_ptr, (png_infopp)NULL, (png_infopp)NULL); return (ERROR); } if (setjmp(png_jmpbuf(png_ptr))) { png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL); return (ERROR); } /* This one's new. You can provide functions to be called when the header info is valid, when each row is completed, and when the image is finished. If you aren't using all functions, you can specify NULL parameters. Even when all three functions are NULL, you need to call png_set_progressive_read_fn(). You can use any struct as the user_ptr (cast to a void pointer for the function call), and retrieve the pointer from inside the callbacks using the function png_get_progressive_ptr(png_ptr); which will return a void pointer, which you have to cast appropriately. */ png_set_progressive_read_fn(png_ptr, (void *)user_ptr, info_callback, row_callback, end_callback); return 0; } /* A code fragment that you call as you receive blocks of data */ int process_data(png_bytep buffer, png_uint_32 length) { if (setjmp(png_jmpbuf(png_ptr))) { png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL); return (ERROR); } /* This one's new also. Simply give it a chunk of data from the file stream (in order, of course). On machines with segmented memory models machines, don't give it any more than 64K. The library seems to run fine with sizes of 4K. Although you can give it much less if necessary (I assume you can give it chunks of 1 byte, I haven't tried less than 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); /* At this point you can call png_process_data_skip if you want to handle data the library will skip yourself; it simply returns the number of bytes to skip (and stops libpng skipping that number of bytes on the next png_process_data call). return 0; } /* This function is called (as set by png_set_progressive_read_fn() above) when enough data has been supplied so all of the header has been read. */ void info_callback(png_structp png_ptr, png_infop info) { /* Do any setup here, including setting any of the transformations mentioned in the Reading PNG files section. For now, you _must_ call either png_start_read_image() or png_read_update_info() after all the transformations are set (even if you don't set any). You may start getting rows before png_process_data() returns, so this is your last chance to prepare for that. This is where you turn on interlace handling, assuming you don't want to do it yourself. If you need to you can stop the processing of your original input data at this point by calling png_process_data_pause. This returns the number of unprocessed bytes from the last png_process_data call - it is up to you to ensure that the next call sees these bytes again. If you don't want to bother with this you can get libpng to cache the unread bytes by setting the 'save' parameter (see png.h) but then libpng will have to copy the data internally. */ } /* 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. If you did not turn on interlace handling then the callback is called for each row of each sub-image when the image is interlaced. In this case 'row_num' is the row in the sub-image, not the row in the output image as it is in all other cases. For the non-NULL rows of interlaced images when you have switched on libpng interlace handling, 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 if you switch on interlace handling; */ png_progressive_combine_row(png_ptr, old_row, new_row); /* where old_row is what was displayed 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. You can also call png_process_data_pause in this callback - see above. */ } void end_callback(png_structp png_ptr, png_infop info) { /* This function is called after the whole image has been read, including any chunks after the image (up to and including the IEND). You will usually have the same info chunk as you had in the header, although some data may have been added to the comments and time fields. Most people won't do much here, perhaps setting a flag that marks the image as finished. */ }

IV. Writing

Much of this is very similar to reading. However, everything of importance is repeated here, so you won't have to constantly look back up in the reading section to understand writing.

Setup

You will want to do the I/O initialization before you get into libpng, so if it doesn't work, you don't have anything to undo. If you are not using the standard I/O functions, you will need to replace them with custom writing functions. See the discussion under Customizing libpng.

FILE *fp = fopen(file_name, "wb"); if (!fp) return (ERROR);

Next, png_struct and png_info need to be allocated and initialized. As these can be both relatively large, you may not want to store these on the stack, unless you have stack space to spare. Of course, you will want to check if they return NULL. If you are also reading, you won't want to name your read structure and your write structure both png_ptr; you can call them anything you like, such as read_ptr and write_ptr. Look at pngtest.c, for example.

png_structp png_ptr = png_create_write_struct (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, user_error_fn, user_warning_fn); if (!png_ptr) return (ERROR); png_infop info_ptr = png_create_info_struct(png_ptr); if (!info_ptr) { png_destroy_write_struct(&png_ptr, (png_infopp)NULL); return (ERROR); }

If you want to use your own memory allocation routines, define PNG_USER_MEM_SUPPORTED and use png_create_write_struct_2() instead of png_create_write_struct():

png_structp png_ptr = png_create_write_struct_2(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, user_error_fn, user_warning_fn, (png_voidp)user_mem_ptr, user_malloc_fn, user_free_fn);

After you have these structures, you will need to set up the error handling. When libpng encounters an error, it expects to longjmp() back to your routine. Therefore, you will need to call setjmp() and pass the png_jmpbuf(png_ptr). If you write the file from different routines, you will need to update the png_jmpbuf(png_ptr) every time you enter a new routine that will call a png_*() function. See your documentation of setjmp/longjmp for your compiler for more information on setjmp/longjmp. See the discussion on libpng error handling in the Customizing Libpng section below for more information on the libpng error handling.

if (setjmp(png_jmpbuf(png_ptr))) { png_destroy_write_struct(&png_ptr, &info_ptr); fclose(fp); return (ERROR); } ... return;

If you would rather avoid the complexity of setjmp/longjmp issues, you can compile libpng with PNG_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.

Checking for invalid palette index on write was added at libpng 1.5.10. If a pixel contains an invalid (out-of-range) index libpng issues a benign error. This is enabled by default because this condition is an error according to the PNG specification, Clause 11.3.2, but the error can be ignored in each png_ptr with

png_set_check_for_invalid_index(png_ptr, 0);

If the error is ignored, or if png_benign_error() treats it as a warning, any invalid pixels are written as-is by the encoder, resulting in an invalid PNG datastream as output. In this case the application is responsible for ensuring that the pixel indexes are in range when it writes a PLTE chunk with fewer entries than the bit depth would allow.

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);

If you are embedding your PNG into a datastream such as MNG, and don't want libpng to write the 8-byte signature, or if you have already written the signature in your application, use

png_set_sig_bytes(png_ptr, 8);

to inform libpng that it should not write a signature.

Write callbacks

At this point, you can set up a callback function that will be called after each row has been written, which you can use to control a progress meter or the like. It's demonstrated in pngtest.c. You must supply a function

void write_row_callback(png_structp 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);

When this function is called the row has already been completely processed and it has also been written out. 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.

You now have the option of modifying how the compression library will run. The following functions are mainly for testing, but may be useful in some cases, like if you need to write PNG files extremely fast and are willing to give up some compression, or if you want to get the maximum possible compression at the expense of slower writing. If you have no special needs in this area, let the library do what it wants by not calling this function at all, as it has been tuned to deliver a good speed/compression ratio. The second parameter to png_set_filter() is the filter method, for which the only valid values are 0 (as of the July 1999 PNG specification, version 1.2) or 64 (if you are writing a PNG datastream that is to be embedded in a MNG datastream). The third parameter is a flag that indicates which filter type(s) are to be tested for each scanline. See the PNG specification for details on the specific filter types.

/* turn on or off filtering, and/or choose specific filters. You can use * either a single PNG_FILTER_VALUE_NAME or the bitwise OR of one or more * PNG_FILTER_NAME masks. */ png_set_filter(png_ptr, 0, PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE | PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB | PNG_FILTER_UP | PNG_FILTER_VALUE_UP | PNG_FILTER_AVG | PNG_FILTER_VALUE_AVG | PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH| PNG_ALL_FILTERS);

If an application wants to start and stop using particular filters during compression, it should start out with all of the filters (to ensure that the previous row of pixels will be stored in case it's needed later), and then add and remove them after the start of compression.

If you are writing a PNG datastream that is to be embedded in a MNG datastream, the second parameter can be either 0 or 64.

The png_set_compression_*() functions interface to the zlib compression library, and should mostly be ignored unless you really know what you are doing. The only generally useful call is png_set_compression_level() which changes how much time zlib spends on trying to compress the image data. See the Compression Library (zlib.h and algorithm.txt, distributed with zlib) for details on the compression levels.

#include zlib.h /* Set the zlib compression level */ png_set_compression_level(png_ptr, Z_BEST_COMPRESSION); /* Set other zlib parameters for compressing IDAT */ 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) /* Set zlib parameters for text compression. If you don't call these, * the parameters fall back on those defined for IDAT chunks */ png_set_text_compression_mem_level(png_ptr, 8); png_set_text_compression_strategy(png_ptr, Z_DEFAULT_STRATEGY); png_set_text_compression_window_bits(png_ptr, 15); png_set_text_compression_method(png_ptr, 8);

Setting the contents of info for output

You now need to fill in the png_info structure with all the data you wish to write before the actual image. Note that the only thing you are allowed to write after the image is the text chunks and the time chunk (as of PNG Specification 1.2, anyway). See png_write_end() and the latest PNG specification for more information on that. If you wish to write them before the image, fill them in now, and flag that data as being valid. If you want to wait until after the data, don't fill them until png_write_end(). For all the fields in png_info and their data types, see png.h. For explanations of what the fields contain, see the PNG specification.

Some of the more important parts of the png_info are:

png_set_IHDR(png_ptr, info_ptr, width, height, bit_depth, color_type, interlace_type, compression_type, filter_method)
widthHolds the width of the image in pixels (up to 231).
heightHolds the height of the image in pixels (up to 231).
bit_depthHolds 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_typeDescribes which color/alpha channels are present.

PNG_COLOR_TYPE_GRAY (bit depths 1, 2, 4, 8, 16)
PNG_COLOR_TYPE_GRAY_ALPHA (bit depths 8, 16)
PNG_COLOR_TYPE_PALETTE (bit depths 1, 2, 4, 8)
PNG_COLOR_TYPE_RGB (bit_depths 8, 16)
PNG_COLOR_TYPE_RGB_ALPHA (bit_depths 8, 16)
PNG_COLOR_MASK_PALETTE
PNG_COLOR_MASK_COLOR
PNG_COLOR_MASK_ALPHA

interlace_typePNG_INTERLACE_NONE or PNG_INTERLACE_ADAM7.
compression_typeMust be PNG_COMPRESSION_TYPE_DEFAULT.
filter_methodMust be PNG_FILTER_TYPE_DEFAULT or, if you are writing a PNG to be embedded in a MNG datastream, can also be PNG_INTRAPIXEL_DIFFERENCING.

If you call png_set_IHDR(), the call must appear before any of the other png_set_*() functions, because they might require access to some of the IHDR settings. The remaining png_set_*() functions can be called in any order.

If you wish, you can reset the compression_type, interlace_type, or filter_method later by calling png_set_IHDR() again; if you do this, the width, height, bit_depth, and color_type must be the same in each call.

png_set_PLTE(png_ptr, info_ptr, palette, num_palette);
paletteThe palette for the file (array of png_color).
num_paletteNumber of entries in the palette.
png_set_gAMA(png_ptr, info_ptr, file_gamma); png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);
file_gammaThe gamma at which the image was created (PNG_INFO_gAMA).
int_file_gamma100,000 times the gamma at which the image was created.
png_set_cHRM(png_ptr, info_ptr, white_x, white_y, red_x, red_y, green_x, green_y, blue_x, blue_y) png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X, green_Y, green_Z, blue_X, blue_Y, blue_Z) png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y, int_red_x, int_red_y, int_green_x, int_green_y, int_blue_x, int_blue_y) png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y, int_red_Z, int_green_X, int_green_Y, int_green_Z, int_blue_X, int_blue_Y, int_blue_Z)
{white,red,green,blue}_{x,y}A color space encoding specified using the chromaticities of the end points and the white point.
{red,green,blue}_{X,Y,Z}A color space encoding specified using the encoding end points - the CIE tristimulus specification of the intended color of the red, green and blue channels in the PNG RGB data. The white point is simply the sum of the three end points.
png_set_sRGB(png_ptr, info_ptr, srgb_intent);
srgb_intentThe 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_intentThe rendering intent (PNG_INFO_sRGB). The presence of the sRGB chunk means that the pixel data is in the sRGB color space. This function also causes gAMA and cHRM chunks with the specific values that are consistent with sRGB to be written.
png_set_iCCP(png_ptr, info_ptr, name, compression_type, profile, proflen);
nameThe profile name.
compression_typeThe compression type; always PNG_COMPRESSION_TYPE_BASE for PNG 1.0. You may give NULL to this argument to ignore it.
profileInternational Color Consortium color profile data. May contain NULs.
proflenLength of profile data in bytes.
png_set_sBIT(png_ptr, info_ptr, sig_bit);
sig_bitThe 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_alpha, num_trans, trans_color);
trans_alphaArray of alpha (transparency) entries for palette (PNG_INFO_tRNS).
num_transNumber of transparent entries (PNG_INFO_tRNS).
trans_colorGraylevel or color sample values (in order red, green, blue) of the single transparent color for non-paletted images (PNG_INFO_tRNS).
png_set_hIST(png_ptr, info_ptr, hist);
histHistogram of palette (array of png_uint_16) (PNG_INFO_hIST).
png_set_tIME(png_ptr, info_ptr, mod_time);
mod_timeTime image was last modified (PNG_VALID_tIME).
png_set_bKGD(png_ptr, info_ptr, background);
backgroundBackground color (of type png_color_16p) (PNG_VALID_bKGD).
png_set_text(png_ptr, info_ptr, text_ptr, num_text);
text_ptrArray of png_text holding image comments.
text_ptr[i].compressionType of compression used on text: PNG_TEXT_COMPRESSION_NONE, PNG_TEXT_COMPRESSION_zTXt, PNG_ITXT_COMPRESSION_NONE, or PNG_ITXT_COMPRESSION_zTXt.
text_ptr[i].keyKeyword for comment. Must contain 1-79 characters.
text_ptr[i].textText comments for current keyword. Can be NULL or empty.
text_ptr[i].text_lengthLength of text string, after decompression, 0 for iTXt.
text_ptr[i].itxt_lengthLength of itxt string, after decompression, 0 for tEXt/zTXt.
text_ptr[i].langLanguage of comment (NULL or empty for unknown).
text_ptr[i].translated_keywordKeyword in UTF-8 (NULL or empty for unknown).
Note that the itxt_length, lang, and lang_key members of the text_ptr structure only exist when the library is built with iTXt chunk support. Prior to libpng-1.4.0 the library was built by default without iTXt support. Also note that when iTXt is supported, they contain NULL pointers when the compression field contains PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt.
num_textnumber of comments
png_set_sPLT(png_ptr, info_ptr, &palette_ptr, num_spalettes);
palette_ptrArray of png_sPLT_struct structures to be added to the list of palettes in the info structure.
num_spalettesNumber of palette structures to be added.
png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y, unit_type);
offset_xPositive offset from the left edge of the screen.
offset_yPositive offset from the top edge of the screen.
unit_typePNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER.
png_set_pHYs(png_ptr, info_ptr, res_x, res_y, unit_type);
res_xPixels/unit physical resolution in x direction.
res_yPixels/unit physical resolution in y direction.
unit_typePNG_RESOLUTION_UNKNOWN, PNG_RESOLUTION_METER.
png_set_sCAL(png_ptr, info_ptr, unit, width, height)
unitPhysical scale units (an integer).
widthWidth of a pixel in physical scale units.
heightHeight of a pixel in physical scale units (width and height are doubles).
png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
unitPhysical scale units (an integer).
widthWidth of a pixel in physical scale units expressed as a string.
heightHeight 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)
unknownsArray of png_unknown_chunk structures holding unknown chunks.
unknowns[i].nameName of unknown chunk.
unknowns[i].dataData of unknown chunk.
unknowns[i].sizeSize of unknown chunk's data.
unknowns[i].locationPosition 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 compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE. Because tEXt and zTXt chunks don't have a language field, if you specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt any language code or translated keyword will not be written out.

Until text gets around a few hundred bytes, it is not worth compressing it. After the text has been written out to the file, the compression type is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR, so that it isn't written out again at the end (in case you are calling png_write_end() with the same struct).

The keywords that are given in the PNG Specification are:

TitleShort (one line) title or caption for image
AuthorName of image's creator
DescriptionDescription of image (possibly long)
CopyrightCopyright notice
Creation TimeTime of original image creation (usually RFC 1123 format, see below)
SoftwareSoftware used to create the image
DisclaimerLegal disclaimer
WarningWarning of nature of content
SourceDevice used to create the image
CommentMiscellaneous 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) character 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 readable, stick with basic ASCII, and avoid machine specific character set extensions like the IBM-PC character set. The keyword must be present, but you can leave off the text string on non-compressed pairs. Compressed pairs must have a text string, as only the text string is compressed anyway, so the compression would be meaningless.

PNG supports modification time via the png_time structure. Two conversion routines are provided, png_convert_from_time_t() for time_t and png_convert_from_struct_tm() for struct tm. The time_t routine uses gmtime(). You don't have to use either of these, but if you wish to fill in the png_time structure directly, you should provide the time in universal time (GMT) if possible instead of your local time. Note that the year number is the full year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and that months start with 1.

If you want to store the time of the original image creation, you should use a plain tEXt chunk with the "Creation Time" keyword. This is necessary because the "creation time" of a PNG image is somewhat vague, depending on whether you mean the PNG file, the time the image was created in a non-PNG format, a still photo from which the image was scanned, or possibly the subject matter itself. In order to facilitate machine-readable dates, it is recommended that the "Creation Time" tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"), although this isn't a requirement. Unlike the tIME chunk, the "Creation Time" tEXt chunk is not expected to be automatically changed by the software. To facilitate the use of RFC 1123 dates, a function png_convert_to_rfc1123_buffer(buffer, png_timep) is provided to convert from PNG time to an RFC 1123 format string. The caller must provide a writeable buffer of at least 29 bytes.

Writing unknown chunks

You can use the png_set_unknown_chunks function to queue up private chunks for writing. You give it a chunk name, location, raw data, and a size. You also must use png_set_keep_unknown_chunks() to ensure that libpng will handle them. 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, depending upon the specified location. Any chunks previously read into the info structure's unknown-chunk list will also be written out in a sequence that satisfies the PNG specification's ordering rules.

Here is an example of writing two private chunks, prVt and miNE:

#ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED /* Set unknown chunk data */ png_unknown_chunk unk_chunk[2]; strcpy((char *) unk_chunk[0].name, "prVt"; unk_chunk[0].data = (unsigned char *) "PRIVATE DATA"; unk_chunk[0].size = strlen(unk_chunk[0].data)+1; unk_chunk[0].location = PNG_HAVE_IHDR; strcpy((char *) unk_chunk[1].name, "miNE"; unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA"; unk_chunk[1].size = strlen(unk_chunk[0].data)+1; unk_chunk[1].location = PNG_AFTER_IDAT; png_set_unknown_chunks(write_ptr, write_info_ptr, unk_chunk, 2); /* Needed because miNE is not safe-to-copy */ png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS, (png_bytep) "miNE", 1); # if PNG_LIBPNG_VER < 10600 /* Deal with unknown chunk location bug in 1.5.x and earlier */ png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR); png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT); # endif # if PNG_LIBPNG_VER < 10500 /* PNG_AFTER_IDAT writes two copies of the chunk prior to libpng-1.5.0, * one before IDAT and another after IDAT, so don't use it; only use * PNG_HAVE_IHDR location. This call resets the location previously * set by assignment and png_set_unknown_chunk_location() for chunk 1. */ png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR); # endif #endif

The high-level write interface

At this point there are two ways to proceed; through the high-level write interface, or through a sequence of low-level write operations. You can use the high-level interface if your image data is present in the info structure. All defined output transformations are permitted, enabled by the following masks.

PNG_TRANSFORM_IDENTITYNo transformation.
PNG_TRANSFORM_PACKINGPack 1, 2 and 4-bit samples.
PNG_TRANSFORM_PACKSWAPChange order of packed pixels to LSB first.
PNG_TRANSFORM_INVERT_MONOInvert monochrome images.
PNG_TRANSFORM_SHIFTNormalize pixels to the sBIT depth.
PNG_TRANSFORM_BGRFlip RGB to BGR, RGBA to BGRA.
PNG_TRANSFORM_SWAP_ALPHAFlip RGBA to ARGB or GA to AG.
PNG_TRANSFORM_INVERT_ALPHAChange alpha from opacity to transparency.
PNG_TRANSFORM_SWAP_ENDIANByte-swap 16-bit samples
PNG_TRANSFORM_STRIP_FILLERStrip out filler bytes (deprecated).
PNG_TRANSFORM_STRIP_FILLER_BEFOREStrip out leading filler bytes.
PNG_TRANSFORM_STRIP_FILLER_AFTERStrip out trailing filler bytes.

If you have valid image data in the info structure (you can use png_set_rows() to put image data in the info structure), simply do this:

png_write_png(png_ptr, info_ptr, png_transforms, NULL)

where png_transforms is an integer containing the bitwise OR of some set of transformation flags. This call is equivalent to png_write_info(), followed the set of transformations indicated by the transform mask, then png_write_image(), and finally png_write_end().

(The final parameter of this call is not yet used. Someday it might point to transformation parameters required by some future output transform.)

You must use png_transforms and not call any png_set_transform() functions when you use png_write_png().

The low-level write interface

If you are going the low-level route instead, you are now ready to write all the file information up to the actual image data. You do this with a call to png_write_info().

png_write_info(png_ptr, info_ptr);

Note that there is one transformation you may need to do before png_write_info(). In PNG files, the alpha channel in an image is the level of opacity. If your data is supplied as a level of transparency, you can invert the alpha channel before you write it, so that 0 is fully transparent and 255 (in 8-bit or paletted images) or 65535 (in 16-bit images) is fully opaque, with

png_set_invert_alpha(png_ptr);

This must appear before png_write_info() instead of later with the other transformations because in the case of paletted images the tRNS chunk data has to be inverted before the tRNS chunk is written. If your image is not a paletted image, the tRNS data (which in such cases represents a single color to be rendered as transparent) won't need to be changed, and you can safely do this transformation after your png_write_info() call.

If you need to write a private chunk that you want to appear before the PLTE chunk when PLTE is present, you can write the PNG info in two steps, and insert code to write your own chunk between them:

png_write_info_before_PLTE(png_ptr, info_ptr); png_set_unknown_chunks(png_ptr, info_ptr, ...); png_write_info(png_ptr, info_ptr);

After you've written the file information, you can set up the library to handle any special transformations of the image data. The various ways to transform the data will be described in the order that they should occur. This is important, as some of these change the color type and/or bit depth of the data, and some others only work on certain color types and bit depths. Even though each transformation checks to see if it has data that it can do something with, you should make sure to only enable a transformation if it will be valid for the data. For example, don't swap red and blue on grayscale data.

PNG files store RGB pixels packed into 3 or 6 bytes. This code tells the library to strip input data that has 4 or 8 bytes per pixel down to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2 bytes per pixel).

png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

where the 0 is unused, and the location is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel is stored XRGB or RGBX.

PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as they can, resulting in, for example, 8 pixels per byte for 1 bit files. If the data is supplied at 1 pixel per byte, use this code, which will correctly pack the pixels into a single byte:

png_set_packing(png_ptr);

PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your data is of another bit depth, you can write an sBIT chunk into the file so that decoders can recover the original data if desired.

/* Set the true bit depth of the image data */ if (color_type & PNG_COLOR_MASK_COLOR) { sig_bit.red = true_bit_depth; sig_bit.green = true_bit_depth; sig_bit.blue = true_bit_depth; } else { sig_bit.gray = true_bit_depth; } if (color_type & PNG_COLOR_MASK_ALPHA) { sig_bit.alpha = true_bit_depth; } png_set_sBIT(png_ptr, info_ptr, &sig_bit);

If the data is stored in the row buffer in a bit depth other than one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG), this will scale the values to appear to be the correct bit depth as is required by PNG.

png_set_shift(png_ptr, &sig_bit);

PNG files store 16-bit pixels in network byte order (big-endian, ie. most significant bits first). This code would be used if they are supplied the other way (little-endian, i.e. least significant bits first, the way PCs store them):

if (bit_depth > 8) png_set_swap(png_ptr);

If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you need to change the order the pixels are packed into bytes, you can use:

if (bit_depth < 8) png_set_packswap(png_ptr);

PNG files store 3 color pixels in red, green, blue order. This code would be used if they are supplied as blue, green, red:

png_set_bgr(png_ptr);

PNG files describe monochrome as black being zero and white being one. This code would be used if the pixels are supplied with this reversed (black being one and white being zero):

png_set_invert_mono(png_ptr);

Finally, you can write your own transformation function if none of the existing ones meets your needs. This is done by setting a callback with

png_set_write_user_transform_fn(png_ptr, write_transform_fn);

You must supply the function

void write_transform_fn(png_structp png_ptr, png_row_infop row_info, png_bytep data)

See pngtest.c for a working example. Your function will be called before any of the other transformations are processed. If supported libpng also supplies an information routine that may be called from your callback:

png_get_current_row_number(png_ptr); png_get_current_pass_number(png_ptr);

This returns the current row passed to the transform. With interlaced images the value returned is the row in the input sub-image image. Use PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).

The discussion of interlace handling above contains more information on how to use these values.

You can also set up a pointer to a user structure for use by your callback function.

png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

The user_channels and user_depth parameters of this function are ignored when writing; you can set them to zero as shown.

You can retrieve the pointer via the function png_get_user_transform_ptr(). For example:

voidp write_user_transform_ptr = png_get_user_transform_ptr(png_ptr);

It is possible to have libpng flush any pending output, either manually, or automatically after a certain number of lines have been written. To flush the output stream a single time call:

png_write_flush(png_ptr);

and to have libpng flush the output stream periodically after a certain number of scanlines have been written, call:

png_set_flush(png_ptr, nrows);

Note that the distance between rows is from the last time png_write_flush() was called, or the first row of the image if it has never been called. So if you write 50 lines, and then png_set_flush 25, it will flush the output on the next scanline, and every 25 lines thereafter, unless png_write_flush() is called before 25 more lines have been written. If nrows is too small (less than about 10 lines for a 640 pixel wide RGB image) the image compression may decrease noticeably (although this may be acceptable for real-time applications). Infrequent flushing will only degrade the compression performance by a few percent over images that do not use flushing.

Writing the image data

That's it for the transformations. Now you can write the image data. The simplest way to do this is in one function call. If you have the whole image in memory, you can just call png_write_image() and libpng will write the image. You will need to pass in an array of pointers to each row. This function automatically handles interlacing, so you don't need to call png_set_interlace_handling() or call this function multiple times, or any of that other stuff necessary with png_write_rows().

png_write_image(png_ptr, row_pointers);

where row_pointers is:

png_byte *row_pointers[height];

You can point to void or char or whatever you use for pixels.

If you don't want to write the whole image at once, you can use png_write_rows() instead. If the file is not interlaced, this is simple:

png_write_rows(png_ptr, row_pointers, number_of_rows);

row_pointers is the same as in the png_write_image() call.

If you are just writing one row at a time, you can do this with a single row_pointer instead of an array of row_pointers:

png_bytep row_pointer = row; png_write_row(png_ptr, row_pointer);

When the file is interlaced, things can get a good deal more complicated. The only currently (as of the PNG Specification version 1.2, dated July 1999) defined interlacing scheme for PNG files is the "Adam7" interlace scheme, that breaks down an image into seven smaller images of varying size. libpng will build these images for you, or you can do them yourself. If you want to build them yourself, see the PNG specification for details of which pixels to write when.

If you don't want libpng to handle the interlacing details, just use png_set_interlace_handling() and call png_write_rows() the correct number of times to write all the sub-images (png_set_interlace_handling() returns the number of 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);

Think carefully before you write an interlaced image. Typically code that reads such images reads all the image data into memory, uncompressed, before doing any processing. Only code that can display an image on the fly can take advantage of the interlacing and even then the image has to be exactly the correct size for the output device, because scaling an image requires adjacent pixels and these are not available until all the passes have been read.

If you do write an interlaced image you will hardly ever need to handle the interlacing yourself. Call png_set_interlace_handling() and use the approach described above.

The only time it is conceivable that you will really need to write an interlaced image pass-by-pass is when you have read one pass by pass and made some pixel-by-pixel transformation to it, as described in the read code above. In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros to determine the size of each sub-image in turn and simply write the rows you obtained from the read code.

Finishing a sequential write

After you are finished writing the image, you should finish writing the file. If you are interested in writing comments or time, you should pass an appropriately filled png_info pointer. If you are not interested, you can pass NULL.

png_write_end(png_ptr, info_ptr);

When you are done, you can free all memory used by libpng like this:

png_destroy_write_struct(&png_ptr, &info_ptr);

It is also possible to individually free the info_ptr members that point to libpng-allocated storage with the following function:

png_free_data(png_ptr, info_ptr, mask, seq)
mask Identifies data to be freed, a mask containing the bitwise OR of one or more of PNG_FREE_PLTE, PNG_FREE_TRNS, PNG_FREE_HIST, PNG_FREE_ICCP, PNG_FREE_PCAL, PNG_FREE_ROWS, PNG_FREE_SCAL, PNG_FREE_SPLT, PNG_FREE_TEXT, PNG_FREE_UNKN, or simply PNG_FREE_ALL.
seq Sequence number of item to be freed (-1 for all items).

This function may be safely called when the relevant storage has already been freed, or has not yet been allocated, or was allocated by the user and not by libpng, and will in those cases do nothing. The seq parameter is ignored if only one item of the selected data type, such as PLTE, is allowed. If seq is not -1, and multiple items are allowed for the data type identified in the mask, such as text or sPLT, only the nth item in the structure is freed, where n is seq.

If you allocated data such as a palette that you passed in to libpng with png_set_*, you must not free it until just before the call to png_destroy_write_struct().

The default behavior is only to free data that was allocated internally by libpng. This can be changed, so that libpng will not free the data, or so that it will free data that was allocated by the user with png_malloc() or png_calloc() and passed in via a png_set_*() function, with

png_data_freer(png_ptr, info_ptr, freer, mask)
freer One of PNG_DESTROY_WILL_FREE_DATA, PNG_SET_WILL_FREE_DATA, or PNG_USER_WILL_FREE_DATA.
mask Which data elements are affected same choices as in png_free_data().

For example, to transfer responsibility for some data from a read structure to a write structure, you could use

png_data_freer(read_ptr, read_info_ptr, PNG_USER_WILL_FREE_DATA, PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST) png_data_freer(write_ptr, write_info_ptr, PNG_DESTROY_WILL_FREE_DATA, PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

thereby briefly reassigning responsibility for freeing to the user but immediately afterwards reassigning it once more to the write_destroy function. Having done this, it would then be safe to destroy the read structure and continue to use the PLTE, tRNS, and hIST data in the write structure.

This function only affects data that has already been 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_calloc() to allocate it.

If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword separately, do not transfer responsibility for freeing text_ptr to libpng, because when libpng fills a png_text structure it combines these members with the key member, and png_free_data() will free only text_ptr.key. Similarly, if you transfer responsibility for free'ing text_ptr from libpng to your application, your application must not separately free those members. For a more compact example of writing a PNG image, see the file example.c.

V. Simplified API

The simplified API, which became available in libpng-1.6.0, hides the details of both libpng and the PNG file format itself. It allows PNG files to be read into a very limited number of in-memory bitmap formats or to be written from the same formats. If these formats do not accommodate your needs then you can, and should, use the more sophisticated APIs above - these support a wide variety of in-memory formats and a wide variety of sophisticated transformations to those formats as well as a wide variety of APIs to manipulate ancilliary information.

To read a PNG file using the simplified API:

  1. Declare a png_image structure (see below) on the stack, set the version field to PNG_IMAGE_VERSION and the opaque pointer to NULL (this is REQUIRED, your program may crash if you don't do it.)
  2. Call the appropriate png_image_begin_read... function.
  3. Set the png_image format member to the required sample format.
  4. Allocate a buffer for the image and, if required, the color-map.
  5. Call png_image_finish_read to read the image and, if required, the color-map into your buffers.

There are no restrictions on the format of the PNG input itself; all valid color types, bit depths, and interlace methods are acceptable, and the input image is transformed as necessary to the requested in-memory format during the png_image_finish_read() step. The only caveat is that if you request a color-mapped image from a PNG that is full-color or makes complex use of an alpha channel the transformation is extremely lossy and the result may look terrible.

To write a PNG file using the simplified API:

  1. Declare a png_image structure on the stack and memset() it to all zero.
  2. Initialize the members of the structure that describe the image, setting the format member to the format of the image samples.
  3. Call the appropriate png_image_write... function with a pointer to the image and, if necessary, the color-map to write the PNG data.

png_image is a structure that describes the in-memory format of an image when it is being read or defines the in-memory format of an image that you need to write. The png_image structure contains the following members:

png_controlp opaque Initialize to NULL, free with png_image_free
png_uint_32 version Set to PNG_IMAGE_VERSION
png_uint_32 width Image width in pixels (columns)
png_uint_32 height Image height in pixels (rows)
png_uint_32 format Image format as defined below
png_uint_32 flags A bit mask containing informational flags
png_uint_32 colormap_entries;Number of entries in the color-map
png_uint_32 warning_or_error;
char message[64];

In the event of an error or warning the warning_or_error field will be set to a non-zero value and the message field will contain a '\0' terminated string with the libpng error or warning message. If both warnings and an error were encountered, only the error is recorded. If there are multiple warnings, only the first one is recorded.

The upper 30 bits of the warning_or_error value are reserved; the low two bits contain a two bit code such that a value more than 1 indicates a failure in the API just called:

0no warning or error
1warning
2error
3error preceded by warning

The pixels (samples) of the image have one to four channels whose components have original values in the range 0 to 1.0:

  1. A single gray or luminance channel (G).
  2. A gray/luminance channel and an alpha channel (GA).
  3. Three red, green, blue color channels (RGB).
  4. Three color channels and an alpha channel (RGBA).

The channels are encoded in one of two ways:

a) As a small integer, value 0..255, contained in a single byte. For the alpha channel the original value is simply value/255. For the color or luminance channels the value is encoded according to the sRGB specification and matches the 8-bit format expected by typical display devices.

The color/gray channels are not scaled (pre-multiplied) by the alpha channel and are suitable for passing to color management software.

b) As a value in the range 0..65535, contained in a 2-byte integer, in the native byte order of the platform on which the application is running. All channels can be converted to the original value by dividing by 65535; all channels are linear. Color channels use the RGB encoding (RGB end-points) of the sRGB specification. This encoding is identified by the PNG_FORMAT_FLAG_LINEAR flag below.

When the simplified API needs to convert between sRGB and linear colorspaces, the actual sRGB transfer curve defined in the sRGB specification (see the article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2 approximation used elsewhere in libpng.

When an alpha channel is present it is expected to denote pixel coverage of the color or luminance channels and is returned as an associated alpha channel: the color/gray channels are scaled (pre-multiplied) by the alpha value.

The samples are either contained directly in the image data, between 1 and 8 bytes per pixel according to the encoding, or are held in a color-map indexed by bytes in the image data. In the case of a color-map the color-map entries are individual samples, encoded as above, and the image data has one byte per pixel to select the relevant sample from the color-map.

PNG_FORMAT_*

The #defines to be used in png_image::format. Each #define identifies a particular layout of channel data and, if present, alpha values. There are separate defines for each of the two component encodings.

A format is built up using single bit flag values. All combinations are valid. Formats can be built up from the flag values or you can use one of the predefined values below. When testing formats always use the FORMAT_FLAG macros to test for individual features - future versions of the library may add new flags.

When reading or writing color-mapped images the format should be set to the format of the entries in the color-map then png_image_{read,write}_colormap called to read or write the color-map and set the format correctly for the image data. Do not set the PNG_FORMAT_FLAG_COLORMAP bit directly!

NOTE: libpng can be built with particular features disabled. If you see compiler errors because the definition of one of the following flags has been compiled out it is because libpng does not have the required support. It is possible, however, for the libpng configuration to enable the format on just read or just write; in that case you may see an error at run time. You can guard against this by checking for the definition of the appropriate _SUPPORTED macro, one of:

PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED

PNG_FORMAT_FLAG_ALPHA Format with an alpha channel.
PNG_FORMAT_FLAG_COLOR Color format: otherwise grayscale.
PNG_FORMAT_FLAG_LINEAR 2-byte channels else 1-byte.
PNG_FORMAT_FLAG_COLORMAP Image data is color-mapped.
PNG_FORMAT_FLAG_BGR BGR colors, else order is RGB.
PNG_FORMAT_FLAG_AFIRST Alpha channel comes first.

Supported formats are as follows. Future versions of libpng may support more formats; for compatibility with older versions simply check if the format macro is defined using #ifdef. These defines describe the in-memory layout of the components of the pixels of the image.

First the single byte (sRGB) formats:

PNG_FORMAT_GRAY PNG_FORMAT_GA PNG_FORMAT_AG PNG_FORMAT_RGB PNG_FORMAT_BGR PNG_FORMAT_RGBA PNG_FORMAT_ARGB PNG_FORMAT_BGRA PNG_FORMAT_ABGR

Then the linear 2-byte formats. When naming these "Y" is used to indicate a luminance (gray) channel. The component order within the pixel is always the same - there is no provision for swapping the order of the components in the linear format. The components are 16-bit integers in the native byte order for your platform, and there is no provision for swapping the bytes to a different endian condition.

PNG_FORMAT_LINEAR_Y PNG_FORMAT_LINEAR_Y_ALPHA PNG_FORMAT_LINEAR_RGB PNG_FORMAT_LINEAR_RGB_ALPHA

With color-mapped formats the image data is one byte for each pixel. The byte is an index into the color-map which is formatted as above. To obtain a color-mapped format it is sufficient just to add the PNG_FOMAT_FLAG_COLORMAP to one of the above definitions, or you can use one of the definitions below.

PNG_FORMAT_RGB_COLORMAP PNG_FORMAT_BGR_COLORMAP PNG_FORMAT_RGBA_COLORMAP PNG_FORMAT_ARGB_COLORMAP PNG_FORMAT_BGRA_COLORMAP PNG_FORMAT_ABGR_COLORMAP

PNG_IMAGE macros

These are convenience macros to derive information from a png_image structure. The PNG_IMAGE_SAMPLE_ macros return values appropriate to the actual image sample values - either the entries in the color-map or the pixels in the image. The PNG_IMAGE_PIXEL_ macros return corresponding values for the pixels and will always return 1 for color-mapped formats. The remaining macros return information about the rows in the image and the complete image.

NOTE: All the macros that take a png_image::format parameter are compile time constants if the format parameter is, itself, a constant. Therefore these macros can be used in array declarations and case labels where required. Similarly the macros are also pre-processor constants (sizeof is not used) so they can be used in #if tests.

PNG_IMAGE_SAMPLE_CHANNELS(fmt)
Returns the total number of channels in a given format: 1..4
PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt)
Returns the size in bytes of a single component of a pixel or color-map entry (as appropriate) in the image: 1 or 2.
PNG_IMAGE_SAMPLE_SIZE(fmt)
This is the size of the sample data for one sample. If the image is color-mapped it is the size of one color-map entry (and image pixels are one byte in size), otherwise it is the size of one image pixel.
PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt)
The maximum size of the color-map required by the format expressed in a count of components. This can be used to compile-time allocate a color-map:

png_uint_16 colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(linear_fmt)]; png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)];
Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro below to use the information from one of the png_image_begin_read_ APIs and dynamically allocate the required memory.
PNG_IMAGE_COLORMAP_SIZE(fmt)
The size of the color-map required by the format; this is the size of the color-map buffer passed to the png_image_{read,write}_colormap APIs. It is a fixed number determined by the format so can easily be allocated on the stack if necessary.

Corresponding information about the pixels

PNG_IMAGE_PIXEL_CHANNELS(fmt)
The number of separate channels (components) in a pixel; 1 for a color-mapped image.
PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)
The size, in bytes, of each component in a pixel; 1 for a color-mapped image.
PNG_IMAGE_PIXEL_SIZE(fmt)
The size, in bytes, of a complete pixel; 1 for a color-mapped image.

Information about the whole row, or whole image

PNG_IMAGE_ROW_STRIDE(image)
Returns the total number of components in a single row of the image; this is the minimum 'row stride', the minimum count of components between each row. For a color-mapped image this is the minimum number of bytes in a row.

If you need the stride measured in bytes, row_stride_bytes is PNG_IMAGE_ROW_STRIDE(image) * PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt) plus any padding bytes that your application might need, for example to start the next row on a 4-byte boundary.
PNG_IMAGE_BUFFER_SIZE(image, row_stride)
Return the size, in bytes, of an image buffer given a png_image and a row stride - the number of components to leave space for in each row.
PNG_IMAGE_SIZE(image)
Return the size, in bytes, of the image in memory given just a png_image; the row stride is the minimum stride required for the image.
PNG_IMAGE_COLORMAP_SIZE(image)
Return the size, in bytes, of the color-map of this image. If the image format is not a color-map format this will return a size sufficient for 256 entries in the given format; check PNG_FORMAT_FLAG_COLORMAP if you don't want to allocate a color-map in this case.

PNG_IMAGE_FLAG_*

Flags containing additional information about the image are held in the flags field of png_image.

PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01
This indicates the the RGB values of the in-memory bitmap do not correspond to the red, green and blue end-points defined by sRGB.
PNG_IMAGE_FLAG_FAST == 0x02
On write emphasise speed over compression; the resultant PNG file will be larger but will be produced significantly faster, particular for large images. Do not use this option for images which will be distributed, only used it when producing intermediate files that will be read back in repeatedly. For a typical 24-bit image the option will double the read speed at the cost of increasing the image size by 25%, however for many more compressible images the PNG file can be 10 times larger with only a slight speed gain.
PNG_IMAGE_FLAG_16BIT_sRGB == 0x04
On read if the image is a 16-bit per component image and there is no gAMA or sRGB chunk assume that the components are sRGB encoded. Notice that images output by the simplified API always have gamma information; setting this flag only affects the interpretation of 16-bit images from an external source. It is recommended that the application expose this flag to the user; the user can normally easily recognize the difference between linear and sRGB encoding. This flag has no effect on write - the data passed to the write APIs must have the correct encoding (as defined above.)

If the flag is not set (the default) input 16-bit per component data is assumed to be linear.

NOTE: the flag can only be set after the png_image_begin_read_ call, because that call initializes the flags field.

READ APIs

The png_image passed to the read APIs must have been initialized by setting the png_controlp field opaque to NULL (or, better, memset the whole thing.)

int png_image_begin_read_from_file( png_imagep image, const char *file_name)
The named file is opened for read and the image header is filled in from the PNG header in the file.
int png_image_begin_read_from_stdio (png_imagep image, FILE* file)
The PNG header is read from the stdio FILE object.
int png_image_begin_read_from_memory(png_imagep image, png_const_voidp memory, png_size_t size)
The PNG header is read from the given memory buffer.
int png_image_finish_read(png_imagep image, png_colorp background, void *buffer, png_int_32 row_stride, void *colormap));
Finish reading the image into the supplied buffer and clean up the png_image structure.

row_stride is the step, in png_byte or png_uint_16 units as appropriate, between adjacent rows. A positive stride indicates that the top-most row is first in the buffer - the normal top-down arrangement. A negative stride indicates that the bottom-most row is first in the buffer.

background need only be supplied if an alpha channel must be removed from a png_byte format and the removal is to be done by compositing on a solid color; otherwise it may be NULL and any composition will be done directly onto the buffer. The value is an sRGB color to use for the background, for grayscale output the green channel is used.

For linear output removing the alpha channel is always done by compositing on black.
void png_image_free(png_imagep image)
Free any data allocated by libpng in image->opaque, setting the pointer to NULL. May be called at any time after the structure is initialized.

When the simplified API needs to convert between sRGB and linear colorspaces, the actual sRGB transfer curve defined in the sRGB specification (see the article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2 approximation used elsewhere in libpng.

WRITE APIS

For write you must initialize a png_image structure to describe the image to be written:

versionMust be set to PNG_IMAGE_VERSION.
opaqueMust be initialized to NULL.
widthImage width in pixels.
heightImage height in rows.
formatThe format of the data you wish to write.
flagsSet to 0 unless one of the defined flags applies; set PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images where the RGB values do not correspond to the colors in sRGB.
colormap_entriesSet to the number of entries in the color-map (0 to 256).
int png_image_write_to_file, (png_imagep image, const char *file, int convert_to_8bit, const void *buffer, png_int_32 row_stride, const void *colormap));
Write the image to the named file.
int png_image_write_to_stdio(png_imagep image, FILE *file, int convert_to_8_bit, const void *buffer, png_int_32 row_stride, const void *colormap)
Write the image to the given (FILE*).

With all write APIs if image is in one of the linear formats with (png_uint_16) data then setting convert_to_8_bit will cause the output to be a (png_byte) PNG gamma encoded according to the sRGB specification, otherwise a 16-bit linear encoded PNG file is written.

With all APIs row_stride is handled as in the read APIs - it is the spacing from one row to the next in component sized units (float) and if negative indicates a bottom-up row layout in the buffer. If you pass zero, libpng will calculate the row_stride for you from the width and number of channels.

Note that the write API does not support interlacing, sub-8-bit pixels, indexed (paletted) images, or most ancillary chunks.

VI. 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. Both of those are compile-time issues; that is, they are generally determined at the time the code is written, and there is rarely a need to provide the user with a means of changing them.

Memory allocation, input/output, and error handling

All of the memory allocation, input/output, and error handling in libpng goes through callbacks that are user-settable. The default routines are in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change these functions, call the appropriate png_set_*_fn() function.

Memory allocation is done through the functions png_malloc(), png_calloc(), and png_free(). The png_malloc() and png_free() functions currently just call the standard C functions and png_calloc() calls png_malloc() and then clears the newly allocated memory to zero; note that png_calloc(png_ptr, size) is not the same as the calloc(number, size) function provided by stdlib.h. There is limited support for certain systems with segmented memory architectures and the types of pointers declared by png.h match this; you will have to use appropriate pointers in your application. If you prefer to use a different method of allocating and freeing data, you can use png_create_read_struct_2() or png_create_write_struct_2() to register your own functions as described above. These functions also provide a void pointer that can be retrieved via

mem_ptr=png_get_mem_ptr(png_ptr);

Your replacement memory functions must have prototypes as follows:

png_voidp malloc_fn(png_structp png_ptr, png_alloc_size_t size); void free_fn(png_structp png_ptr, png_voidp ptr);

Your malloc_fn() must return NULL in case of failure. The png_malloc() function will normally call png_error() if it receives a NULL from the system memory allocator or from your replacement malloc_fn().

Your free_fn() will never be called with a NULL pointer, since libpng's png_free() checks for NULL before calling free_fn().

Input/Output in libpng is done through png_read() and png_write(), which currently just call fread() and fwrite(). The FILE * is stored in png_struct and is initialized via png_init_io(). If you wish to change the method of I/O, the library supplies callbacks that you can set through the function png_set_read_fn() and png_set_write_fn() at run time, instead of calling the png_init_io() function. These functions also provide a void pointer that can be retrieved via the function png_get_io_ptr(). For example:

png_set_read_fn(png_structp read_ptr, voidp read_io_ptr, png_rw_ptr read_data_fn) png_set_write_fn(png_structp write_ptr, voidp write_io_ptr, png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn); voidp read_io_ptr = png_get_io_ptr(read_ptr); voidp write_io_ptr = png_get_io_ptr(write_ptr);

The replacement I/O functions must have prototypes as follows:

void user_read_data(png_structp png_ptr, png_bytep data, png_size_t length); void user_write_data(png_structp png_ptr, png_bytep data, png_size_t length); void user_flush_data(png_structp png_ptr);

The user_read_data() function is responsible for detecting and handling end-of-data errors.

Supplying NULL for the read, write, or flush functions sets them back to using the default C stream functions, which expect the io_ptr to point to a standard *FILE structure. It is probably a mistake to use NULL for one of write_data_fn and output_flush_fn but not both of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined. 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_NO_SETJMP, in which case it is handled via PNG_ABORT()), but you could change this to do things like exit() if you should wish, as long as your function does not return.

On non-fatal errors, png_warning() is called to print a warning message, and then control returns to the calling code. By default png_error() and png_warning() print a message on stderr via fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined (because you don't want the messages) or PNG_NO_STDIO defined (because fprintf() isn't available). If you wish to change the behavior of the error functions, you will need to set up your own message callbacks. These functions are normally supplied at the time that the png_struct is created. It is also possible to redirect errors and warnings to your own replacement functions after png_create_*_struct() has been called by calling:

png_set_error_fn(png_structp png_ptr, png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warning_fn); png_voidp error_ptr = png_get_error_ptr(png_ptr);

If NULL is supplied for either error_fn or warning_fn, then the libpng default function will be used, calling fprintf() and/or longjmp() if a problem is encountered. The replacement error functions should have parameters as follows:

void user_error_fn(png_structp png_ptr, png_const_charp error_msg); void user_warning_fn(png_structp png_ptr, png_const_charp warning_msg);

The motivation behind using setjmp() and longjmp() is the C++ throw and catch exception handling methods. This makes the code much easier to write, as there is no need to check every return code of every function call. However, there are some uncertainties about the status of local variables after a longjmp, so the user may want to be careful about doing anything after setjmp returns non-zero besides returning itself. Consult your compiler documentation for more details. For an alternative approach, you may wish to use the cexcept facility (see http://cexcept.sourceforge.net), which is illustrated in pngvalid.c and in contrib/visupng.

Beginning in libpng-1.4.0, the png_set_benign_errors() API became available. You can use this to handle certain errors (normally handled as errors) as warnings.

png_set_benign_errors (png_ptr, int allowed);
allowed 0: treat png_benign_error() as an error.
1: treat png_benign_error() as a warning.

As of libpng-1.6.0, the default condition is to treat benign errors as warnings while reading and as errors while writing.

Custom chunks

If you need to read or write custom chunks, you may need to get deeper into the libpng code. The library now has mechanisms for storing and writing chunks of unknown type; you can even declare callbacks for custom chunks. However, this may not be good enough if the library code itself needs to know about interactions between your chunk and existing "intrinsic" chunks.

If you need to write a new intrinsic chunk, first read the PNG specification. Acquire a first level of understanding of how it works. Pay particular attention to the sections that describe chunk names, and look at how other chunks were designed, so you can do things similarly. Second, check out the sections of libpng that read and write chunks. Try to find a chunk that is similar to yours and use it as a template. More details can be found in the comments inside the code. It is best to handle private or unknown chunks in a generic method, via callback functions, instead of by modifying libpng functions. This is illustrated in pngtest.c, which uses a callback function to handle a private "vpAg" chunk and the new "sTER" chunk, which are both unknown to libpng.

If you wish to write your own transformation for the data, look through the part of the code that does the transformations, and check out some of the simpler ones to get an idea of how they work. Try to find a similar transformation to the one you want to add and copy off of it. More details can be found in the comments inside the code itself.

Configuring for gui/windowing platforms

You will need to write new error and warning functions that use the GUI interface, as described previously, and set them to be the error and warning functions at the time that png_create_*_struct() is called, in order to have them available during the structure initialization. They can be changed later via png_set_error_fn(). On some compilers, you may also have to change the memory allocators (png_malloc, etc.).

Configuring zlib

There are special functions to configure the compression. Perhaps the most useful one changes the compression level, which currently uses input compression values in the range 0 - 9. The library normally uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests have shown that for a large majority of images, compression values in the range 3-6 compress nearly as well as higher levels, and do so much faster. For online applications it may be desirable to have maximum speed (Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also specify no compression (Z_NO_COMPRESSION = 0), but this would create files larger than just storing the raw bitmap. You can specify the compression level by calling:

#include zlib.h png_set_compression_level(png_ptr, level);

Another useful one is to reduce the memory level used by the library. The memory level defaults to 8, but it can be lowered if you are short on memory (running DOS, for example, where you only have 640K). Note that the memory level does have an effect on compression; among other things, lower levels will result in sections of incompressible data being emitted in smaller stored blocks, with a correspondingly larger relative overhead of up to 15% in the worst case.

#include zlib.h 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.

#include zlib.h png_set_compression_strategy(png_ptr, strategy); png_set_compression_window_bits(png_ptr, window_bits); png_set_compression_method(png_ptr, method);

This controls the size of the IDAT chunks (default 8192):

png_set_compression_buffer_size(png_ptr, size);

As of libpng version 1.5.4, additional APIs became available to set these separately for non-IDAT compressed chunks such as zTXt, iTXt, and iCCP:

#include zlib.h #if PNG_LIBPNG_VER >= 10504 png_set_text_compression_level(png_ptr, level); png_set_text_compression_mem_level(png_ptr, level); png_set_text_compression_strategy(png_ptr, strategy); png_set_text_compression_window_bits(png_ptr, window_bits); png_set_text_compression_method(png_ptr, method); #endif

Controlling row filtering

If you want to control whether libpng uses filtering or not, which filters are used, and how it goes about picking row filters, you can call one of these functions. The selection and configuration of row filters can have a significant impact on the size and encoding speed and a somewhat lesser impact on the decoding speed of an image. Filtering is enabled by default for RGB and grayscale images (with and without alpha), but not for paletted images nor for any images with bit depths less than 8 bits/pixel.

The method parameter sets the main filtering method, which is currently only '0' in the PNG 1.2 specification. The filters parameter sets which filter(s), if any, should be used for each scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS to turn filtering on and off, respectively.

Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB, PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise ORed together with '|' to specify one or more filters to use. These filters are described in more detail in the PNG specification. If you intend to change the filter type during the course of writing the image, you should start with flags set for all of the filters you intend to use so that libpng can initialize its internal structures appropriately for all of the filter types. (Note that this means the first row must always be adaptively filtered, because libpng currently does not allocate the filter buffers until png_write_row() is called for the first time.)

filters = PNG_FILTER_NONE | PNG_FILTER_SUB PNG_FILTER_UP | PNG_FILTER_AVG | PNG_FILTER_PAETH | PNG_ALL_FILTERS; png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE, filters);

The second parameter can also be PNG_INTRAPIXEL_DIFFERENCING if you are writing a PNG to be embedded in a MNG datastream. This parameter must be the same as the value of filter_method used in png_set_IHDR().

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)

in which level is compared to PNG_DEBUG to decide whether to print the message, message is the formatted string to be printed, and p1 and p2 are parameters that are to be embedded in the string according to printf-style formatting directives. For example,

png_debug1(2, "foo=%d", foo);

is expanded to

if (PNG_DEBUG > 2) fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);

When PNG_DEBUG is defined but is zero, the macros aren't defined, but you can still use PNG_DEBUG to control your own debugging:

#ifdef PNG_DEBUG fprintf(stderr, ... #endif

When PNG_DEBUG = 1, the macros are defined, but only png_debug statements having level = 0 will be printed. There aren't any such statements in this version of libpng, but if you insert some they will be printed.

VII. MNG support

The MNG specification (available at http://www.libpng.org/pub/mng) allows certain extensions to PNG for PNG images that are embedded in MNG datastreams. Libpng can support some of these extensions. To enable them, use the png_permit_mng_features() function:

feature_set = png_permit_mng_features(png_ptr, mask)
mask A png_uint_32 containing the bitwise OR of the features you want to enable. These include PNG_FLAG_MNG_EMPTY_PLTE, PNG_FLAG_MNG_FILTER_64, and PNG_ALL_MNG_FEATURES.
feature_set A png_uint_32 that is the bitwise AND of your mask with the set of MNG features that is supported by the version of libpng that you are using.

It is an error to use this function when reading or writing a standalone PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped in a MNG datastream. As a minimum, it must have the MNG 8-byte signature and the MHDR and MEND chunks. Libpng does not provide support for these or any other MNG chunks; your application must provide its own support for them. You may wish to consider using libmng (available at http://www.libmng.com) instead.

VIII. Changes to Libpng from version 0.88

It should be noted that versions of libpng later than 0.96 are not distributed by the original libpng author, Guy Schalnat, nor by Andreas Dilger, who had taken over from Guy during 1996 and 1997, and distributed versions 0.89 through 0.96, but rather by another member of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are still alive and well, but they have moved on to other things.

The old libpng functions png_read_init(), png_write_init(), png_info_init(), png_read_destroy(), and png_write_destroy() have been moved to PNG_INTERNAL in version 0.95 to discourage their use. These functions will be removed from libpng version 1.4.0.

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 initialization, which the old functions do not. The functions png_read_destroy() and png_write_destroy() do not actually free the memory that libpng allocated for these structs, but just reset the data structures, so they can be used instead of png_destroy_read_struct() and png_destroy_write_struct() if you feel there is too much system overhead allocating and freeing the png_struct for each image read.

Setting the error callbacks via png_set_message_fn() before png_read_init() as was 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.

Support for the sCAL, iCCP, iTXt, and sPLT chunks was added at libpng-1.0.6; however, iTXt support was not enabled by default.

Starting with version 1.0.7, you can find out which version of the library you are using at run-time:

png_uint_32 libpng_vn = png_access_version_number();

The number libpng_vn is constructed from the major version, minor version with leading zero, and release number with leading zero, (e.g., libpng_vn for version 1.0.7 is 10007).

Note that this function does not take a png_ptr, so you can call it before you've created one.

You can also check which version of png.h you used when compiling your application:

png_uint_32 application_vn = PNG_LIBPNG_VER;

IX. Changes to Libpng from version 1.0.x to 1.2.x

Support for user memory management was enabled by default. To accomplish this, the functions png_create_read_struct_2(), png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(), png_malloc_default(), and png_free_default() were added.

Support for the iTXt chunk has been enabled by default as of version 1.2.41.

Support for certain MNG features was enabled.

Support for numbered error messages was added. However, we never got around to actually numbering the error messages. The function png_set_strip_error_numbers() was added (Note: the prototype for this function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE builds of libpng-1.2.15. It was restored in libpng-1.2.36).

The png_malloc_warn() function was added at libpng-1.2.3. This issues a png_warning and returns NULL instead of aborting when it fails to acquire the requested memory allocation.

Support for setting user limits on image width and height was enabled by default. The functions png_set_user_limits(), png_get_user_width_max(), and png_get_user_height_max() were added at libpng-1.2.6.

The png_set_add_alpha() function was added at libpng-1.2.7.

The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9. Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is deprecated.

A number of macro definitions in support of runtime selection of assembler code features (especially Intel MMX code support) were added at libpng-1.2.0:

PNG_ASM_FLAG_MMX_SUPPORT_COMPILED PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU PNG_ASM_FLAG_MMX_READ_COMBINE_ROW PNG_ASM_FLAG_MMX_READ_INTERLACE PNG_ASM_FLAG_MMX_READ_FILTER_SUB PNG_ASM_FLAG_MMX_READ_FILTER_UP PNG_ASM_FLAG_MMX_READ_FILTER_AVG PNG_ASM_FLAG_MMX_READ_FILTER_PAETH PNG_ASM_FLAGS_INITIALIZED PNG_MMX_READ_FLAGS PNG_MMX_FLAGS PNG_MMX_WRITE_FLAGS PNG_MMX_FLAGS

We added the following functions in support of runtime selection of assembler code features:

png_get_mmx_flagmask() png_set_mmx_thresholds() png_get_asm_flags() png_get_mmx_bitdepth_threshold() png_get_mmx_rowbytes_threshold() png_set_asm_flags()

We replaced all of these functions with simple stubs in libpng-1.2.20, when the Intel assembler code was removed due to a licensing issue.

These macros are deprecated:

PNG_READ_TRANSFORMS_NOT_SUPPORTED PNG_PROGRESSIVE_READ_NOT_SUPPORTED PNG_NO_SEQUENTIAL_READ_SUPPORTED PNG_WRITE_TRANSFORMS_NOT_SUPPORTED PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED

They have been replaced, respectively, by:

PNG_NO_READ_TRANSFORMS PNG_NO_PROGRESSIVE_READ PNG_NO_SEQUENTIAL_READ PNG_NO_WRITE_TRANSFORMS PNG_NO_READ_ANCILLARY_CHUNKS PNG_NO_WRITE_ANCILLARY_CHUNKS

PNG_MAX_UINT was replaced with PNG_UINT_31_MAX. It has been deprecated since libpng-1.0.16 and libpng-1.2.6.

The function

png_check_sig(sig, num)

was replaced with

!png_sig_cmp(sig, 0, num)

It has been deprecated since libpng-0.90.

The function

png_set_gray_1_2_4_to_8()

which also expands tRNS to alpha was replaced with

png_set_expand_gray_1_2_4_to_8()

which does not. It has been deprecated since libpng-1.0.18 and 1.2.9.

X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x

Private libpng prototypes and macro definitions were moved from png.h and pngconf.h into a new pngpriv.h header file.

Functions png_set_benign_errors(), png_benign_error(), and png_chunk_benign_error() were added.

Support for setting the maximum amount of memory that the application will allocate for reading chunks was added, as a security measure. The functions png_set_chunk_cache_max() and png_get_chunk_cache_max() were added to the library.

We implemented support for I/O states by adding png_ptr member io_state and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c

We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level input transforms.

Checking for and reporting of errors in the IHDR chunk is more thorough.

Support for global arrays was removed, to improve thread safety.

Some obsolete/deprecated macros and functions have been removed.

Typecasted NULL definitions such as

#define png_voidp_NULL (png_voidp)NULL

were eliminated. If you used these in your application, just use NULL instead.

The png_struct and info_struct members trans and trans_values were changed to trans_alpha and trans_color, respectively.

The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles were removed.

The PNG_1_0_X and PNG_1_2_X macros were eliminated.

The PNG_LEGACY_SUPPORTED macro was eliminated.

Many WIN32_WCE #ifdefs were removed.

The functions png_read_init(info_ptr), png_write_init(info_ptr), png_info_init(info_ptr), png_read_destroy(), and png_write_destroy() have been removed. They have been deprecated since libpng-0.95.

The png_permit_empty_plte() was removed. It has been deprecated since libpng-1.0.9. Use png_permit_mng_features() instead.

We removed the obsolete stub functions png_get_mmx_flagmask(), png_set_mmx_thresholds(), png_get_asm_flags(), png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(), png_set_asm_flags(), and png_mmx_supported().

We removed the obsolete png_check_sig(), png_memcpy_check(), and png_memset_check() functions. Instead use !png_sig_cmp(), memcpy(), and memset(), respectively.

The function png_set_gray_1_2_4_to_8() was removed. It has been deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with png_set_expand_gray_1_2_4_to_8() because the former function also expanded any tRNS chunk to an alpha channel.

Macros for png_get_uint_16, png_get_uint_32, and png_get_int_32 were added and are used by default instead of the corresponding functions. Unfortunately, from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the function) incorrectly returned a value of type png_uint_32.

We changed the prototype for png_malloc() from

png_malloc(png_structp png_ptr, png_uint_32 size)

to

png_malloc(png_structp png_ptr, png_alloc_size_t size)

This also applies to the prototype for the user replacement malloc_fn().

The png_calloc() function was added and is used in place of of "png_malloc(); memset();" except in the case in png_read_png() where the array consists of pointers; in this case a for loop is used after the png_malloc() to set the pointers to NULL, to give robust behavior in case the application runs out of memory part-way through the process.

We changed the prototypes of png_get_compression_buffer_size() and png_set_compression_buffer_size() to work with png_size_t instead of png_uint_32.

Support for numbered error messages was removed by default, since we never got around to actually numbering the error messages. The function png_set_strip_error_numbers() was removed from the library by default.

The png_zalloc() and png_zfree() functions are no longer exported. The png_zalloc() function no longer zeroes out the memory that it allocates. Applications that called png_zalloc(png_ptr, number, size) can call png_calloc(png_ptr, number*size) instead, and can call png_free() instead of png_zfree().

Support for dithering was disabled by default in libpng-1.4.0, because it has not been well tested and doesn't actually "dither". The code was not removed, however, and could be enabled by building libpng with PNG_READ_DITHER_SUPPORTED defined. In libpng-1.4.2, this support was re-enabled, but the function was renamed png_set_quantize() to reflect more accurately what it actually does. At the same time, the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED was renamed to PNG_READ_QUANTIZE_SUPPORTED.

We removed the trailing '.' from the warning and error messages.

XI. Changes to Libpng from version 1.4.x to 1.5.x

From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the function) incorrectly returned a value of type png_uint_32. The incorrect macro was removed from libpng-1.4.5.

Checking for invalid palette index on write was added at libpng 1.5.10. If a pixel contains an invalid (out-of-range) index libpng issues a benign error. This is enabled by default because this condition is an error according to the PNG specification, Clause 11.3.2, but the error can be ignored in each png_ptr with

png_set_check_for_invalid_index(png_ptr, allowed);
allowedOne of:
0: disable benign error (accept the invalid data without warning).
1: enable benign error (treat the invalid data as an error or a warning).

If the error is ignored, or if png_benign_error() treats it as a warning, any invalid pixels are decoded as opaque black by the decoder and written as-is by the encoder.

Retrieving the maximum palette index found was added at libpng-1.5.15. This statement must appear after png_read_png() or png_read_image() while reading, and after png_write_png() or png_write_image() while writing.

int max_palette = png_get_palette_max(png_ptr, info_ptr);

This will return the maximum palette index found in the image, or "-1" if the palette was not checked, or "0" if no palette was found. Note that this does not account for any palette index used by ancillary chunks such as the bKGD chunk; you must check those separately to determine the maximum palette index actually used.

There are no substantial API changes between the non-deprecated parts of the 1.4.5 API and the 1.5.0 API; however, the ability to directly access members of the main libpng control structures, png_struct and png_info, deprecated in earlier versions of libpng, has been completely removed from libpng 1.5.

We no longer include zlib.h in png.h. The include statement has been moved to pngstruct.h, where it is not accessible by applications. Applications that need access to information in zlib.h will need to add the #include "zlib.h" directive. It does not matter whether this is placed prior to or after the "#include png.h" directive.

The png_sprintf(), png_strcpy(), and png_strncpy() macros are no longer used and were removed.

We moved the png_strlen(), png_memcpy(), png_memset(), and png_memcmp() macros into a private header file (pngpriv.h) that is not accessible to applications.

In png_get_iCCP, the type of profile was changed from png_charpp to png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep.

There are changes of form in png.h, including new and changed macros to declare parts of the API. Some API functions with arguments that are pointers to data not modified within the function have been corrected to declare these arguments with PNG_CONST.

Much of the internal use of C macros to control the library build has also changed and some of this is visible in the exported header files, in particular the use of macros to control data and API elements visible during application compilation may require significant revision to application code. (It is extremely rare for an application to do this.)

Any program that compiled against libpng 1.4 and did not use deprecated features or access internal library structures should compile and work against libpng 1.5, except for the change in the prototype for png_get_iCCP() and png_set_iCCP() API functions mentioned above.

libpng 1.5.0 adds PNG_PASS macros to help in the reading and writing of interlaced images. The macros return the number of rows and columns in each pass and information that can be used to de-interlace and (if absolutely necessary) interlace an image.

libpng 1.5.0 adds an API png_longjmp(png_ptr, value). This API calls the application-provided png_longjmp_ptr on the internal, but application initialized, longjmp buffer. It is provided as a convenience to avoid the need to use the png_jmpbuf macro, which had the unnecessary side effect of resetting the internal png_longjmp_ptr value.

libpng 1.5.0 includes a complete fixed point API. By default this is present along with the corresponding floating point API. In general the fixed point API is faster and smaller than the floating point one because the PNG file format used fixed point, not floating point. This applies even if the library uses floating point in internal calculations. A new macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether the library uses floating point arithmetic (the default) or fixed point arithmetic internally for performance critical calculations such as gamma correction. In some cases, the gamma calculations may produce slightly different results. This has changed the results in png_rgb_to_gray and in alpha composition (png_set_background for example). This applies even if the original image was already linear (gamma == 1.0) and, therefore, it is not necessary to linearize the image. This is because libpng has not been changed to optimize that case correctly, yet.

Fixed point support for the sCAL chunk comes with an important caveat; the sCAL specification uses a decimal encoding of floating point values and the accuracy of PNG fixed point values is insufficient for representation of these values. Consequently a string API (png_get_sCAL_s and png_set_sCAL_s) is the only reliable way of reading arbitrary sCAL chunks in the absence of either the floating point API or internal floating point calculations. Starting with libpng-1.5.0, both of these functions are present when PNG_sCAL_SUPPORTED is defined. Prior to libpng-1.5.0, their presence also depended upon PNG_FIXED_POINT_SUPPORTED being defined and PNG_FLOATING_POINT_SUPPORTED not being defined.

Applications no longer need to include the optional distribution header file pngusr.h or define the corresponding macros during application build in order to see the correct variant of the libpng API. From 1.5.0 application code can check for the corresponding _SUPPORTED macro:

#ifdef PNG_INCH_CONVERSIONS_SUPPORTED /* code that uses the inch conversion APIs. */ #endif

This macro will only be defined if the inch conversion functions have been compiled into libpng. The full set of macros, and whether or not support has been compiled in, are available in the header file pnglibconf.h. This header file is specific to the libpng build. Notice that prior to 1.5.0 the _SUPPORTED macros would always have the default definition unless reset by pngusr.h or by explicit settings on the compiler command line. These settings may produce compiler warnings or errors in 1.5.0 because of macro redefinition.

Applications can now choose whether to use these macros or to call the corresponding function by defining PNG_USE_READ_MACROS or PNG_NO_USE_READ_MACROS before including png.h. Notice that this is only supported from 1.5.0; defining PNG_NO_USE_READ_MACROS prior to 1.5.0 will lead to a link failure.

Prior to libpng-1.5.4, the zlib compressor used the same set of parameters when compressing the IDAT data and textual data such as zTXt and iCCP. In libpng-1.5.4 we reinitialized the zlib stream for each type of data. We added five png_set_text_*() functions for setting the parameters to use with textual data.

Prior to libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED option was off by default, and slightly inaccurate scaling occurred. This option can no longer be turned off, and the choice of accurate or inaccurate 16-to-8 scaling is by using the new png_set_scale_16_to_8() API for accurate scaling or the old png_set_strip_16_to_8() API for simple chopping. In libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED macro became PNG_READ_SCALE_16_TO_8_SUPPORTED, and the PNG_READ_16_TO_8 macro became PNG_READ_STRIP_16_TO_8_SUPPORTED, to enable the two png_set_*_16_to_8() functions separately.

Prior to libpng-1.5.4, the png_set_user_limits() function could only be used to reduce the width and height limits from the value of PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said that it could be used to override them. Now this function will reduce or increase the limits.

Starting in libpng-1.5.10, the user limits can be set en masse with the configuration option PNG_SAFE_LIMITS_SUPPORTED. If this option is enabled, a set of "safe" limits is applied in pngpriv.h. These can be overridden by application calls to png_set_user_limits(), png_set_user_chunk_cache_max(), and/or png_set_user_malloc_max() that increase or decrease the limits. Also, in libpng-1.5.10 the default width and height limits were increased from 1,000,000 to 0x7fffffff (i.e., made unlimited). Therefore, the limits are now

defaultsafe
png_user_width_max 0x7fffffff 1,000,000
png_user_height_max 0x7fffffff 1,000,000
png_user_chunk_cache_max 0 (unlimited) 128
png_user_chunk_malloc_max0 (unlimited)8,000,000

The png_set_option() function (and the "options" member of the png struct) was added to libpng-1.5.15, with option PNG_ARM_NEON.

The library now supports a complete fixed point implementation and can thus be used on systems that have no floating point support or very limited or slow support. Previously gamma correction, an essential part of complete PNG support, required reasonably fast floating point.

As part of this the choice of internal implementation has been made independent of the choice of fixed versus floating point APIs and all the missing fixed point APIs have been implemented.

The exact mechanism used to control attributes of API functions has changed, as described in the INSTALL file.

A new test program, pngvalid, is provided in addition to pngtest. pngvalid validates the arithmetic accuracy of the gamma correction calculations and includes a number of validations of the file format. A subset of the full range of tests is run when make check is done (in the configure build.) pngvalid also allows total allocated memory usage to be evaluated and performs additional memory overwrite validation.

Many changes to individual feature macros have been made. The following are the changes most likely to be noticed by library builders who configure libpng:

1) All feature macros now have consistent naming:

#define PNG_NO_feature /* turns the feature off */ #define PNG_feature_SUPPORTED /* turns the feature on */

pnglibconf.h contains one line for each feature macro which is either:

#define PNG_feature_SUPPORTED

if the feature is supported or:

/*#undef PNG_feature_SUPPORTED*/

if it is not. Library code consistently checks for the SUPPORTED macro. It does not, and libpng applications should not, check for the NO macro which will not normally be defined even if the feature is not supported. The NO macros are only used internally for setting or not setting the corresponding SUPPORTED macros.

Compatibility with the old names is provided as follows:

PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED

And the following definitions disable the corresponding feature:

PNG_SETJMP_NOT_SUPPORTED disables SETJMP
PNG_READ_TRANSFORMS_NOT_SUPPORTED disables READ_TRANSFORMS
PNG_NO_READ_COMPOSITED_NODIV disables READ_COMPOSITE_NODIV
PNG_WRITE_TRANSFORMS_NOT_SUPPORTED disables WRITE_TRANSFORMS
PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED disables READ_ANCILLARY_CHUNKS
PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables WRITE_ANCILLARY_CHUNKS

Library builders should remove use of the above, inconsistent, names.

2) Warning and error message formatting was previously conditional on the STDIO feature. The library has been changed to use the CONSOLE_IO feature instead. This means that if CONSOLE_IO is disabled the library no longer uses the printf(3) functions, even though the default read/write implementations use (FILE) style stdio.h functions.

3) Three feature macros now control the fixed/floating point decisions:

PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs

PNG_FIXED_POINT_SUPPORTED enables the fixed point APIs; however, in practice these are normally required internally anyway (because the PNG file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT merely stops the function from being exported.

PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating point implementation or the fixed point one. Typically the fixed point implementation is larger and slower than the floating point implementation on a system that supports floating point; however, it may be faster on a system which lacks floating point hardware and therefore uses a software emulation.

4) Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED. This allows the functions to read and write ints to be disabled independently of PNG_USE_READ_MACROS, which allows libpng to be built with the functions even though the default is to use the macros - this allows applications to choose at app buildtime whether or not to use macros (previously impossible because the functions weren't in the default build.)

XII. Changes to Libpng from version 1.5.x to 1.6.x

A "simplified API" has been added (see documentation in png.h and a simple example in contrib/examples/pngtopng.c). The new publicly visible API includes the following:

Macros:

PNG_FORMAT_* PNG_IMAGE_*

Structures:

png_control png_image

Read functions:

png_image_begin_read_from_file() png_image_begin_read_from_stdio() png_image_begin_read_from_memory() png_image_finish_read() png_image_free()

Write functions:

png_image_write_to_file() png_image_write_to_stdio()

Starting with libpng-1.6.0, you can configure libpng to prefix all exported symbols, using the PNG_PREFIX macro.

We no longer include string.h in png.h. The include statement has been moved to pngpriv.h, where it is not accessible by applications. Applications that need access to information in string.h must add an #include directive. It does not matter whether this is placed prior to or after the #include "png.h" directive.

The following API are now DEPRECATED:

The following have been removed:

The signatures of many exported functions were changed, such that

where "rp" indicates a "restricted pointer".

Dropped support for 16-bit platforms. The support for FAR/far types has been eliminated and the definition of png_alloc_size_t is now controlled by a flag so that small size_t systems can select it if necessary.

Error detection in some chunks has improved; in particular the iCCP chunk reader now does pretty complete validation of the basic format. Some bad profiles that were previously accepted are now accepted with a warning or rejected, depending upon the png_set_benign_errors() setting, in particular the very old broken Microsoft/HP 3144-byte sRGB profile. Starting with libpng-1.6.11, recognizing and checking sRGB profiles can be avoided by means of

#if defined(PNG_SKIP_sRGB_CHECK_PROFILE) && \ defined(PNG_SET_OPTION_SUPPORTED) png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE, PNG_OPTION_ON); #endif

It's not a good idea to do this if you are using the "simplified API", which needs to be able to recognize sRGB profiles conveyed via the iCCP chunk.

The PNG spec requirement that only grayscale profiles may appear in images with color type 0 or 4 and that even if the image only contains gray pixels, only RGB profiles may appear in images with color type 2, 3, or 6, is now enforced. The sRGB chunk is allowed to appear in images with any color type and is interpreted by libpng to convey a one-tracer-curve gray profile or a three-tracer-curve RGB profile as appropriate.

Libpng 1.5.x erroneously used /MD for Debug DLL builds; if you used the debug builds in your app and you changed your app to use /MD you will need to change it back to /MDd for libpng 1.6.x.

Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk contained an empty language field or an empty translated keyword. Both of these are allowed by the PNG specification, so these warnings are no longer issued.

The library now issues an error if the application attempts to set a transform after it calls png_read_update_info() or if it attempts to call both png_read_update_info() and png_start_read_image() or to call either of them more than once.

The default condition for benign_errors is now to treat benign errors as warnings while reading and as errors while writing.

The library now issues a warning if both background processing and RGB to gray are used when gamma correction happens. As with previous versions of the library the results are numerically very incorrect in this case.

There are some minor arithmetic changes in some transforms such as png_set_background(), that might be detected by certain regression tests.

Unknown chunk handling has been improved internally, without any API change. This adds more correct option control of the unknown handling, corrects a pre-existing bug where the per-chunk keep setting is ignored, and makes it possible to skip IDAT chunks in the sequential reader.

The machine-generated configure files are no longer included in branches libpng16 and later of the GIT repository. They continue to be included in the tarball releases, however.

Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning of the IDAT stream to set the size of the sliding window for reading instead of using the default 32-kbyte sliding window size. It was discovered that there are hundreds of PNG files in the wild that have incorrect CMF bytes that caused zlib to issue the "invalid distance too far back" error and reject the file. Libpng-1.6.3 and later calculate their own safe CMF from the image dimensions, provide a way to revert to the libpng-1.5.x behavior (ignoring the CMF bytes and using a 32-kbyte sliding window), by using

png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW, PNG_OPTION_ON);

and provide a tool (contrib/tools/pngfix) for rewriting a PNG file while optimizing the CMF bytes in its IDAT chunk correctly.

Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks with the wrong length, which resulted in PNG files that cannot be read beyond the bad iTXt chunk. This error was fixed in libpng-1.6.3, and a tool (called contrib/tools/png-fix-itxt) has been added to the libpng distribution.

Starting with libpng-1.6.17, the PNG_SAFE_LIMITS macro was eliminated and safe limits are used by default (users who need larger limits can still override them at compile time or run time, as described above).

The new limits are

defaultspec limit
png_user_width_max 1,000,0002,147,483,647
png_user_height_max 1,000,0002,147,483,647
png_user_chunk_cache_max 128unlimited
png_user_chunk_malloc_max8,000,000unlimited

Starting with libpng-1.6.18, a PNG_RELEASE_BUILD macro was added, which allows library builders to control compilation for an installed system (a release build). It can be set for testing debug or beta builds to ensure that they will compile when the build type is switched to RC or STABLE. In essence this overrides the PNG_LIBPNG_BUILD_BASE_TYPE definition which is not directly user controllable.

Starting with libpng-1.6.19, attempting to set an over-length PLTE chunk is an error. Previously this requirement of the PNG specification was not enforced, and the palette was always limited to 256 entries. An over-length PLTE chunk found in an input PNG is silently truncated.

XIII. Detecting libpng

The png_get_io_ptr() function has been present since libpng-0.88, has never changed, and is unaffected by conditional compilation macros. It is the best choice for use in configure scripts for detecting the presence of any libpng version since 0.88. In an autoconf configure.in you could use

AC_CHECK_LIB(png, png_get_io_ptr, ...

XIV. Source code repository

Since about February 2009, version 1.2.34, libpng has been under git source control. The git repository was built from old libpng-x.y.z.tar.gz files going back to version 0.70. You can access the git repository (read only) at

git://git.code.sf.net/p/libpng/code

or you can browse it with a web browser by selecting the code button at https://sourceforge.net/projects/ libpng

Patches can be sent to glennrp at users.sourceforge.net or to png-mng-implement at lists.sourceforge.net or you can upload them to the libpng bug tracker at http://libpng.sourceforge.net.

We also accept patches built from the tar or zip distributions, and simple verbal discriptions of bug fixes, reported either to the SourceForge bug tracker, to the png-mng-implement at lists.sf.net mailing list, or directly to glennrp.

XV. Coding style

Our coding style is similar to the "Allman" style (See http://en.wikipedi a.org/wiki/Indent_style#Allman_style), with curly braces on separate lines:

if (condition) { action; } else if (another condition) { another action; }

The braces can be omitted from simple one-line actions:

if (condition) return (0);

We use 3-space indentation, except for continued statements which are usually indented the same as the first line of the statement plus four more spaces.

For macro definitions we use 2-space indentation, always leaving the "#" in the first column.

#ifndef PNG_NO_FEATURE # ifndef PNG_FEATURE_SUPPORTED # define PNG_FEATURE_SUPPORTED # endif #endif

Comments appear with the leading "/*" at the same indentation as the statement that follows the comment:

/* Single-line comment */ statement; /* This is a multiple-line * comment. */ statement;

Very short comments can be placed after the end of the statement to which they pertain:

statement; /* comment */

We don't use C++ style ("//") comments. We have, however, used them in the past in some now-abandoned MMX assembler code.

Functions and their curly braces are not indented, and exported functions are marked with PNGAPI:

/* This is a public function that is visible to * application programmers. It does thus-and-so. */ void PNGAPI png_exported_function(png_ptr, png_info, foo) { body; }

The return type and decorations are placed on a separate line ahead of the function name, as illustrated above.

The prototypes for all exported functions appear in png.h, above the comment that says

/* Maintainer: Put new public prototypes here ... */

We mark all non-exported functions with "/* PRIVATE */"":

void /* PRIVATE */ png_non_exported_function(png_ptr, png_info, foo) { body; }

The prototypes for non-exported functions (except for those in pngtest) appear in pngpriv.h above the comment that says

/* Maintainer: Put new private prototypes here ^ */

To avoid polluting the global namespace, the names of all exported functions and variables begin with "png_", and all publicly visible C preprocessor macros begin with "PNG". We request that applications that use libpng not begin any of their own symbols with either of these strings.

We put a space after the sizeof operator and we omit the optional parentheses around its argument when the argument is an expression, not a type name, and we always enclose the sizeof operator, with its argument, in parentheses:

(sizeof (png_uint_32)) (sizeof array)

Prior to libpng-1.6.0 we used a png_sizeof() macro, formatted as though it were a function.

Control keywords if, for, while, and switch are always followed by a space to distinguish them from function calls, which have no trailing space.

We put a space after each comma and after each semicolon in for statements, and we put spaces before and after each C binary operator and after for or while, and before ?. We don't put a space between a typecast and the expression being cast, nor do we put one between a function name and the left parenthesis that follows it:

for (i = 2; i > 0; --i) y[i] = a(x) + (int)b;

We prefer #ifdef and #ifndef to #if defined() and #if !defined() when there is only one macro being tested. We always use parentheses with defined.

We express integer constants that are used as bit masks in hex format, with an even number of lower-case hex digits, and to make them unsigned (e.g., 0x00U, 0xffU, 0x0100U) and long if they are greater than 0x7fff (e.g., 0xffffUL).

We prefer to use underscores rather than camelCase in names, except for a few type names that we inherit from zlib.h.

We prefer "if (something != 0)" and "if (something == 0)" over "if (something)" and "if (!something)", respectively.

We do not use the TAB character for indentation in the C sources.

Lines do not exceed 80 characters.

Other rules can be inferred by inspecting the libpng source.

XVI. Y2K Compliance in libpng

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 1.6.21 are Y2K compliant. It is my belief that earlier versions were also Y2K compliant.

Libpng only has two year fields. One is a 2-byte unsigned integer that will hold years up to 65535. The other, which is deprecated, holds the date in text format, and will hold years up to 9999.

The integer is png_uint_16 year in png_time_struct. The string is char time_buffer[29]" in png_struct. This is no longer used in libpng-1.6.x and will be removed from libpng-1.7.0.

There are seven time-related functions:

All appear to handle dates properly in a Y2K environment. The png_convert_from_time_t() function calls gmtime() to convert from system clock time, which returns (year - 1900), which we properly convert to the full 4-digit year. There is a possibility that applications using libpng are not passing 4-digit years into the png_convert_to_rfc_1123() function, or that they are incorrectly passing only a 2-digit year instead of "year - 1900" into the png_convert_from_struct_tm() function, but this is not under our control. The libpng documentation has always stated that it works with 4-digit years, and the APIs have been documented as such.

The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned integer to hold the year, and can hold years as large as 65535.

zlib, upon which libpng depends, is also Y2K compliant. It contains no date-related code.

Glenn Randers-Pehrson
libpng maintainer
PNG Development Group