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libpng.txt - A description on how to use and modify libpng libpng version 1.2.19 - August 18, 2007 Updated and distributed by Glenn Randers-Pehrson <glennrp at users.sourceforge.net> Copyright (c) 1998-2007 Glenn Randers-Pehrson For conditions of distribution and use, see copyright notice in png.h. based on: libpng 1.0 beta 6 version 0.96 May 28, 1997 Updated and distributed by Andreas Dilger Copyright (c) 1996, 1997 Andreas Dilger libpng 1.0 beta 2 - version 0.88 January 26, 1996 For conditions of distribution and use, see copyright notice in png.h. Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc. Updated/rewritten per request in the libpng FAQ Copyright (c) 1995, 1996 Frank J. T. Wojcik December 18, 1995 & January 20, 1996 I. Introduction This file describes how to use and modify the PNG reference library (known as libpng) for your own use. There are five sections to this file: introduction, structures, reading, writing, and modification and configuration notes for various special platforms. In addition to this file, example.c is a good starting point for using the library, as it is heavily commented and should include everything most people will need. We assume that libpng is already installed; see the INSTALL file for instructions on how to install libpng. For examples of libpng usage, see the files "example.c", "pngtest.c", and the files in the "contrib" directory, all of which are included in the libpng distribution. Libpng was written as a companion to the PNG specification, as a way of reducing the amount of time and effort it takes to support the PNG file format in application programs. The PNG specification (second edition), November 2003, is available as a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2003 (E)) at <http://www.w3.org/TR/2003/REC-PNG-20031110/ The W3C and ISO documents have identical technical content. The PNG-1.2 specification is available at <http://www.libpng.org/pub/png/documents/> The PNG-1.0 specification is available as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a W3C Recommendation <http://www.w3.org/TR/REC.png.html>. Some additional chunks are described in the special-purpose public chunks documents at <http://www.libpng.org/pub/png/documents/>. Other information about PNG, and the latest version of libpng, can be found at the PNG home page, <http://www.libpng.org/pub/png/>. Most users will not have to modify the library significantly; advanced users may want to modify it more. All attempts were made to make it as complete as possible, while keeping the code easy to understand. Currently, this library only supports C. Support for other languages is being considered. Libpng has been designed to handle multiple sessions at one time, to be easily modifiable, to be portable to the vast majority of machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy to use. The ultimate goal of libpng is to promote the acceptance of the PNG file format in whatever way possible. While there is still work to be done (see the TODO file), libpng should cover the majority of the needs of its users. Libpng uses zlib for its compression and decompression of PNG files. Further information about zlib, and the latest version of zlib, can be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>. The zlib compression utility is a general purpose utility that is useful for more than PNG files, and can be used without libpng. See the documentation delivered with zlib for more details. You can usually find the source files for the zlib utility wherever you find the libpng source files. Libpng is thread safe, provided the threads are using different instances of the structures. Each thread should have its own png_struct and png_info instances, and thus its own image. Libpng does not protect itself against two threads using the same instance of a structure. Note: thread safety may be defeated by use of some of the MMX assembler code in pnggccrd.c, which is only compiled when the user defines PNG_THREAD_UNSAFE_OK. II. Structures There are two main structures that are important to libpng, png_struct and png_info. The first, png_struct, is an internal structure that will not, for the most part, be used by a user except as the first variable passed to every libpng function call. The png_info structure is designed to provide information about the PNG file. At one time, the fields of png_info were intended to be directly accessible to the user. 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. The fields of png_info are still available for older applications, but it is suggested that applications use the new interfaces if at all possible. Applications that do make direct access to the members of png_struct (except for png_ptr->jmpbuf) must be recompiled whenever the library is updated, and applications that make direct access to the members of png_info must be recompiled if they were compiled or loaded with libpng version 1.0.6, in which the members were in a different order. In version 1.0.7, the members of the png_info structure reverted to the old order, as they were in versions 0.97c through 1.0.5. Starting with version 2.0.0, both structures are going to be hidden, and the contents of the structures will only be accessible through the png_get/png_set functions. The png.h header file is an invaluable reference for programming with libpng. And while I'm on the topic, make sure you include the libpng header file: #include <png.h> III. Reading We'll now walk you through the possible functions to call when reading in a PNG file sequentially, briefly explaining the syntax and purpose of each one. See example.c and png.h for more detail. While progressive reading is covered in the next section, you will still need some of the functions discussed in this section to read a PNG file. Setup You will want to do the I/O initialization(*) before you get into libpng, so if it doesn't work, you don't have much to undo. Of course, you will also want to insure that you are, in fact, dealing with a PNG file. Libpng provides a simple check to see if a file is a PNG file. To use it, pass in the first 1 to 8 bytes of the file to the function png_sig_cmp(), and it will return 0 if the bytes match the corresponding bytes of the PNG signature, or nonzero otherwise. Of course, the more bytes you pass in, the greater the accuracy of the prediction. If you are intending to keep the file pointer open for use in libpng, you must ensure you don't read more than 8 bytes from the beginning of the file, and you also have to make a call to png_set_sig_bytes_read() with the number of bytes you read from the beginning. Libpng will then only check the bytes (if any) that your program didn't read. (*): If you are not using the standard I/O functions, you will need to replace them with custom functions. See the discussion under Customizing libpng. FILE *fp = fopen(file_name, "rb"); if (!fp) { return (ERROR); } fread(header, 1, number, fp); is_png = !png_sig_cmp(header, 0, number); if (!is_png) { return (NOT_PNG); } Next, png_struct and png_info need to be allocated and initialized. In order to ensure that the size of these structures is correct even with a dynamically linked libpng, there are functions to initialize and allocate the structures. We also pass the library version, optional pointers to error handling functions, and a pointer to a data struct for use by the error functions, if necessary (the pointer and functions can be NULL if the default error handlers are to be used). See the section on Changes to Libpng below regarding the old initialization functions. The structure allocation functions quietly return NULL if they fail to