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libpng-manual.txt - A description on how to use and modify libpng Copyright (c) 2018-2023 Cosmin Truta Copyright (c) 1998-2018 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 version 1.6.36, December 2018, through 1.6.40 - June 2023 Updated and distributed by Cosmin Truta Copyright (c) 2018-2023 Cosmin Truta libpng versions 0.97, January 1998, through 1.6.35 - July 2018 Updated and distributed by Glenn Randers-Pehrson Copyright (c) 1998-2018 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 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 <https://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 <https://png-mng.sourceforge.io/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 at <https://png-mng.sourceforge.io/pub/png/spec/1.0/> and as a W3C Recommendation at <https://www.w3.org/TR/REC-png-961001>. Some additional chunks are described in the special-purpose public chunks documents at <http://www.libpng.org/pub/png/spec/register/> Other information about PNG, and the latest version of libpng, can be found at the PNG home page, <http://www.libpng.org/pub/png/>. Most users will not have to modify the library significantly; advanced users may want to modify it more. All attempts were made to make it as complete as possible, while keeping the code easy to understand. Currently, this library only supports C. Support for other languages is being considered. Libpng has been designed to handle multiple sessions at one time, to be easily modifiable, to be portable to the vast majority of machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy to use. The ultimate goal of libpng is to promote the acceptance of the PNG file format in whatever way possible. While there is still work to be done (see the TODO file), libpng should cover the majority of the needs of its users. Libpng uses zlib for its compression and decompression of PNG files. Further information about zlib, and the latest version of zlib, can be found at the zlib home page, <https://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 bu