README for libpng version 1.6.43

See the note about version numbers near the top of png.h. See INSTALL for instructions on how to install libpng.

Libpng comes in several distribution formats. Get libpng-*.tar.gz or libpng-*.tar.xz if you want UNIX-style line endings in the text files, or lpng*.7z or lpng*.zip if you want DOS-style line endings.

For a detailed description on using libpng, read libpng-manual.txt. For examples of libpng in a program, see example.c and pngtest.c. For usage information and restrictions (what little they are) on libpng, see png.h. For a description on using zlib (the compression library used by libpng) and zlib's restrictions, see zlib.h.

You should use zlib 1.0.4 or later to run this, but it may work with versions as old as zlib 0.95. Even so, there are bugs in older zlib versions which can cause the output of invalid compression streams for some images.

You should also note that zlib is a compression library that is useful for more things than just PNG files. You can use zlib as a drop-in replacement for fread() and fwrite(), if you are so inclined.

zlib should be available at the same place that libpng is, or at https://zlib.net .

You may also want a copy of the PNG specification. It is available as an RFC, a W3C Recommendation, and an ISO/IEC Standard. You can find these at http://www.libpng.org/pub/png/pngdocs.html .

This code is currently being archived at https://libpng.sourceforge.io in the download area, and at http://libpng.download/src .

This release, based in a large way on Glenn's, Guy's and Andreas' earlier work, was created and will be supported by myself and the PNG development group.

Send comments, corrections and commendations to png-mng-implement at lists.sourceforge.net. (Subscription is required; visit https://lists.sourceforge.net/lists/listinfo/png-mng-implement to subscribe.)

Send general questions about the PNG specification to png-mng-misc at lists.sourceforge.net. (Subscription is required; visit https://lists.sourceforge.net/lists/listinfo/png-mng-misc to subscribe.)

Historical notes

The libpng library has been in extensive use and testing since mid-1995. Version 0.89, published a year later, was the first official release. By late 1997, it had finally gotten to the stage where there hadn't been significant changes to the API in some time, and people have a bad feeling about libraries with versions below 1.0. Version 1.0.0 was released in March 1998.

Note that some of the changes to the png_info structure render this version of the library binary incompatible with libpng-0.89 or earlier versions if you are using a shared library. The type of the filler parameter for png_set_filler() has changed from png_byte to png_uint_32, which will affect shared-library applications that use this function.

To avoid problems with changes to the internals of the info_struct, new APIs have been made available in 0.95 to avoid direct application access to info_ptr. These functions are the png_set_<chunk> and png_get_<chunk> functions. These functions should be used when accessing/storing the info_struct data, rather than manipulating it directly, to avoid such problems in the future.

It is important to note that the APIs did not make current programs that access the info struct directly incompatible with the new library, through libpng-1.2.x. In libpng-1.4.x, which was meant to be a transitional release, members of the png_struct and the info_struct can still be accessed, but the compiler will issue a warning about deprecated usage. Since libpng-1.5.0, direct access to these structs is not allowed, and the definitions of the structs reside in private pngstruct.h and pnginfo.h header files that are not accessible to applications. It is strongly suggested that new programs use the new APIs (as shown in example.c and pngtest.c), and older programs be converted to the new format, to facilitate upgrades in the future.

The additions since 0.89 include the ability to read from a PNG stream which has had some (or all) of the signature bytes read by the calling application. This also allows the reading of embedded PNG streams that do not have the PNG file signature. As well, it is now possible to set the library action on the detection of chunk CRC errors. It is possible to set different actions based on whether the CRC error occurred in a critical or an ancillary chunk.

The additions since 0.90 include the ability to compile libpng as a Windows DLL, and new APIs for accessing data in the info_struct. Experimental functions included the ability to set weighting and cost factors for row filter selection, direct reads of integers from buffers on big-endian processors that support misaligned data access, faster methods of doing alpha composition, and more accurate 16-to-8 bit color conversion. Some of these experimental functions, such as the weighted filter heuristics, have since been removed.

Files included in this distribution

ANNOUNCE      =>  Announcement of this version, with recent changes
AUTHORS       =>  List of contributing authors
CHANGES       =>  Description of changes between libpng versions
INSTALL       =>  Instructions to install libpng
LICENSE       =>  License to use and redistribute libpng
README        =>  This file
TODO          =>  Things not implemented in the current library
TRADEMARK     =>  Trademark information
example.c     =>  Example code for using libpng functions
libpng.3      =>  Manual page for libpng (includes libpng-manual.txt)
libpng-manual.txt  =>  Description of libpng and its functions
libpngpf.3    =>  Manual page for libpng's private functions (deprecated)
png.5         =>  Manual page for the PNG format
png.c         =>  Basic interface functions common to library
png.h         =>  Library function and interface declarations (public)
pngpriv.h     =>  Library function and interface declarations (private)
pngconf.h     =>  System specific library configuration (public)
pngstruct.h   =>  png_struct declaration (private)
pnginfo.h     =>  png_info struct declaration (private)
pngdebug.h    =>  debugging macros (private)
pngerror.c    =>  Error/warning message I/O functions
pngget.c      =>  Functions for retrieving info from struct
pngmem.c      =>  Memory handling functions
pngbar.png    =>  PNG logo, 88x31
pngnow.png    =>  PNG logo, 98x31
pngpread.c    =>  Progressive reading functions
pngread.c     =>  Read data/helper high-level functions
pngrio.c      =>  Lowest-level data read I/O functions
pngrtran.c    =>  Read data transformation functions
pngrutil.c    =>  Read data utility functions
pngset.c      =>  Functions for storing data into the info_struct
pngtest.c     =>  Library test program
pngtest.png   =>  Library test sample image
pngtrans.c    =>  Common data transformation functions
pngwio.c      =>  Lowest-level write I/O functions
pngwrite.c    =>  High-level write functions
pngwtran.c    =>  Write data transformations
pngwutil.c    =>  Write utility functions
arm/          =>  Optimized code for ARM Neon
intel/        =>  Optimized code for INTEL SSE2
loongarch/    =>  Optimized code for LoongArch LSX
mips/         =>  Optimized code for MIPS MSA and MIPS MMI
powerpc/      =>  Optimized code for PowerPC VSX
ci/           =>  Scripts for continuous integration
contrib/      =>  External contributions
    arm-neon/     =>  Optimized code for the ARM-NEON platform
    mips-msa/     =>  Optimized code for the MIPS-MSA platform
    powerpc-vsx/  =>  Optimized code for the POWERPC-VSX platform
    examples/     =>  Examples of libpng usage
    gregbook/     =>  Source code for PNG reading and writing, from
                      "PNG: The Definitive Guide" by Greg Roelofs,
                      O'Reilly, 1999
    libtests/     =>  Test programs
    oss-fuzz/     =>  Files used by the OSS-Fuzz project for fuzz-testing
                      libpng
    pngexif/      =>  Program to inspect the EXIF information in PNG files
    pngminim/     =>  Minimal decoder, encoder, and progressive decoder
                      programs demonstrating the use of pngusr.dfa
    pngminus/     =>  Simple pnm2png and png2pnm programs
    pngsuite/     =>  Test images
    testpngs/     =>  Test images
    tools/        =>  Various tools
    visupng/      =>  VisualPng, a Windows viewer for PNG images
projects/     =>  Project files and workspaces for various IDEs
    owatcom/      =>  OpenWatcom project
    visualc71/    =>  Microsoft Visual C++ 7.1 workspace
    vstudio/      =>  Microsoft Visual Studio workspace
scripts/      =>  Scripts and makefiles for building libpng
                  (see scripts/README.txt for the complete list)
tests/        =>  Test scripts

Good luck, and happy coding!

• Cosmin Truta (current maintainer, since 2018)
• Glenn Randers-Pehrson (former maintainer, 1998-2018)
• Andreas Eric Dilger (former maintainer, 1996-1997)
• Guy Eric Schalnat (original author and former maintainer, 1995-1996) (formerly of Group 42, Inc.)