README for libpng version 1.6.56
================================

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
    riscv/        =>  Optimized code for the RISC-V platform
    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
        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
        riscv-rvv/    =>  Optimized code for the RISC-V Vector platform
        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.)
