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README for libpng version 1.6.35beta01 - September 29, 2017 (shared library 16.0)
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 or if you want UNIX-style line endings in the text files,
or lpng*.7z or lpng*.zip if you want DOS-style line endings.
Version 0.89 was the first official release of libpng. Don't let the
fact that it's the first release fool you. The libpng library has been in
extensive use and testing since mid-1995. 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 < 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 png 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.
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 include 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->8 bit color
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 changes made to the library, and bugs fixed are based on discussions
on the PNG-implement mailing list and not on material submitted
privately to Guy, Andreas, or Glenn. They will forward any good
suggestions to the list.
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
I have included a general makefile, as well as several machine and
compiler specific ones, but you may have to modify one for your own needs.
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 will definitely need zlib 1.0.4 or later if you are
taking advantage of the MS-DOS "far" structure allocation for the small
and medium memory models. 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
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 .
This code is currently being archived at in the
[DOWNLOAD] area, and at . If you
can't find it in any of those places, e-mail me, and I'll help you find it.
I am not a lawyer, but I believe that the Export Control Classification
Number (ECCN) for libpng is EAR99, which means not subject to export
controls or International Traffic in Arms Regulations (ITAR) because it
is open source, publicly available software, that does not contain any
encryption software. See the EAR, paragraphs 734.3(b)(3) and 734.7(b).
If you have any code changes, requests, problems, etc., please e-mail
them to me. Also, I'd appreciate any make files or project files,
and any modifications you needed to make to get libpng to compile,
along with a #define variable to tell what compiler/system you are on.
If you needed to add transformations to libpng, or wish libpng would
provide the image in a different way, drop me a note (and code, if
possible), so I can consider supporting the transformation.
Finally, if you get any warning messages when compiling libpng
(note: not zlib), and they are easy to fix, I'd appreciate the
fix. Please mention "libpng" somewhere in the subject line. Thanks.
This release was created and will be supported by myself (of course
based in a large way on Guy's and Andreas' earlier work), and the PNG
development group.
Send comments/corrections/commendations to png-mng-implement at (subscription required; visit
to subscribe) or to glennrp at
You can't reach Guy, the original libpng author, at the addresses
given in previous versions of this document. He and Andreas will
read mail addressed to the png-implement list, however.
Please do not send general questions about PNG. Send them to
png-mng-misc at (subscription required; visit to
subscribe). If you have a question about something
in the PNG specification that is related to using libpng, send it
to me. Send me any questions that start with "I was using libpng,
and ...". If in doubt, send questions to me. I'll bounce them
to others, if necessary.
Please do not send suggestions on how to change PNG. We have
been discussing PNG for twenty years now, and it is official and
finished. If you have suggestions for libpng, however, I'll
gladly listen. Even if your suggestion is not used immediately,
it may be used later.
Files in this distribution:
ANNOUNCE => Announcement of this version, with recent changes
CHANGES => Description of changes between libpng versions
KNOWNBUG => List of known bugs and deficiencies
LICENSE => License to use and redistribute libpng
README => This file
TODO => Things not implemented in the current library
Y2KINFO => Statement of Y2K compliance
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
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 => Contains optimized code for the ARM platform
powerpc => Contains optimized code for the PowerPC platform
contrib => Contributions
arm-neon => Optimized code for ARM-NEON platform
powerpc-vsx => Optimized code for POWERPC-VSX platform
examples => Example programs
gregbook => source code for PNG reading and writing, from
Greg Roelofs' "PNG: The Definitive Guide",
O'Reilly, 1999
libtests => Test programs
mips-msa => Optimized code for MIPS-MSA platform
pngminim => Minimal decoder, encoder, and progressive decoder
programs demonstrating use of pngusr.dfa
pngminus => Simple pnm2png and png2pnm programs
pngsuite => Test images
tools => Various tools
visupng => Contains a MSVC workspace for VisualPng
intel => Optimized code for INTEL-SSE2 platform
mips => Optimized code for MIPS platform
projects => Contains project files and workspaces for
building a DLL
owatcom => Contains a WATCOM project for building libpng
visualc71 => Contains a Microsoft Visual C++ (MSVC)
workspace for building libpng and zlib
vstudio => Contains a Microsoft Visual C++ (MSVC)
workspace for building libpng and zlib
scripts => Directory containing scripts for building libpng:
(see scripts/README.txt for the list of scripts)
Good luck, and happy coding.
-Glenn Randers-Pehrson (current maintainer, since 1998)
Internet: glennrp at
-Andreas Eric Dilger (former maintainer, 1996-1997)
Internet: adilger at
-Guy Eric Schalnat (original author and former maintainer, 1995-1996)
(formerly of Group 42, Inc)
Internet: gschal at