README for libpng version 1.6.21 - January 15, 2016 (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 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</chunk>

png_get_<chunk> functions.  These functions should be used when</chunk>

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

conversion.

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 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/documents/

This code is currently being archived at libpng.sf.net in the

[DOWNLOAD] area, and at ftp://ftp.simplesystems.org.  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

lists.sourceforge.net (subscription required; visit

https://lists.sourceforge.net/lists/listinfo/png-mng-implement

to subscribe) or to glennrp at users.sourceforge.net

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 lists.sf.net (subscription required; visit

https://lists.sourceforge.net/lists/listinfo/png-mng-misc 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

      contrib       =>  Contributions

       examples         =>  Example programs

       gregbook         =>  source code for PNG reading and writing, from

                            Greg Roelofs' "PNG: The Definitive Guide",

                            O'Reilly, 1999

       libtests         =>  Test programs

       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

      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 users.sourceforge.net

-Andreas Eric Dilger (former maintainer, 1996-1997)

 Internet: adilger at enel.ucalgary.ca

 Web: http://www-mddsp.enel.ucalgary.ca/People/adilger/

-Guy Eric Schalnat (original author and former maintainer, 1995-1996)

 (formerly of Group 42, Inc)

 Internet: gschal at infinet.com