pngcheck version 2.4.0 of 31 October 2020

This version adds basic eXIf-chunk support (for EXIF metadata), plus some
extremely simple man pages (essentially identical to the corresponding usage
screens) kindly provided by Ben Beasley of the Fedora Linux project.

Here's a list of the major enhancements since version 1.98, which was the
last release before I took over maintenance:

 - zlib support (to test the compressed stream and, optionally, to print
   out the image's row filters)
 - support for all remaining known PNG chunks (conformance)
 - complete support for all known MNG and JNG chunks (informational)
 - extended support for printing palettes (includes transparency info and
   histograms)
 - optional color (text) output
 - improved error-checking
 - info on the compression factor of the image (expressed as a percentage,
   where 0% is no compression and 100% would be total compression; note that
   this can be negative since it counts PNG's chunk overhead against the
   compression factor)
 - png-fix-IDAT-windowsize utility
 - pngsplit utility
 - compilation support for Win32 (using MSVC), RISC OS, and Amiga

There are also many fixes, of course, including ones from Tom Lane, Glenn
Randers-Pehrson, Tom Zerucha, Paul Matzke, Darren Salt, John Bowler, and
others.  Thanks also to Chris Nokleberg (brokensuite), Tim Pritlove, Bob
Friesenhahn, the GraalOnline folks, and others for test images.  See the
included CHANGELOG file for the complete, detailed list of who did what.

Note that while MNG support is now complete in the sense of covering all
registered chunk types, there are still numerous error conditions that
pngcheck won't detect, plus a few non-error conditions that it will flag
erroneously.  Some of those can and will be fixed (particularly the latter
class), but many of them involve complex interactions between different
chunk types and would require virtually a full MNG decoder engine, something
that is unlikely ever to happen in pngcheck.  In other words, consider
pngcheck a handy MNG debugging tool but not a full validator.  Use it in
conjunction with the MNG specification and a libmng-based viewer for best
results.  (PNG support, on the other hand, is pretty solid.)  Also use
zlib 1.2.x for best results--older versions failed to detect a number of
invalid deflate/zlib conditions, including out-of-range LZ77 distance codes.

Originally I had hoped to add support for EBCDIC-based systems (and perhaps
UTF-16 and UTF-32-based ones, if there are any for which "char" defaults to
more than 8 bits), but there doesn't seem to be much point in that anymore.
I'd still kind of like to extend the zlib support to include zTXt, iTXt, iCCP,
etc., but given the pace of recent years ("nonexistent" would be fair), folks
should definitely not hold their breath.  Similarly, the code could also do a
better job with chunks whose data exceed the buffer size, and in general,
immense if-else blocks (e.g., > 3000 lines) are extremely nasty and should be
rewritten, but...yeah.  The gap between this release and the previous one
(2.3.0) was bigger than that between the previous one and the creation of the
PNG format itself. :-/

But if there ever are additional updates, you might find them here:

    http://www.libpng.org/pub/png/apps/pngcheck.html

Greg Roelofs
http://gregroelofs.com/greg_contact.html
