Getting APNG support into libpng

The APNG extensions failed their original vote, and an attempt to 
restart discussions some years ago failed from lack of discussion.

Thus, libpng does not have any APNG support and unofficial patches are 
merged in by various Linux distros, individual maintainers of image 
editing tools, and Web browsers.

However, APNG is now very widely supported so this is a bad situation.
https://caniuse.com/apng

Meanwhile the PNG specification is badly needing maintenance. There are 
many errata reported since 2003. To help with that, I forked the W3C PNG 
specification to make an Editors Draft.
https://w3c.github.io/PNG-spec/

Each reported erratum is raised as a GitHub issue and the clear and 
obvious ones I have rolled into the Editors Draft so the changes can be 
viewed in place. The others need to be discussed.
https://github.com/w3c/PNG-spec/issues

I don't think the original PNG mailing list is the right way to handle 
specification maintenance anymore. The discussion procedure is baroque, 
there is a poor record of discussing or adopting proposals, and momentum 
has ground to a halt. I think an open W3C Working Group is the better 
way to handle this.

Beyond just maintenance though, there is a clear need to re-align the 
PNG specification with widely-implemented reality, especially as regards 
APNG.

An updated PNG Specification which incorporates APNG would then allow 
libpng to incorporate support directly, rather than a plethora of ad-hoc 
patches, forks, or other means to support it.

Cosmin, I would really value your comments on this path forwards. 
Continued alignment between the PNG specification and libpng is very 
important. libpng is being held back by the brokenness of the current 
PNG spec maintenance process. It should not be.

(I posted this earlier to
  APNG support on main libpng library? #267
  https://github.com/glennrp/libpng/issues/267 )

-- 
Chris Lilley
@svgeesus
Technical Director @ W3C
W3C Strategy Team, Core Web Design
W3C Architecture & Technology Team, Core Web & Media



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