#1143: [wg/png] PNG Working Group Charter

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Opened Aug 26, 2025

This issue was created because the 'horizontal review requested' label was added to § https://github.com/w3c/strategy/issues/514

This review is requested prior to the Advisory Committee Review.

New charter proposal, reviewers please take note.

Charter Review

Charter

Expected end of charter refinement phase: unknown

If applicable:

Horizontal Reviews: apply the Github label "Horizontal review requested" to request reviews for accessibility (a11y), internationalization (i18n), privacy, security, and TAG. Also add a "card" for this issue to the Strategy Funnel.

Communities suggested for outreach

Known or potential areas of concern

Where would charter proponents like to see issues raised? (this strategy funnel issue, a different github repo, email, ...)

This issue

Anything else we should think about as we review?

Note: proposed chairs should be copied @programmax

Charter facilitator(s)

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Discussions

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Comment by @christianliebel Aug 28, 2025 (See Github)

@svgeesus Thanks a lot for the details! 😊

Discussed Sep 1, 2025 (See Github)

Christian: charter looks OK to me. Jeffrey is concerned that the new compression algorithm is not backwards compatible. Should they introduce a new media type for that?

Martin: I think Jeffrey is entirely right. If you can't consume the content without new code, it's a new format.

Discussed Sep 8, 2025 (See Github)

Christian: 4th ed includes HDR/SDR. 5th edition includes new compression algorithm that will break compatibility with existing image files. Thoughts so far are that we're fine with it, but they need to make sure you can treat the new images files safely. Martin suggestetd that if you need new code to interpret it, then it's a breaking change.

Yves: Same issue with all the containers that embed differnt formats. E.g. video like Matroska. I treat this as the same case.

Matthew: My read was that if it needs new code to interpret it, it's a new format. A way to make it safe is to duplicate the old format, but then you've lost the benefit. Don't see how this is fine if that's the way to solve it. That would be hard.

Jeffrey: There's a comment Christian linked to in the discusion that makes the point I would make: if existing decoders can't decode new files, then it's a new format. It's true that's been done in video codecs, but I think it's new for images, and we should hold that line. We also have 2 new generations and 3 new formats for images WebP, AVI..., JPEG XL that all do better than PNG. Any suggestion to break compatibility with PNG needs to be compared to those new formats and justify itself. We should be sceptical of any backwards-incompatible changes.

Christian: Do the same review resolutions apply to charter reviews?

Jeffrey: There are no rules; we can use what's appropriate. The text of the charter review is more appropriate than the code - it'll be pasted into the strategy repo thread.

Christian: I'd remove the resolution, add what you said to the proposed comment, and then we can discuss again. Will post to (TAG's) private repo for discussion.

Comment by @svgeesus Sep 22, 2025 (See Github)

@christianliebel anything else I can help you with, regarding this charter review?

Comment by @christianliebel Sep 22, 2025 (See Github)

@svgeesus Thanks for your patience. Here is our review:

This is a charter update for the PNG Working Group, which maintains one of the key graphics formats used on the web and beyond.

Specifically, the Working Group plans to deliver two new editions of the PNG specification:

  • An improvement of the interoperability of High Dynamic Range (HDR) and Standard Dynamic Range (SDR) in PNG Fourth Edition, an update expected to be minor.
  • An improvement of compression and decompression in PNG Fifth Edition, a larger effort that may lead to smaller image files and faster opening times, but may break compatibility with existing image creation and consuming software.

The TAG welcomes the improvement in HDR/SDR compatibility but is concerned about the breaking change regarding the compression algorithm. We agree with https://github.com/w3c/png/issues/39#issuecomment-2674690324 that introducing such a change would basically create a new format. For this new compression to be worth it, it needs to clearly outperform other newer image formats like WebP and JPEG XL. The benefits need to be clear and strong enough to make a breaking change worthwhile.

Comment by @svgeesus Sep 22, 2025 (See Github)

Thanks to the TAG for their comments.

We agree about any breaking change being a new format (whose introduction would need to be justified): as the [charter says:](https://w3c.github.io/charter-drafts/2025/png-wg.html#section-out-of-scope)

The following features are out of scope, and will not be addressed by this Working Group.

  • Changes that will invalidate existing files, editors, or viewers that conform to Portable Network Graphics (PNG) Specification (Third Edition).

If no changes to the draft charter are proposed, please have someone add the "TAG Review Completed" tag to the strategy issue