CSS Color: lab(), lch()

[felipeerias]

Hola TAG!

I'm requesting a TAG review of:

• Name: lab() and lch() CSS functions for colors.
• Specification URL: https://drafts.csswg.org/css-color-4/#specifying-lab-lch
• Tests: https://github.com/web-platform-tests/wpt/tree/master/css/css-color
• Primary contacts:
  • CSSWG for the specification
  • @felipeerias for the Chromium implementation
• Organization driving the specification: CSSWG
• External issue trackers:
  • Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=1026287
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1352757
  • Safari: https://bugs.webkit.org/show_bug.cgi?id=205675

We'd prefer the TAG provide feedback as:

💬 leave review feedback as a comment in this issue and @-notify

[dbaron]

Do you mean lch() rather than lhr()?

I'd also cc @svgeesus here as the primary contact for the spec.

[felipeerias]

Do you mean lch() rather than lhr()?

Sorry, yes, of course :)

[dbaron]

I think one thing we should perhaps look at is whether other parts of the Web platform (e.g., canvas) need things to prepare for colors that are outside of the sRGB gamut.

[felipeerias]

There is an ongoing discussion at the intent-to-prototype thread about which other parts of the spec still need some work:

https://groups.google.com/a/chromium.org/d/topic/blink-dev/iwsT-jkCQcI/discussion

And there are open issues at the CSSWG with different levels of consensus for topics such as:

• working color spaces ([css-color] Working Color Space w3c/csswg-drafts#300)
• rendering colors outside the sRGB gamut ([css-color-4] Should lab() colors which are outside the sRGB gamut be rendered on capable displays? w3c/csswg-drafts#4646)
• interpolation in non-sRGB color spaces ([css-color-4] Do gradients/animations using lab/lch colors interpolate in the Lab colorspace? w3c/csswg-drafts#4647)
• etc.

My main question at the moment is whether it would make sense to implement Lab-like selection and interpolation using the browser's existing color infrastructure (i.e. with clamped outputs), or whether it would be necessary to spec and implement general support for wider gamuts first.

A related question is whether there could be a path from those clamped lab() and lch() functions to their wide-gamut versions (if/when they are supported by the browser).

[svgeesus]

@dbaron, thanks for cc'ing me into this issue.

I think one thing we should perhaps look at is whether other parts of the Web platform (e.g., canvas) need things to prepare for colors that are outside of the sRGB gamut.

Yes, they do. This was discussed previously in TAG review although that issue stalled on waiting for responses from the original poster.

The Color on the Web Community Group is working on a draft report for HDR on the Web; although the focus is on HDR, Wide Color Gamut is also being examined and all parts of the web platform will need some changes to get outside the sRGB gamut while maintaining both backawrds compatibility and future extensibility.

@felipeerias wrote:

My main question at the moment is whether it would make sense to implement Lab-like selection and interpolation using the browser's existing color infrastructure (i.e. with clamped outputs), or whether it would be necessary to spec and implement general support for wider gamuts first.

Good question From discussions with @tabatkins I understood that Chromium infrastructure had an internal datatype, a 16bit scRGB which (because it can go below 0.0 and above 1.0) could in principle be used to express a wider gamut than sRGB?