[csswg-drafts] Powerless a,b and chroma for Lightness >= 100% (#7924)

svgeesus has just created a new issue for https://github.com/w3c/csswg-drafts:

== Powerless a,b and chroma for Lightness >= 100% ==
In a somewhat handwavy piece of spec text that was intended to provide an escape hatch for HDR, we have, for [CIE Lab](http://w3c.github.io/csswg-drafts/css-color-4/#specifying-lab-lch)

> If the lightness of a Lab color is 0%, both the a and b components are [powerless](http://w3c.github.io/csswg-drafts/css-color-4/#powerless-color-component).

and for [Oklab](http://w3c.github.io/csswg-drafts/css-color-4/#specifying-oklab-oklch)

> If the lightness of an Oklab color is 0%, both the a and b components are [powerless](http://w3c.github.io/csswg-drafts/css-color-4/#powerless-color-component).

In the midst of a [long discussion about powerless components, `none` and `NaN`](https://github.com/w3c/csswg-drafts/issues/6107), @tabatkins [wrote](https://github.com/w3c/csswg-drafts/issues/6107#issuecomment-834785792):

> Just black and white. Those colors are singularities for chroma (all chroma values return the same color), while the rest of the neutral axis does have the color change as chroma changes.

I'm wondering it it would be better to also say (as @tabatkins seems to be suggesting, in that older thread) that 

a) Lightness of 100% clamps to 100% 
b) If the lightness of a Lab color is 100%, both the a and b components are [powerless](http://w3c.github.io/csswg-drafts/css-color-4/#powerless-color-component).

Oklab was defined by the author to be specifically SDR; and although Lab has been experimentally extended by Fairchild to L=400, he gave that a different name (`Lab-HDR`) and it didn't really catch on. In practice, HDR colors are likely to be specified in `rec-2100-hlg` or `rec-2100-pq`.


Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/7924 using your GitHub account


-- 
Sent via github-notify-ml as configured in https://github.com/w3c/github-notify-ml-config

Received on Wednesday, 19 October 2022 21:18:02 UTC