Powerless a,b and chroma for Lightness >= 100% #7924
Description
In a somewhat handwavy piece of spec text that was intended to provide an escape hatch for HDR, we have, for CIE Lab
If the lightness of a Lab color is 0%, both the a and b components are powerless.
and for Oklab
If the lightness of an Oklab color is 0%, both the a and b components are powerless.
In the midst of a long discussion about powerless components, none and NaN, @tabatkins wrote:
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.
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 (hdr-CIELAB) and it didn't really catch on. In practice, HDR colors are likely to be specified in rec-2100-hlg or rec-2100-pq.