Re: [w3ctag/design-reviews] CSS Color: lab(), lch() (#488)

@dbaron wrote:

> discussion (in last week's breakout and plenary and between them) on whether the L component of lab() and lch() would be useful for color contrast calculations: our conclusion was that it's not, but only because it differs by a gamma correction from a value that would be useful (see the conversion from y to L in XYZ_to_Lab(), particularly the Math.cbrt). (Although it differs from the value currently used by both that gamma correction and by the difference between the D50 and D65 whitepoints, but the whitepoint difference seems much less significant.) (I also noticed w3c/wcag21#815 on the way.)

I agree with this conclusion. The L in Lab is certainly useful for many things (unlike the L in HSL, which is almost meaningless) but for color contrast calculations one needs a linear-light-intensity color space. The Y in XYZ (which is a step along the way to calculating Lab) is luminance, and this is used in the current WCAG contrast calculation. Which means that, when WCAG is ready to add this, the color contrast between, say, a foreground color in `display-p3` and a background color in `sRGB` can easily be specified.

There is also a [very interesting and detailed study on color contrast](https://github.com/w3c/wcag/issues/695) which may be used in WCAG 3, replacing the simpler formula in WCAG 2; but again, starting from the luminance of the foreground and background colors.


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