- From: Chris Lilley via GitHub <sysbot+gh@w3.org>
- Date: Wed, 13 Jul 2022 13:14:20 +0000
- To: public-css-archive@w3.org
> The contrast algorithms we've seen, whether WCAG 2.1 or APCA, take two Ys as input Not exactly. WCAG 2.1 takes two luminances (CIE Y) as input. APCA uses a non-standard transfer function of linearizing sRGB and thus the `Ys` term is not the same as CIE Y, and cannot be computed from it. It requires the individual non-linear color component values as starting values. I [reported that here](https://github.com/w3c/silver/issues/643) and the conclusion was that this is intentional, nt an approximation error. > Y and L* are the same physical quantity. No. > They are pure functions of each other. They can be converted to and from each other with no other inputs. Yes. > Thus, given a L* and contrast measure, we can find the L* that contrasts Yes. We convert L* to Y, compute the other Y, and then can if we want convert that back to L* > Given that, the L* and the L* needed for contrast, we can calculate delta L* required for contrast For _that color_, yes, _if_ we want to replicate the WCAG 2.1 algorithm. Doing so seems pointless because the problem with WCAG 2.1 is that is uses a non perceptually-uniform measure (CIE Luminance) instead of a perceptually uniform measure (CIE Lightness). -- GitHub Notification of comment by svgeesus Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/7357#issuecomment-1183209237 using your GitHub account -- Sent via github-notify-ml as configured in https://github.com/w3c/github-notify-ml-config
Received on Wednesday, 13 July 2022 13:14:22 UTC