Re: [csswg-drafts] [css-color] color-contrast() should allow specifying multiple contrast algorithms that need to be satisfied (#7357)

> .... _HCT which uses the L axis from CIE Lab (Tone)_ ....
> _Note that the minimum contrasts for small and large text (50 and 40) are different from the thresholds for WCAG 2.1 and for APCA; thresholds are algorithm-specific._

I've been watching James' developments with interest. I'm a little surprised at the use of L* instead of J.

∆L* has an interesting attribute in that is sort of lines up with WCAG 2 — the implication is, that contrast using a plain L* suffers the same issues as WCAG2.

Digging up some of the early comparison tables, here's one with ∆L*:

<img width="440" alt="Simple WCAG Weber ModWeb Lstar Mich SAPC98G" src="https://user-images.githubusercontent.com/42009457/174482102-e83d35d0-a251-4e90-9e51-46032e2bcfed.png">

An L* simple difference, when the lightest color is white, lines up with a little less than 40 = WCAG 3:!, 50 = 4.5:1, and 62 = 7:1 ... and still as colors get darker, contrast is over reported. I've also tried this with multiple offsets, but that still is not polarity sensitive. 


### But wait, isn't L* perceptually uniform?

L* is based on Munsell value, so it _is_ perceptually based.... _on diffuse reflected light using large color patches, when the observer is in the defined ambient/adaptation conditions._

Viewing **_text_** on a self-illuminated monitor is a different matter, and a different perception. "Middle contrast" for text and other high spatial frequency stimuli isn't at 18%, it's up between 34 and 42 (ish).

As such, ∆L* without some offsets and massaging is not much different than WCAG 2 as this chart shows:

<img width="509" alt="WCGACLstarSAPC98Gat60" src="https://user-images.githubusercontent.com/42009457/174482738-b9348aca-d83f-46d1-a9e7-75211063dfa7.png">



-- 
GitHub Notification of comment by Myndex
Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/7357#issuecomment-1159723228 using your GitHub account


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

Received on Sunday, 19 June 2022 13:12:44 UTC