Re: [csswg-drafts] [css-color-6] color-contrast() should allow specifying multiple contrast algorithms that need to be satisfied (#7357)

TL;DR: The source for the delta L measure _is_ contrast ratio. They are equivalent. And APCA can be measured that way too

The conversations around contrast tend to assume too much and go too far too quick, something very simple and tremendously helpful has gotten missed: 

#1) all* the contrast measures are based on relative luminance, Y in XYZ (all? pedantically, contrast ratio/WCAG through 2.x, APCA as documented for WCAG 3 draft)
#2) Y and L* are pure functions of eachother, they are the same physical quantity on different scales
#3) using #2, the equations for the measures, and given a desired contrast ratio/APCA measure, we can find the darker/lighter L* that contrasts with a given L* 
#4) given #3, we can calculate the delta between any L* and its darker/lighter contrasting L*
#5) given #4, we can find the maximum delta needed, and thus, an equivalent delta L* for a contrast ratio

The intellectual hole there is the rule of thumb we give designers is contrast ratio 3.0 is a L* delta of 40...but the actual maximum is 38.3, and its as low as ~31. This is the effect Andrew mentions in message that starts with "I've been watching James' developments with interest."

I'd prefer to use J or something more advanced, but CAM16 J is dependent on more than luminance, and I haven't see anything _remotely_ convincing that says we can count on non-luminance contrast (I know Andrew is thinking about / working towards a measure that includes hue/chroma in contrast measure, but in a purely a11y context, I'm not sure it's relevant. you'd need to know if a user had a CVD, what it was, and how severe it was to make it relevant for a11y. and that's even before we start talking about how perceptually weak chroma is compared to luminance)




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Received on Monday, 11 July 2022 16:27:33 UTC