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

Hi David @dbaron, 

I just now noticed this:

> * _discussion (in [last week's breakout].... 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 ...._

I wanted to address: the new contrast method in Silver/WCAG3 is [**APCA** (Advanced Perceptual Contrast Algorithm).](https://github.com/Myndex/SAPC-APCA) it does use perceptually-based power curves, though with different exponents depending on the context and polarity.

The problem with using just ∆L*  (Lab difference) is that ***color difference is not contrast.***

Like color, "contrast" is not real, that is, it is a perception, and not absolute relative to perception. Contrast is extremely context dependent, and in some ways our perception of contrast is more non-linear than our perception of a color or luminance. Our perception of visual contrast is much more than a color difference: for instance spatial frequency is critical, and has a stronger effect on contrast perception at high spatial frequencies than luminance (for web, this translates to very thin small fonts).

Here is my favorite demonstration of context. Both yellow dots, and the squares they are on, **have the same absolute color coming out of your monitor:**

![checkerYellowDotVersion](https://user-images.githubusercontent.com/42009457/100090344-e2d21680-2e07-11eb-8713-a9c1005fd874.png)

And the yellow of course is not yellow, it is separate red and green that stimulates the L and M cones in a similar way as spectral yellow does (which is between red and green). The red and green do not mix in the air like paint: they "mix" in your neurological system, starting with the ganglion cells behind the retina ( _'opponent color'_), and then through the multistage filtering and processing of the visual cortex.

What you see all day is not exactly reality: it is your brain's filtered and deconstructed perception, which can be quite different from absolute values. Context needs to be accounted for or estimated to predict a perception as complicated as contrast.

APCA is focused on luminance contrast for fluent readability: getting whole words and letter-pairs into the Visual Word Form Area (VWFA) — this needs ample luminance contrast. But other stimuli, for spot reading, object recognition, state change.... are processed via different routes and in some different areas of the brain, and have different requirements.

Andy
_A Guy Too Obsessed with Color_

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