APCA (Accessible Perceptual Contrast Algorithm) is a new method for predicting contrast for use in emerging web standards (WCAG 3) for determining readability contrast. APCA is derived form the SAPC (S-LUV Advanced Predictive Color) which is an accessibility-oriented color appearance model designed for self-illuminated displays.
As I read through this thread, particularly one titled "shootout", there's two aspects I'm not getting that I hope someone can help with. These come from various experiences from usability testing, where I've seen quite a variety in which colour combinations cause issues for various people. I get uncomfortable when the visibility of a colour combination to people without an impairment is used as evidence for it's visibility from an accessibility point of view.
The two main questions I have are:
When talking about percentages of colour combinations that aren't right with a particular model, is that out of the whole colour space of combinations, or some sub-set?
Is it in comparison to a theoretical model, or the variety of human sight?
I think I need to show my current mental modal about this. I am assuming that if you looked at a colour space of hues, and mapped which people (with no visual impairment) could read against white text, it might look something like this:
Where anything under the line didn't have enough contrast against a white background. That line would probably wave up and down a bit with different hues, but just take that as a baseline for regular vision.
Then, if someone had low-vision with less acuity, presumably that line needs to move up?
Then, if we accounted for various colour deficiencies (e.g. protanopia) you'd need to adjust that line in various ways depending on the hue used.
Then, to produce an overall model that accounted for both you'd want to be failing anything under either line:
I'm also assuming that the WCAG 2 formula is similar to the straight line, and the formula behind APCA is more bendy (apologies for the technical language ;-) ) to better match (non-impaired?) perception.
However, when talking about percentages, what is it if you include all the colours that would fail both? Because (from my mental model above) there are a ton of combinations that are terrible and should fail both, so a ~50% incorrect rate just doesn't sound right.
As I read through this thread, particularly one titled "shootout", there's two aspects I'm not getting that I hope someone can help with. These come from various experiences from usability testing, where I've seen quite a variety in which colour combinations cause issues for various people. I get uncomfortable when the visibility of a colour combination to people without an impairment is used as evidence for it's visibility from an accessibility point of view.
The two main questions I have are:
I think I need to show my current mental modal about this. I am assuming that if you looked at a colour space of hues, and mapped which people (with no visual impairment) could read against white text, it might look something like this:
Where anything under the line didn't have enough contrast against a white background. That line would probably wave up and down a bit with different hues, but just take that as a baseline for regular vision.
Then, if someone had low-vision with less acuity, presumably that line needs to move up?
Then, if we accounted for various colour deficiencies (e.g. protanopia) you'd need to adjust that line in various ways depending on the hue used.
Then, to produce an overall model that accounted for both you'd want to be failing anything under either line:
I'm also assuming that the WCAG 2 formula is similar to the straight line, and the formula behind APCA is more bendy (apologies for the technical language ;-) ) to better match (non-impaired?) perception.
However, when talking about percentages, what is it if you include all the colours that would fail both? Because (from my mental model above) there are a ton of combinations that are terrible and should fail both, so a ~50% incorrect rate just doesn't sound right.
Originally posted by @alastc in https://github.com/w3c/low-vision-a11y-tf/discussions/131#discussioncomment-1946983