Re: Color contrast between semi-transparent colors

Oops, looks like I replied to the wrong mail! That reply was for David Wooley!

Lea Verou
W3C developer relations
http://w3.org/people/all#lea ✿ http://lea.verou.me ✿ @leaverou






On Sep 7, 2012, at 19:00, Phill Jenkins wrote:

> Marc said: 
> "You have a background and a foreground and you use a semi transparent 
> layer between them. So just make sure (if text is dark) that the darkest 
> possible background (black) will be still bright enough to produce a 
> satisfying contrast ratio.
> 
> Or use a semi transparent layer which darkens even a white background so 
> that you can use a white font. If the font should be not white, you must 
> darken the background even more...
> 
> Marc" 
> 
> and Lea said: 
> "I've started another thread about variable color backgrounds [1]. We cannot assume that just because the background is semi-transparent, it will be overlaid over a variable color background. For example, many designers use semi-transparent white or black on top of solid colors, to create lighter and darker variants while still being able to change the base color with one edit.
> 
> [1]: http://lists.w3.org/Archives/Public/w3c-wai-ig/2012JulSep/0329.html
> 
> Lea Verou" 
> 
> 
> The question I think that needs to be asked and answered  is who is the "you"?  in other words, what is or should be the role of the user agent vs the responsibility of the developer? 
>   
> I believe the user agent should be able to change the middle "semi-transparent layer" to imporve contrast accessibility.  If user agents can already "turn off" images, why not also be able to place a 'semi-transparent layer' if one doesn't exist, or re-place the one with a darker or lighter one.  Again my point being that desired solution belongs with the user agent and the responsibility of the developer is to enable it.   
> 
> Also, is there a grey color that at some point can be contrasted with either white or black background and achieve the same contrast level either way?  And did WCAG 2.0 minimum contrast level of 3.5 to 1 then in effect prohibit certain grey fonts becasue they can't achieve the minimum level?
> ____________________________________________
> Regards,
> Phill Jenkins, 
> IBM Research - Human Ability & Accessibility Center
> http://www.ibm.com/able
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> http://www.linkedin.com/in/philljenkins