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Re: Color contrast between semi-transparent colors

From: Lea Verou <lea@w3.org>
Date: Fri, 7 Sep 2012 17:07:55 +0300
Cc: w3c-wai-ig@w3.org
Message-Id: <648F2685-EDDE-4C35-BE21-643559485495@w3.org>
To: David Woolley <forums@david-woolley.me.uk>
Hi David,

Thank you for your input. 

I believe this should be addressed as a separate guideline. 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
W3C developer relations
http://w3.org/people/all#leahttp://lea.verou.me ✿ @leaverou

On Sep 6, 2012, at 23:06, David Woolley wrote:

> Lea Verou wrote:
>> On Sep 6, 2012, at 14:48, Marc Haunschild wrote:
>>> Hi Lea,
>>> now I got it, I guess - its about how to calculate it, to change the semitransparent layer automatically to produce a satisfying contrast ratio?
>> Kind of. Think of it this way:
>> - We know the text color
>> - We know the (semi-transparent) background color
>> - We DON’T know what's going to be under that. 
> I think you are missing a factor here.  The standard colour contrast rules assume a background with no high spatial frequencies.  If you overlay on an image, without using alpha 1 for both the text and a halo, any high spatial frequencies in the image will make it more difficult to read the text, so you need a higher minimum contrast.
> -- 
> David Woolley
> Emails are not formal business letters, whatever businesses may want.
> RFC1855 says there should be an address here, but, in a world of spam,
> that is no longer good advice, as archive address hiding may not work.
Received on Friday, 7 September 2012 14:08:04 UTC

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