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Re: Color contrast of text on variable color background

From: Gregg Vanderheiden <gv@trace.wisc.edu>
Date: Tue, 04 Sep 2012 16:51:24 -0500
To: Lea Verou <lea@w3.org>
Cc: w3c-wai-ig@w3.org
Message-id: <468694F7-CE0C-4A4C-8D46-910D714E6BEB@trace.wisc.edu>
Since it is not good to post text over a pattern -- or photo -- the best would be either to not do it or to halo the text with white/very light. 

Gregg Vanderheiden Ph.D.
Director Trace R&D Center
Professor Industrial & Systems Engineering
and Biomedical Engineering
University of Wisconsin-Madison

Technical Director - Cloud4all Project - http://Cloud4all.info
Co-Director, Raising the Floor - International
and the Global Public Inclusive Infrastructure Project
http://Raisingthefloor.org   ---   http://GPII.net

On Sep 4, 2012, at 4:06 PM, Lea Verou <lea@w3.org> wrote:

> Hi all,
> I was studying the color contrast section of WCAG 2.0 [1] and I observed that the algorithm presented assumes opaque colors. In modern web design, authors often want to place text over a variable color background, such as a photograph, a gradient or even shadows of the text itself. I was wondering how to calculate the contrast ratio for such use cases. 
> One possible approach would be to take the minimum contrast that occurs. However, this could often be unnecessarily restrictive, as it might only occur in a small number of pixels that don’t hinder readability. Especially in the case of photographs, where practically every pixel has a different color.
> Another possible approach would be to take the average color and calculate the contrast ratio for that. However, this might yield many false positives. A simple example would be black text on a black & white checkerboard. Although the average background color is gray, which has an acceptable 5.3:1 ratio, the text would still be unreadable.
> It appears that there are multiple factors affecting the readability of text on such cases, so I'm not sure what kind of algorithm could be followed. Thoughts?
> [1]: http://www.w3.org/TR/WCAG/#visual-audio-contrast-contrast
> Lea Verou
> W3C developer relations
> http://w3.org/people/all#leahttp://lea.verou.me ✿ @leaverou

Received on Tuesday, 4 September 2012 21:51:56 UTC

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