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Re: Color contrast of text on variable color background

From: Phill Jenkins <pjenkins@us.ibm.com>
Date: Tue, 4 Sep 2012 21:18:17 -0500
To: w3c-wai-ig@w3.org
Message-ID: <OF3E0CD898.AF1C7529-ON86257A70.0009D3A7-86257A70.000CAA8E@us.ibm.com>
I believe this is another example of a requirement for the browser with or 
without the Assistive Technology (AT), not the web content 
author/developer responsibility alone. 

Some users want and need the "transparency" background common on many web 
sits today - not the assumed "opaque" background that was in place back in 
2008 when WCAG 2.0 was finalized.  Same UI technique (and issue or 
benefit) is used in television programming guides where you still see the 
TV show in the background via slightly transparent text on top.  Some love 
it, some hate it, others find it critically beneficial while others see it 
as an absolute barrier.  One size (contrast ratio) does not fit all here. 
The browser should have a user setting to make the transparency more (or 
less) transparent per the user's preference and needs, not some "majority 
rules" ratio.  Visual usability improving cognition is conflicting with 
vision impairment needs. 

Of course this would NOT apply to authored content where the text is 
placed over an image at authoring time and saved as an image - this one 
size fits all approach is still under control of the author, not the 
browser.  Again, I'm referring to the translucent so called "pop-ups" in 
the newer web 2.0 stile websites that use JavaScript and CSS.  Measuring 
contrast ratios here does not make sense to me when the browser is (or 
should be) controlling the rendering, not the author/developer.

Phill Jenkins, 
IBM Research - Human Ability & Accessibility Center




From:   Gregg Vanderheiden <gv@trace.wisc.edu>
To:     Lea Verou <lea@w3.org>, 
Cc:     w3c-wai-ig@w3.org
Date:   09/04/2012 04:54 PM
Subject:        Re: Color contrast of text on variable color background

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 Wednesday, 5 September 2012 02:18:50 UTC

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