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Re: Color contrast of text on variable color background

From: Jim Allan <jimallan@tsbvi.edu>
Date: Wed, 5 Sep 2012 11:02:54 -0500
Message-ID: <CA+=z1WmW5-h9JFc3PjUsxNRgGbFdFxt47HerNKpCvO8TSBuZ0Q@mail.gmail.com>
To: Phill Jenkins <pjenkins@us.ibm.com>
Cc: w3c-wai-ig@w3.org, WAI-ua <w3c-wai-ua@w3.org>
I see this as a possible success criteria for User Agent Accessibility
Guidelines - next version.
The question is can/will browsers implement a feature to control the
transparency of gif, jpg, png (are the ones that come to mind). While
at the same time they have no control over flash or shockwave images.
I have no idea if browsers can control the images that are background
coming from css, or images called from javascript, or something
created in canvas, or some new way we haven't thought of yet..
This gets complicated quickly for the browser. It is also confusing
for the user, when the transparency works sometimes depending on image
type, and how the author chose to get the image into the browser and
how the transparency was set in graphics tool, and etc.. The user
doesn't care how this transparent image appeared, or who did it in
what tool, or pick your problem...they just want to control the

If you have more information or resources (people or documents) that
would help the User Agent Working Group craft a success criteria, they
would be much appreciated and used.

Jim Allan, co-Chair UAWG

On Tue, Sep 4, 2012 at 9:18 PM, Phill Jenkins <pjenkins@us.ibm.com> wrote:
> 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.
> ____________________________________________
> Regards,
> Phill Jenkins,
> IBM Research - Human Ability & Accessibility Center
> http://www.ibm.com/able
> http://www.facebook.com/IBMAccessibility
> http://twitter.com/IBMAccess
> http://www.linkedin.com/in/philljenkins
> 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
> --------------------------------------------------------
> 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

Jim Allan, Accessibility Coordinator & Webmaster
Texas School for the Blind and Visually Impaired
1100 W. 45th St., Austin, Texas 78756
voice 512.206.9315    fax: 512.206.9264  http://www.tsbvi.edu/
"We shape our tools and thereafter our tools shape us." McLuhan, 1964
Received on Wednesday, 5 September 2012 16:03:27 UTC

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