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Re: Colour contrasts (was: WCAG2 Conformance Questions)

From: Jon Gunderson <jongund@illinois.edu>
Date: Fri, 22 Aug 2008 09:21:19 -0500 (CDT)
To: Gregg Vanderheiden <gv@trace.wisc.edu>, David Woolley <forums@david-woolley.me.uk>
Cc: "'wai-ig list'" <w3c-wai-ig@w3.org>
Message-Id: <20080822092119.BJC38982@expms1.cites.uiuc.edu>
The Firefox Accessibility Extension has a color contrast analysis featue:



---- Original message ----
>Date: Thu, 21 Aug 2008 17:20:51 -0500
>From: Gregg Vanderheiden <gv@trace.wisc.edu>  
>Subject: Re: Colour contrasts (was: WCAG2 Conformance Questions)  
>To: David Woolley <forums@david-woolley.me.uk>
>Cc: "'wai-ig list'" <w3c-wai-ig@w3.org>
>   there are contrast tools available as a plug in tool
>   for Firefox.   They may also create one for your
>   platform if you approach them.
>   I would check with faulkner.steve@gmail.com
>   Gregg
>   -----------------------
>   Gregg Vanderheiden Ph.D.
>   Director Trace R&D Center
>   Professor Ind and Biomed Engr
>   University of Wisconsin-Madison
>   On Aug 21, 2008, at 5:11 PM, David Woolley wrote:
>     Peter Thiessen wrote:
>       ORCA.  But as for contrast testing, an automated
>       tool, or any sort of tool would be helpful
>       start. Or hell, even a few examples wouldn’t
>       hurt.
>     What the formulae are doing is converting the
>     image to greyscale and
>     then requiring a certain ratio between the
>     absolute intensities of
>     adjacent shades of grey.  The numerator and
>     demoninator are offset
>     slightly so that you need a significant difference
>     at low intensities.
>     Unfortunately image manipulation tools don't seem
>     to provide an sRGB to
>     linear conversion function, so you will need to
>     perform a gamma
>     correction (mid level slider, or gamma function),
>     then simply convert
>     the result to grayscale.  (sRGB isn't a gamma
>     curve at low intensities.)
>     The one thing that this doesn't automate is
>     looking the the contrasts in
>     the resulting grayscale image.  To this
>     subjectively, you should first
>     convert back to the colour profile of your
>     monitor.
>     The complexity in doing the greyscale conversion
>     is in that it does it
>     reasonably accurately, whereas your image
>     manipulation tool probably
>     goes straight into the weighted sum.
>     --
>     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.
Jon Gunderson, Ph.D.
Coordinator Information Technology Accessibility
Disability Resources and Educational Services

Rehabilitation Education Center
Room 86
1207 S. Oak Street
Champaign, Illinois 61821

Voice: (217) 244-5870

WWW: http://www.cita.uiuc.edu/

WWW: https://netfiles.uiuc.edu/jongund/www/

Received on Friday, 22 August 2008 14:22:14 UTC

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