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Re: Colour contrasts (was: WCAG2 Conformance Questions)

From: Gregg Vanderheiden <gv@trace.wisc.edu>
Date: Thu, 21 Aug 2008 17:20:51 -0500
Cc: "'wai-ig list'" <w3c-wai-ig@w3.org>
Message-Id: <48F1C750-1C0F-4582-8D19-97DBDA343FA3@trace.wisc.edu>
To: David Woolley <forums@david-woolley.me.uk>
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 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.
Received on Thursday, 21 August 2008 22:21:47 UTC

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