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[TECH] Colour Difference Algorithm

From: Chris Ridpath <chris.ridpath@utoronto.ca>
Date: Tue, 26 Aug 2003 09:45:22 -0400
Message-ID: <014201c36bd8$4bad9180$b040968e@WILDDOG>
To: "WAI WCAG List" <w3c-wai-gl@w3.org>

The current WCAG2 draft [1] checkpoint 1.6 is still missing an algorithm for
determining when "foreground content is easily differentiable from
background" regarding the use of colour.

The AERT document [2], technique 2.2.1 for the WCAG1 proposed an algorithm
that worked reasonably well but was not perfect. Some colour combinations
were passed, though many people would find them unacceptable, and other
colours were failed though many people would find them acceptable.

Note: I'm suggesting that the general term "colour difference" includes, but
is not limited to, a difference in brightness and a difference in colour
(separation along the colour spectrum). Other factors such as saturation may
also come into play.

Listed below are several sites dealing with colour on the web:

I thought there was a WAI site where all these sort of links were collected
but can't find it. Anyone know of a current collection of colour related

I'm still digging for other colour testing algorithms and will post more as
I find them.

My current feeling is that we're not going to find a perfect algorithm.
Maybe the best we can hope for is a test that will give a qualified result.
Perhaps something like "75% of people surveyed found the colour combination
to be difficult to read".


Can we disregard the colour guidelines for text and background as most user
agents allow the user to set their own colours?

Do the colour guidelines only apply to images and/or visual technologies
such as Java applets, Flash etc.?

Do these colour guidelines only apply to text?


[1] http://www.w3.org/WAI/GL/WCAG20/WD-WCAG20-20030616.html
[2] http://www.w3.org/TR/AERT#color

Received on Tuesday, 26 August 2003 09:45:33 UTC

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