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Colour contrasts (was: WCAG2 Conformance Questions)

From: David Woolley <forums@david-woolley.me.uk>
Date: Sat, 16 Aug 2008 13:15:29 +0100
Message-ID: <48A6C4E1.6020002@david-woolley.me.uk>
To: "w3c-wai-ig@w3.org" <w3c-wai-ig@w3.org>

Peter Thiessen wrote:

> Also, for guideline "1.4.3 Contrast (Minimum)", to help determine the
> color ratio, a tool/calculator where a user can plug in RGB values would
> simplify the process/understanding; rather than only giving the formula
> <http://www.w3.org/WAI/GL/WCAG20/WD-UNDERSTANDING-WCAG20-20080505/visual-audio-contrast-contrast.html#contrast-ratiodef>
> The time commitment to understand contrast ratios through the guidelines
> is rather high. A few example calculations would help, or at least help
> me <grin />. Would you or anyone have any example contrast calculations?

The difficult part of these formulae is undoing the sRGB colour space to 
get a linear relation between light energy and numerical value. 
Although my impression is that too few web designers understand gamma 
correction, an understanding of it is essential for all but the simplest 
of uses of colours on web pages (e.g. there are many pages with images 
that are not sRGB and proper handling of anti-aliasing of image 
replacements of text requires that you understand it).

However, I wonder if doing the final ratio in linear energy is actually 
correct, as I understand that the eye is less able to distinguish 
intensity differences at high intensities than at low intensities. 
This, like CRT responses, and therefore sRGB, can be approximated with a 
gamma value, and I understand that the Mac actually uses that gamma 

I would have thought, for a pure component colour, one ought to be 
comparing values corrected to that effective gamma (or some more 
accurate model of the eye).  I suspect that there may be complexities in 
combining colours - if nothing else phosphor colours don't uniquely 
select cones for just one colour.  Colour combination effects may well 
be too fine detail for a guideline, but the eye's gamma curve probably 

The other problem is that it assumes that the viewer actually has all 
three sorts of cone working properly and that they are within a 
reasonable tolerance of having the correct sensitivity ratio.

Does it allow for yellowing of the eye with age?

Overall, I also had the concern that this is all about being barely 
legal, when one should be encouraging the use of colour contrasts that 
are so obvious that there is no doubt about them.

David Woolley
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Received on Saturday, 16 August 2008 12:14:43 UTC

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