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Addendum: Background Image readability

From: Alan Chuter <achuter@technosite.es>
Date: Thu, 05 Mar 2009 10:12:17 +0100
Message-ID: <49AF9771.8080707@technosite.es>
To: Mobile Web Best Practices Working Group WG <public-bpwg@w3.org>
Scheppe, Kai-Dietrich wrote:
> +3.4 Background Image readability
> The Example should perhaps be an image (remembering
> STYLE_SHEETS_SUPPORT). Without CSS it is black on white.
> The WCAG 2.0 Techniques [2] give a list of tools to check this,
> including one developed especially for WCAG 2.0.  I think that the
> Ishihara Test for Color Blindness isn't very useful as it consists of
> very specific examples. If people aren't using exactly those colours it
> won't help them.
> WCAG success criterion 1.4.3 Contrast (Minimum) [3] gives a definition,
> and exceptions to this which might be worth mentioning.
> -- I disagree on using a picture as this is a test for contrast.  White
> on black is a good way to demonstrate this.

I mean that if style sheets are turned off (which I do quite often) the 
text appears as normal. A picture is more or less fixed.

>    I am not aware of the Ishihara Test for Color Blindness being limited
> to colors, but rather to contrast levels of two adjascent colors.
The Ishihara Test images are not really useful unless the viewer is 
colourblind; to others they appear as normal. Such people will be aware 
of the problem anyway so they don't need an example. What's needed 
perhaps is a screenshot in which contrast is poor for people who are 
*not* colourblind (like all the themes supplied with my phone).

I found a couple of examples [1]. It wouldn't be difficult to create 
some to include in the document. There are free images out there (for 
example [2]) although I'm not sure how one can be sure they are truly 
"free" to use.


[2] http://www.fg-a.com/backgrounds.htm

Alan Chuter
Departamento de Usabilidad y Accesibilidad
Technosite - Grupo Fundosa
FundaciĆ³n ONCE
Tfno.: 91 121 03 30
Fax: 91 375 70 51
Received on Thursday, 5 March 2009 09:14:39 UTC

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