Re: Color Contrast and Transparency | WCAG 2.1

The colour contrast analyser has a way to make a bigger than 1 pixel
sample....

Cheers,
David MacDonald



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On Fri, Jul 29, 2016 at 5:02 AM, Detlev Fischer <detlev.fischer@testkreis.de
> wrote:

> In testing, we often encounter text over photos which have some parts of
> text with sufficient contrast, others not so.
> The problem may be minor (just some small patches with insufficient
> contrast). If the background is not largely uniform / blurred but
> irregular, chosing the reference point for the colour picker gets
> difficult. In many cases, picking the worst bit of background (as David
> suggests) seems subjectively unfair if we are dealing with a small and
> isolated patch. Its main benefit is that it is rule-like ("always go for
> the point with the weakest contrast even if it is just one pixel wide").
>
> Theoretically one could create a mix of bg colours (use Gaussian blur) to
> get the 'average' bg colour but that would not account for the perceived
> difficulties for reading on vivid backgrounds where the averaged value
> would pass but the text is nevertheless hard to read.
>
> In sum: measuring contrast of text on photo backgrounds involves
> subjective judgment. Tools help, but whether you pass or fail an edge case
> around 4,5:1 will be down to your handling of the colour picker...
>
> --
> Detlev Fischer
> testkreis c/o feld.wald.wiese
> Thedestr. 2, 22767 Hamburg
>
> Mobil +49 (0)157 57 57 57 45
> Fax +49 (0)40 439 10 68-5
>
> http://www.testkreis.de
> Beratung, Tests und Schulungen für barrierefreie Websites
>
> Alastair Campbell schrieb am 29.07.2016 10:39:
>
> > JF: “the impact of Alpha Transparency of colors and the impact on color
> > contrast and accessibility?”
> >
> > I guess this is mostly a matter of how it is measured?
> >
> > So if designer/developer specifies a foreground and/or background colour
> with
> > some transparency, you shouldn’t use the CSS defined colour to test
> contrast
> > without accounting for the transparency. (I know transparency is
> generally
> > defined with the colour, but that doesn’t mean a testing tool accounts
> for
> > it.)
> > If the transparent colour of a box (the background for text) is on top
> of a
> > white background it will be lighter than on a black background, which
> needs to
> > be accounted for.
> >
> > However, if you use a tool with an eye-dropper style mechanism on the
> resulting
> > colours, that should be accurate. Unless I’m missing something?
> >
> > The contrast adjuster function in the CSS-color-4 spec looks cool (if
> it’s
> > implemented?), it references WCAG2 1.4.3 so should line up.
> >
> > Perhaps we need to make G18 more specific about testing the result
> rather than
> > the colour definition?
> >
> > And perhaps we could create a test page that would pass if transparency
> is not
> > accounted for, but should fail, and check that each of the tools listed
> in G18
> > accounts for the transparency? (Perhaps the browser based ones don’t?)
> >
> > Cheers,
> >
> > -Alastair
> >
> >
>
>
>
>