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Re: Color contrast between semi-transparent colors

From: David Woolley <forums@david-woolley.me.uk>
Date: Thu, 06 Sep 2012 21:06:38 +0100
Message-ID: <5049024E.5020600@david-woolley.me.uk>
To: w3c-wai-ig@w3.org
Lea Verou wrote:
> On Sep 6, 2012, at 14:48, Marc Haunschild wrote:
> 
>> Hi Lea,
>>
>> now I got it, I guess - its about how to calculate it, to change the semitransparent layer automatically to produce a satisfying contrast ratio?
> Kind of. Think of it this way:
> - We know the text color
> - We know the (semi-transparent) background color
> - We DONíT know what's going to be under that. 
> 
I think you are missing a factor here.  The standard colour contrast 
rules assume a background with no high spatial frequencies.  If you 
overlay on an image, without using alpha 1 for both the text and a halo, 
any high spatial frequencies in the image will make it more difficult to 
read the text, so you need a higher minimum contrast.
> 


-- 
David Woolley
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Received on Thursday, 6 September 2012 20:07:10 UTC

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