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Re: [css3-images] cross-fade()

From: fantasai <fantasai.lists@inkedblade.net>
Date: Thu, 05 May 2011 17:06:35 -0700
Message-ID: <4DC33B8B.2070804@inkedblade.net>
To: www-style@w3.org, Robert O'Callahan <robert@ocallahan.org>
On 05/04/2011 02:43 PM, Rik Cabanier wrote:
>  David Baron wrote:
>> I think the current definition of cross-fade() in
>> http://dev.w3.org/csswg/css3-images/#cross-fade-function is
>> incorrect.  It defines cross-fade() in terms of the porter-duff over
>> operator, which is not symmetric.  This means that cross-fade(A, B,
>> 30%) is different from cross-fade(B, A, 70%).
>>
>> I *think*, though I'm not sure, that the right way to define
>> cross-fade is in terms of the plus operator described in section 4.5
>> of the original Porter-Duff paper:
>> http://keithp.com/~keithp/porterduff/
>
> I *think* you're right.
>
> It should read either:
>
>   Then, the start image has a global alpha applied to it equal to
>   (1-p), the end image has a global alpha applied to it equal to p,
>   and the end image is then composited over the start image with
>   the plus operation
>
> or
>
>   Then, the end image has a global alpha applied to it equal to p,
>   and the end image is then composited over the start image
>   with the source-over operation
>
> but I could be wrong...

Ok, I've updated the spec with s/source-over/plus/ per dbaron's instructions.
But as I'm not a graphics person /at all/, I would like someone to confirm
whether the resulting is correct. :)
   http://dev.w3.org/csswg/css3-images/#cross-fade-function

~fantasai
Received on Friday, 6 May 2011 00:07:07 GMT

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