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Re: [CSS4 color][CSS4 transition] color pre-multiplied vs non pre-multiplied was (Re: [CSSWG] Minutes TPAC Tue 2012-10-30 AM I: Abstract Directions, Transforms, Transitions)

From: Alan Gresley <alan@css-class.com>
Date: Tue, 05 Feb 2013 14:53:51 +1100
Message-ID: <5110824F.2050400@css-class.com>
To: Brad Kemper <brad.kemper@gmail.com>
CC: Simon Fraser <smfr@me.com>, Rik Cabanier <cabanier@gmail.com>, fantasai <fantasai.lists@inkedblade.net>, "www-style@w3.org" <www-style@w3.org>
On 3/02/2013 4:01 AM, Alan Gresley wrote:
> On 2/02/2013 1:50 AM, Brad Kemper wrote:
>> On Jan 31, 2013, at 11:18 PM, Simon Fraser <smfr@me.com> wrote:
>>
>>> We rely on Core Graphics to render gradients on Mac, and that can't
>>> do pre-multiplied. That's the main reason why gradients don't match
>>> transitions.
>>
>> If I understand correctly, this means that if an author wants a nice
>> clean-looking 3-stop gradient that goes from yellow to transparent to
>> blue, he will have to use two same-position transparent color stops,
>> so that it is actually yellow to transparent-yellow, and then
>> transparent-blue to blue. Couldn't WebKit just perform this extra
>> conversion step for us before passing it on to Core Graphics?
>
> It still won't work. A pre-multiplied gradient of yellow to transparent
> to blue is not the same as a non pre-multiplied gradient of yellow to
> transparent-yellow, and then transparent-blue to blue.
>
> That is why I have suggested pre-transparent (or pm-transparent) as a
> way to do this when support for pre-multiplied gradients is universal.

Brad. I'm just wondering what type of background you plan to use under 
any gradient with alpha transparency?

A pre-multiplied gradient of yellow to transparent to blue is identical 
to a non pre-multiplied gradient of yellow to transparent-white to blue 
when composite on a white background. There is _no need_ for an extra 
color stop.


-- 
Alan Gresley
http://css-3d.org/
http://css-class.com/
Received on Tuesday, 5 February 2013 03:54:21 GMT

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