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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: Rik Cabanier <cabanier@gmail.com>
Date: Tue, 5 Feb 2013 15:08:09 +1100
Message-ID: <CAGN7qDBvEeNneDGgtjSvKh7jVmP5AtF4FiCGs6TDAZM=gf33yQ@mail.gmail.com>
To: Alan Gresley <alan@css-class.com>
Cc: Brad Kemper <brad.kemper@gmail.com>, Simon Fraser <smfr@me.com>, fantasai <fantasai.lists@inkedblade.net>, "www-style@w3.org" <www-style@w3.org>
On Tue, Feb 5, 2013 at 2:53 PM, Alan Gresley <alan@css-class.com> wrote:

> 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.


No, it's different (see attached)
In order to emulate the bottom gradient, you need to create an extra color
stop under the hood.


mulvspremul.png
(image/png attachment: mulvspremul.png)

Received on Tuesday, 5 February 2013 04:08:36 GMT

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