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Re: [CSS3-images] premultiplied alpha (was rotate(<angle>[, <translation-value>, <translation-value>]))

From: Rik Cabanier <cabanier@gmail.com>
Date: Tue, 21 Feb 2012 11:12:05 -0800
Message-ID: <CAGN7qDBrZgaHAuJKe4AxFEZpnRX_NyYYBxyoRrHT8qzm4Qk29w@mail.gmail.com>
To: "L. David Baron" <dbaron@dbaron.org>
Cc: "Tab Atkins Jr." <jackalmage@gmail.com>, www-style list <www-style@w3.org>
On Tue, Feb 21, 2012 at 10:53 AM, L. David Baron <dbaron@dbaron.org> wrote:

> On Tuesday 2012-02-21 10:40 -0800, Rik Cabanier wrote:
> > specifying premultiplied alpha to fix the issue seems odd.
> > It seems that the issue is really how to you handle the  'transparent'
> > keyword. Because transparent is specified as (0,0,0,0) you transition the
> > color values  as well.
> > So, non-premultiplied red->transparent, the midpoint of your gradient
> > becomes (0, .5, 0, .5) which after compositing with white becomes (.5,
> .75,
> > .5) which is too dark.
> > In premultiplied space, you end up with (.5, 1, .5) which is what you
> want.
> >
> > However by forcing premultiplied space, you can no longer go from
> 'yellow'
> > to 'transparent red' because the 'red' value is removed when you multiply
> > by 0.
> >
> > Was this discussed and deemed OK? I tried to find it in the archives but
> > couldn't find it.
> > Sorry if this was already decided.
>
> I believe it has been discussed a number of times.
>
> There are effects that one can achieve with either premultiplied
> alpha or nonpremultiplied alpha that cannot be achieved with the
> other.  Choosing premultiplied means optimizing for the normal case
> rather than the odd feature of transition to transparent + color.
>
>
Well, you *could* special case 'transparent' to say use a transparent
version of the previous or next color stop which is really what the author
means.
I don't think going from red to transparent yellow is an odd case at all.
It gives you a similar effect to InDesign's gradient feather.

Rik
Received on Tuesday, 21 February 2012 19:12:39 GMT

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