Re: [css3-transitions] animation of color values should mention premultiplication

On Wednesday 2009-07-01 16:48 -0700, L. David Baron wrote:
> http://dev.w3.org/csswg/css3-transitions/#animation-of-property-types-
> currently says:
>   # color: interpolated via red, green, blue and alpha components
>   # (treating each as a number, see below). 
> 
> This bullet point should probably mention that the red, green, and
> blue values should NOT be premultiplied with the alpha.  (I think
> that's the correct behavior, anyway.)

Actually, I'm not so sure this is correct (although WebKit's
implementation does do non-premultiplied).

I wrote a testcase:
  http://dbaron.org/css/test/2009/transitions-alpha
(also attached) and I think it looks better with premultiplied
colors, although authors have less control.  (It's the first case
that I'm particularly concerned about.)

If I do premultiplication before interpolation, all of the first
three tests look the same.  If I don't, only the second one looks
"right"; the first and third show colors other than red (black and
green respectively) in the intermediate stages.  The fourth test is
the same either way; the fifth crosses through yellow hue more
quickly if I do premultiplication.

There is a control tradeoff here (giving authors more control versus
doing the right thing in more cases).  I think it's simpler if
authors get the right thing when they animate 'transparent' to 'red'
rather than having to specify 'rgba(255, 0, 0, 0)' to 'red', though.

-David

-- 
L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/