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Posted by Koncept on 01/12/07 23:15
In article <l01gq2lh85bevfdktu3erk9obeoo1bkrlm@4ax.com>, Onideus Mad
Hatter <usenet@backwater-productions.net> wrote:
> :If an alpha channel is used in an image, it is common to also multiply
> :the color by the alpha value, in order to save on additional
> :multiplications during the compositing process. This is usually
> :referred to as premultiplied alpha. Thus, assuming that the pixel
> :color is expressed using RGB triples, a pixel value of (0.0, 0.5, 0.0,
> :0.5) implies a pixel which is fully green and has 50% coverage.
>
> ...wow...even Wikipedia FAILS to explain it with anything other than
> the most simplistic glazing jive of redundant incoherency. Tsch,
> tsch, tsch...a simple formula/equation is just TOO MUCH to ask
> apparently. Oh how the mighty web has fallen.
I came across this: http://www.gimp.org/docs/plug-in/appendix-alpha.html
Premultiplied alpha is just a different way of representing alphified
pixels. If the separate alpha pixel is (r, g, b, a), then the
premultiplied alpha pixel is (ar, ag, ab, a).
The reason why it's interesting is that linear combinations of pixels
(i.e. a1p1 + a2p2) work better in premultiplied alpha space than in
separate alpha space.
For example, taking the 50/50 blend of white and transparent works like
this: White is (1, 1, 1, 1) and transparent is (0, 0, 0, 0) in both
spaces. So the 50/50 blend is (0.5, 0.5, 0.5, 0.5). In separated space,
that's half-transparent gray, but in premultiplied space, that's
half-transparent white, which is what you expect.
--
Koncept <<
"The snake that cannot shed its skin perishes. So do the spirits who are
prevented from changing their opinions; they cease to be a spirit." -Nietzsche
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