Posted by Jeckyl on 01/16/07 23:48

>>> :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
>>It just did .. READ IT
> I did read it, it says nothing of premultiplied RGB values. Yeesh, is
> yer stupid ass even READING this thread or are you just skimming it?

That is exactly what it is talking about. Its not that it doesn't say
anything .. its that you don't understand it.

If only for the sake of other readers of this thread, I'll spell it out more
clearly:

Premultiplying rgb means that you multiple each of the R,G,B values by (in
this case) the alpha value and keep that result (ie the premultiplied RGB
value). This saves you having to do that multiplication again when
processing the image. its an optimisation.
--
Jeckyl

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