Re: Compositing math in SVG

Let's say you have red at 50% alpha. Premultiplied that becomes (.5, 0, 0,
.5) but this is not what you send to the display since it can't display
alpha.
So, what you do is matte (=composite) with white to get rid of the alpha:
 (.5, 0, 0) + (1-.5)*(1,1,1) = (1, .5, .5)

On Sat, Jul 21, 2012 at 4:59 PM, Calculemus <calculemus1988@gmail.com>wrote:

> Yes we are using pre-multiplied values so I don't need to
> divide the result with alpha. But yes I also calculate the result
> alpha as: aa + ba(1-aa)
>
> Hmm, what exactly do you mean by "matte with white"?
>
> Thanks
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