bear ([EMAIL PROTECTED]) wrote:
> Assume there are four layers, all of which are in Normal mode and grayscale.
> for a specific pixel in each layer,
> Their intensity and alpha values(mapped to [0,1]) are
> i1,a1
> i2,a2
> i3,a3
> i4(background)
> the merged intensity is ((i1*a1+i2*(1-a1))*a2+i3*(1-a2))*a3+i4*(1-a3)
> am I correct? Thanks!

Yes. You might want to look for the paper
  T. Porter & T. Duff - Compositing Digital Images
for an explantation and the reasoning behind these rules.

The paper itself is available at e.g.

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