For regular 2D images, the "holdout" operation (or just "out" in Porter/Duff
speak) is the compositing operation where (A out B) is that B blocks A by B's
alpha amount, but it doesn't contribute any color or opacity of its own. That
is, A with a B-shaped hole cut in it. If B has alpha=1, the resulting pixel
will be completely transparent. If B has alpha=0.75, the result will have A,
but with its color and opacity multiplied by 0.25.
We're on the same page, yes?
OK, so my real question is what the "deep holdout" operation actually does. If
A is the "source" image, and B is the "holdout" image, we have the intuition
that B's samples should block any farther samples of A, but without
contributing samples itself. But I'm having real trouble figuring out the
correct behavior for a stack of interleaved A and B samples. Nuke supports a
deep holdout operation, but the docs aren't specific about exactly what it does.
I've tried walking front to back, keeping an accumulated holdout alpha and
multiplying by that as we copy source to result. Here's the sketch:
holdout_alpha = 0 // accumulated holdout-ness
walking near to far in the combined set of source and holdout samples:
S = next closest sample
if S from holdout:
holdout_alpha += (1-holdout_alpha)*S.alpha
else : // S is from source image
S' = S * (1-holdout_alpha)
copy S' to result image
For a "flat-equivalent" image of one holdout sample closer than one source
sample, it matches what I expect for the 2D flat holdout operation. But it
doesn't seem to give me what I'm looking for in many circumstances involving
multiple depth samples of the source.
Can somebody bail me out here and spell out what the correct behavior of a deep
holdout operation ought to be?
--
Larry Gritz
[email protected]
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