Short Film Compositing,

[evilboz]

Looks like a double premult problem to me. The easiest way to solve it would be to render the background without a holdout for the foreground, then comp the tree on top. I hope that helps!

[nedgin]

thank you so much for the reply,

If i render with whole scene with back and foreground,

wouldn't it double the rendering time?

at this point the render time for each passes takes 1min~5min

I am doing 4min animation project, and the rendering time is becoming really difficult problem to solve :(

[sundialsvc4]

No... it won't radically change your times.

"Premul," short for premultiplication, is a step that multiplies pixel-values by the "alpha" setting in advance. Which is fine for the "final, finished output" but ... it is an arithmetic process that you only want to be applied once.

Here's the rub: every layer that you prepare, with the intent of subsequently compositing it with something else, is, by definition, "not final." Rather, it is "the input to some down-stream processing stage." For all of those compositing steps, you want the "edges" of everything that you are compositing to be "razor-sharp." You don't want anti-aliasing and you especially don't want pre-multiplication, because, for those final-output pixels, they will very quickly "multiply all the way down to zero."

(Mathematically, it's every bit as simple as "0.25 x 0.25 = 0.0625, and if you do that even one more time it's too-damn-close-to, so-let's-just-round-down-to, (heh, heh) 0.000." Whups! Which is more-or-less why that green background shows through.)

Only after all of these steps have been performed do you (perhaps...) want to "soften the edges of" the final result ... which you can easily do with the appropriate node in your rendering chain.

What this means is that... the various intermediate layers, when viewed separately, will appear to have sharp, jaggy edges ... but, when they are subsequently composited, everything looks fine. You just gotta resist that little voice that's screaming to you, "OMG! Jaggies!!"

[DoubleSupercool]

Definitely try and avoid rendering occluded passes (passes with alphas cut out of them). It really doesn't add much to your render time and limits your options. Say you want to scale or move your background slightly? Nope, you have a stonking great hole cut out of it! :)

If you need to do some particular corrections, render yourself out a matte pass to isolate the element.

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