why if I render premult background color affects the comp?

#1

why if I render premult alpha background color affects the comp? For example the background color of the background where the alpha equals 0 is blue, and the alpha there is 0. Why it’s not 0 in the end? Shouldn’t it be scaled down to 0? If we mult any color by 0, it’s 0. Why does the opposite happens?

#2

Which file format you are saving to?

#3

it’s open EXR. Butit’s the same with tif. It’s simply how it behaves. But why?

#4

Maybe the reason is default additive merge mode in fusion? I read from manual it “just adds pixels together if images are premutiplied”.

#5

mainly its because fusion (and nearly every other comp app) expects that you premult against black.

if you do a premult against an other color you will get that nasty edges like in your other picture.

there is a macro on vfxpedia which can also do a premult against any color (but vfxpedia is currently down)

for the first start check the ‘premult’ option in the loader

#6

Thank you. But if I have a premult image, should I check “Post-Multiply By Alpha”? I though not, as it’s already multiplied, so will result in double mult. But if I check this box, the background becomes transparent in merging.

#7

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