why if I render premult background color affects the comp?


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?


Which file format you are saving to?


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


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


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


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.


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