Premultiply and Unpremultiply

Hi guys! I’m a little confused and am unsure if I’m doing things right with premultiply. Normally when I get a CG render, I just inject into the action node and premultiply inside it and it works (with no black edges). But recently I have come across some instances when I tried to premultiply, white edges appears around the CG layer. Also sometimes if i have to post blur the cg layer, premultiply doesnt work too. So I just use it unpremultiplied and pixelspread to remove the black edges. Not sure if this is the right way. Would love some advice! Thanks!

I always like to do the unpremultiply at the very start using a comp node.

I like to do it this way because the alpha passes through and comes out of the alpha on the comp node :hugs:

Unpremultiplying is important if I am doing any grading or mixing up of the AOVs to the CG.

If I need to blur then I multiply back again, blur both RGB and Alpha the same way and then unpremultiply directly afterwards.

Doing it inside action is perfectly fine but I love for it to be explicit in my setups.


Sometimes errors occur in action when you scale or reposition surfaces that require unpremultiplying.

I believe that the order of events does the resampling before the unpremultiply


this works awesome! thanks man!

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This bites me allll the time with imported PS layers. So annoying!

The other related thing I just learned - Adobe apparently has a weird way of handling premult in After Effects. I had some premult renders and still go some weird halos.

In After Effects there is a background color which is set on a per-comp basis. Might default to white or black. Apparently when AE premults on export it does some blending with that background color for odd reasons I don’t totally understand, and that can come through.

So on AE comps you definitely want to receive straight alpha renders.


Or just tell your AE artists to premult only with black like every other app uses and stop being iconoclast nutjobs.

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The More You Know Nbc GIF by For(bes) The Culture

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