Hi, I am struggling with the compositing of two mantra passes :

The first one is some objects rendered with a smoke volume forced as matte in mantra rop node,
and the other is the same smoke volume this time with all others objects rendered as matte.

For the compositing, my assumption was that simply “adding” the two sets of image planes together should produce the right result.
But I have some weird small artifacts in the resulting alpha channel.
see in the right bottom square ( I have exagerated constrast a little to make it obvious )


I tried to premult, unpremult , I tried adding, screening, atop.
I also tried to make use of smoke_mask variable in pyro, but nothing works for now.
I always have these grainy things screwing up my alpha channel.

Am I doing this wrong ?

I think I have the begginning of an answer.
I was using shadow maps on the smoke pass, I just made shadows raytraced, and the issue dissapears … good.