Object & Material Index / ID Mask nodes - do not motion blur properly!!

Hi there,

I am rendering a file where things move around with Motion Blur.

But if I try to use Object & Material Index / ID Mask nodes to isolate just particular materials on a mesh of the thing that is moving, the motion blur comes out as really really grainy, and if you tick or untick ‘Anti-Aliasing’ on the ID Mask node, it just changes one really grainy image for a slightly different really grainy image!!

Is there a way to solve this?
Is it like this on purpose for a good and logical reason?
Is it something that is yet to be coded?

What I have done instead is duplicate the scene, delete all amps, swap all materials to a non-glossy black material apart from the material I want to mask which is swapped to a white emission (now the only thing emitting light)…this then creates a mask with proper motion blur, but it is a lot of work to do something so simple!

Unfortunately the way you described is THE way to go. ID masks don’t work properly. They are not antialiased and “antialiasing” option in the node doesn’t antialias. It only blurs the edges.

Ad 1.: No. The way you do it is the only kind of working way.
Ad 2.: I tried to figure out some reasoning for such behavior and didn’t find any
Ad 3.: I’ve been hoping for this for ages, but I don’t thing this matter is taken care of by anybody at the moment. Compositor as such is now being completely re-written and there probably are more important things to address now.

BTW: Current behavior of ID’s drives me crazy as well, but I somehow got used to it and simply shut up and do what I have to.