Cryptomatte or Index pass with volume objects or simulations? (vdb)

We have a “complex” scene where some volumes are mixed with a plane with alphas plus a background.

We need to extract a mask of the volumes but we tried with Cryptomatte and with Pass Index and none of them give us any mask of the volumes.
I also tried with Custom AOV without luck.

I’m unable to extract a mask for the volumes.

Is there a solution for this that I’m missing?

I imagine this is someething that @brecht or specially @StefanW may know, because in Tangent you may have been forced to mix volumes with other things obviously.


Allow me to insist in this question, since we are going to encounter this a lot, and I’m not sure if it’s a current limitation or a bug or what :slight_smile:

Bumping this, no one knows what’s happening here?

Same issue here, this is a huge problem in a pipeline. Any news??

Hi guys, just came across this thread, because I just had the exact same problem.
So far my dirty workaround is by using a negative mask of cryptomatte.
Since the volume doesnt show up on the cryptomatte pass but everything else does, what Im doing right now is basically select every material on cryptomatte and then invert that mask, which gives me basically the volume.
Of course this only works well if you dont have a transparent background, only if basically all the volume has surfaces on the back.
Hope this idea helps out, I know its messy, but like I said , just a dirty workaround