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?