Masking Smoke

Hi there!

I would appreciate if somebody knows a solution for masking smoke
e.g. with “sky-materials”.

My issue is that the smoke mustn’t be rendered where some objects are,
so I assigned a material with the sky-option checked to these objects.
The objects should also not be rendered, neither in view nor in alpha.
But it won’t work out, trying for 4 hours now.
The scene is a hollow with an opening and a “rocket” is rushing out
of that opening with camera view behind the “rocket”.
So the trail of smoke of the “rocket” must be masked e.g. where there are some
stalagmites and the stalagmites mustn’t be rendered.
Another problem is the depth, because where the stalagmites are behind the
rocket, smoke must not be masked and vice versa.

Thank you very much for your help!

My guess is that you cannot do a volumetric pass, so you may need to make another version of the scene with altered materials on all objects. Eg light the smoke but make all the stalagmites black but cast shadows (on smoke). Then use that to comp in smoke to master scene.