I have been struggling with a problem that seemed trivial at first, but that I can’t seem to find an easy solution for. So, I thought, I’d ask for help.
To make a long story story: I would like to render a pass that shows the volumetric light in an interior scene. I already set up the scene and a sun light and added a cube that encloses the complete scene to define the volume. That works pretty well and so far I’m pretty happy with how the composite turned out. (I’m using Cycles in Blender 2.76, by the way.)
Anyway, I now need a render pass that shows only the volumetric light in the scene, but excludes the window. I would later like to use this pass like a mask to comp the light “on top” of an existing photograph (along with a separate shadow pass and the object itself), so, ideally, I would like to create a render pass for the light that “travels through the air” only. I don’t mind the pass including the spots where the sun light hits the walls and floor (I would prefer not to have these, but I don’t really mind):
But I really need the window to be dark in this mask, as otherwise the original window in the photograph would be blown in the comp’ing step. Yes, I could manually mask out the window, but… hmm…
Is there another way that I’m overlooking? Any help would be much appreciated. Thanks in advance!