Render Pass for Volumetric Light excluding Window?

Hi all,

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!

That’s really easy: all you need to do is to put a plane behind the window with a dark material and remove the shadow.

Thanks for the quick reply. That looks good. But how comes the plane is not blocking the rays of light? I tried roughly the same thing (with a window), but lost the volumetric light rays in the process… I got the impression that by closing the window, I lost the “volume” that was defined by the enclosing box.

Oh, now I see: You turned off “Shadow” for “Ray Visibility”… very clever! Thanks, that works perfectly. :slight_smile:

Yes, just disable the shadow of the plane. Very simple, but I use that trick in production and that’s working :slight_smile: