Suppose I am rendering a smoking torus from above in Cycles:
(at laughably bad resolution, of course, but that's not the point)
Cycles spends a lot of time rendering the empty space, even though the
0 (and therefore, the volume should have no effect on light). I don't quite know why Cycles has to think so much.
In fact, the render times can be almost halved (from 11 min to 6 min) by using an old BI trick and masking empty parts of the volume with a shadeless object (which can be made here).
This is fine for simple test renders, but now I'd like to make actual scenes with smoke and fire in them, and obviously my little masking trick won't work because there are objects behind the smoke that can't be masked. (Also, shrinking the domain is not an option)
How can I optimize scenes with large empty volumetric sections?