Bidir vs Path
Posted: Wed Oct 31, 2018 2:02 am
I realize there are fundamental differences in how bidir and path (and thus CPU vs GPU) process a scene, and that convergence on a solution will require different amounts of time depending on how the scene is lit (compounded by the fact that GPU is faster when the scene is optimized for it)... but I always assumed that ultimately the images should converge on roughly the same solution given enough time (ignoring any global illumination issues).
So here are two "simple" images rendered for 12+ hrs with the only difference that one used bidir and the other used path. Only lighting in the scene is a single sun/sky lamp. The syringes consist of a few different glass materials with volume priority correctly set, and tiny air gaps between all objects anyway. I realize that the existence of layers of different volume materials perhaps violates the definition of a simple lighting setup, but is this really the expected output of a path render? If so, I would suggest some additional restrictions on recommending path rendering "for scenes with simple lighting". Or did I just not understand how to set up such a scene for a path render?
Here's the bidir render, which is what I'd expect the scene to look like:
And here's what it looks like with path:
I tried all sorts of adjustments to get the path render to look "correct" (overlapping volumes with volume priority, completely air gapped volumes, turning subsurf modifiers on/off, etc), but it always looks like the render above. I typically use bidir for all my scenes anyway, but was excited about the idea of speeding things up by throwing my GPUs at it... Is there a fundamental reason this won't ever work?
So here are two "simple" images rendered for 12+ hrs with the only difference that one used bidir and the other used path. Only lighting in the scene is a single sun/sky lamp. The syringes consist of a few different glass materials with volume priority correctly set, and tiny air gaps between all objects anyway. I realize that the existence of layers of different volume materials perhaps violates the definition of a simple lighting setup, but is this really the expected output of a path render? If so, I would suggest some additional restrictions on recommending path rendering "for scenes with simple lighting". Or did I just not understand how to set up such a scene for a path render?
Here's the bidir render, which is what I'd expect the scene to look like:
That was definitely the problem. So just out of curiosity, why does Path default to lower path depth than BiDir? I would naively assume the opposite - since Path is handicapped by only shooting rays from the camera, shouldn't it use more rays by default to make up for that?