Denoising Monte Carlo Renders With Diffusion Models

Physically-based renderings contain Monte-Carlo noise, with variance that increases as the number of rays per pixel decreases. This noise, while zero-mean for good modern renderers, can have heavy tails (most notably, for scenes containing specular or refractive objects). Learned methods for restoring low fidelity renders are highly developed, because suppressing render noise means one can save compute and use fast renders with few rays per pixel. We demonstrate that a diffusion model can denoise low fidelity renders successfully. Furthermore, our method can be conditioned on a variety of natural render information, and this conditioning helps performance. Quantitative experiments show that our method is competitive with SOTA across a range of sampling rates, but current metrics slightly favor competitor methods. Qualitative examination of the reconstructions suggests that the metrics themselves may not be reliable. The image prior applied by a diffusion method strongly favors reconstructions that are "like" real images -- so have straight shadow boundaries, curved specularities, no "fireflies" and the like -- and metrics do not account for this. We show numerous examples where methods preferred by current metrics produce qualitatively weaker reconstructions than ours.