ReSTIR Subsurface Scattering for Real-Time Path Tracing

Subsurface scattering is an important visual cue and in real-time rendering it is often approximated using screen-space algorithms. Path tracing with the diffusion approximation can easily overcome the limitations of these algorithms, but increases image noise. We improve its efficiency by applying reservoir-based spatiotemporal importance resampling (ReSTIR) to subsurface light transport paths. For this, we adopt BSSRDF importance sampling for generating candidates. Further, spatiotemporal reuse requires shifting paths between domains. We observe that different image regions benefit most from either reconnecting through the translucent object (reconnection shift), or one vertex later (delayed reconnection shift). We first introduce a local subsurface scattering specific criterion for a hybrid shift that deterministically selects one of the two shifts for a path. Due to the locality, it cannot always choose the most efficient shift, e.g. near shadow boundaries. Therefore, we additionally propose a novel sequential shift to combine multiple shift mappings: We execute subsequent resampling passes, each one using a different shift, which does not require to deterministically choose a shift for a path. Instead, resampling can pick the most successful shift implicitly. Our method achieves realtime performance and significantly reduces noise and denoising artifacts in regions with visible subsurface scattering compared to standard path tracing with equal render time.