ReSTIR
A technique that reuses and combines light samples across neighbouring pixels and frames to make ray-traced lighting noise-free in real time.
ReSTIR (Reservoir-based Spatiotemporal Importance Resampling) was presented by Bitterli et al. at SIGGRAPH 2020. The core idea builds on reservoir sampling: each pixel keeps a small reservoir of the best light samples seen so far and updates it in a streaming fashion. The pixel then borrows samples from its spatial neighbours and from the previous frame in time — hence spatiotemporal.
The effect is dramatic. Scenes with thousands of light sources, once requiring minutes of path tracing, can be shaded with a handful of rays per pixel and still come out clean. A follow-up, ReSTIR GI (2021), extended the method to indirect global illumination. It is a cornerstone of modern real-time ray tracing.