Spatiotemporal reservoir resampling

A 3D model (figurine of a dancer), zoomed in on just the head and neck, rendered using simple Monte Carlo integration (all areas of the object are speckled with random pixels, referred to as "noise"), resampled importance sampling (RIS) (shadowed areas are still noisy, brighter areas less noisy), ReSTIR spatial reuse (similar to RIS, but shadows are less noisy), and denoised ReSTIR (noise has been smoothed away).
A 3D model (figurine of a dancer) rendered using simple Monte Carlo integration, resampled importance sampling (RIS), ReSTIR spatial reuse, and denoised ReSTIR. Only direct light was rendered, using an environment map to simulate a scene with many lights. All images except the denoised image are unbiased. At most 5 rays were traced per pixel in each image.
Top left: (Monte Carlo integration) 5 light samples per pixel, up to 5 rays traced to test shadowing
Top right: (RIS) 32 light samples, 1 ray traced to test shadowing; faster if light samples are cheap and tracing rays is expensive
Bottom left: Two ReSTIR spatial reuse passes (after initial light sampling using RIS), 4 additional rays per pixel
Bottom right: ReSTIR output processed using Intel Open Image Denoise
A 3D model (figurine of a dancer) rendered using simple Monte Carlo integration (all areas of the object are speckled with random pixels, referred to as "noise"), resampled importance sampling (RIS) (shadowed areas are still noisy, brighter areas less noisy), ReSTIR spatial reuse (similar to RIS, but shadows are less noisy), and denoised ReSTIR (noise has been smoothed away).
A zoomed out version of the above image

Spatiotemporal reservoir resampling, commonly known as ReSTIR (from "Reservoir-based SpatioTemporal Importance Resampling"), is a collection of computer graphics techniques for reusing samples during rendering. It was developed primarily to allow more realistic lighting in real-time rendering, because relatively few rays can be traced per pixel while maintaining an acceptable frame rate. It can also be used to speed up off-line path tracing.

The first ReSTIR paper, published in 2020, provided algorithms for direct lighting, allowing scenes containing thousands of lights to be rendered in real time on a high-end GPU. Researchers later proposed versions for rendering indirect lighting (and more recently, motion blur and depth of field) and built up a framework of mathematical concepts and notation conventions that help analyze such algorithms.

A major focus of this work is removing or reducing the bias that could be introduced when samples from other pixels or frames are reused—or selectively allowing some bias in order to speed up rendering and reduce variance (visible as "noise" in the image). Versions for path tracing apply transformations called shift mappings to samples, typically reusing parts of paths closer to the light and modifying the portion closer to the camera.

ReSTIR-related papers and talks have been presented every year at the SIGGRAPH conference since 2020.

One of the first games to incorporate ReSTIR into its rendering was Cyberpunk 2077.

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