Computer Graphics Group

Temporal Sample Reuse for Next Event Estimation and Path Guiding for Real-Time Path Tracing

Addis Dittebrandt, Johannes Hanika, Carsten Dachsbacher

Eurographics Symposium on Rendering 2020

Temporal Sample Reuse for Next Event Estimation and Path Guiding for Real-Time Path Tracing
(Left) Comparison of Visibility Light Cache (VLC) against Light Hierarchy in Q2RTX. Through implicit handling of the visibility term by explicitly tracking and sampling visible lights, we gain significant variance reduction. (Right) Optimal sampling density for radiance field of a given surface point and approximation by Compressed Directional Quadtree (CDQ) in 16 bytes. It is used for guiding rays towards high-energy directions.

Abstract

Good importance sampling is crucial for real-time path tracing where only low sample budgets are possible. We present two efficient sampling techniques tailored for massively-parallel GPU path tracing which improve next event estimation (NEE) for rendering with many light sources and sampling of indirect illumination. As sampling density need to vary spatially, we use an octree structure in world space and introduce algorithms to continuously adapt the partitioning and distribution of the sampling bugdet. Both sampling techniques exploit temporal coherence by reusing samples from the previous frame: For NEE we collect unoccluded samples on light sources and show how to deduplicate, but also diffuse this information to efficiently sample light sources in the subsequent frame. For sampling indirect illumination, we present a compressed directional quadtree structure which is iteratively adapted towards high-energy directions using samples from the previous frame. The updates and rebuilding of all data structures takes about 1 ms in our test scenes, and adds about 6 ms at 1080p to the path tracing time compared to using state-of-the-art light hierarchies and BRDF sampling. We show that this additional effort reduces noise in terms of mean squared error by at least one order of magnitude in many situations.

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Bibtex

@inproceedings {2020_tsr_nee_pg_rtpt,
 booktitle = {Eurographics Symposium on Rendering - DL-only Track},
 editor = {Dachsbacher, Carsten and Pharr, Matt},
 title = {{Temporal Sample Reuse for Next Event Estimation and
           Path Guiding for Real-Time Path Tracing}},
 author = {Dittebrandt, Addis and
           Hanika, Johannes and
           Dachsbacher, Carsten},
 year = {2020},
 publisher = {The Eurographics Association},
 DOI = {10.2312/sr.20201135}
}