We present two new distance-sampling methods for production volume path tracing. We extend the null-collision integral formulation to efficiently gather heterogeneous volumetric emission, achieving higher-quality results. Additionally, we propose a tabulation-based approach to importance sample volumetric in-scattering through a spatial guiding data structure. Our methods improve the sampling efficiency for scenarios where low-order heterogeneous scattering dominates, which tends to cause high variance renderings with existing null-collision methods.

Wei-Feng Wayne Huang, Peter Kutz, Yining Karl Li, and Matt Jen-Yuan Chiang. Unbiased Emission and Scattering Importance Sampling For Heterogeneous Volumes. ACM SIGGRAPH 2021 Talks. Article 3, Aug 2021.

@article{Huang2021VolumeEmissionScatterSampling,
    author = {Huang, Wei-Feng Wayne and Kutz, Peter and Li, Yining Karl and Chiang, Matt Jen-Yuan},
    title = {Unbiased Emission and Scattering Importance Sampling For Heterogeneous Volumes},
    journal = {ACM SIGGRAPH 2021 Talks},
    month = aug,
    year = {2021},
    articleno = {3},
    doi = {10.1145/3450623.3464644},
}

We thank the Hyperion development team for supporting this project, the many artists at Walt Disney Animation Studios who tested these techniques, the leadership of Raya and the Last Dragon for adopting these techniques in production, and finally our paper reviewers for providing invaluable comments and suggestions.

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