Combining point and line samples for direct illumination

In Computer Graphics Forum (Proceedings of EGSR), 2019

We compare BSDF and solid-angle (SA) point-sampling to our line-sampling and its MIS with BSDF sampling at equal time (three seconds). As shown by the mean relative squared error (MRSE), line samples provide a clear benefit in soft shadows, but the glossy material is handled better by MISing additionally with BSDF sampling.

Abstract

We develop a unified framework for combining point and line samples in direct lighting calculations. While line samples have proven beneficial in a variety of rendering contexts, their application in direct lighting has been limited due to a lack of formulas for evaluating advanced BRDFs along a line and performance tied to the orientation of occluders in the scene. We lift these limitations by elevating line samples to a shared higher-dimensional space with point samples. Our key insight is to separate the probability distribution functions of line samples and points that lie along a line sample. This simple conceptual change allows us to apply multiple importance sampling (MIS) between points and lines, and lines with each other, in order to leverage their respective strengths. We also show how to improve the convergence rate of the MIS combination in unoccluded regions in an unbiased way using a novel discontinuity-smoothing balance heuristic. We verify through a set of rendering experiments that our proposed MISing of points and lines, and lines with each other, reduces variance of the direct lighting estimate while supporting an increased range of BSDFs compared to analytic line integration.

Downloads

• Publication

  • Paper, PDF (8MB)
  • Supplemental document, PDF (3MB)
  • Publisher's official version (may require a subscription)

• Slides

  • Presentation slides, Keynote (9MB)
  • Presentation slides, MP4 (37MB)
  • Presentation slides, PDF (3MB)

• Data

  • Supplemental archive, ZIP (220MB) - offline version of the interactive comparisons, also with EXR renderings.

• Code

  • GitHub project - Full source code for our method built on top of PBRTv3.

Videos

Interactive result comparisons

Below you can view interactive comparisons of the full OpenEXR rendered results from the paper. You can also find low-dynamic range versions below each thumbnail if you prefer.

Shiny Monolith
(LDR version)

Cornell box
(LDR version)

Veach
(LDR version)

Bathroom
(LDR version)

Kitchen
(LDR version)

Acknowledgements

We are grateful to the anonymous reviews for their suggestions on improving the paper. We also thank past and present members of the Dartmouth Visual Computing Lab. This work was partially supported by NSF grant IIS-181279.

Cite

@article{salesin19combining,
    author    = {Salesin, Katherine and Jarosz, Wojciech},
    title     = {Combining point and line samples for direct illumination},
    journal   = {Computer Graphics Forum (Proceedings of EGSR)},
    year      = {2019},
    volume    = {38},
    number    = {4},
    month     = jul,
    pages     = {159--169},
    doi       = {10/gf6rx6},
    publisher = {The Eurographics Association}
}

Related Projects

• 2019 Paper: Fourier analysis of correlated Monte Carlo importance sampling
• 2017 Paper: Variance and convergence analysis of Monte Carlo line and segment sampling
• 2017 Paper: Convergence analysis for anisotropic Monte Carlo sampling spectra

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