Integral formulations of volumetric transmittance

In ACM Transactions on Graphics (Proceedings of SIGGRAPH Asia), 2019

Equal-lookup comparisons of various estimators in a scene with both homogeneous (chess board) and heterogeneous (chess pieces) media. Our proposed estimators (in bold) trade more extinction lookups for lower-variance estimates of transmittance. RMSE and render times are for the entire image.

Abstract

Computing the light attenuation between two given points is an essential yet expensive task in volumetric light transport simulation. Existing unbiased transmittance estimators are all based on "null-scattering" random walks enabled by augmenting the media with fictitious matter. This formulation prevents the use of traditional Monte Carlo estimator variance analysis, thus the efficiency of such methods is understood from a mostly empirical perspective. In this paper, we present several novel integral formulations of volumetric transmittance in which existing estimators arise as direct Monte Carlo estimators. Breaking from physical intuition, we show that the null-scattering concept is not strictly required for unbiased transmittance estimation, but is a form of control variates for effectively reducing variance. Our formulations bring new insight into the problem and the efficiency of existing estimators. They also provide a framework for devising new types of transmittance estimators with distinct and complementary performance tradeoffs, as well as a clear recipe for applying sample stratification.

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• Slides

  • Presentation slides, Keynote (42MB)
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  • Presentation slides, PDF (18MB)

• Data

  • Supplemental - website with interactive image comparisons and error data.

• Code

  • GitHub project - C++ implementation of various transmittance estimators.

Videos

Acknowledgements

All images have been rendered using PBRT. The chessboard and pieces in Figs. 1 and 8 are from TurboSquid. The OpenVDB cloud model in Fig. 9 is from Disney Research. The Fox model in Fig. 11 is from Thingiverse.

This work was supported by a gift from Autodesk, National Science Foundation Grant IIS-1812796, Czech Science Foundation Grant 19-07626S, and Charles University Grant SVV-2017-260452.

Cite

@article{georgiev19integral,
    author   = {Georgiev, Iliyan and Misso, Zackary and Hachisuka, Toshiya and Nowrouzezahrai, Derek and K\v
                riv\'anek, Jaroslav and Jarosz, Wojciech},
    title    = {Integral formulations of volumetric transmittance},
    journal  = {ACM Transactions on Graphics (Proceedings of SIGGRAPH Asia)},
    volume   = {38},
    number   = {6},
    year     = {2019},
    month    = nov,
    doi      = {10/dffn},
    keywords = {participating media, transmittance, null collision, null scattering, stochastic sampling, Monte Carlo
                integration}
}

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• 2022 Paper: Unbiased and consistent rendering using biased estimators
• 2019 Paper: A null-scattering path integral formulation of light transport
• 2018 Paper: Monte Carlo methods for volumetric light transport simulation
• 2014 Paper: Residual ratio tracking for estimating attenuation in participating media

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