Spectral and Decomposition Tracking for Rendering Heterogeneous Volumes

Peter Kutz, Ralf Habel, Yining Karl Li, and Jan Novák

ACM Transactions on Graphics (Proceedings of SIGGRAPH 2017), vol. 36, no. 4

A cloudscape rendered with a combination of our spectral and decomposition tracking techniques, which gracefully handle chromatic media and reduce collision coefficient evaluations. The insets on the right were computed in equal time, with our method yielding 3.5× lower MSE than delta tracking.

abstract

We present two novel unbiased techniques for sampling free paths in heterogeneous participating media. Our decomposition tracking accelerates free-path construction by splitting the medium into a control component and a residual component and sampling each of them separately. To minimize expensive evaluations of spatially varying collision coefficients, we define the control component to allow constructing free paths in closed form. The residual heterogeneous component is then homogenized by adding a fictitious medium and handled using weighted delta tracking, which removes the need for computing strict bounds of the extinction function. Our second contribution, spectral tracking, enables efficient light transport simulation in chromatic media. We modify free-path distributions to minimize the fluctuation of path throughputs and thereby reduce the estimation variance. To demonstrate the correctness of our algorithms, we derive them directly from the radiative transfer equation by extending the integral formulation of null-collision algorithms recently developed in reactor physics. This mathematical framework, which we thoroughly review, encompasses existing trackers and postulates an entire family of new estimators for solving transport problems; our algorithms are examples of such. We analyze the proposed methods in canonical settings and on production scenes, and compare to the current state of the art in simulating light transport in heterogeneous participating media.

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bibtex

@article{Kutz2017SDT,
    title     = {Spectral and Decomposition Tracking for Rendering Heterogeneous Volumes},
    author    = {Kutz, Peter and Habel, Ralf and Li, Yining Karl, and Nov\'ak, Jan},
    journal   = {ACM Transactions on Graphics (Proceedings of SIGGRAPH 2017)},
    volume    = {36},
    number    = {4},
    year      = {2017},
    articleno = {111},
    pages     = {111:1--111:16},
    publisher = {ACM},
    address   = {New York, NY, USA},
    doi       = {10.1145/3072959.3073665},
}