Second-order occlusion-aware volumetric radiance caching

1Universidad de Zaragoza, I3A 2Dartmouth College

In ACM Transactions on Graphics (Presented at SIGGRAPH), 2018

Teaser
An equal-time comparison of path tracing, progressive photon beams (PPB), occlusion-unaware gradients [Jarosz et al. 2008], and our second-order occlusion-aware solution on the Staircase scene. We include a fully converged solution for path tracing. Each cache in both our method and Jarosz et al. is computed using 16k stratified angular samples, and rendered using 16 samples per pixel. The progressive photon beams solution was obtained using the publicly available Tungsten renderer. Note how the occlusion unaware method creates visible artifacts in the patterns created by the shadows crossing from different windows, while our method correctly captures those details in equal time. Due to distant lighting, progressive photon beams fails to densely sample the light shafts coming through the windows, resulting in visible variance after 400 iterations of 1M beams/iteration.
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Abstract

We present a second-order gradient analysis of light transport in participating media and use this to develop an improved radiance caching algorithm for volumetric light transport. We adaptively sample and interpolate radiance from sparse points in the medium using a second-order Hessian-based error metric to determine when interpolation is appropriate. We derive our metric from each point's incoming lightfield, computed by using a proxy triangulation-based representation of the radiance reflected by the surrounding medium and geometry. We use this representation to efficiently compute the first- and second-order derivatives of the radiance at the cache points while accounting for occlusion changes. We also propose a self-contained two-dimensional model for light transport in media and use it to validate and analyze our approach, demonstrating that our method outperforms previous radiance caching algorithms both in terms of accurate derivative estimates and final radiance extrapolation. We generalize these findings to practical three-dimensional scenarios, where we show improved results while reducing computation time by up to 30% compared to previous work.

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Julio Marco, Adrian Jarabo, Wojciech Jarosz, Diego Gutierrez. Second-order occlusion-aware volumetric radiance caching. ACM Transactions on Graphics (Presented at SIGGRAPH), 37(2), April 2018. 
@article{marco18second,
    author  = {Marco, Julio and Jarabo, Adrian and Jarosz, Wojciech and Gutierrez, Diego},
    title   = {Second-order occlusion-aware volumetric radiance caching},
    journal = {ACM Transactions on Graphics (Presented at SIGGRAPH)},
    volume  = {37},
    number  = {2},
    year    = {2018},
    month   = apr,
    doi     = {10/gdv86k}
}
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