position-free monte carlo simulation for arbitrary layered...
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Position-Free Monte Carlo Simulation for Arbitrary Layered BSDFsYu Guo1, Miloš Hašan2 and Shuang Zhao1
1University of California, Irvine 2Autodesk Inc.
ACM Transactions on Graphics (SIGGRAPH Asia 2018), 37(6), 2018
Introduction
In this paper, we introduced a new BSDFmodel to capture the appearanceof layered materials. Inside the evaluation and sampling routines of thelayered BSDF, we run a Monte Carlo simulation of light transport withinflat slabs. This is substantially faster than explicitly constructing the layergeometry, but also allows constructing light transport paths that wouldnot easily be available to a generic light transport algorithm, due to ournew position-free path formulation.
Within this framework, we introduced unbiased Monte Carlo techniquesanalogous to a forward path tracer with next event estimation (NEE) anda fully bidirectional estimator. We demonstrated the capabilities of oursolution on a number of examples, featuring multiple layers with surfaceand volumetric scattering, surface and phase function anisotropy, andspatial variation in all parameters. This leads to the first BSDF layeringsolution that offers unbiased accuracy and full flexibility in setting thelayer properties.
Assumption
Small displacement assumption: Since the geometric thickness hof the layer is small, we assume the displacements (e.g., ∆x) of light’sentrance and departure locations can be neglected.
MethodsBSDF sampling: Forward path tracing in flat slab configurationBSDF evaluation:
(c) (d) (e)(a) (b)Unidirectional estimator
(c) (d) (e)(a) (b)Bidirectional estimator
Pdf estimation:
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Groundtruth pdf
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Unbiased pdf
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Approximate pdf
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ComparisonEqual-time comparison:
98 spp 35 spp 280 spp 56 spp 26 spp
60 spp 25 spp 80 spp 15 spp 19 sppReference PT BDPT MLT Our uni-dir Our bi-dir
Comparison to previous work:
not available
expensiveprecomputation
not available not available
Ours [Zeltner 2018] [Belcour 2018]
Comparison to volumetric cloth:
Volume rendering
Our BSDF + fiber-direction map
More resultsTransmission (Spectrally varying optical densities):
Transmission (Different phase function):
HG (g = 0.9) HG (g = 0.99) vMF (κ = 10) vMF (κ = 100)
Anisotropic media within layers:Satin Twill Velvet
Multi-layer BSDF:
References[1] Tizian Zeltner and Wenzel Jakob.
The layer laboratory: A calculus for additive and subtractive composition ofanisotropic surface reflectance.ACM Trans. Graph., 2018.
[2] Laurent Belcour.Efficient rendering of layered materials using an atomic decomposition withstatistical operators.ACM Trans. Graph., 2018.