Abstract
Rendering translucent materials is costly: light transport algorithms need to simulate a large number of scattering events inside the material before reaching convergence. The cost is especially high for materials with a large albedo or a small mean-free-path, where higher-order scattering effects dominate. In simple terms, the paths get lost in the medium. Path guiding has been proposed for surface rendering to make convergence faster by guiding the sampling process. In this paper, we introduce a path guiding solution for translucent materials. We learn an adaptive approximate representation of the radiance distribution in the volume and use it to sample the scattering direction, combining it with phase function sampling by resampled importance sampling. The proposed method significantly improves the performance of light transport simulation in participating media, especially for small lights and media with refractive boundaries. Our method can handle any homogeneous participating medium, with high or low scattering, with high or low absorption, and from isotropic to highly anisotropic.
Highlights
Computing illumination simulation in scenes with participating media is still a costly process, even for algorithms designed to handle them.Manuscript received: 2020-02-06; accepted: 2020-02-19To render high-albedo materials, such as milk or skin, we need to simulate many scattering events before convergence
We propose a resampling method, combining this incoming radiance distribution with the phase function to better sample the product of two high frequency functions
We propose a representation for incoming radiance distribution in participating media with SD-trees: a spatial tree for discrete samples in the volume and a directional tree in each leaf node for angular distribution
Summary
Computing illumination simulation in scenes with participating media is still a costly process, even for algorithms designed to handle them.Manuscript received: 2020-02-06; accepted: 2020-02-19To render high-albedo materials, such as milk or skin, we need to simulate many scattering events before convergence. The main reason is that sampling the out-scattering direction only depends on importance sampling of the phase function, and has no knowledge of the location of the light. This results in many low contribution paths being followed. We consider a scene containing objects with translucent materials Each of these is assumed to be made of a homogeneous material, with index of refraction η, scattering coefficient σs, absorption coefficient σa, and phase function p(ω, ωt). Li(xs, ω) is the in-scattering radiance at xt from all directions ωt over the sphere of directions Ω4π using the phase function p, defined as L(xs, ω) is the exit radiance from the nearest surface, which is governed by the rendering equation [26].
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