Tizian Zeltner

Specular Manifold Sampling for Rendering
High-Frequency Caustics and Glints

In Transactions on Graphics (Proceedings of SIGGRAPH 2020)

Rendering of a shop window featuring a combination of challenging-to-sample light paths with specular-diffuse-specular ("SDS") interreflection: the two golden normal-mapped pedestals are illuminated by spot lights and project intricate caustic patterns following a single reflection from the metallic surface, while the transparent center pedestal generates caustics via double refraction. The glinty appearance of the shoes arises due to specular microgeometry encoded in a high-frequency normal map. This image was rendered by an ordinary unidirectional path tracer using our new specular manifold sampling strategy. The remaining noise is due to indirect lighting by caustics, which is not explicitly sampled by our technique.

Abstract

Scattering from specular surfaces produces complex optical effects that are frequently encountered in realistic scenes: intricate caustics due to focused reflection, multiple refraction, and high-frequency glints from specular microstructure. Yet, despite their importance and considerable research to this end, sampling of light paths that cause these effects remains a formidable challenge.

In this article, we propose a surprisingly simple and general path sampling strategy for specular light paths including the above examples, unifying the previously disjoint areas of caustic and glint rendering into a single framework. Given two path vertices, our algorithm stochastically finds a specular subpath connecting the endpoints. In contrast to prior work, our method supports high-frequency normal- or displacement-mapped geometry, samples specular-diffuse-specular ("SDS") paths, and is compatible with standard Monte Carlo methods including unidirectional path tracing. Both unbiased and biased variants of our approach can be constructed, the latter often significantly reducing variance, which may be appealing in applied settings (e.g. VFX). We demonstrate our method on a range of challenging scenes and evaluate it against state-of-the-art methods for rendering caustics and glints.

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