Bidirectional path tracing using backward stochastic light culling

Bidirectional path tracing (BPT) produces noticeable variance for specular-diffuse-specular reflections even if they are not perfectly specular. This is because sampling of the connection between a light vertex and eye vertex does not take bidirectional reflectance distribution functions (BRDFs) into account. This paper presents a novel unbiased sampling method referred to as backward stochastic light culling which addresses the problem of specular-diffuse-glossy reflections. Our method efficiently performs Russian roulette for many light vertices according to the glossy BRDF at a given eye vertex using a hierarchical culling algorithm. We combine our method with light vertex cache-based BPT using multiple importance sampling to significantly reduce variance when rendering caustics reflected on highly glossy surfaces.