Bi-directional Rays in Global Illumination

Csaba Kelemen, Balázs Benedek, László Szirmay-Kalos, László Szécsi 
Budapest University of Technology and Economics
Department of Control Engineering and Information Technology, Magyar Tudósok krt. 2.
1117 Budapest



Keywords: Global illumination, finite-element techniques, Monte-Carlo methods


Global illumination algorithms use rays or ray-bundles to transfer the light in the scene. A ray connects two points that are mutually visible from each other. Although it seems intuitive to transfer the light into both directions along the ray, only a few algorithms applying global sampling or restricting the solution to the diffuse case have taken advantage of bi-directional rays so far. Using bi-directional rays doubles the samples of single reflection, may multiply the samples by four for double reflections if we can connect them in an optimal way, and generally bi-directional rays potentially increase the number of paths used for the estimation of d bounces in the order of 2^d. Thus at minimal additional cost, the number of samples is significantly increased. In this paper we propose a non-diffuse global illumination algorithm that exploits this promising alternative. The algorithm is easy to implement but robust and is able to render complex scenes within a few tens of seconds.