Ryusuke Villemin, Magnus Wrenninge, Julian Fong
In recent years, path tracing has become the dominant image synthesis technique for production rendering. Unbiased methods for volume intergration have followed, and techniques such as delta tracking, ratio tracking and spectral decomposition tracking are all in active use, and this paper is focused on optimizing the underlying mechanics of these. We present a method for reducing the number of calls to the random number generator and show how modifications to the distance sampling strategy and interaction probabilities can help reduce variance when rendering thin homogeneous and heterogeneous volumes. Our methods are implemented in version 21.7 of Pixar's RenderMan software.