Accelerating shadow rays using volumetric occluders and modified kd-tree traversal

Monte Carlo ray tracing remains a simple and elegant method for generating robust shadows. This approach, however, is often hampered by the time needed to evaluate the numerous shadow ray queries required to generate a high-quality image. We propose the use of volumetric occluders stored within a kd-tree in order to accelerate shadow rays cast on a closed, watertight mesh. Intersection with a volumetric occluder is much cheaper than intersection with mesh geometry, although performing these intersections requires modification to the traversal order through the kd-tree. We propose two such modifications, both of which enable the use of volumetric occluders for cheap shadow ray termination. We also propose using a software-managed cache to store and reuse volumetric occluders for even earlier termination. Our approach provides a performance improvement of up to 2.0x for our test scenes while producing images identical to those produced by the unaccelerated baseline.