SRDH: Specializing BVH Construction and Traversal Order Using Representative Shadow Ray Sets

Abstract

We derive the Shadow Ray Distribution Heuristic (SRDH), an accurate cost estimator for shadow ray traversal through a bounding volume hierarchy (BVH). The SRDH leverages up-front knowledge of the distribution and intersection results of previously traced shadow rays to construct a shadow-ray-specialized BVH and choose an associated traversal order policy which together promote early termination by quickly finding occlusions. In scenes containing large amounts of occlusion, SRDH reduces the number of BVH node traversal steps needed for shadow computations between 22% and 56% compared to average-case traversal through SAH-constructed trees. Evaluating the SRDH using a sparse shadow ray set recorded from a 16 x16 pixel rendering of the scene consistently produces BVHs whose traversal cost is within 6% of those built when all shadow rays are available to the metric at the time of construction. The benefits of the SRDH come at the cost of storing an additional BVH in memory and a 2.4x increase (on average) in BVH construction time.