Ray Distribution Aware Heuristics For BVHs Construction In Ray Tracing

The Bounding Volume Hierarchy (BVH) is a fundamental data structure in the ray tracing algorithms to accelerate the detection of ray-scene intersections. The Surface Area Heuristic (SAH), employed during the construction of the BVH, is based on the hypothesis that the ray distribution in the scene is uniform. In this paper, we show that the SAH hypothesis is not valid when importance sampling is used, and propose two novel heuristics. With the Projected Area Heuristic (PAH) we demonstrate how it is possible to estimate better the cost of a BVH. In particular, we replace the approximation of the probability an AABB is hit by a ray, from the ratio between the surface areas of the node and the root of the BVH to the ratio of their projected areas. The plane the AABBs are projected on and the kind of projection (either orthographic or perspective) are chosen based on the local ray distribution. With the Splitting Plane Facing Heuristic (SPFH) we show how we can build higher-quality BVHs by taking into account the knowledge of the ray distribution in a local region of the scene during the selection of the orientation of the plane used to split a node.