Improved two-level BVHs using partial re-braiding

We propose a novel approach for improving the quality of two-level BVHs (i.e., a two-level data structure that uses a top-level BVH built over second-level object BVHs). After building an individual, high-quality BVH for each object, our new top-level BVH build approach selectively re-braids (opens and merges) object BVHs during the build process to reduce overlap and improve SAH quality. We demonstrate that compared to the two main state-of-the-art techniques---brute-force re-construction of a single, flat BVH; and building a traditional two-level BVH over objects, respectively---the proposed approach achieves build times significantly faster than the former, while simultaneously yielding traversal performance that is much higher than the latter.