Real‐Time Ray Tracing of Micro‐Poly Geometry with Hierarchical Level of Detail

Abstract

In recent work, Nanite has demonstrated how to rasterize virtualized micro‐poly geometry in real time, thus enabling immense geometric complexity. We present a system that employs similar methods for real‐time ray tracing of micro‐poly geometry. The geometry is preprocessed in almost the same fashion: Nearby triangles are clustered together and clusters get merged and simplified to obtain hierarchical level of detail (LOD). Then these clusters are compressed and stored in a GPU‐friendly data structure. At run time, Nanite selects relevant clusters, decompresses them and immediately rasterizes them. Instead of rasterization, we decompress each selected cluster into a small bounding volume hierarchy (BVH) in the format expected by the ray tracing hardware. Then we build a complete BVH on top of the bounding volumes of these clusters and use it for ray tracing. Our BVH build reaches more than 74% of the attainable peak memory bandwidth and thus it can be done per frame. Since LOD selection happens per frame at the granularity of clusters, all triangles cover a small area in screen space.