Texture Level of Detail Strategies for Real-Time Ray Tracing

Unlike rasterization, where one can rely on pixel quad partial derivatives, an alternative approach must be taken for filtered texturing during ray tracing. We describe two methods for computing texture level of detail for ray tracing. The first approach uses ray differentials, which is a general solution that gives high-quality results. It is rather expensive in terms of computations and ray storage, however. The second method builds on ray cone tracing and uses a single trilinear lookup, a small amount of ray storage, and fewer computations than ray differentials. We explain how ray differentials can be implemented within DirectX Raytracing (DXR) and how to combine them with a G-buffer pass for primary visibility. We present a new method to compute barycentric differentials. In addition, we give previously unpublished details about ray cones and provide a thorough comparison with bilinearly filtered mip level 0, which we consider as a base method.