Figure 1: Left: ray casting with shadows (RCS). Middle: Whitted-style ray tracing (WRT). Right: distribution ray tracing (DRT) with 64 samples per pixel. This paper investigates interactive WRT on current hardware and the prospects for interactive DRT on future hardware. ABSTRACTMuch progress has been made toward interactive ray tracing, but most research has focused specifically on ray casting. A common approach is to use "packets" of rays to amortize cost across sets of rays. Whether "packets" can be used to speed up the cost of reflection and refraction rays is unclear. The issue is complicated since such rays do not share common origins and often have less directional coherence than viewing and shadow rays. Since the primary advantage of ray tracing over rasterization is the computation of global effects, such as accurate reflection and refraction, this lack of knowledge should be corrected. We are also interested in exploring whether distribution ray tracing, due to its stochastic properties, further erodes the effectiveness of techniques used to accelerate ray casting. This paper addresses the question of whether packet-based ray tracing algorithms can be effectively used for more than visibility computation. We show that by choosing an appropriate data structure and a suitable packet assembly algorithm we can extend the idea of "packets" from ray casting to Whitted-style and distribution ray tracing, while maintaining efficiency.

We render tree foliage levels of detail (LODs) using a new adaptation of billboard clouds. Our contributions are a simple and efficient billboard cloud creation algorithm designed specifically for tree foliage, and a method for smooth transitions between LODs. The cloud creation algorithm performs stochastic search to find a set of billboards that approximate the base mesh. Billboard clouds offer an alternative to traditional triangle reduction methods, which break down for foliage with its small, disconnected pieces of geometry. By projecting foliage geometry onto large precomputed textures, our method shifts the bulk of the runtime rendering to the fragment processing stage. This results in higher framerates for most viewing distances, with adjustable visual accuracy. We give results for foliage from two fully detailed tree models and discuss implementation issues.