SIMD Friendly Ray Tracing on GPU

In this paper, we present a novel BVH tracing method on GPU, which can achieve better SIMD utilization than traditional method. In the traditional way, thread usually sticks to a ray until the closest hit is found. When the threads of the same warp follow very divergent ray paths, SIMD utilization drops significantly. The idea of our method is to redefine the way of work distribution by binding the ray and the data to be tested together, in order to spread the computation of the single ray to multi threads. We also separate the tracing process into three steps to collect the work units of the same type and process them in a stream-like manner. The first step is ray traversal whose task is to do ray-box testing for the ray-node pairs. Its output is an stack of ray-triangle pairs, which is then fed to intersecting step to form an stack of ray-hit pairs. The last step is to use ray-hit pairs to update the closest hits for each ray of the same warp. The experiment shows our method can efficiently improve the SIMD utilization and result in less tracing time.