Figure 1: We present a method that enables fast, per-frame and from-scratch re-builds of a bounding volume hierarchy, thus completely removing a BVH-based ray tracer's reliance on updating or re-fitting. On a dual-2.6GHz Clovertown system (8 cores total), our method renders the exploding dragon model (252K triangles) at around 13-21 frames per second (1024x1024 pixels) including animating the triangles, per-frame rebuilds, shading, shadows, and display. The build itself takes less than 20ms, and is nearly agnostic to the distribution of the triangles; thus, the variation in frame rate (21 fps for the initial, smooth frame, and 13 fps for the timestep corresponding to the fourth image) is due only to varying traversal cost, without any deterioration in BVH quality at all (i.e., when starting with the last frame, the frame rate actually increases). ABSTRACTWith ray traversal performance reaching the point where real-time ray tracing becomes practical, ray tracing research is now shifting away from faster traversal, and towards the question what has to be done to use it in truly interactive applications such as games. Such applications are problematic because when geometry changes every frame, the ray tracer's internal index data structures are no longer valid. Fully rebuilding all data structures every frame is the most general approach to handling changing geometry, but was long considered impractical except for grid-based grid based ray tracers, trivial scenes, or reduced quality of the index structure. In this paper, we investigate how some of the fast, approximate construction techniques that have recently been proposed for kd-trees can also be applied to bounding volume hierarchies (BVHs). We argue that these work even better for BVHs than they do for kd-trees, and demonstrate that when using those techniques, BVHs can be rebuilt up to 10× faster than competing kd-tree based techniques.
Ray tracing has long been a method of choice for off-line rendering, but traditionally was too slow for interactive use. With faster hardware and algorithmic improvements this has recently changed, and real-time ray tracing is finally within reach. However, real-time capability also opens up new problems that do not exist in an off-line environment. In particular real-time ray tracing offers the opportunity to interactively ray trace moving/animated scene content. This presents a challenge to the data structures that have been developed for ray tracing over the past few decades. Spatial data structures crucial for fast ray tracing must be rebuilt or updated as the scene changes, and this can become a bottleneck for the speed of ray tracing. This bottleneck has recently received much attention by researchers and that has resulted in a multitude of different algorithms, data structures and strategies for handling animated scenes. The effectiveness of techniques for ray tracing dynamic scenes vary dramatically depending on details such as scene complexity, model structure, type of motion and the coherency of the rays. Consequently, there is so far no approach that is best in all cases, and determining the best technique for a particular problem can be a challenge. In this State of the Art Report (STAR), we aim to survey the different approaches to ray tracing animated scenes, discussing their strengths and weaknesses, and their relationship to other approaches. The overall goal is to help the reader choose the best approach depending on the situation, and to expose promising areas where there is potential for algorithmic improvements.
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