Figure 1: Dress simulation: Four different images of a 210 step sequence taken from a dynamic cloth simulation and consisting of 40K triangles. By updating in real-time instead of rebuilding the BVH of the deforming model according to our heuristic, we are able to render the animation at 13 frames per second with 512 2 screen resolution using a dual-core P4 processor at 2.8 GHz. We present an efficient approach for interactive ray tracing of deformable or animated models. Unlike many of the recent approaches for ray tracing static scenes, we use bounding volume hierarchies (BVHs) instead of kd-trees as the underlying acceleration structure. Our algorithm makes no assumptions about the simulation or the motion of objects in the scene and dynamically updates or recomputes the BVHs. We also describe a method to detect BVH quality degradation during the simulation in order to determine when the hierarchy needs to be rebuilt. Furthermore, we show that the ray coherence techniques introduced for kd-trees can be naturally extended to BVHs and yield similar improvements. Finally, we compare BVHs to spatial kd-trees, which have been used recently as a replacement for AABB hierarchies. Our algorithm has been applied to different scenarios arising in animation and simulation and consisting of tens of thousands to a million triangles. In practice, our system can ray trace these models at 3-13 frames a second on a desktop PC including secondary rays.

A major reason for the recent advancements in ray tracing performance is the use of optimized acceleration structures, namely kdtrees based on the surface area heuristic (SAH). Though algorithms exist to build these search trees in O(nlogn), the construction times for larger scenes are still high and do not allow for rebuilding the kd-tree every frame to support dynamic changes. In this paper we propose modifications to previous kd-tree construction algorithms that significantly increase the coherence of memory accesses during construction of the kd-tree. Additionally we provide theoretical and practical results regarding conservatively sub-sampling of the SAH cost function.

Figure 1: Dynamic scenes ray traced using parallel fast construction of kd-tree. The scenes were rendered with shadows, 1 reflection (except HAND) and textures at 512x512 resolution on a 2-way Intel R○Core TM 2 Duo machine. a) HAND- a static model of a man with a dynamic hand; 47K static and 8K dynamic triangles; 2 lights; 46.5 FPS. b) GOBLIN- a static model of a hall with a dynamic model of a goblin; 297K static and 153K dynamic triangles; 2 lights; 9.2 FPS. c) BAR- a static model of bar Carta Blanca with a dynamic model of a man; 239K static and 53K dynamic triangles; 2 lights; 12.6 FPS. d) OPERA TEAM- a static model of an opera house with a dynamic model of 21 men without instancing; 78K static and 1105K dynamic triangles; 4 lights; 2.0 FPS. We present a highly parallel, linearly scalable technique of kd-tree construction for ray tracing of dynamic geometry. We use conventional kd-tree compatible with the high performing algorithms such as MLRTA or frustum tracing. Proposed technique offers exceptional construction speed maintaining reasonable kd-tree quality for rendering stage. The algorithm builds a kd-tree from scratch each frame, thus prior knowledge of motion/deformation or motion constraints are not required. We achieve nearly real-time performance of 7-12 FPS for models with 200K of dynamic triangles at 1024x1024 resolution with shadows and textures. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Three-Dimensional

Recent developments have produced several techniques for interactive ray tracing of dynamic scenes. In particular, bounding volume hierarchies (BVHs) are efficient acceleration structures that handle complex triangle distributions and can accommodate deformable scenes by updating (refitting) the bounding primitive without restructuring the entire tree. Unfortunately, updating only the bounding primitive can result in a degradation of the quality of the BVH, and in some scenes will result in a dramatic deterioration of rendering performance. The typical method to avoid this degradation is to rebuild the BVH when a heuristic determines the tree is no longer efficient, but this rebuild results in a disruption of interactive system response. We present a method that removes this gradual decline in performance while enabling consistently fast BVH performance. We accomplish this by asynchronously rebuilding the BVH concurrently with rendering and animation, allowing the BVH to be restructured within a handful of frames.

Abstract. Many computer graphics rendering algorithms and techniques use ray tracing for generation of natural and photo-realistic images. The efficiency of the ray tracing algorithms depends, among other techniques, upon the data structures used in the background. kd-trees are some of the most commonly used data structures for accelerating ray tracing algorithms. Data structures using cost optimization techniques based upon Surface Area Heuristics (SAH) are generally considered to be best and of high quality. During the last decade, the trend has been moved from off-line rendering towards real time rendering with the introduction of high speed computers and dedicated Graphical Processing Units (GPUs). In this situation, SAH-optimized structures have been considered too slow to allow real-time rendering of complex scenes. Our goal is to demonstrate an accelerated approach in building SAH-based data structures to be used in real time rendering algorithms. The quality of SAH-based data structures heavily depends upon split-plane locations and the major bottleneck of SAH techniques is the time consumed to find those optimum split locations. We present a parabolic interpolation technique combined with a golden section search criteria for predicting kd-tree split plane locations. The resulted structure is 30 % faster with 6 % quality degradation as compared to a standard SAH approach for reasonably complex scenes with around 170k polygons. 1

Bounding volume hierarchies have become a very popular way to speed up ray tracing. In this paper we present a novel traversal and approximation scheme for bounding volume hierarchies, which is comparable in speed, has a very compact traversal algorithm and uses only 25 % of the memory, compared to a standard bounding volume hierarchy. 1

Bounding volume hierarchies (BVH) are a commonly used method for speeding up ray tracing. Even though the memory footprint of a BVH is relatively low compared to other acceleration data structures, they still can consume a large amount of memory for complex scenes and exceed the memory bounds of the host system. This can lead to a tremendous performance decrease on the order of several magnitudes. In this paper we present a novel scheme for construction and storage of BVHs that can reduce the memory consumption to less than 1 % of a standard BVH. We show that our representation, which uses only 2 bits per node, is the smallest possible representation on a per node basis that does not produce empty space deadlocks. Our data structure, called the Minimal Bounding Volume Hierarchy (MVH) reduces the memory requirements in two important ways: using implicit indexing and preset surface reduction factors. Obviously, this scheme has a non-negligible computational overhead, but this overhead can be compensated to a large degree by shooting larger ray bundles instead of single rays, using a simpler intersection scheme and a two-level representation of the hierarchy. These measure enable interactive ray tracing performance without the necessity to rely on out-of-core techniques that would be inevitable for a standard BVH.

The use of SIMD ray packets [24] has been an important step forward in ray tracing performance. It is the first significant acceleration process based on a strategy that deals with “small ” ray bundles. In order to improve the outcome for wider bundles, very powerful data structures have been used. However, these new data structures are not the most powerful when dealing with single rays or SIMD rays. They are therefore unsuitable, and this incompatibility raised the problem of incoherent rays which, such as secondary rays or global illumination rays (i.e. most the rays traced in real life ray tracing) are impossible to process efficiently. An original coupled use of BSP and BVH trees is proposed to overcome this disadvantage. Each tree, used individually, can efficiently boost both small ray bundles and wide ray bundles. Used together in a coupled strategy, they can achieve a global speedup of +50%. Index Terms: I.3.7 [Computer Graphics]: Ray Tracing— 1

Abstract. The generation of natural and photorealistic images in computer graphics, normally make use of a well known method called ray tracing. Ray tracing is being adopted as a primary image rendering method in the research community for the last few years. With the advent of todays high speed processors, the method has received much attention over the last decade. Modern power of GPUs/CPUs and the accelerated data structures are behind the success of ray tracing algorithms. kd-tree is one of the most widely used data structures based on surface area heuristics (SAH). The major bottleneck in kd-tree construction is the time consumed to find optimum split locations. In this paper, we propose a prediction algorithm for animated ray tracing based on Kalman and Wiener filters. Both the algorithms successfully predict the split locations for the next consecutive frame in the animation sequence. Thus, giving good initial starting points for one dimensional search algorithms to find optimum split locations – in our case parabolic interpolation combined with golden section search. With our technique implemented, we have reduced the “running kd-tree construction ” time by between 78 % and 87 % for dynamic scenes with 16.8K and 252K polygons respectively. 1

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 received much recent attention by researchers 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 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. 1.