Interactive Isosurface Ray Tracing of Large Octree Volumes

Abstract

Figure 1: Large volume data ray-traced at 512 2 using octrees for compression and acceleration. From left to right: (1) LLNL Richtmyer-Meshkov instability field (shown at timestep 270, with an isovalue of 100). (2) Closer view of the previous scene. (3) Utah CSAFE heptane simulation (timestep 152, isovalue 42). Data is losslessly compressed into an octree volume to occupy less than one quarter the size of the original 3D array. Our approach permits storage of large data such as the LLNL simulation, and full sequences of medium-size data such as the heptane, in main memory of consumer machines. Frame rates on an Intel Core Duo 2.16 GHz laptop with 2 GB RAM are 2.4, 1.3, and 3.3 fps respectively. On a 16-node NUMA 2.4 GHz Opteron workstation, these images render at 17.9, 9.8, and 22.0 fps. ABSTRACTWe present a technique for ray tracing isosurfaces of large compressed structured volumes. Data is first converted into a losslesscompression octree representation that occupies a fraction of the original memory footprint. An isosurface is then dynamically rendered by tracing rays through a min/max hierarchy inside interior octree nodes. By embedding the acceleration tree and scalar data in a single structure and employing optimized octree hash schemes, we achieve competitive frame rates on common multicore architectures, and render large time-variant data that could not otherwise be accomodated.

Coherent multiresolution isosurface ray tracing

“…Knoll et al [1] implemented a single-ray traversal scheme for rendering compressed octree volume data. By employing one structure for both the min/max acceleration tree and the voxel data itself, the authors were able to render large volumes given limited main memory.…”

“…In this scenario, a coherent system may perform worse than a single-ray tracer. On scenes with poor coherence, coherent isosurface ray tracing using conservative 2x2 ray packets [20] has produced performance generally on par with a single-ray system [1].…”

“…Alternately, the octree can be built from a scalar field in a 3D array. Such a build is detailed by Knoll et al [1]; this paper only discusses extensions to the construction technique that allow for multiresolution. In brief, construction is a bottom-up procedure in which identical or similar voxels are merged together into a single voxel within a parent node.…”

“…In practice however, the memory savings of this approach were too small to justify the added computation. Our multiresolution system is constructed identically to the staticresolution implementation [1], with separate eight-value arrays for both child pointers and scalar leaves. The only difference is that non-leaf scalars are actually employed in multiresolution rendering.…”

“…This flexibility enables use of an adaptive-resolution octree, which we can use as both a natively compressed data format and an acceleration structure for rendering. Previous work raytraced large octree volumes interactively, but on substantial workstation hardware [1]. In this work, we optimize isosurface ray tracing with a coherent octree traversal technique, then employ a multiresolution level of detail scheme to ensure coherence and hence performance.…”