Threaded octree structures for fast neighbor voxel processing in a parallel ray tracer

Abstract

In the field of Computer Graphics, Ray Tracing has so far been the the best algorithm for rendering of realistic three dimensional images created by mathematical models. Ray Tracing is also known for its very large computation times, where the computation depends on the picture resolution, the number of objects and the complexity of the scene.In the last few years , sewr<tl algorithms and special computer architectures have been proposed making Ray Tra(:i.ng comp·.ttationally less expensive. Parallelizing the algorithm has immensely speeded up the process. 2Rendering images whose entities are built by mathematical models involves scene description, object definition and the rendering process. Ray Tracing implements the camera model which is very similar to human perception. The camera's view point is the focal point, and the film in the camera is the image plane. Ray Tracing computes the paths of light rays, and evaluates the objects described in the scene as seen from the view point. It computes reflection and refraction of light from the objects and the color of the object as perceived from the view point.One of the computationally most expensive process in the algorithm is finding intersections of the ray and the objects described in the scene. Several methods for reducing these intersection calculations have been proposed. The method of using Bounding Volumes has given the best results in reducing the number of Ray-Object intersection calculations. In this method complex objects are bounded by simple bounding Volumes, or a number of objects can be bounded by one simple bounding volume. The intersection of the ray can be tested with this simple bounding volume.If there is an intersection with the bounding volume in the direction of the ray, only then does the intersection of the ray with the objects need to be considered.An excellent scheme of creating bounding volumes is the Octree technique. It allows for dynamic subdivision of Object space into cubes of decreasing volume.These cubes also called as voxels (volume elements) can hold a certain number of objects described in the scene. Ray intersections are first tested with a voxel in the direction of the ray, and if an intersection exists, Ray-Object intersections are calculated with the objects contained in the voxel. One of the major problems here, is, finding the next neighboring voxel in the direction of the ray. Algorithms which have been proposed so far, address this problem by finding the voxel address from the top of the octree structure. This idea of finding the neighboring voxel from the top of the octree structure has been found to introduce a very significant computation overhead. In this thesis, image space parallelism of the ray tracer was implemented on a 4 Cogent XTM parallel workstation using Cogent's Kernel Linda parallel programming language. Nine T800 processors each with 4MB of memory were available. Processing times from one to nine processors have been registered. Five experimental images were developed, where each image was built to i...