CS 551/651: Advanced Computer Graphics

Goal: Experiment with different bounding volume strategies to accelerating ray tracing

Assigned: Tuesday, February 9, 1999

Due: Tuesday, February 23, 1999

Relevant reading/files/software:

• See Watt and Watt, Chapter 9
• The class web page will note any issues that arise.
• Some many-triangle test scenes will also be posted on the course web page.
• Your modified version of RSRT, which can output PPM files.
• [Advanced] A overwhelmingly comprehensive ray-tracing resource on the web is Ray Tracing News.
• [Advanced] A software package which creates OBB hierarchies (albeit for collision detection, not ray tracing) is RAPID. With some work, you should be able to use code from RAPID to create OBB hierarchies.

Synopsis: You will continue modifying the RSRT source code, this time to implement ray-tracing acceleration via bounding volume hierarchies. RSRT already supports spherical bounding volumes, though not organized into a hierarchy. In addition, RSRT already supports axis-aligned bounding boxes as a primitive, so the code to use AABBs as a bounding volume should be straightforward. Your modified RSRT should read the input file, specified using the RSRT input format described in the RSRT documentation. This input format allows many different primitives, including spheres, discs, boxes, triangles, and so on. You need only organize triangles into a bounding volume hierarchy—all other primitives are optional. Ideally, your modified ray-tracer will permit other types of primitives even if they are not organized into the hierarchy, but this is not required. You must support two types of hierarchical bounding volumes: spheres and axis-aligned bounding boxes. Command-line options should be provided to choose which type of volume is used. You may optionally support other bounding volumes, e.g. oriented bounding boxes, slabs, ellipsoids, cylinders, etc., or hybrid schemes using multiple types of bounding volumes.

Accelerating a ray tracer with bounding volume hierarchies involves two basic tasks:

• Building the hierarchy. You should build the hierarchy automatically upon reading the input file. You will probably wish to use an octree, K-D tree, or BSP tree as discussed in class to partition the triangles into a hierarchy of clusters, then assign tight-fitting bounding volumes to each cluster; octrees are the simplest to code. There is a tradeoff between the number of triangles in the smallest cluster and the depth of the hierarchy; part of the assignment is to experiment with this tradeoff and try to produce an optimal clustering strategy. After creating the hierarchy your program should print out the number of nodes and maximum depth of the hierarchy.
• Intersecting rays and volumes. This is a frequent task and needs to be fast.

You should include a README file that documents what strategy or strategies you used for these tasks, how well they worked, and anything else you learned in the process. Also describe what command-line options are necessary to run your version of RSRT with different bounding volumes and (perhaps) different clustering strategies.

Grading: The usual 10-point scale. An assignment which does everything required perfectly will receive 8 points. An assignment implementing "extras" such as bounding volumes beyond spheres and AABBs, or clustering strategies more sophisticated than an octree, or additional primitives besides triangles, can get up to 9 points. The single fastest ray tracer (according to our test scenes, which will contain many triangles inconveniently distributed) will get 10 points.

Turning in the assignment: Before class starts on Tuesday, February 23, you should:

• Tar up a directory containing your code, and gzip the tar file.
• Send the TA (Dale Newfield) an e-mail message containing the gzipped tar file as an attachment.
• Dale will compile your ray tracer and test the result against some sample scenes we will provide on the web page, and some additional test scenes

• Start early. Don’t underestimate how long this will take, or the difficult of debugging the dreaded "black screen" effect.
• Get spheres working first, then AABBs, then work on any extras.
• You will need to modify the basic ray-tracing function to traverse a hierarchy depth-first, rather than simply comparing the ray with a list of objects. Don’t forget to modify the shadow ray traversal as well.
• You may want to have a "debugging" command line option which shows the bounding volumes as real (perhaps transparent) objects.