A mixed dataflow algorithm for ray tracing on the CRAY T3E

The ray tracing scheme is one of the most complete and efficient rendering methods. A major drawback of this model is its high computational cost which limits its practical use. Moreover, the quest for realistic rendering requires larger and larger databases to describe scenes. With the development of distributed memory parallel computers such as the CRAY T3E, the most promising way to improve ray traced pictures productions seems to be parallelization which offers both increased CPU power and memory facilities. In the ray tracing algorithm, each pixel of the screen is processed independently. A natural way of parallelization is to distribute pixels over the machine nodes. However, since we want to deal with large scenes, objects also have to be distributed among processors, so a modified parallel algorithm is necessary. Strategies based on object dataflow have been proposed, but their communication load is too high. More efficient algorithms have to reduce the number of messages. Therefore we propose a mixed dataflow approach : each message will contain several pieces of information on both objects and rays. By this way, we hope to limit the communication load and to ensure a dynamic load balancing. A parallel ray tracing algorithm on the T3E using our mixed dataflow approach is implemented. The results are very encouraging since the computation time and the communication flows may be reduced by significant factors. Scalability is globally improved, mainly because saturation occurs for larger problem sizes.