A system rapidly builds bounding volume hierarchies for ray tracing using both the CPU cores and an integrated graphics processor. The hierarchy is built directly into shared memory (between the CPU and GPU). The method starts by sorting the triangles along a space-filling curve, and then quickly sets up a number of mini-trees with a small number of triangles in them, which includes computing the bounding boxes of the mini-trees. This makes it possible to build the mini-trees using a surface-area heuristic in parallel on the graphics processor, while at the same time, the trees above the mini-trees are built in a top-down fashion using the CPU cores.