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eosofsky

Just to summarize what this tree is showing- the space is divided up in increasingly smaller bounding boxes so that (in this case) the algorithm will have to check a maximum of 2 primitives (triangles) instead of all of them. This allows for increased efficiency since only the primitives that have a chance of being hit are actually checked for collision.

The idea of an active ray is a ray that at some node still has a chance of hitting a primitive lower down in the tree. As soon as a ray does not intersect the bounding box represented by a node, it can stop being considered.