I'm a noob to DS&A, but I'm writing a little add-on for Blender and have come up against BVH and kd-trees. What makes one more popular for 'within objects' and the other for 'between'?

BVHes handle sparsity and very large scenes with variable density very well, so they are good to be able to see which objects you might intersect. kd-Trees are much better for objects because they handle the high density of geometry very well and are thus very effective if you want to test against a million triangles in close proximity, for example.

Thanks!