BVH leaf's hit() function may be called twice

[kyamant]

During the construction of the BVH tree, the following case will create a leaf's parent node whose left and right pointers being equal:

if (object_span == 1) {
        left = right = objects[start];

Which will result in the leaf object's hit() function getting invoked twice in bvh_node::hit():

bool hit_left = left->hit(r, t_min, t_max, rec);
bool hit_right = right->hit(r, t_min, hit_left ? rec.t : t_max, rec);

This can be remedied by checking if right is not equal to left before executing right->hit(...).

[trevordblack]

The bvh_node::hit function is designed to be as fast as possible.

We expect (and want) the BVH to be filled as evenly as possible.
A node with only a single object_span should either be a scene hittable_list with a single object, or a leaf of a much bigger tree.

Optimizing a bvh with 1 node, or the leaves of a BVH, is an O(1) optimization.
We would make the O(1) case faster.

We would unfortunately be adding more checks to every other node. So we would have a O(LogN) loss.

This is a bad trade off.
Furthermore, checking the same node twice in a row won't actually be twice as long. With caching it'll take meaningfully less than that, maybe ~1.5-1.7x

If you're familiar with profiling code, I strongly suggest profiling your own code and getting an intuition for where the optimization and bottlenecks are in the code.

This was an awesome catch.
It's just not necessarily a performance bug.

[kyamant]

Thanks for the clarification.
Closing the issue...