Bounding Volumes

Enclose complex objects within a simple-to-intersect object. If the ray does not intersect the simple object then its contents can be ignored. If the ray does intersect the bounding volume it may or may not intersect the enclosed object. The likelihood that it will strike the object depends on how tightly the volume surrounds the object.

Spheres were one of the first bounding volumes used in raytracing, because of their simple ray-intersection and the fact that only one is required to enclose a volume.
	However, spheres do not usually give a very tight fitting bounding volume. More frequently, axis-aligned bounding boxes are used. Clearly, hierarchical or nested bounding volumes can be used for even greater advantage.