Everyone says the method of obtaining a bounding sphere by using the AABB's center as the sphere center and half diagnal length as the radius will not give an optimal bounding sphere.
But I can't think of a case where it's not optimal. It looks pretty optimal to me: the radius can't be any smaller otherwise one of the AABBs' corner vertices will come out.
But I can't think of a case where it's not optimal. It looks pretty optimal to me: the radius can't be any smaller otherwise one of the AABBs' corner vertices will come out.