1. A method of forming an acceleration structure for intersection testing of rays, comprising: receiving, from a computer readable medium, data defining at least a portion of a 3-D scene; identifying, based on the scene data, an object in the 3-D scene, the object having a geometric span; determining 3-D spatial locations and radii for a plurality of spherical acceleration elements, each respective location and radius selected so that each spherical acceleration element bounds the object, has a radius larger than the geometric span of the object, and the radius of each acceleration element is offset from the geometric span of the object; and storing data defining the 3-D spatial locations and radii of the spherical acceleration elements on a computer readable medium with an indication that positive intersection results between a ray and each of the plurality of acceleration elements are required to conclude that the ray may intersect the object.