1. A method of ray tracing, comprising: accessing an acceleration structure for a 3-D scene that is composed of primitives and in which rays are to be traced, the acceleration structure comprising a plurality of nodes, each defining a surface bounding a volume in the 3-D scene; receiving a plurality of rays; identifying a plurality of nodes of the acceleration structure to be tested for intersection with a selection of one or more of the rays; loading definition data for different of the plurality of nodes into different test cells of a plurality of test cells; testing, in its respective test cell, each of the nodes for intersection with each ray of the selection of rays; and outputting results of the intersection testing.