1. A method of forming an acceleration structure for intersection testing of rays, comprising: receiving data defining a plurality of objects located in a 3-D scene, each of the objects being composed of primitives; analysing the primitives to determine acceleration structure elements for the objects for inclusion in a hierarchical acceleration structure for use in intersection testing of rays in the 3-D scene, wherein an acceleration structure element for an object is defined using data that locates one or more individual constituent 3-D volumes and specifies a respective extent of each of the constituent 3-D volumes, wherein determining the acceleration structure element for the object comprises selecting how many constituent 3-D volumes will be used and a location and an extent of each constituent 3-D volume based on a number of objects which are to be bounded by that acceleration structure element; and defining relationships between the determined acceleration structure elements in the hierarchical acceleration structure.