1. A method of performing ray tracing image processing for a three-dimensional scene, comprising: providing a spatial index having a first portion and a second portion and having at least one branch from a node in the first portion to a node in the second portion, wherein nodes in the first portion correspond to bounding volumes which partition the three-dimensional scene with respect to a first coordinate system, and wherein nodes in the second portion correspond to bounding volumes which partition the three-dimensional scene with respect to a second coordinate system; issuing a ray into the three-dimensional scene; tracing the ray through the three-dimensional scene by taking branches to nodes corresponding to bounding volumes intersected by the ray until the at least one branch from a node in the first portion to a node in the second portion is reached; and selecting a coordinate system to use when tracing the ray through the portion of the spatial index which partitions the three-dimensional scene with respect to the second coordinate system.