GRAPHICS PROCESSING SYSTEMS

A set of plural primitives to be processed (140) for a frame for output in a graphics processing system is divided into plural subsets of primitives (141), such that each primitive is allocated into one subset of primitives only, and each subset of primitives contains only contiguous primitives. For each subset of primitives that the set of primitives has been divided into, data representative of the primitives of the subset of primitives, and data indicating the volume of space that the subset of primitives falls within, is generated and stored (143). This data is stored as a tree representation of the set of primitives with each leaf node of the tree representing a respective sub-set of primitives that the set of primitives has been divided into (142). The tree representation of the subsets of primitives is then used, e.g., to determine respective sets of subsets of primitives that could cast a shadow from a light source for respective regions of an output frame to be generated.