Compression and decompression of sub-primitive presence indications for use in a rendering system

Abstract

Compressed data is decompressed to determine sub-primitive presence indications in a rendering system. A received compressed block of sub-primitive presence indications is subdivided into a plurality of parent regions, each subdivided into a plurality of child regions. A hierarchical representation of the block of sub-primitive presence indications is provided, wherein: (i) for each parent region whose child regions all have the same presence state, parent-level data is included to represent the presence state of the parent region without child-level data for the child regions within the parent region being included in the hierarchical representation, and (ii) for each parent region whose child regions do not all have the same presence state, child-level data for the child regions within the parent region is included in the hierarchical representation to represent the presence states for the child regions within the parent region. If child-level data is included in the hierarchical representation, child-level data for said one of the parent regions is used to determine one or more sub-primitive presence indications in the parent region. If child-level data is not included in the hierarchical representation, parent-level data is used, without child-level data, to determine one or more sub-primitive presence indications in the parent region.