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

Decompressing or decoding data representing a block of sub-primitive presence indicators which indicate opacity states of sub-primitives or sub-triangles in a primitive for intersection testing in graphics processing. In embodiments the opacity states are: wholly opaque or present, wholly transparent, or that the sub-primitive is partially present i.e. partly opaque and partly transparent. This is useful for rendering alpha or masked textures in punch-through primitives representing complex shapes like leaves, where part of the primitive is given a see-through or transparent texture. By encoding sub-primitive presence indicators, the ray tracer or intersection tester (e.g. in a shader) can run with less interruption for detailed testing of whether an intersection is at a transparent location, hence introducing less latency. In this invention the block of presence indicators is split into sub-blocks, and the compressed data comprises, for each sub-block, an index to indicate one of a list of candidate arrangements of presence indicators in that sub-block. Preferably, there are two palettes or lists of candidates, one comprising only present and partially present values and one comprising only absent and partially present values. The compressed block may also comprise transformations to be applied to the indicated candidate to obtain the final pattern.