Graphics processors

A graphics processor comprising a processing circuit for processing primitives, wherein the graphics processor is configured to:

• for a sequence of primitives to be rendered for a render output:
• cause the processing circuit to:
• perform a first, pre-pass operation in which primitives in the sequence of primitives to be rendered are processed into respective sets of one or more fragments, each fragment associated with a respective set of one or more sampling positions within the render output, and wherein fragments are depth tested against a depth buffer for the sequence of primitives, wherein when a fragment survives the depth testing, a corresponding entry for the sampling position or positions to which the fragment relates is updated accordingly to record the depth value of the fragment;
• and
• thereafter perform a second, main pass operation in which primitives that were processed by the first, pre-pass operation and which could potentially have updated the depth buffer in the first, pre-pass operation are processed again into their respective sets of one or more fragments for processing for respective sampling positions within the render output, wherein the processing for a fragment that was processed by the first, pre-pass operation and which could potentially have updated the depth buffer in the first, pre-pass operation comprises testing whether the fragment's depth value matches a corresponding entry in the depth buffer for the sampling position or positions to which the fragment relates to determine whether the fragment should be further processed for the render output;
• wherein during the first, pre-pass operation, the processing circuit is configured to perform fragment depth testing using a specified depth test function for the fragment, whereas during the second, main pass operation, the processing circuit is configured to use an equals depth test function for all fragments for primitives that were processed by the first, pre-pass operation and which could potentially have updated the depth buffer in the first, pre-pass operation regardless of the specified depth test function for the fragment;
• and
• wherein for fragments for primitives that were processed by the first, pre-pass operation and for which an exclusive depth test function was used for the depth testing during the first, pre-pass operation, the processing circuit is configured to further control the further processing of the fragments during the second, main pass operation based on a tracking bit associated with the sampling position or positions to which the fragments relate, wherein the tracking bit can be set/cleared during the respective first, pre-pass and second, main pass operations and used during the second, main pass operation to determine for a fragment whose depth value matches the corresponding entry in the depth buffer for the sampling position or positions to which the fragment relates whether or not the fragment should nonetheless be culled.