Graphics processors

A method of operating a graphics processor, the method comprising:

• for a sequence of primitives to be processed for a render output:
• performing a first, pre-pass operation in which primitives in the sequence of primitives are processed by rasterising the primitives 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 as part of the first, pre-pass operation fragments for the primitives in the sequence of primitives are processed to determine visibility information for the sequence of primitives, the visibility information being usable to determine whether or not fragments for a primitive in the sequence of primitives should subsequently be processed further for the render output;
• and
• thereafter performing a second, main pass operation for which the further processing of fragments for primitives that were processed during the first, pre-pass operation is controlled based on the determined visibility information for the sequence of primitives such that fragments for which the visibility information indicates that the fragments should be processed further for the render output are processed again during the second, main pass operation by rendering the fragments to determine rendered output data for sampling positions associated with the fragments, whereas for fragments for which the visibility information indicates that the fragments should not be processed further for the render output some or all of the processing during the second, main pass is omitted;
• the method further comprising:
• determining that a primitive in the sequence of primitives to be processed by the first, pre-pass operation is incompatible with being processed by the first, pre-pass operation;
• and
• omitting some or all of the first, pre-pass operation in respect of that primitive and subsequently processing the primitive that was determined to be incompatible with the first, pre-pass operation by a third, fallback operation.

A graphics processor configured to perform tile-based rendering in which a render output is sub-divided into a plurality of tiles for rendering,

• the graphics processor comprising:
• a rendering circuit that is operable to render tiles;
• and
• a command processing circuit that is operable to receive commands for graphics processing work and schedule processing work for the rendering circuit accordingly;
• wherein in response to a command to render a tile, the command processing circuit causes the rendering circuit to:
• for a sequence of primitives that has been identified to be processed for the tile using a respective primitive list or lists applying to the tile;
• perform a first, pre-pass operation in which primitives in the sequence of primitives for the tile are processed by rasterising the primitives 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 as part of the first, pre-pass operation fragments for primitives in the sequence of primitives are processed to determine visibility information for the sequence of primitives, the visibility information being usable to determine whether or not the fragments for a primitive in the sequence of primitives should subsequently be processed further for the render output;
• and
• thereafter perform a second, main pass operation for which the further processing of fragments for primitives that were processed during the first, pre-pass operation is controlled based on the determined visibility information for the sequence of primitives such that fragments for primitives for which the visibility information indicates that the fragments should be processed further for the render output are processed again during the second, main pass operation by rendering the fragments to determine rendered output data for sampling positions associated with the fragments, whereas for fragments for which the visibility information indicates that the fragments should not be processed further for the render output some or all of the processing during the second, main pass is omitted.

A graphics processor that is configured to generate a render output, the graphics processor comprising a rendering circuit that is operable to render sequences of primitives for the render output, wherein the graphics processor is configured to:

• for a sequence of primitives to be processed for a render output:
• cause the rendering circuit to:
• perform a first, pre-pass operation in which primitives in the sequence of primitives are processed by rasterising the primitives 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 as part of the first, pre-pass operation fragments for the primitives in the sequence of primitives are processed to determine visibility information for the sequence of primitives, the visibility information being usable to determine whether or not fragments for a primitive in the sequence of primitives should subsequently be processed further for the render output;
• and
• thereafter perform a second, main pass operation for which the further processing of fragments for primitives that were processed during the first, pre-pass operation is controlled based on the determined visibility information for the sequence of primitives such that fragments for which the visibility information indicates that the fragments should be processed further for the render output are processed again during the second, main pass operation by rendering the fragments to determine rendered output data for sampling positions associated with the fragments, whereas for fragments for which the visibility information indicates that the fragments should not be processed further for the render output some or all of the processing during the second, main pass is omitted;
• the rendering circuit further configured to:
• determine whether primitives in the sequence of primitives to be processed by the first, pre-pass operation are incompatible with being processed by the first, pre-pass operation;
• and
• when it is determined that a primitive is incompatible with being processed by the first, pre-pass operation, omit some or all the first, pre-pass operation in respect of that primitive and subsequently process the primitive that was determined to be incompatible with the first, pre-pass operation by a third, fallback operation.