Rendering of primitives with pre-pass and fallback operations
When performing tile-based rendering a first, pre-pass operation in which primitives in a sequence of primitives for a tile 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, is performed. Thereafter a second, main pass operation is performed in 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 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 first, pre-pass can be stopped to process primitives in a fail-safe manner if necessary.
1 . A method of operating a graphics processor, the method comprising:
for a sequence of primitives to be rendered for a render output:
performing a first, pre-pass operation in which primitives in the sequence of primitives are processed to determine visibility information for the sequence of primitives;
determining whilst performing the first, pre-pass operation 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;
stopping the first, pre-pass operation for the sequence of primitives such that some or all of the first, pre-pass operation is omitted in respect of the incompatible primitive and thereafter performing a second, main pass operation in which primitives in the sequence of primitives that have been processed so far by the first, pre-pass operation, including the primitive in the sequence of primitives immediately before the incompatible primitive, are processed again to generate rendered output data for sampling positions within the render output based on the determined visibility information for the sequence of primitives; and
when the second, main pass operation reaches the incompatible primitive: switching to a third, fallback operation and processing a subsequent set of one or more primitives including the incompatible primitive by the third, fallback operation.
2 . The method of claim 1 , further comprising:
determining whilst processing primitives by the third, fallback operation that a further primitive in the sequence of primitives should be processed by the first, pre-pass operation; and
switching from the third, fallback operation to the first, pre-pass operation and processing by the first, pre-pass operation a subsequent set of one or more primitives including the further primitive that has been determined should be processed by the first, pre-pass operation.
3 . The method of claim 1 , wherein when the graphics processor switches between operations, the method comprises storing suitable information to allow the subsequent processing operation to continue processing the sequence of primitives including the primitive that triggered the switching of operations.
4 . The method of claim 1 , wherein the first, pre-pass operation comprises performing depth testing to update a depth buffer for the sequence of primitives, and wherein when the first, pre-pass operation is stopped for the sequence of primitives, the content of the depth buffer remains available for the subsequent third, fallback operation.
5 . The method of claim 2 , wherein the third, fallback operation comprises performing depth testing to update a depth buffer for the sequence of primitives, and wherein when the graphics processor is switched from the third, fallback operation back to the first, pre-pass operation, the content of the depth buffer remains available for the subsequent first, pre-pass operation.
6 . The method of claim 2 , comprising tracking which primitives in the sequence of primitives have been processed so far either by the first, pre-pass operation or by the third, fallback operation, and wherein when the graphics processor switches from the third, fallback operation to the first, pre-pass operation, the method comprises using the tracking to avoid some or all repeated processing of primitives that have been processed already either by the first, pre-pass operation or the third, fallback operation during the resumed first, pre-pass operation.
7 . The method of claim 1 , wherein primitives in the sequence of primitives are associated with respective indicators indicating whether or not the primitive is compatible with being processed by the first, pre-pass operation and wherein the first, pre-pass operation comprises checking the respective indicators to determine whether or not a primitive is compatible with the first, pre-pass, and wherein the step of stopping the first, pre-pass operation is based on the first, pre-pass operation determining from its indicator that a primitive is incompatible with the first, pre-pass operation.
8 . The method of claim 7 , wherein the second, main pass operation also comprises checking the respective indicators to determine whether or not a primitive is compatible with the first, pre-pass operation, and wherein the switching to the third, fallback operation is performed based on the second, main pass operation determining from its indicator that a primitive is incompatible with the first, pre-pass operation.
9 . The method of claim 1 , further comprising a step of classifying primitives in the sequence of primitives as being compatible or not with being processed by the first, pre-pass operation.
10 . The method of claim 2 , wherein the step of determining whilst processing primitives by the third, fallback operation that a further primitive in the sequence of primitives should be processed by the first, pre-pass operation comprises determining that the further primitive is compatible with the first, pre-pass operation.
11 . The method of claim 10 , wherein the step of determining whilst processing primitives by the third, fallback operation that a further primitive in the sequence of primitives should be processed by the first, pre-pass operation comprises determining that the further primitive is compatible with the first, pre-pass operation and determining whether one or more other conditions for triggering a switch back to the first, pre-pass operation are met.
12 . 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 rendered for a render output:
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;
determine whilst performing the first, pre-pass operation 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;
stop the first, pre-pass operation for the sequence of primitives such that some or all of the first, pre-pass operation is omitted in respect of the incompatible primitive and thereafter performing a second, main pass operation in which primitives in the sequence of primitives that have been processed so far by the first, pre-pass operation, including the primitive in the sequence of primitives immediately before the incompatible primitive, are processed again to generate rendered output data for sampling positions within the render output and in which second, main pass operation 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; and
when the second, main pass operation reaches the incompatible primitive: switch to a third, fallback operation and process a subsequent set of one or more primitives including the incompatible primitive by the third, fallback operation.
13 . The graphics processor of claim 12 , being further configured to:
determine whilst processing primitives by the third, fallback operation that a further primitive in the sequence of primitives should be processed by the first, pre-pass operation; and
switch from the third, fallback operation to the first, pre-pass operation and processing by the first, pre-pass operation a subsequent set of one or more primitives including the further primitive that has been determined should be processed by the first, pre-pass operation.
14 . The graphics processor of claim 12 , wherein when the graphics processor switches between operations, the graphics processor is configured to store suitable information to allow the subsequent processing operation to continue processing the sequence of primitives including the primitive that triggered the switching of operations.
15 . The graphics processor of claim 12 wherein the graphics processor when performing the first, pre-pass operation is configured to perform depth testing to update a depth buffer for the sequence of primitives, and wherein when the first, pre-pass operation is stopped for the sequence of primitives, the content of the depth buffer remains available for the subsequent third, fallback operation.
16 . The graphics processor of claim 13 , wherein the graphics processor when performing the third, fallback operation is configured to perform depth testing to update a depth buffer for the sequence of primitives, and wherein when the graphics processor is switched from the third, fallback operation back to the first, pre-pass operation, the content of the depth buffer remains available for the subsequent first, pre-pass operation.
17 . The graphics processor of claim 13 , wherein the graphics processor is configured to track which primitives in the sequence of primitives have been processed so far either by the first, pre-pass operation or by the third, fallback operation, and wherein when the graphics processor switches from the third, fallback operation to the first, pre-pass operation, the graphics processor uses the tracking to avoid some or all repeated processing of primitives that have been processed already either by the first, pre-pass operation or the third, fallback operation during the resumed first, pre-pass operation.
18 . The graphics processor of claim 12 , wherein primitives in the sequence of primitives are associated with respective indicators indicating whether or not the primitive is compatible with being processed by the first, pre-pass operation and wherein the first, pre-pass operation comprises checking the respective indicators to determine whether or not a primitive is compatible with the first, pre-pass, and wherein the step of stopping the first, pre-pass operation is based on the first, pre-pass operation determining from its indicator that a primitive is incompatible with the first, pre-pass operation.
19 . The graphics processor of claim 18 , wherein the graphics processor is also configured to check the respective indicators to determine whether or not a primitive is compatible with the first, pre-pass operation during the second, main pass operation, and wherein the switching to the third, fallback operation is performed based on the second, main pass operation determining from its indicator that a primitive is incompatible with the first, pre-pass operation.
20 . The graphics processor of claim 13 , wherein the step of determining whilst processing primitives by the third, fallback operation that a further primitive in the sequence of primitives should be processed by the first, pre-pass operation comprises determining whether one or more conditions for triggering a switch back to the first, pre-pass operation are met.