SPV_MESA_fragment_coverage_mask

SPV_MESA_fragment_coverage_mask

To report problems with this extension, please open a new issue at:

https://github.com/KhronosGroup/SPIRV-Registry

Complete

This extension is written against the SPIR-V Specification, Version 1.6 Revision 6.

This extension requires SPIR-V 1.0.

This extension introduces a new built-in decoration, FragmentCoverageMaskMESA, for fragment shader inputs. This new capability addresses limitations in how the existing SampleMask built-in behaves during per-sample fragment shader execution, providing a more direct and consistent mechanism for accessing the full coverage mask of the fragment across all samples.

The existing input decoration SampleMask is intended to provide the set of samples that contribute to a fragment. However, when a fragment shader executes in a per-sample shading mode (e.g. when a fragment shader input is decorated with SampleId or SamplePosition), the SampleMask built-in is masked to only include the bits corresponding to the specific sample(s) currently being processed by that invocation. In the case of full per-sample shading (one invocation per sample), this results in only a single bit being set. When a sample shading rate is specified (e.g. via Vulkan’s minSampleShading parameter), an invocation may process a subset of samples, and SampleMask will contain the corresponding subset of bits.

This constrained behaviour makes it impossible for the shader to determine the entire set of samples covered by the fragment. Specifically, it is impossible to directly map the functionality of input coverage mask in D3D10 API, making it very difficult to port some D3D10 shaders to SPIR-V. Furthermore, when per-sample shading is triggered by a sample shading rate rather than by the use of SampleId or SamplePosition, those built-in variables are not available, making it impossible to derive the full coverage mask from SampleMask alone.

The FragmentCoverageMaskMESA built-in decoration provides a fragment shader input that is guaranteed to contain the complete set of samples covered by the fragment, regardless of the fragment’s execution mode.

Specifically, when a fragment shader is executing in per-sample mode:

Note that when SampleId is available, fragment shaders using FragmentCoverageMaskMESA can recover the value of SampleMask from FragmentCoverageMaskMESA by computing a bitwise AND with a bit set at position SampleId.

The new functionality is enabled under the FragmentCoverageMESA capability.

To use this extension within a SPIR-V module, the following OpExtension must be present in the module:

This extension introduces a new capability:

Builtin IDs added:

None.

An OpExtension must be added to the SPIR-V for validation layers to check legal use of this extension: