KhronosGroup / KhronosGroup/Vulkan-Portability
VkPipelineMultisampleStateCreateInfo::minSampleShading cannot be supported in Metal
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Description
Vulkan requires VkPipelineMultisampleStateCreateInfo::minSampleShading to be honoured if sampleShadingEnable is enabled:
If sample shading is enabled, an implementation must invoke the fragment shader at least max(VkPipelineMultisampleStateCreateInfo::minSampleShading × VkPipelineMultisampleStateCreateInfo::rasterizationSamples, 1) times per fragment.
Metal does not support forced running of Per-Sample shading unless the shader actually uses Per-Sample built-ins.
I think we need to add a VkPhysicalDevicePortabilitySubsetFeaturesKHR::supportMinSampleShading capability, in order to allow an app to query whether the implementation honours the above Vulkan requirement.
I am not aware of any Vulkan CTS tests that covers this and will fail, but it has appeared as a user request in MoltenVK.
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Research direction
Start by reviewing the Vulkan requirement quoted in the issue and the referenced MoltenVK user request. Check existing Vulkan CTS coverage for minSampleShading, then define what capability reporting and conformance coverage would be needed; done means the portability limitation is queryable and validated.
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Assessment
- Domain
- api, computer-graphics
- Issue type
- Feature
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 25/100