SPV_EXT_fragment_shader_interlock

SPV_EXT_fragment_shader_interlock

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

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

This extension is written against the SPIR-V Specification, Version 1.4, Revision 1, Unified.

This extension interacts with the SPV_NV_shading_rate and SPV_EXT_fragment_invocation_density extensions.

This extension requires SPIR-V 1.0.

This extension provides SPIR-V support for the GL_ARB_fragment_shader_interlock GLSL extension.

This extension provides SPIR-V support for the shading_rate_interlock_ordered and shading_rate_interlock_unordered layouts defined in the GL_NV_shading_rate_image GLSL extension.

This extension provides new instructions OpBeginInvocationInterlockEXT and OpEndInvocationInterlockEXT that delimit a critical section of fragment shader code. For pairs of shader invocations with "overlapping" coverage in a given pixel, the implementation will guarantee that the critical section of the fragment shader will be executed for only one fragment at a time.

There are six different interlock execution modes supported by this extension. The execution modes PixelInterlockOrderedEXT and PixelInterlockUnorderedEXT provide mutual exclusion in the critical section for any pair of fragments corresponding to the same pixel, or pixels if the fragment covers more than one pixel. When using multisampling, the execution modes SampleInterlockOrderedEXT and SampleInterlockUnorderedEXT only provide mutual exclusion for pairs of fragments that both cover at least one common sample in the same pixel; these are recommended for performance if shaders use per-sample data structures. When using the ShadingRateNV or FragmentDensityEXT capabilities, the execution modes ShadingRateInterlockOrderedEXT and ShadingRateInterlockUnorderedEXT provide mutual exclusion for pairs of fragments with at least one or more associated samples in common from all pixels belonging to the fragments.

Additionally, when the PixelInterlockOrderedEXT, SampleInterlockOrderedEXT or ShadingRateInterlockOrderedEXT execution mode is used, the interlock also guarantees that the critical section for multiple shader invocations with "overlapping" coverage will be executed in primitive order, as defined by the Primitive Order section of the Vulkan specification. Such a guarantee is useful for applications that perform blending in the fragment shader, where the application requires fragment values to be composited in primitive order.

This extension can be useful for algorithms that need to access per-pixel data structures via shader loads and stores. Algorithms using this extension can access per-pixel data structures in critical sections without other invocations accessing the same per-pixel data. Additionally, the ordering guarantees are useful for cases where the API ordering of fragments is meaningful. For example, applications may be able to execute programmable blending operations in the fragment shader, where the destination buffer is read via image loads and the final value is written via image stores.

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

This extension introduces these new execution modes:

which can be used with the FragmentShaderSampleInterlockEXT capability.

This extension introduces these new execution modes:

which can be used with the FragmentShaderPixelInterlockEXT capability.

This extension introduces these new execution modes:

which can be used with the FragmentShaderShadingRateInterlockEXT capability.

This extension introduces new capabilities:

Instructions added under the FragmentShaderSampleInterlockEXT, FragmentShaderPixelInterlockEXT, or FragmentShaderShadingRateInterlockEXT capabilities.

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