484 lines
22 KiB
Plaintext
484 lines
22 KiB
Plaintext
// Copyright (c) 2015-2016 The Khronos Group Inc.
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// Copyright notice at https://www.khronos.org/registry/speccopyright.html
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[[shaders]]
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= Shaders
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A shader specifies programmable operations that execute for each vertex,
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control point, tessellated vertex, primitive, fragment, or workgroup in
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the corresponding stage(s) of the graphics and compute pipelines.
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Graphics pipelines include vertex shader execution as a result of
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<<drawing,primitive assembly>>, followed, if enabled, by tessellation
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control and evaluation shaders operating on
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<<drawing-primitive-topologies-patches,patches>>, geometry shaders, if
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enabled, operating on primitives, and fragment shaders, if present,
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operating on fragments generated by <<primsrast,Rasterization>>. In this
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specification, vertex, tessellation control, tessellation evaluation and
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geometry shaders are collectively referred to as vertex processing stages
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and occur in the logical pipeline before rasterization. The fragment shader
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occurs logically after rasterization.
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Only the compute shader stage is included in a compute pipeline. Compute
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shaders operate on compute invocations in a workgroup.
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Shaders can: read from input variables, and read from and write to
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output variables. Input and output variables can: be used to transfer data
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between shader stages, or to allow the shader to interact with values that
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exist in the execution environment. Similarly, the execution environment
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provides constants that describe capabilities.
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Shader variables are associated with execution environment-provided inputs
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and outputs using _built-in_ decorations in the shader. The available
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decorations for each stage are documented in the following subsections.
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[[shader-modules]]
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== Shader Modules
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_Shader modules_ contain _shader code_ and one or more entry points. Shaders
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are selected from a shader module by specifying an entry point as part of
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<<pipelines,pipeline>> creation. The stages of a pipeline can: use shaders
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that come from different modules. The shader code defining a shader module
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must: be in the SPIR-V format, as described by the <<spirvenv,Vulkan
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Environment for SPIR-V>> appendix.
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A shader module is created by calling:
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include::../protos/vkCreateShaderModule.txt[]
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* pname:device is the logical device that creates the shader module.
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* pname:pCreateInfo parameter is a pointer to an instance of the
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sname:VkShaderModuleCreateInfo structure.
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* pname:pAllocator controls host memory allocation as described in the
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<<memory-allocation, Memory Allocation>> chapter.
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* pname:pShaderModule points to a sname:VkShaderModule handle in which the
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resulting render pass object is returned.
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include::../validity/protos/vkCreateShaderModule.txt[]
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The sname:VkShaderModuleCreateInfo structure is defined as:
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include::../structs/VkShaderModuleCreateInfo.txt[]
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* pname:sType is the type of this structure.
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* pname:pNext is `NULL` or a pointer to an extension-specific structure.
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* pname:flags is reserved for future use.
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* pname:codeSize is the size, in bytes, of the code pointed to by
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pname:pCode.
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* pname:pCode points to code that is used to create the shader
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module. The type and format of the code is determined from the content
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of the memory addressed by pname:pCode.
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include::../validity/structs/VkShaderModuleCreateInfo.txt[]
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Once a shader module has been created, any entry points it contains can: be
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used in pipeline shader stages as described in <<pipelines-compute,Compute
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Pipelines>> and <<pipelines-graphics,Graphics Pipelines>>.
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To destroy a shader module, call:
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include::../protos/vkDestroyShaderModule.txt[]
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* pname:device is the logical device that destroys the shader module.
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* pname:shaderModule is the handle of the shader module to destroy.
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* pname:pAllocator controls host memory allocation as described in the
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<<memory-allocation, Memory Allocation>> chapter.
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A shader module can: be destroyed while pipelines created using its
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shaders are still in use.
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include::../validity/protos/vkDestroyShaderModule.txt[]
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[[shaders-execution]]
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== Shader Execution
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At each stage of the pipeline, multiple invocations of a shader may: execute
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simultaneously. Further, invocations of a single shader produced as the
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result of different commands may: execute simultaneously. The relative
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execution order of invocations of the same shader type is undefined. Shader
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invocations may: complete in a different order than that in which the
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primitives they originated from were drawn or dispatched by the application.
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However, fragment shader outputs are written to attachments in
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<<fundamentals-queueoperation-apiorder,API order>>.
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The relative order of invocations of different shader types is largely
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undefined. However, when invoking a shader whose inputs are generated from a
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previous pipeline stage, the shader invocations from the previous stage are
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guaranteed to have executed far enough to generate input values for all
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required inputs.
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[[shaders-execution-memory-ordering]]
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== Shader Memory Access Ordering
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The order in which image or buffer memory is read or written by shaders is
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largely undefined. For some shader types (vertex, tessellation evaluation,
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and in some cases, fragment), even the number of shader invocations that
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may: perform loads and stores is undefined.
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In particular, the following rules apply:
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* <<shaders-vertex-execution,Vertex>> and
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<<shaders-tessellation-evaluation-execution,tessellation evaluation>>
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shaders will be invoked at least once for each unique vertex, as defined
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in those sections.
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* <<shaders-fragment-execution,Fragment>> shaders will be invoked zero or
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more times, as defined in that section.
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* The relative order of invocations of the same shader type are undefined.
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A store issued by a shader when working on primitive B might complete
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prior to a store for primitive A, even if primitive A is specified prior
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to primitive B. This applies even to fragment shaders; while fragment
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shader outputs are always written to the framebuffer
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<<fundamentals-queueoperation-apiorder,in primitive order>>, stores
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executed by fragment shader invocations are not.
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* The relative order of invocations of different shader types is largely
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undefined.
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[NOTE]
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.Note
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====
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The above limitations on shader invocation order make some forms of
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synchronization between shader invocations within a single set of primitives
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unimplementable. For example, having one invocation poll memory written by
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another invocation assumes that the other invocation has been launched and
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will complete its writes in finite time.
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====
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Stores issued to different memory locations within a single shader
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invocation may: not be visible to other invocations in the order they were
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performed. The code:OpMemoryBarrier instruction can: be used to provide
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stronger ordering of reads and writes performed by a single invocation.
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code:OpMemoryBarrier guarantees that any memory transactions issued by the
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shader invocation prior to the instruction complete prior to the memory
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transactions issued after the instruction. Memory barriers are needed for
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algorithms that require multiple invocations to access the same memory and
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require the operations to be performed in a partially-defined relative
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order. For example, if one shader invocation does a series of writes,
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followed by an code:OpMemoryBarrier instruction, followed by another write,
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then the results of the series of writes before the barrier become visible to
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other shader invocations at a time earlier or equal to when the results of
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the final write become visible to those invocations. In practice it means
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that another invocation that sees the results of the final write would also
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see the previous writes. Without the memory barrier, the final write may: be
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visible before the previous writes.
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The built-in atomic memory transaction instructions can: be used to read and
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write a given memory address atomically. While built-in atomic functions
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issued by multiple shader invocations are executed in undefined order
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relative to each other, these functions perform both a read and a write of a
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memory address and guarantee that no other memory transaction will write to
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the underlying memory between the read and write.
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[NOTE]
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.Note
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====
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Atomics allow shaders to use shared global addresses for mutual exclusion or
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as counters, among other uses.
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====
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[[shaders-inputs]]
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== Shader Inputs and Outputs
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Data is passed into and out of shaders using variables with input or output
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storage class, respectively. User-defined inputs and outputs are connected
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between stages by matching their code:Location decorations. Additionally,
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data can: be provided by or communicated to special functions provided by
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the execution environment using code:BuiltIn decorations.
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In many cases, the same code:BuiltIn decoration can: be used in multiple
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shader stages with similar meaning. The specific behavior of variables
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decorated as code:BuiltIn is documented in the following sections.
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[[shaders-vertex]]
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== Vertex Shaders
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Each vertex shader invocation operates on one vertex and its associated
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<<fxvertex-attrib,vertex attribute>> data, and outputs one vertex and
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associated data. Graphics pipelines must: include a vertex shader, and
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the vertex shader stage is always the first shader stage in the graphics
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pipeline.
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[[shaders-vertex-execution]]
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=== Vertex Shader Execution
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A vertex shader must: be executed at least once for each vertex specified by
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a draw command. During execution, the shader is presented with the index of
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the vertex and instance for which it has been invoked. Input variables
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declared in the vertex shader are filled by the implementation with the
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values of vertex attributes associated with the invocation being executed.
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If a vertex is a part of more than one input primitive, for example
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by including the same index value multiple times in an index buffer, the
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vertex shader may: be invoked only once and the results shared amongst the
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resulting primitives. This is known as _vertex reuse_.
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ifdef::implementation-guide[]
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.Implementor's Note
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****
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If a vertex is repeated in a draw command (i.e. the same index is repeated
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in an indexed draw), the shader may: be executed anywhere from one to the
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number of repetitions times for that vertex, depending on the
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implementation's ability to reuse shader results.
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****
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endif::implementation-guide[]
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[[shaders-tessellation-control]]
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== Tessellation Control Shaders
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The tessellation control shader is used to read an input patch provided by
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the application and to produce an output patch. Each tessellation control
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shader invocation operates on an input patch (after all control points in
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the patch are processed by a vertex shader) and its associated data, and
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outputs a single control point of the output patch and its associated data,
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and can: also output additional per-patch data. The input patch is sized
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according to the pname:patchControlPoints member of
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slink:VkPipelineTessellationStateCreateInfo, as part of input assembly. The
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size of the output patch is controlled by the code:OpExecutionMode
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code:OutputVertices specified in the tessellation control or tessellation
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evaluation shaders, which must: be specified in at least one of the shaders.
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The size of the input and output patches must: each be greater than zero and
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less than or equal to
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sname:VkPhysicalDeviceLimits::pname:maxTessellationPatchSize.
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[[shaders-tessellation-control-execution]]
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=== Tessellation Control Shader Execution
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A tessellation control shader is invoked at least once for each _output_
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vertex in a patch.
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Inputs to the tessellation control shader are generated by the vertex
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shader. Each invocation of the tessellation control shader can: read the
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attributes of any incoming vertices and their associated data. The
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invocations corresponding to a given patch execute logically in parallel,
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with undefined relative execution order. However, the code:OpControlBarrier
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instruction can: be used to provide limited control of the execution order
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by synchronizing invocations within a patch, effectively dividing
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tessellation control shader execution into a set of phases. Tessellation
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control shaders will read undefined values if one invocation reads a
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per-vertex or per-patch attribute written by another invocation at any point
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during the same phase, or if two invocations attempt to write different
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values to the same per-patch output in a single phase.
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[[shaders-tessellation-evaluation]]
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== Tessellation Evaluation Shaders
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The Tessellation Evaluation Shader operates on an input patch of control
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points and their associated data, and a single input barycentric coordinate
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indicating the invocation's relative position within the subdivided patch,
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and outputs a single vertex and its associated data.
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[[shaders-tessellation-evaluation-execution]]
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=== Tessellation Evaluation Shader Execution
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A tessellation evaluation shader is invoked at least once for each
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unique vertex generated by the tessellator.
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[[shaders-geometry]]
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== Geometry Shaders
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The geometry shader operates on a group of vertices and their associated
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data assembled from a single input primitive, and emits zero or more
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output primitives and the group of vertices and their associated data
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required for each output primitive.
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[[shaders-geometry-execution]]
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=== Geometry Shader Execution
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A geometry shader is invoked at least once for each primitive produced by
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the tessellation stages, or at least once for each primitive generated by
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<<drawing,primitive assembly>> when tessellation is not in use. The number
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of geometry shader invocations per input primitive is determined from the
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invocation count of the geometry shader specified by the
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code:OpExecutionMode code:Invocations in the geometry shader. If the
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invocation count is not specified, then a default of one invocation is
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executed.
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[[shaders-fragment]]
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== Fragment Shaders
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Fragment shaders are invoked as the result of rasterization in a graphics
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pipeline. Each fragment shader invocation operates on a single fragment and
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its associated data. With few exceptions, fragment shaders do not have
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access to any data associated with other fragments and are considered to
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execute in isolation of fragment shader invocations associated with other
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fragments.
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[[shaders-fragment-execution]]
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=== Fragment Shader Execution
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For each fragment generated by rasterization, a fragment shader may: be
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invoked. A fragment shader mustnot: be invoked if the <<fragops-early,Early
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Per-Fragment Tests>> cause it to have no coverage.
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Furthermore, if it is determined that a fragment generated as the result of
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rasterizing a first primitive will have its outputs entirely overwritten by
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a fragment generated as the result of rasterizing a second primitive in the
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same subpass, and the fragment shader used for the fragment has no other
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side effects, then the fragment shader may: not be executed for the fragment
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from the first primitive.
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Relative ordering of execution of different fragment shader invocations is
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not defined.
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The number of fragment shader invocations produced per-pixel is determined
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as follows:
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- If per-sample shading is enabled, the fragment shader is invoked once
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per covered sample.
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- Otherwise, the fragment shader is invoked at least once per fragment but
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no more than once per covered sample.
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In addition to the conditions outlined above for the invocation of a
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fragment shader, a fragment shader invocation may: be produced as a _helper
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invocation_. A helper invocation is a fragment shader invocation that is
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created solely for the purposes of evaluating derivatives for use in
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non-helper fragment shader invocations. Stores and atomics performed by
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helper invocations mustnot: have any effect on memory, and values returned
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by atomic instructions in helper invocations are undefined.
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[[shaders-fragment-earlytest]]
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=== Early Fragment Tests
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An explicit control is provided to allow fragment shaders to enable early
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fragment tests. If the fragment shader specifies the
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code:EarlyFragmentTests code:OpExecutionMode, the per-fragment tests
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described in <<fragops-early-mode,Early Fragment Test Mode>> are
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performed prior to fragment shader execution. Otherwise, they are performed
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after fragment shader execution.
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[[shaders-compute]]
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== Compute Shaders
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Compute shaders are invoked via flink:vkCmdDispatch and
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flink:vkCmdDispatchIndirect commands. In general, they have access to
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similar resources as shader stages executing as part of a graphics pipeline.
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Compute workloads are formed from groups of work items called workgroups
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and processed by the compute shader in the current compute pipeline. A
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workgroup is a collection of shader invocations that execute the same
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shader, potentially in parallel. Compute shaders execute in _global
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workgroups_ which are divided into a number of _local workgroups_ with a size
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that can: be set by assigning a value to the code:LocalSize execution mode or
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via an object decorated by the code:WorkgroupSize decoration. An invocation
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within a local workgroup can: share data with other members of the local
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workgroup through shared variables and issue memory and control flow barriers
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to synchronize with other members of the local workgroup.
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[[shaders-interpolation-decorations]]
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== Interpolation Decorations
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Interpolation decorations control the behavior of attribute interpolation in
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the fragment shader stage. Interpolation decorations can: be applied to
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code:Input storage class variables in the fragment shader stage's interface,
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and control the interpolation behavior of those variables.
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Inputs that could be interpolated can: be decorated by at most one
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of the following decorations:
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* code:Flat: no interpolation
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* code:NoPerspective: linear interpolation (for
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<<line_noperspective_interpolation,lines>> and
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<<triangle_noperspective_interpolation,polygons>>).
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Fragment input variables decorated with neither code:Flat nor
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code:NoPerspective use perspective-correct interpolation (for
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<<line_perspective_interpolation,lines>> and
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<<triangle_perspective_interpolation,polygons>>).
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The presence of and type of interpolation is controlled by the above
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interpolation decorations as well as the auxiliary decorations code:Centroid
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and code:Sample.
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A variable decorated with code:Flat will not be interpolated. Instead, it
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will have the same value for every fragment within a triangle. This value
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will come from a single <<vertexpostproc-flatshading,provoking vertex>>. A
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variable decorated with code:Flat can: also be decorated with code:Centroid
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or code:Sample, which will mean the same thing as decorating it only as
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code:Flat.
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For fragment shader input variables decorated with neither code:Centroid nor
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code:Sample, the assigned variable may: be interpolated
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anywhere within the pixel and a single value may: be assigned to each sample
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within the pixel.
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code:Centroid and code:Sample can: be used to control the location and
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frequency of the sampling of the decorated fragment shader input. If a
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fragment shader input is decorated with code:Centroid, a single value may:
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be assigned to that variable for all samples in the pixel, but that value
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must: be interpolated to a location that lies in both the pixel and in the
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primitive being rendered, including any of the pixel's samples covered by
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the primitive. Because the location at which the variable is interpolated
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may: be different in neighboring pixels, and derivatives may: be computed by
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computing differences between neighboring pixels, derivatives of
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centroid-sampled inputs may: be less accurate than those for non-centroid
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interpolated variables. If a fragment shader input is decorated with
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code:Sample, a separate value must: be assigned to that variable for each
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covered sample in the pixel, and that value must: be sampled at the location
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of the individual sample. When pname:rasterizationSamples is
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ename:VK_SAMPLE_COUNT_1_BIT, the pixel center must: be used for
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code:Centroid, code:Sample, and undecorated attribute interpolation.
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Fragment shader inputs that are signed or unsigned integers, integer
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vectors, or any double-precision floating-point type must: be decorated with
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code:Flat.
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[[shaders-staticuse]]
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== Static Use
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A SPIR-V module declares a global object in memory using the code:OpVariable
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instruction, which results in a pointer code:x to that object. A specific
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entry point in a SPIR-V module is said to _statically use_ that object if
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that entry-point's call tree contains a function that contains a memory
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instruction or image instruction with code:x as an code:id operand. See the
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``Memory Instructions'' and ``Image Instructions'' subsections of section 3
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``Binary Form'' of the SPIR-V specification for the complete list of SPIR-V
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memory instructions.
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Static use is not used to control the behavior of variables with code:Input
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and code:Output storage. The effects of those variables are applied based
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only on whether they are present in a shader entry point's interface.
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[[shaders-invocationgroups]]
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== Invocation and Derivative Groups
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An _invocation group_ (see the subsection ``Control Flow'' of section 2 of the
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SPIR-V specification) for a compute shader is the set of invocations in a
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single local workgroup. For graphics shaders, an invocation group is an
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implementation-dependent subset of the set of shader invocations of a given
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shader stage which are produced by a single drawing command. For indirect
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drawing commands with pname:drawCount greater than one, invocations from
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separate draws are in distinct invocation groups.
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[NOTE]
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.Note
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====
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Because the partitioning of invocations into invocation groups is
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implementation-dependent and not observable, applications generally need to
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assume the worst case of all invocations in a draw belonging to a single
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invocation group.
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====
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A _derivative group_ (see the subsection ``Control Flow'' of section 2 of the
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SPIR-V 1.00 Revision 4 specification) for a fragment shader is the set of
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invocations generated by a single primitive (point, line, or triangle),
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including any helper invocations generated by that primitive. Derivatives are
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undefined for a sampled image instruction if the instruction is in flow
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control that is not uniform across the derivative group.
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