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