126 lines
5.3 KiB
Plaintext
126 lines
5.3 KiB
Plaintext
[[mesh]]
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= Mesh Shading
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<<shaders-task,Task>> and <<shaders-mesh,mesh shaders>> operate in
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workgroups to produce a collection of primitives that will be processed by
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subsequent stages of the graphics pipeline.
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Work on the mesh pipeline is initiated by the application
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<<drawing-mesh-shading,drawing>> a set of mesh tasks organized in global
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workgroups.
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If the optional task shader is active, each workgroup triggers the execution
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of task shader invocations that will create a new set of mesh workgroups
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upon completion.
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Each of these created workgroups, or each of the original workgroups if no
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task shader is present, triggers the execution of mesh shader invocations.
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Each mesh shader workgroup emits zero or more output primitives along with
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the group of vertices and their associated data required for each output
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primitive.
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[[mesh-task-input]]
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== Task Shader Input
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For every workgroup issued via the drawing commands a group of task shader
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invocations is executed.
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There are no inputs other than the builtin workgroup identifiers.
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[[mesh-task-output]]
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== Task Shader Output
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The task shader can emit zero or more mesh workgroups to be generated using
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the <<interfaces-builtin-variables,built-in variable>> code:TaskCountNV.
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This value must: be less than or equal to
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sname:VkPhysicalDeviceMeshShaderPropertiesNV::pname:maxTaskOutputCount.
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It can also output user-defined data that is passed as input to all mesh
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shader invocations that the task creates.
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These outputs are decorated as code:PerTaskNV.
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[[mesh-generation]]
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== Mesh Generation
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If a task shader exists, the mesh assembler creates a variable amount of
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mesh workgroups depending on each task's output.
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If there is no task shader, the drawing commands emit the mesh shader
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invocations directly.
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[[mesh-input]]
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== Mesh Shader Input
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The only inputs available to the mesh shader are variables identifying the
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specific workgroup and invocation and, if applicable, any outputs written as
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code:PerTaskNV by the task shader that spawned the mesh shader's workgroup.
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The mesh shader can operate without a task shader as well.
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[[mesh-output]]
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== Mesh Shader Output Primitives
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A mesh shader generates primitives in one of three output modes: points,
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lines, or triangles.
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The primitive mode is specified in the shader using an code:OpExecutionMode
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instruction with the code:OutputPoints, code:OutputLinesNV, or
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code:OutputTrianglesNV modes, respectively.
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Each mesh shader must: include exactly one output primitive mode.
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The maximum output vertex count is specified as a literal in the shader
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using an code:OpExecutionMode instruction with the mode set to
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code:OutputVertices and must: be less than or equal to
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sname:VkPhysicalDeviceMeshShaderPropertiesNV::pname:maxMeshOutputVertices.
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The maximum output primitive count is specified as a literal in the shader
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using an code:OpExecutionMode instruction with the mode set to
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code:OutputPrimitivesNV and must: be less than or equal to
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sname:VkPhysicalDeviceMeshShaderPropertiesNV::pname:maxMeshOutputPrimitives.
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The number of primitives output by the mesh shader is provided via writing
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to the <<interfaces-builtin-variables,built-in variable>>
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code:PrimitiveCountNV and must: be less than or equal to the maximum output
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primitive count specified in the shader.
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A variable decorated with code:PrimitiveIndicesNV is an output array of
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local index values into the vertex output arrays from which primitives are
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assembled according to the output primitive type.
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These resulting primitives are then further processed as described in
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<<primsrast>>.
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[[mesh-output-perview]]
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== Mesh Shader Per-View Outputs
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The mesh shader outputs decorated with the code:PositionPerViewNV,
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code:ClipDistancePerViewNV, code:CullDistancePerViewNV, code:LayerPerViewNV,
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and code:ViewportMaskPerViewNV built-in decorations are the per-view
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versions of the single-view variables with equivalent names (that is
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code:Position, code:ClipDistance, code:CullDistance, code:Layer, and
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code:ViewportMaskNV, respectively).
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If a shader statically assigns a value to any element of a per-view array it
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must: not statically assign a value to the equivalent single-view variable.
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Each of these outputs is considered arrayed, with separate values for each
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view.
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The view number is used to index the first dimension of these arrays.
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The second dimension of the code:ClipDistancePerViewNV, and
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code:CullDistancePerViewNV arrays have the same requirements as the
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code:ClipDistance, and code:CullDistance arrays.
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If a mesh shader output is _per-view_, the corresponding fragment shader
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input is taken from the element of the per-view output array that
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corresponds to the view that is currently being processed by the fragment
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shader.
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[[mesh-ordering]]
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== Mesh Shader Primitive Ordering
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Following guarantees are provided for the relative ordering of primitives
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produced by a mesh shader, as they pertain to <<drawing-primitive-order,
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primitive order>>.
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* When a task shader is used, mesh workgroups spawned from lower tasks
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will be ordered prior those workgroups from subsequent tasks.
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* All output primitives generated from a given mesh workgroup are passed
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to subsequent pipeline stages before any output primitives generated
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from subsequent input workgroups.
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* All output primitives within a mesh workgroup, will be generated in the
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ordering provided by the builtin primitive indexbuffer (from low address
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to high address).
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