264 lines
12 KiB
Plaintext
264 lines
12 KiB
Plaintext
// Copyright (c) 2015-2018 Khronos Group. This work is licensed under a
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// Creative Commons Attribution 4.0 International License; see
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// http://creativecommons.org/licenses/by/4.0/
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[[geometry]]
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= Geometry Shading
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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 output
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primitives and the group of vertices and their associated data required for
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each output primitive.
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Geometry shading is enabled when a geometry shader is included in the
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pipeline.
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[[geometry-input]]
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== Geometry Shader Input Primitives
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Each geometry shader invocation has access to all vertices in the primitive
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(and their associated data), which are presented to the shader as an array
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of inputs.
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The input primitive type expected by the geometry shader is specified with
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an code:OpExecutionMode instruction in the geometry shader, and must: be
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compatible with the primitive topology used by primitive assembly (if
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tessellation is not in use) or must: match the type of primitive generated
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by the tessellation primitive generator (if tessellation is in use).
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Compatibility is defined below, with each input primitive type.
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The input primitive types accepted by a geometry shader are:
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Points::
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Geometry shaders that operate on points use an code:OpExecutionMode
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instruction specifying the code:InputPoints input mode.
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Such a shader is valid only when the pipeline primitive topology is
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ename:VK_PRIMITIVE_TOPOLOGY_POINT_LIST (if tessellation is not in use) or if
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tessellation is in use and the tessellation evaluation shader uses
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code:PointMode.
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There is only a single input vertex available for each geometry shader
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invocation.
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However, inputs to the geometry shader are still presented as an array, but
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this array has a length of one.
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Lines::
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Geometry shaders that operate on line segments are generated by including an
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code:OpExecutionMode instruction with the code:InputLines mode.
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Such a shader is valid only for the ename:VK_PRIMITIVE_TOPOLOGY_LINE_LIST,
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and ename:VK_PRIMITIVE_TOPOLOGY_LINE_STRIP primitive topologies (if
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tessellation is not in use) or if tessellation is in use and the
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tessellation mode is code:Isolines.
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There are two input vertices available for each geometry shader invocation.
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The first vertex refers to the vertex at the beginning of the line segment
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and the second vertex refers to the vertex at the end of the line segment.
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Lines with Adjacency::
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Geometry shaders that operate on line segments with adjacent vertices are
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generated by including an code:OpExecutionMode instruction with the
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code:InputLinesAdjacency mode.
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Such a shader is valid only for the
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ename:VK_PRIMITIVE_TOPOLOGY_LINE_LIST_WITH_ADJACENCY and
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ename:VK_PRIMITIVE_TOPOLOGY_LINE_STRIP_WITH_ADJACENCY primitive topologies
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and must: not be used when tessellation is in use.
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+
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In this mode, there are four vertices available for each geometry shader
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invocation.
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The second vertex refers to attributes of the vertex at the beginning of the
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line segment and the third vertex refers to the vertex at the end of the
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line segment.
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The first and fourth vertices refer to the vertices adjacent to the
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beginning and end of the line segment, respectively.
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Triangles::
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Geometry shaders that operate on triangles are created by including an
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code:OpExecutionMode instruction with the code:Triangles mode.
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Such a shader is valid when the pipeline topology is
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ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST,
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ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP, or
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ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN (if tessellation is not in use) or
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when tessellation is in use and the tessellation mode is code:Triangles or
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code:Quads.
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+
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In this mode, there are three vertices available for each geometry shader
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invocation.
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The first, second, and third vertices refer to attributes of the first,
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second, and third vertex of the triangle, respectively.
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Triangles with Adjacency::
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Geometry shaders that operate on triangles with adjacent vertices are
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created by including an code:OpExecutionMode instruction with the
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code:InputTrianglesAdjacency mode.
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Such a shader is valid when the pipeline topology is
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ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST_WITH_ADJACENCY or
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ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY, and must: not be
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used when tessellation is in use.
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+
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In this mode, there are six vertices available for each geometry shader
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invocation.
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The first, third and fifth vertices refer to attributes of the first, second
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and third vertex of the triangle, respectively.
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The second, fourth and sixth vertices refer to attributes of the vertices
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adjacent to the edges from the first to the second vertex, from the second
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to the third vertex, and from the third to the first vertex, respectively.
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[[geometry-output]]
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== Geometry Shader Output Primitives
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A geometry shader generates primitives in one of three output modes: points,
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line strips, or triangle strips.
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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:OutputLineStrip or
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code:OutputTriangleStrip modes, respectively.
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Each geometry shader must: include exactly one output primitive mode.
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The vertices output by the geometry shader are assembled into points, lines,
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or triangles based on the output primitive type and the resulting primitives
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are then further processed as described in <<primsrast>>.
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If the number of vertices emitted by the geometry shader is not sufficient
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to produce a single primitive, vertices corresponding to incomplete
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primitives are not processed by subsequent pipeline stages.
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The number of vertices output by the geometry shader is limited to a maximum
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count specified in the shader.
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The maximum output vertex count is specified in the shader using an
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code:OpExecutionMode instruction with the mode set to code:OutputVertices
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and the maximum number of vertices that will be produced by the geometry
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shader specified as a literal.
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Each geometry shader must: specify a maximum output vertex count.
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[[geometry-invocations]]
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== Multiple Invocations of Geometry Shaders
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Geometry shaders can: be invoked more than one time for each input
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primitive.
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This is known as _geometry shader instancing_ and is requested by including
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an code:OpExecutionMode instruction with code:mode specified as
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code:Invocations and the number of invocations specified as an integer
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literal.
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In this mode, the geometry shader will execute at least [eq]#n# times for
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each input primitive, where [eq]#n# is the number of invocations specified
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in the code:OpExecutionMode instruction.
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The instance number is available to each invocation as a built-in input
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using code:InvocationId.
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[[geometry-ordering]]
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== Geometry Shader Primitive Ordering
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Limited guarantees are provided for the relative ordering of primitives
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produced by a geometry shader, as they pertain to <<drawing-primitive-order,
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primitive order>>.
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* For instanced geometry shaders, the output primitives generated from
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each input primitive are passed to subsequent pipeline stages using the
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invocation number to order the primitives, from least to greatest.
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* All output primitives generated from a given input primitive are passed
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to subsequent pipeline stages before any output primitives generated
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from subsequent input primitives.
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ifdef::VK_NV_geometry_shader_passthrough[]
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[[geometry-passthrough]]
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== Geometry Shader Passthrough
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A geometry shader that uses the code:PassthroughNV decoration on a variable
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in its input interface is considered a _passthrough geometry shader_.
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Output primitives in a passthrough geometry shader must: have the same
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topology as the input primitive and are not produced by emitting vertices.
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The vertices of the output primitive have two different types of attributes,
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per-vertex and per-primitive.
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Geometry shader input variables with code:PassthroughNV decoration are
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considered to produce per-vertex outputs, where values for each output
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vertex are copied from the corresponding input vertex.
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Any built-in or user-defined geometry shader outputs are considered
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per-primitive in a passthrough geometry shader, where a single output value
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is copied to all output vertices.
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The remainder of this section details the usage of the code:PassthroughNV
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decoration and modifications to the interface matching rules when using
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passthrough geometry shaders.
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[[geometry-passthrough-passthrough]]
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=== code:PassthroughNV Decoration
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Decorating a geometry shader input variable with the code:PassthroughNV
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decoration indicates that values of this input are copied through to the
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corresponding vertex of the output primitive.
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Input variables and block members which do not have the code:PassthroughNV
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decoration are consumed by the geometry shader without being passed through
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to subsequent stages.
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The code:PassthroughNV decoration must: only be used within a geometry
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shader.
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Any variable decorated with code:PassthroughNV must: be declared using the
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code:Input storage class.
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The code:PassthroughNV decoration must: not be used with any of:
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* an input primitive type other than code:InputPoints, code:InputLines, or
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code:Triangles, as specified by the mode for code:OpExecutionMode.
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* an invocation count other than one, as specified by the code:Invocations
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mode for code:OpExecutionMode.
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* an code:OpEntryPoint which statically uses the code:OpEmitVertex or
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code:OpEndPrimitive instructions.
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* a variable decorated with the code:InvocationId built-in decoration.
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* a variable decorated with the code:PrimitiveId built-in decoration that
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is declared using the code:Input storage class.
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[[geometry-passthrough-interface]]
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=== Passthrough Interface Matching
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When a passthrough geometry shader is in use, the
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<<interfaces-iointerfaces-matching,Interface Matching>> rules involving the
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geometry shader input and output interfaces operate as described in this
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section.
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For the purposes of matching passthrough geometry shader inputs with outputs
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of the previous pipeline stages, the code:PassthroughNV decoration is
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ignored.
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For the purposes of matching the outputs of the geometry shader with
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subsequent pipeline stages, each input variable with the code:PassthroughNV
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decoration is considered to add an equivalent output variable with the same
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type, decoration (other than code:PassthroughNV), number, and declaration
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order on the output interface.
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The output variable declaration corresponding to an input variable decorated
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with code:PassthroughNV will be identical to the input declaration, except
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that the outermost array dimension of such variables is removed.
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The output block declaration corresponding to an input block decorated with
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code:PassthroughNV or having members decorated with code:PassthroughNV will
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be identical to the input declaration, except that the outermost array
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dimension of such declaration is removed.
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If an input block is decorated with code:PassthroughNV, the equivalent
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output block contains all the members of the input block.
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Otherwise, the equivalent output block contains only those input block
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members decorated with code:PassthroughNV.
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All members of the corresponding output block are assigned code:Location and
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code:Component decorations identical to those assigned to the corresponding
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input block members.
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Output variables and blocks generated from inputs decorated with
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code:PassthroughNV will only exist for the purposes of interface matching;
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these declarations are not available to geometry shader code or listed in
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the module interface.
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For the purposes of component counting, passthrough geometry shaders count
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all statically used input variable components declared with the
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code:PassthroughNV decoration as output components as well, since their
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values will be copied to the output primitive produced by the geometry
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shader.
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endif::VK_NV_geometry_shader_passthrough[]
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