150 lines
7.0 KiB
Plaintext
150 lines
7.0 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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[[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
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output primitives and the group of vertices and their associated data
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required for each output primitive. Geometry shading is enabled when a
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geometry shader is included in the 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. The input primitive type expected by the geometry shader is
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specified with an code:OpExecutionMode instruction in the geometry shader,
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and must: be compatible with the primitive topology used by primitive
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assembly (if tessellation is not in use) or must: match the type of
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primitive generated by the tessellation primitive generator (if tessellation
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is in use). 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 specifing the code:InputPoints input mode. Such a shader is
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valid only when the pipeline primitive topology is
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code: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. There is only a single input vertex available for each
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geometry shader invocation. However, inputs to the geometry shader are still
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presented as an array, but 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. Such a
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shader is valid only for the code:VK_PRIMITIVE_TOPOLOGY_LINE_LIST, and
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code:VK_PRIMITIVE_TOPOLOGY_LINE_STRIP primitive topologies (if tessellation
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is not in use) or if tessellation is in use and the tessellation mode is
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code:Isolines. There are two input vertices available for each geometry
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shader invocation. The first vertex refers to the vertex at the beginning of
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the line segment and the second vertex refers to the vertex at the end of
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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. Such a shader is valid only for the
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code:VK_PRIMITIVE_TOPOLOGY_LINES_WITH_ADJACENCY and
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code:VK_PRIMITIVE_TOPOLOGY_LINE_STRIP_WITH_ADJACENCY primitive topologies
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and mustnot: 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. The second vertex refers to attributes of the vertex at the
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beginning of the line segment and the third vertex refers to the vertex at
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the end of the line segment. The first and fourth vertices refer to the
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vertices adjacent to the beginning and end of the line segment,
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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. Such a
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shader is valid when the pipeline topology is
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code:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST,
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code:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP, or
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code: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. The first, second, and third vertices refer to attributes of the
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first, 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. Such a shader is valid when the pipeline
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topology is code:VK_PRIMITIVE_TOPOLOGY_TRIANGLES_WITH_ADJACENCY or
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code:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY, and mustnot: 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. The first, third and fifth vertices refer to attributes of the
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first, second and third vertex of the triangle, respectively. The second,
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fourth and sixth vertices refer to attributes of the vertices adjacent to
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the edges from the first to the second vertex, from the second to the third
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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. The primitive mode is specified in the
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shader using an code:OpExecutionMode instruction with the code:OutputPoints,
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code:OutputLineStrip or code:OutputTriangleStrip modes, respectively. Each
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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>>. If the number of
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vertices emitted by the geometry shader is not sufficient to produce a
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single primitive, vertices corresponding to incomplete primitives are not
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processed by subsequent pipeline stages. The number of vertices output by
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the geometry shader is limited to a maximum 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. Each geometry shader must: specify a maximum
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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. This is known as _geometry shader instancing_ and is requested by
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including 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 latexmath:[$n$] times for
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each input primitive, where latexmath:[$n$] is the number of invocations
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specified in the code:OpExecutionMode instruction. The instance number is
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available to each invocation as a built-in input 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.
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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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