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23. Geometry Shading

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. Geometry shading is enabled when a geometry shader is included in the pipeline.

23.1. Geometry Shader Input Primitives

Each geometry shader invocation has access to all vertices in the primitive (and their associated data), which are presented to the shader as an array of inputs.

The input primitive type expected by the geometry shader is specified with an OpExecutionMode instruction in the geometry shader, and must match the incoming primitive type specified by either the pipeline’s primitive topology if tessellation is inactive, or the tessellation mode if tessellation is active, as follows:

  • An input primitive type of InputPoints must only be used with a pipeline topology of VK_PRIMITIVE_TOPOLOGY_POINT_LIST, or with a tessellation shader that specifies PointMode. The input arrays always contain one element, as described by the point list topology or tessellation in point mode.

  • An input primitive type of InputLines must only be used with a pipeline topology of VK_PRIMITIVE_TOPOLOGY_LINE_LIST or VK_PRIMITIVE_TOPOLOGY_LINE_STRIP, or with a tessellation shader specifying IsoLines that does not specify PointMode. The input arrays always contain two elements, as described by the line list topology or line strip topology, or by isoline tessellation.

  • An input primitive type of InputLinesAdjacency must only be used when tessellation is inactive, with a pipeline topology of VK_PRIMITIVE_TOPOLOGY_LINE_LIST_WITH_ADJACENCY or VK_PRIMITIVE_TOPOLOGY_LINE_STRIP_WITH_ADJACENCY. The input arrays always contain four elements, as described by the line list with adjacency topology or line strip with adjacency topology.

  • An input primitive type of Triangles must only be used with a pipeline topology of VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST, VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP, or VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN; or with a tessellation shader specifying Quads or Triangles that does not specify PointMode. The input arrays always contain three elements, as described by the triangle list topology, triangle strip topology, or triangle fan topology, or by triangle or quad tessellation. Vertices may be in a different absolute order to that specified by the topology, but must adhere to the specified winding order.

  • An input primitive type of InputTrianglesAdjacency must only be used when tessellation is inactive, with a pipeline topology of VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST_WITH_ADJACENCY or VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY. The input arrays always contain six elements, as described by the triangle list with adjacency topology or triangle strip with adjacency topology. Vertices may be in a different absolute order to that specified by the topology, but must adhere to the specified winding order, and the vertices making up the main primitive must still occur at the first, third, and fifth index.

23.2. Geometry Shader Output Primitives

A geometry shader generates primitives in one of three output modes: points, line strips, or triangle strips. The primitive mode is specified in the shader using an OpExecutionMode instruction with the OutputPoints, OutputLineStrip or OutputTriangleStrip modes, respectively. Each geometry shader must include exactly one output primitive mode.

The vertices output by the geometry shader are assembled into points, lines, or triangles based on the output primitive type and the resulting primitives are then further processed as described in Rasterization. If the number of vertices emitted by the geometry shader is not sufficient to produce a single primitive, vertices corresponding to incomplete primitives are not processed by subsequent pipeline stages. The number of vertices output by the geometry shader is limited to a maximum count specified in the shader.

The maximum output vertex count is specified in the shader using an OpExecutionMode instruction with the mode set to OutputVertices and the maximum number of vertices that will be produced by the geometry shader specified as a literal. Each geometry shader must specify a maximum output vertex count.

23.3. Multiple Invocations of Geometry Shaders

Geometry shaders can be invoked more than one time for each input primitive. This is known as geometry shader instancing and is requested by including an OpExecutionMode instruction with mode specified as Invocations and the number of invocations specified as an integer literal.

In this mode, the geometry shader will execute at least n times for each input primitive, where n is the number of invocations specified in the OpExecutionMode instruction. The instance number is available to each invocation as a built-in input using InvocationId.

23.4. Geometry Shader Primitive Ordering

Limited guarantees are provided for the relative ordering of primitives produced by a geometry shader, as they pertain to primitive order.

  • For instanced geometry shaders, the output primitives generated from each input primitive are passed to subsequent pipeline stages using the invocation number to order the primitives, from least to greatest.

  • All output primitives generated from a given input primitive are passed to subsequent pipeline stages before any output primitives generated from subsequent input primitives.

23.5. Geometry Shader Passthrough

A geometry shader that uses the PassthroughNV decoration on a variable in its input interface is considered a passthrough geometry shader. Output primitives in a passthrough geometry shader must have the same topology as the input primitive and are not produced by emitting vertices. The vertices of the output primitive have two different types of attributes, per-vertex and per-primitive. Geometry shader input variables with PassthroughNV decoration are considered to produce per-vertex outputs, where values for each output vertex are copied from the corresponding input vertex. Any built-in or user-defined geometry shader outputs are considered per-primitive in a passthrough geometry shader, where a single output value is copied to all output vertices.

The remainder of this section details the usage of the PassthroughNV decoration and modifications to the interface matching rules when using passthrough geometry shaders.

23.5.1. PassthroughNV Decoration

Decorating a geometry shader input variable with the PassthroughNV decoration indicates that values of this input are copied through to the corresponding vertex of the output primitive. Input variables and block members which do not have the PassthroughNV decoration are consumed by the geometry shader without being passed through to subsequent stages.

The PassthroughNV decoration must only be used within a geometry shader.

Any variable decorated with PassthroughNV must be declared using the Input storage class.

The PassthroughNV decoration must not be used with any of:

  • an input primitive type other than InputPoints, InputLines, or Triangles, as specified by the mode for OpExecutionMode.

  • an invocation count other than one, as specified by the Invocations mode for OpExecutionMode.

  • an OpEntryPoint which statically uses the OpEmitVertex or OpEndPrimitive instructions.

  • a variable decorated with the InvocationId built-in decoration.

  • a variable decorated with the PrimitiveId built-in decoration that is declared using the Input storage class.

23.5.2. Passthrough Interface Matching

When a passthrough geometry shader is in use, the Interface Matching rules involving the geometry shader input and output interfaces operate as described in this section.

For the purposes of matching passthrough geometry shader inputs with outputs of the previous pipeline stages, the PassthroughNV decoration is ignored.

For the purposes of matching the outputs of the geometry shader with subsequent pipeline stages, each input variable with the PassthroughNV decoration is considered to add an equivalent output variable with the same type, decoration (other than PassthroughNV), number, and declaration order on the output interface. The output variable declaration corresponding to an input variable decorated with PassthroughNV will be identical to the input declaration, except that the outermost array dimension of such variables is removed. The output block declaration corresponding to an input block decorated with PassthroughNV or having members decorated with PassthroughNV will be identical to the input declaration, except that the outermost array dimension of such declaration is removed.

If an input block is decorated with PassthroughNV, the equivalent output block contains all the members of the input block. Otherwise, the equivalent output block contains only those input block members decorated with PassthroughNV. All members of the corresponding output block are assigned Location and Component decorations identical to those assigned to the corresponding input block members.

Output variables and blocks generated from inputs decorated with PassthroughNV will only exist for the purposes of interface matching; these declarations are not available to geometry shader code or listed in the module interface.

For the purposes of component counting, passthrough geometry shaders count all statically used input variable components declared with the PassthroughNV decoration as output components as well, since their values will be copied to the output primitive produced by the geometry shader.