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 be
compatible with the primitive topology used by primitive assembly (if
tessellation is not in use) or must match the type of primitive generated
by the tessellation primitive generator (if tessellation is in use).
Compatibility is defined below, with each input primitive type.
The input primitive types accepted by a geometry shader are:
 Points

Geometry shaders that operate on points use an
OpExecutionMode
instruction specifying theInputPoints
input mode. Such a shader is valid only when the pipeline primitive topology isVK_PRIMITIVE_TOPOLOGY_POINT_LIST
(if tessellation is not in use) or if tessellation is in use and the tessellation evaluation shader usesPointMode
. There is only a single input vertex available for each geometry shader invocation. However, inputs to the geometry shader are still presented as an array, but this array has a length of one.  Lines

Geometry shaders that operate on line segments are generated by including an
OpExecutionMode
instruction with theInputLines
mode. Such a shader is valid only for theVK_PRIMITIVE_TOPOLOGY_LINE_LIST
, andVK_PRIMITIVE_TOPOLOGY_LINE_STRIP
primitive topologies (if tessellation is not in use) or if tessellation is in use and the tessellation mode isIsolines
. There are two input vertices available for each geometry shader invocation. The first vertex refers to the vertex at the beginning of the line segment and the second vertex refers to the vertex at the end of the line segment.  Lines with Adjacency

Geometry shaders that operate on line segments with adjacent vertices are generated by including an
OpExecutionMode
instruction with theInputLinesAdjacency
mode. Such a shader is valid only for theVK_PRIMITIVE_TOPOLOGY_LINE_LIST_WITH_ADJACENCY
andVK_PRIMITIVE_TOPOLOGY_LINE_STRIP_WITH_ADJACENCY
primitive topologies and must not be used when tessellation is in use.In this mode, there are four vertices available for each geometry shader invocation. The second vertex refers to attributes of the vertex at the beginning of the line segment and the third vertex refers to the vertex at the end of the line segment. The first and fourth vertices refer to the vertices adjacent to the beginning and end of the line segment, respectively.
 Triangles

Geometry shaders that operate on triangles are created by including an
OpExecutionMode
instruction with theTriangles
mode. Such a shader is valid when the pipeline topology isVK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST
,VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP
, orVK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN
(if tessellation is not in use) or when tessellation is in use and the tessellation mode isTriangles
orQuads
.In this mode, there are three vertices available for each geometry shader invocation. The first, second, and third vertices refer to attributes of the first, second, and third vertex of the triangle, respectively.
 Triangles with Adjacency

Geometry shaders that operate on triangles with adjacent vertices are created by including an
OpExecutionMode
instruction with theInputTrianglesAdjacency
mode. Such a shader is valid when the pipeline topology isVK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST_WITH_ADJACENCY
orVK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY
, and must not be used when tessellation is in use.In this mode, there are six vertices available for each geometry shader invocation. The first, third and fifth vertices refer to attributes of the first, second and third vertex of the triangle, respectively. The second, fourth and sixth vertices refer to attributes of the vertices adjacent to the edges from the first to the second vertex, from the second to the third vertex, and from the third to the first vertex, respectively.
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 builtin 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,
pervertex and perprimitive.
Geometry shader input variables with PassthroughNV
decoration are
considered to produce pervertex outputs, where values for each output
vertex are copied from the corresponding input vertex.
Any builtin or userdefined geometry shader outputs are considered
perprimitive 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
, orTriangles
, as specified by the mode forOpExecutionMode
. 
an invocation count other than one, as specified by the
Invocations
mode forOpExecutionMode
. 
an
OpEntryPoint
which statically uses theOpEmitVertex
orOpEndPrimitive
instructions. 
a variable decorated with the
InvocationId
builtin decoration. 
a variable decorated with the
PrimitiveId
builtin decoration that is declared using theInput
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.