The tessellation control shader is used to read an input patch provided by
the application and to produce an output patch.
Each tessellation control shader invocation operates on an input patch
(after all control points in the patch are processed by a vertex shader) and
its associated data, and outputs a single control point of the output patch
and its associated data, and can also output additional per-patch data.
The input patch is sized according to the patchControlPoints
member of
VkPipelineTessellationStateCreateInfo
, as part of input assembly.
The size of the output patch is controlled by the OpExecutionMode
OutputVertices
specified in the tessellation control or tessellation
evaluation shaders, which must be specified in at least one of the shaders.
The size of the input and output patches must each be greater than zero and
less than or equal to
VkPhysicalDeviceLimits
::maxTessellationPatchSize
.
A tessellation control shader is invoked at least once for each output vertex in a patch.
Inputs to the tessellation control shader are generated by the vertex
shader.
Each invocation of the tessellation control shader can read the attributes
of any incoming vertices and their associated data.
The invocations corresponding to a given patch execute logically in
parallel, with undefined relative execution order.
However, the OpControlBarrier
instruction can be used to provide
limited control of the execution order by synchronizing invocations within a
patch, effectively dividing tessellation control shader execution into a set
of phases.
Tessellation control shaders will read undefined values if one invocation
reads a per-vertex or per-patch attribute written by another invocation at
any point during the same phase, or if two invocations attempt to write
different values to the same per-patch output in a single phase.