PrimitiveId
PrimitiveId
Decorating a variable with the PrimitiveId built-in decoration will
make that variable contain the index of the current primitive.
The index of the first primitive generated by a drawing command is zero, and the index is incremented after every individual point, line, or triangle primitive is processed.
For triangles drawn as points or line segments (see Polygon Mode), the primitive index is incremented only once, even if multiple points or lines are eventually drawn.
Variables decorated with PrimitiveId are reset to zero between each
instance drawn.
Restarting a primitive topology using primitive restart has no effect on the
value of variables decorated with PrimitiveId.
In tessellation control and tessellation evaluation shaders, it will contain the index of the patch within the current set of rendering primitives that corresponds to the shader invocation.
In a geometry shader, it will contain the number of primitives presented as input to the shader since the current set of rendering primitives was started.
In a fragment shader, it will contain the primitive index written by the mesh shader if a mesh shader is present, or the primitive index written by the geometry shader if a geometry shader is present, or with the value that would have been presented as input to the geometry shader had it been present.
In an intersection, any-hit, or closest hit shader, it will contain the index within the geometry of the triangle or bounding box being processed.
When the PrimitiveId decoration is applied to an output variable in the
mesh shader or
geometry shader, the resulting value is seen through the PrimitiveId
decorated input variable in the fragment shader.
The fragment shader using PrimitiveId will need to declare either the
MeshShadingNV,
MeshShadingEXT,
Geometry or Tessellation capability to satisfy the requirement
SPIR-V has to use PrimitiveId.
Valid Usage
VUID-PrimitiveId-PrimitiveId-04330
The PrimitiveId decoration must be used only within the
MeshEXT, MeshNV, IntersectionKHR, AnyHitKHR,
ClosestHitKHR, TessellationControl,
TessellationEvaluation, Geometry, or Fragment
Execution Model
VUID-PrimitiveId-Fragment-04331
If pipeline contains both the Fragment and Geometry
Execution Model and a variable decorated with PrimitiveId is read
from Fragment shader, then the Geometry shader must write to
the output variables decorated with PrimitiveId in all execution
paths
VUID-PrimitiveId-Fragment-04332
If pipeline contains both the Fragment and MeshEXT or
MeshNV Execution Model and a variable decorated with
PrimitiveId is read from Fragment shader, then the
MeshEXT or MeshNV shader must write to the output variables
decorated with PrimitiveId in all execution paths
VUID-PrimitiveId-Fragment-04333
If Fragment Execution Model contains a variable decorated with
PrimitiveId, then either the MeshShadingEXT,
MeshShadingNV, Geometry or Tessellation capability must
also be declared
VUID-PrimitiveId-PrimitiveId-04334
The variable decorated with PrimitiveId within the
TessellationControl, TessellationEvaluation, Fragment,
IntersectionKHR, AnyHitKHR, or ClosestHitKHR
Execution Model must be declared using the Input Storage Class
VUID-PrimitiveId-PrimitiveId-04335
The variable decorated with PrimitiveId within the Geometry
Execution Model must be declared using the Input or Output
Storage Class
VUID-PrimitiveId-PrimitiveId-04336
The variable decorated with PrimitiveId within the MeshEXT or
MeshNV Execution Model must be declared using the Output
Storage Class
VUID-PrimitiveId-PrimitiveId-04337
The variable decorated with PrimitiveId must be declared as a
scalar 32-bit integer value
VUID-PrimitiveId-PrimitiveId-07040
The variable decorated with PrimitiveId within the MeshEXT
Execution Model must also be decorated with the PerPrimitiveEXT
decoration