Description

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.

Note

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

See Also

No cross-references are available

Document Notes

For more information, see the Vulkan Specification

This page is extracted from the Vulkan Specification. Fixes and changes should be made to the Specification, not directly.

Copyright

Copyright 2014-2024 The Khronos Group Inc.

SPDX-License-Identifier: CC-BY-4.0