SPV_KHR_ray_tracing_position

Name Strings

SPV_KHR_ray_tracing_position_fetch

Contact

See Issues list in the Khronos SPIRV-Headers repository: https://github.com/KhronosGroup/SPIRV-Headers

Contributors

Eric Werness, NVIDIA
Daniel Koch, NVIDIA
Alan Baker, Google

Status

2023-02-22 Approved by the SPIR Working Group
2023-04-14 Ratified by the Khronos Board

Version

Last Modified Date

2023-04-21

Revision

Dependencies

This extension is written against the Unified SPIR-V Specification, Version 1.5, Revision 1.

This extension requires SPIR-V 1.4.

This extension requires SPV_KHR_ray_tracing or SPV_KHR_ray_query.

Overview

This extension adds functionality to ray pipelines and ray query to allow fetching the vertex positions for the current hit.

Extension Name

To use this extension within a SPIR-V module, the following OpExtension must be present in the module:

OpExtension "SPV_KHR_ray_tracing_position_fetch"

New Capabilities

This extension introduces two new capabilities:

RayTracingPositionFetchKHR
RayQueryPositionFetchKHR

New Builtins

Builtin added under the RayTracingPositionFetchKHR capability

HitTriangleVertexPositionsKHR

New Instructions

Instruction added under the RayQueryPositionFetchKHR capability

OpRayQueryGetIntersectionTriangleVertexPositionsKHR

Modifications to the SPIR-V Specification

(Modify Section 3.21, BuiltIn, adding rows to the BuiltIn table)

BuiltIn	Enabling Capabilities
5335	HitTriangleVertexPositionsKHR The vertex positions for the triangle hit for the ray being traced in the AnyHitKHR or ClosestHitKHR execution models.+ + Refer to the Ray Tracing chapter of Vulkan API specification for more details.	RayTracingPositionFetchKHR

BuiltIn

Enabling Capabilities

5335

HitTriangleVertexPositionsKHR
The vertex positions for the triangle hit for the ray being traced in the AnyHitKHR or ClosestHitKHR execution models.+ + Refer to the Ray Tracing chapter of Vulkan API specification for more details.

RayTracingPositionFetchKHR

(Modify Section 3.31, Capability, adding rows to the Capability table)

Capability	Implicitly Declares
5336	RayTracingPositionFetchKHR Uses the HitTriangleVertexPositionsKHR builtin.	RayTracingKHR
5391	RayQueryPositionFetchKHR Uses the OpRayQueryGetIntersectionTriangleVertexPositionsKHR instruction.	RayQueryKHR

Capability

Implicitly Declares

5336

RayTracingPositionFetchKHR
Uses the HitTriangleVertexPositionsKHR builtin.

RayTracingKHR

5391

RayQueryPositionFetchKHR
Uses the OpRayQueryGetIntersectionTriangleVertexPositionsKHR instruction.

RayQueryKHR

(Add to sub section 3.32.RQInstructions, Ray Query Instructions)

OpRayQueryGetIntersectionTriangleVertexPositionsKHR

Gets the vertex positions for the triangle at the current intersection.

Result is the returned vertex positions.

Result Type must be an array with a Length of 3, and an Element Type that is a vector type with a Component Type that is a 32-bit floating-point type and a Component Count of 3.

Intersection must be the <id> of a constant instruction with a 32-bit scalar integer type.

Intersection identifies which intersection values should be returned for, either the current candidate or the closest recorded hit so far; see Ray Query Intersection.

Ray Query is a pointer to the ray query object.

If Intersection is RayQueryCandidateIntersectionKHR, behavior is undefined if OpRayQueryProceedKHR was not executed on the same ray query object, or if the last value returned by such an execution of OpRayQueryProceedKHR was not true.
If Intersection is RayQueryCommittedIntersectionKHR, behavior is undefined if there is no current committed intersection (see OpRayQueryCommittedTypeKHR).

Capability:
RayQueryPositionFetchKHR

5340

<id> Result Type

Result <id>

<id> Ray Query

<id> Intersection

Validation Rules

An OpExtension must be added to the SPIR-V for validation layers to check legal use of this extension:

OpExtension "SPV_KHR_ray_tracing_position_fetch"

Interactions with SPV_KHR_ray_tracing

The RayTracingPositionFetchKHR capability and the HitTriangleVertexPositionsKHR builtin are only supported if SPV_KHR_ray_tracing and the RayTracingKHR capability are supported.

Interactions with SPV_KHR_ray_query

The RayQueryPositionFetchKHR capability and the OpRayQueryGetIntersectionTriangleVertexPositionsKHR instruction are only supported if SPV_KHR_ray_query and the RayQueryKHR capability are supported.

Issues

1) Should triangle be in the name somewhere?

RESOLVED: Yes, though OpRayQueryGetIntersectionTriangleVertexPositionsKHR seems a bit long.

2) Where should the functionality of the new builtin and instruction be defined?

RESOLVED: Following precedent, ray tracing (pipeline) relies more on "Refer to the Ray Tracing chapter of Vulkan API" language while ray query inlines more of the functionality definition directly in the SPIR-V extensions.

Revision History

Rev	Date	Author	Changes
1	2022-05-12	Eric Werness	First draft
2	2022-12-14	Daniel Koch	Use two capabilities and other spec cleanup.
3	2023-01-06	Daniel Koch	Follow SPIR-V conventions for undefined behavior.
4	2023-04-21	Daniel Koch	Add ratification status