31. Ray Traversal

Ray traversal is the process which handles intersections between a traced ray and geometry in an acceleration structure.

Ray traversal cannot be started by a Vulkan API command directly - a shader must execute OpRayQueryProceedKHR or OpTraceRayKHR. When the rayTracing feature is enabled, OpTraceRayKHR can be used for ray tracing in a ray tracing pipeline. When the rayQuery feature is enabled, OpRayQueryProceedKHR can be used in any shader stage.

31.1. Ray Intersection Candidate Determination

Once tracing begins, rays will be tested against geometries in an acceleration structure to determine if a hit occurred between them, initially based only on their geometric properties (i.e. their vertices). The implementation performs similar operations to that of rasterization, but with the effective viewport determined by the parameters of the ray, and the geometry transformed into a space determined by that viewport.

The acceleration structure space coordinates of each primitive are transformed according to the origin and direction of the ray as follows:

a is the axis of rotation between k and d

s and c are the sine and cosine of the angle α between k and d

k is the unit vector

o and d are the ray origin and direction, respectively; the vector described by xas, yas, and zas is any position in acceleration structure space; and the vector described by xr, yr, and zr is the same position in ray space.

An intersection candidate is a unique point of intersection between a ray and a geometric primitive. For any primitive that has within its bounds a position xyzas such that

an intersection candidate exists.

The determination of this condition is performed in an implementation specific manner, and may be performed with floating point operations. Due to the complexity and number of operations involved, inaccuracies are expected, particularly as the scale of values involved begins to diverge. Implementations should take efforts to maintain as much precision as possible.

Note

One very common case is when geometries are close to each other at some distance from the origin in acceleration structure space, where an effect similar to “z-fighting” is likely to be observed. Applications can mitigate this by ensuring their detailed geometries remain close to the origin.

Another likely case is when the origin of a ray is set to a position on a previously intersected surface, and its tmin is zero or near zero; an intersection may be detected on the emitting surface. This case can usually be mitigated by offsetting tmin slightly.

In the case of AABB geometries, implementations may increase their size in an acceleration structure in order to mitigate precision issues. This may result in false positives being reported to the application.

For triangle intersection candidates, the a and b barycentric coordinates on the triangle where the above condition is met are made available to future shading. If the ray was traced with OpTraceRayKHR, these values are available as a vector of 2 32-bit floating point values in the HitAttributeKHR storage class.

Once an intersection candidate is determined, it proceeds through the following operations, in order:

The sections below describe the exact details of these tests. There is no ordering guarantee between operations performed on different intersection candidates.

31.1.1. Watertightness

For a set of triangles with identical transforms, within a single instance:

  • Any set of two or more triangles where all triangles have one vertex with an identical position value, that vertex is a shared vertex.

  • Any set of two triangles with two shared vertices that were specified in the same winding order in each triangle have a shared edge defined by those vertices.

A closed fan is a set of three or more triangles where:

  • All triangles in the set have the same shared vertex as one of their vertices.

  • All edges that include the above vertex are shared edges.

  • All above shared edges are shared by exactly two triangles from the set.

  • No two triangles in the set intersect, except at shared edges.

  • Every triangle in the set is joined to every other triangle in the set by a series of the above shared edges.

Implementations should not double-hit or miss when a ray intersects a shared edge, or a shared vertex of a closed fan.

31.2. Ray Intersection Culling

Candidate intersections go through several phases of culling before confirmation as an actual hit. There is no particular ordering dependency between the different culling operations.

31.2.1. Ray Primitive Culling

If the rayTracingPrimitiveCulling feature is enabled, the SkipTrianglesKHR and SkipAABBsKHR ray flags can be specified when tracing a ray.

If SkipTrianglesKHR was included in the Ray Flags operand of the ray trace instruction, and the intersection is with a triangle primitive, the intersection is dropped, and no further processing of this intersection occurs. If VK_PIPELINE_CREATE_RAY_TRACING_SKIP_TRIANGLES_BIT_KHR was included in the pipeline, traversal with OpTraceKHR calls will all behave as if SkipTrianglesKHR was included in its Ray Flags operand.

If SkipAABBsKHR was included in the Ray Flags operand of the ray trace instruction, and the intersection is with an AABB primitive, the intersection is dropped, and no further processing of this intersection occurs. If VK_PIPELINE_CREATE_RAY_TRACING_SKIP_AABBS_BIT_KHR was included in the pipeline, traversal with OpTraceKHR calls will all behave as if SkipTrianglesKHR was included in its Ray Flags operand.

31.2.2. Ray Mask Culling

Instances can be made invisible to particular rays based on the value of VkAccelerationStructureInstanceKHR::mask used to add that instance to a top-level acceleration structure, and the Cull Mask parameter used to trace the ray.

For the instance which is intersected, if mask & Cull Mask == 0, the intersection is dropped, and no further processing occurs.

31.2.3. Ray Face Culling

As in polygon rasterization, one of the stages of ray traversal is to determine if a triangle primitive is back- or front-facing, and primitives can be culled based on that facing.

If the intersection candidate is with an AABB primitive, this operation is skipped.

Determination

When a ray intersects a triangle primitive, the order that vertices are specified for the polygon affects whether the ray intersects the front or back face. Front or back facing is determined in the same way as they are for rasterization, based on the sign of the polygon’s area but using the ray space coordinates instead of framebuffer coordinates. One way to compute this area is:

where and are the x and y ray space coordinates of the ith vertex of the n-vertex polygon (vertices are numbered starting at zero for the purposes of this computation) and i ⊕ 1 is (i + 1) mod n.

If VK_GEOMETRY_INSTANCE_TRIANGLE_FRONT_COUNTERCLOCKWISE_BIT_KHR is included in VkAccelerationStructureInstanceKHR::flags for the instance containing the intersected triangle, if a is positive then the intersection is with the front face of the triangle, otherwise it is with the back face. If VK_GEOMETRY_INSTANCE_TRIANGLE_FRONT_COUNTERCLOCKWISE_BIT_KHR was not included, this determination is reversed. Additionally, if a is 0, the intersection candidate is treated as not intersecting with any face, irrespective of the sign.

If the ray was traced with OpTraceRayKHR, the HitKindKHR built-in is set to HitKindFrontFacingTriangleKHR if the intersection is with front-facing geometry, and HitKindBackFacingTriangleKHR if the intersection is with back-facing geometry, for shader stages considering this intersection.

If the ray was traced with OpRayQueryProceedKHR, OpRayQueryGetIntersectionFrontFaceKHR will return true for intersection candidates with front faces, or false for back faces.

Culling

If CullBackFacingTrianglesKHR was included in the Ray Flags parameter of the ray trace instruction, and the intersection is determined as with the back face of a triangle primitive, the intersection is dropped, and no further processing of this intersection occurs.

If CullFrontFacingTrianglesKHR was included in the Ray Flags parameter of the ray trace instruction, and the intersection is determined as with the front face of a triangle primitive, the intersection is dropped, and no further processing of this intersection occurs.

This culling is disabled if VK_GEOMETRY_INSTANCE_TRIANGLE_FACING_CULL_DISABLE_BIT_KHR was included in VkAccelerationStructureInstanceKHR::flags for the instance which the intersected geometry belongs to.

Intersection candidates that have not intersected with any face (a == 0) are unconditionally culled, irrespective of ray flags and geometry instance flags.

31.2.4. Ray Opacity Culling

Each geometry in the acceleration structure may be considered either opaque or not. Opaque geometries continue through traversal as normal, whereas non-opaque geometries need to be either confirmed or discarded by shader code. Intersection candidates can also be culled based on their opacity.

Determination

Each individual intersection candidate is initally determined as opaque if VK_GEOMETRY_OPAQUE_BIT_KHR was included in the VkAccelerationStructureGeometryKHR::flags when the geometry it intersected with was built, otherwise it is considered non-opaque.

If the intersection candidate was generated by an intersection shader, the intersection is initially considered to have opacity matching the AABB candidate that it was generated from.

However, this opacity can be overridden when it is built into an instance. Setting VK_GEOMETRY_INSTANCE_FORCE_OPAQUE_BIT_KHR in VkAccelerationStructureInstanceKHR::flags will force all geometries in the instance to be considered opaque. Similarly, setting VK_GEOMETRY_INSTANCE_FORCE_NO_OPAQUE_BIT_KHR will force all geometries in the instance to be considered non-opaque.

This can again be overridden by including OpaqueKHR or NoOpaqueKHR in the Ray Flags parameter when tracing a ray. OpaqueKHR forces all geometries to behave as if they are opaque, regardless of their build parameters. Similarly, NoOpaqueKHR forces all geometries to behave as if they are non-opaque.

If the ray was traced with OpRayQueryProceedKHR, to determine the opacity of AABB intersection candidates, OpRayQueryGetIntersectionCandidateAABBOpaqueKHR can be used. This instruction will return true for opaque intersection candidates, and false for non-opaque intersection candidates.

Culling

If CullOpaqueKHR is included in the Ray Flags parameter when tracing a ray, an intersection with a geometry that is considered opaque is dropped, and no further processing occurs.

If CullNoOpaqueKHR is included in the Ray Flags parameter when tracing a ray, an intersection with a geometry that is considered non-opaque is dropped, and no further processing occurs.

31.3. Ray Intersection Confirmation

Depending on the opacity of intersected geometry and whether it is a triangle or an AABB, candidate intersections are further processed to determine the eventual hit result. Candidates generated from AABB intersections run through the same confirmation process as triangle hits.

31.3.1. AABB Intersection Candidates

For intersection candidates with an AABB geometry generated by Ray Intersection Candidate Determination, shader code is executed to determine whether any hits should be reported to the traversal infrastructure; no further processing of this intersection candidate occurs.

If the ray was traced with OpTraceRayKHR, an intersection shader is invoked from the Shader Binding Table according to the specified indexing for the intersected geometry. If this shader calls OpReportIntersectionKHR, a new intersection candidate is generated as described below.

Each new candidate generated as a result of this processing is a generated intersection candidate that intersects the AABB geometry, with a t value equal to the Hit parameter of the OpReportIntersectionKHR instruction. The new generated candidate is then independently run through Ray Intersection Confirmation as a generated intersection.

If the ray was traced with OpRayQueryProceedKHR, control is returned to the shader which executed OpRayQueryProceedKHR, returning true. The resulting ray query has a candidate intersection type of RayQueryCandidateIntersectionAABBKHR. OpRayQueryGenerateIntersectionKHR can be called to commit a new intersection candidate with committed intersection type of RayQueryCommittedIntersectionGeneratedKHR. Further ray query processing can be continued by executing OpRayQueryProceedKHR with the same ray query, or intersection can be terminated with OpRayQueryTerminateKHR. Unlike rays traced with OpTraceKHR, candidates generated in this way skip generated intersection candidate confirmation; applications should make this determination before generating the intersection.

This operation may be executed multiple times for the same intersection candidate.

31.3.2. Triangle and Generated Intersection Candidates

For triangle and generated intersection candidates, additional shader code may be executed based on the intersections opacity.

If the intersection is opaque, the candidate is immediately confirmed as a valid hit and passes to the next stage of processing.

For non-opaque intersection candidates, shader code is executed to determine whether a hit occurred or not.

If the ray was traced with OpTraceRayKHR, an any hit shader is invoked from the Shader Binding Table according to the specified indexing. If this shader calls OpIgnoreIntersectionKHR, the candidate is dropped and no further processing of the candidate occurs. If the any hit shader identified is VK_SHADER_UNUSED_KHR, the candidate is immediately confirmed as a valid hit and passes to the next stage of processing.

If the ray was traced with OpRayQueryProceedKHR, control is returned to the shader which executed OpRayQueryProceedKHR, returning true. As only triangle candidates participate in this operation with ray queries, the resulting candidate intersection type is always RayQueryCandidateIntersectionTriangleKHR. OpRayQueryConfirmIntersectionKHR can be called on the ray query to confirm the candidate as a hit with committed intersection type of RayQueryCommittedIntersectionTriangleKHR. Further ray query processing can be continued by executing OpRayQueryProceedKHR with the same ray query, or intersection can be terminated with OpRayQueryTerminateKHR. If OpRayQueryConfirmIntersectionKHR has not been executed, the candidate is dropped and no further processing of the candidate occurs.

This operation may be executed multiple times for the same intersection candidate unless VK_GEOMETRY_NO_DUPLICATE_ANY_HIT_INVOCATION_BIT_KHR was specified for the intersected geometry.

31.4. Ray Closest Hit Determination

Unless the ray was traced with the TerminateOnFirstHitKHR ray flag, the implementation must track the closest confirmed hit until all geometries have been tested and either confirmed or dropped.

After an intersection candidate is confirmed, its t value is compared to tmax to determine which intersection is closer, where t is the parametric distance along the ray at which the intersection occurred.

  • If t < tmax, tmax is set to t and the candidate is set as the current closest hit.

  • If t > tmax, the candidate is dropped and no further processing of that candidate occurs.

  • If t = tmax, the candidate may be set as the current closest hit or dropped.

If TerminateOnFirstHitKHR was included in the Ray Flags used to trace the ray, once the first hit is confirmed, the ray trace is terminated.

31.5. Ray Result Determination

Once all candidates have finished processing the prior stages, or if the ray is forcibly terminated, the final result of the ray trace is determined.

If a closest hit result was identified by Ray Closest Hit Determination, a closest hit has occurred, otherwise the final result is a miss.

For rays traced with OpTraceRayKHR, if a closest hit result was identified, a closest-hit shader is invoked from the Shader Binding Table according to the specified indexing for the intersected geometry. Control returns to the shader that executed OpTraceRayKHR once this shader returns. This shader is skipped if either the ray flags included SkipClosestHitShaderKHR, or if the closest-hit shader identified is VK_SHADER_UNUSED_NV.

For rays traced with OpTraceRayKHR where no hit result was identified, the miss shader identified by the Miss Index parameter of OpTraceRayKHR is invoked. Control returns to the shader that executed OpTraceRayKHR once this shader returns.

If the ray was traced with OpRayQueryProceedKHR, control is returned to the shader which executed OpRayQueryProceedKHR, returning false. If a closest hit was identified by Ray Closest Hit Determination, the ray query will now have a committed intersection type of RayQueryCommittedIntersectionGeneratedKHR or RayQueryCommittedIntersectionTriangleKHR. If no closest hit was identified, the committed intersection type will be RayQueryCommittedIntersectionNoneKHR.

No further processing of a ray query occurs after this result is determined.