Ray Traversal

The ray traversal process identifies and handles intersections between a ray and geometries in an acceleration structure.

Ray traversal cannot be started by a Vulkan API command directly - a shader must execute OpRayQueryProceedKHR or a pipeline trace ray instruction . When the rayTracingPipeline 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.

Ray Intersection Candidate Determination

Once tracing begins, rays are first tested against instances in a top-level acceleration structure. A ray that intersects an instance will be transformed into the space of the instance to continue traversal within that instance; therefore the transform matrix stored in the instance must be invertible.

In case multiple instances are intersected by a ray, the ray transformation into the space of the instance is invariant under the order in which these instances are encountered in the top-level acceleration structure.

Applying multiple forward and reverse transforms to a ray to transition from one instance to another could result in accumulated errors. Thus an implementation should behave as if the ray is transformed from the origin for each instance independently.

Next, rays are tested against geometries in a bottom-level 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 vertices of each primitive are transformed from acceleration structure space _as to ray space _r according to the ray origin and direction as follows:

\left( \begin{array}{c} x\_{r} \\\ y\_{r}\\\ z\_{r} \end{array} \right) = \left( \begin{matrix} a\_x^2(1-c) + c & a\_xa\_y(1-c) - sa\_z & a\_xa\_z(1-c) + sa\_y \\\ a\_xa\_y(1-c) + sa\_z & a\_y^2(1-c) + c & a\_ya\_z(1-c) - sa\_x \\\ a\_xa\_z(1-c) - sa\_y & a\_ya\_z(1-c) + sa\_x & a\_z^2(1-c) + c \end{matrix} \right) \left( \begin{array}{c} x\_{as} - o\_x \\\ y\_{as} - o\_y \\\ z\_{as} - o\_z \end{array} \right)

$\mathbf{a}$ is the axis of rotation from the unnormalized ray direction vector $\mathbf{d}$ to the axis vector $\mathbf{k}$ :

\mathbf{a} = \begin{cases} \frac{\mathbf{d} \times \mathbf{k}}{|| \mathbf{d} \times \mathbf{k} ||} & \mathrm{if}\\; || \mathbf{d} \times \mathbf{k} || \ne 0 \\\ \left(\begin{array}{c} 0 \\\ 1 \\\ 0 \end{array} \right) & \mathrm{if}\\; || \mathbf{d} \times \mathbf{k} || = 0 \\\ \end{cases}

$\mathit{s}$ and $\mathit{c}$ are the sine and cosine of the angle of rotation about $\mathbf{a}$ from $\mathbf{d}$ to $\mathbf{k}$ :

\begin{aligned} c &= {{\mathbf{d} \cdot \mathbf{k}}\over{||\mathbf{d}||}} \\\ s &= \sqrt{1 - c^2} \end{aligned}

$\mathbf{k}$ is the unit vector:

\mathbf{k} = \left( \begin{array}{c} 0 \\\ 0 \\\ -1 \end{array} \right)

$\mathbf{o}$ and $\mathbf{d}$ are the ray origin and unnormalized direction, respectively; the vector described by x_as, y_as, and z_as is any position in acceleration structure space; and the vector described by x_r, y_r, and z_r 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 $\mathbf{xyz\_{as}}$ such that

\begin{aligned} x\_r &= 0 \\\ y\_r &= 0 \\\ t\_\mathit{min} \lt {-{z\_r}\over{||\mathbf{d}||}} &\lt t\_\mathit{max} & \text{if the primitive is a triangle,} \\\ t\_\mathit{min} \leq {-{z\_r}\over{||\mathbf{d}||}} &\leq t\_\mathit{max} & \text{otherwise} \\\ \end{aligned}

(where $t = {-{z\_r}\over{||\mathbf{d}||}}$ ), an intersection candidate exists.

Triangle primitive bounds consist of all points on the plane formed by the three vertices and within the bounds of the edges between the vertices, subject to the watertightness constraints below. AABB primitive bounds consist of all points within an implementation-defined bound which includes the specified box.

The bounds of the AABB including all points internal to the bound implies that a ray started within the AABB will hit that AABB.

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.

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 t_min is zero or near zero; an intersection may be detected on the emitting surface. This case can usually be mitigated by offsetting t_min slightly.

For a motion primitive or a motion instance, the positions for intersection are evaluated at the time specified in the time parameter to OpTraceRayMotionNV by interpolating between the two endpoints as specified for the given motion type. If a motion acceleration structure is traced with OpTraceRayKHR, it behaves as a OpTraceRayMotionNV with time of 0.0.

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 positive intersections being reported to the application.

For triangle intersection candidates, the b and c

barycentric coordinates on the triangle where the above condition is met are made available to future shading. If the ray was traced with a pipeline trace ray instruction, 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.

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.

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.

Ray Primitive Culling

If the rayTraversalPrimitiveCulling or rayQuery features are enabled, the SkipTrianglesKHR and SkipAABBsKHR ray flags can be specified when tracing a ray. SkipTrianglesKHR and SkipAABBsKHR are mutually exclusive. SkipTrianglesKHR is also mutually exclusive with CullBackFacingTrianglesKHR and CullFrontFacingTrianglesKHR.

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 pipeline trace ray instructions will all behave as if SkipTrianglesKHR was included in their 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 pipeline trace ray instructions will all behave as if SkipAABBsKHR was included in their Ray Flags operand.

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.

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:

a = -{1 \over 2}\sum\_{i=0}^{n-1} x\_r^i y\_r^{i \oplus 1} - x\_r^{i \oplus 1} y\_r^i

where $x\_r^i$ and $y\_r^i$ 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.

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

In a left-handed coordinate system, an intersection will be with the front face of a triangle if the vertices of the triangle, as defined in index order, appear from the ray’s perspective in a clockwise rotation order. VK_GEOMETRY_INSTANCE_TRIANGLE_FLIP_FACING_BIT_KHR was previously annotated as VK_GEOMETRY_INSTANCE_TRIANGLE_FRONT_COUNTERCLOCKWISE_BIT_KHR because of this.

If the ray was traced with a pipeline trace ray instruction, 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.

The CullBackFacingTrianglesKHR and CullFrontFacingTrianglesKHR

Ray Flags are mutually exclusive.

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 initially 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 geometry includes an opacity micromap, the opacity of the intersection at this point is instead derived as described in Ray Opacity Micromap.

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.

The OpaqueKHR, NoOpaqueKHR, CullOpaqueKHR, and CullNoOpaqueKHR Ray Flags are mutually exclusive.

Ray Opacity Micromap

A VK_GEOMETRY_TYPE_TRIANGLES_KHR geometry in the acceleration structure may have an opacity micromap associated with it to give finer-grained opacity information.

If the intersection candidate is with a geometry with an associated opacity micromap and VK_GEOMETRY_INSTANCE_DISABLE_OPACITY_MICROMAPS_EXT is not set in its instance then the micromap is used to determine geometry opacity instead of the VK_GEOMETRY_OPAQUE_BIT_KHR flag in the geometry.

The opacity information in the micromap object is accessed using the candidate intersection u and v coordinates. The integer u and v are computed from ⌊u⌋ + ⌊v⌋, clamping ⌊u⌋ as needed to keep the sum less than or equal to 1 << subdivisionlevel. These values are mapped into a linear index with a space filling curve which is defined recursively by traversing into the sub-triangle nearest vertex 0, then the middle triangle with ordering flipped, then nearest vertex 1 then nearest vertex 2.

This encoding is spatially coherent, purely hierarchical, and allows a bit-parallel conversion between barycentric address and index values.

See the appendix for reference code implementing this mapping.

The result of the opacity micromap lookup and operations is to treat the intersection as opaque, non-opaque, or ignored. The interpretation of the values depends on VK_GEOMETRY_INSTANCE_FORCE_OPACITY_MICROMAP_2_STATE_EXT in the instance of the candidate intersection or ForceOpacityMicromap2StateEXT ray flags on the ray. If either is set, the opacity micromap information is interpreted in 2 state override mode. If the result of the micromap lookup is to treat the intersection candidate as ignored, no further processing of that candidate is done.

If the associated opacity micromap has format VK_OPACITY_MICROMAP_FORMAT_2_STATE_EXT, each element of the micromap is represented by a single bit at the index derived above.

If the associated opacity micromap has format VK_OPACITY_MICROMAP_FORMAT_4_STATE_EXT, each element is represented by a two bit value at the index derived above.

4 State value	2 State value	Special index value	2 State override	Result
0	0	VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_TRANSPARENT_EXT	Y	Ignored
0	0	VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_TRANSPARENT_EXT	N	Ignored
1	1	VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_OPAQUE_EXT	Y	Opaque
1	1	VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_OPAQUE_EXT	N	Opaque
2		VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_TRANSPARENT_EXT	Y	Ignored
2		VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_TRANSPARENT_EXT	N	Non-opaque
3		VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_OPAQUE_EXT	Y	Opaque
3		VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_OPAQUE_EXT	N	Non-opaque

4 State value

2 State value

Special index value

2 State override

Result

VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_TRANSPARENT_EXT

Ignored

VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_TRANSPARENT_EXT

Ignored

VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_OPAQUE_EXT

Opaque

VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_OPAQUE_EXT

Opaque

VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_TRANSPARENT_EXT

Ignored

VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_TRANSPARENT_EXT

Non-opaque

VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_OPAQUE_EXT

Opaque

VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_OPAQUE_EXT

Non-opaque

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.

AABB Intersection Candidates

For an intersection candidate 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. The occurrence of an AABB intersection candidate does not guarantee the ray intersects the primitive bounds. To avoid propagating false intersections the application should verify the intersection candidate before reporting any hits and only report intersections within the bounds of the AABB. Reporting an intersection outside the AABB either through the implementation giving a conservative bound or reporting a t out of range is legal but may result in unpredictable closest hit results.

If the ray was traced with a pipeline trace ray instruction, 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. If the intersection shader is VK_SHADER_UNUSED_KHR (which is only allowed for a zero shader group) then no further processing of the intersection candidate occurs.

Each new candidate generated as a result of this processing is a generated intersection candidate from the intersection with 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 a pipeline trace ray instruction, 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.

Triangle and Generated Intersection Candidates

For triangle and generated intersection candidates, additional shader code may be executed based on the intersection’s 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 a pipeline trace ray instruction, 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.

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 t_max to determine which intersection is closer, where t is the parametric distance along the ray at which the intersection occurred.

If t < t_max, t_max is set to t and the candidate is set as the current closest hit.
If t > t_max, the candidate is dropped and no further processing of that candidate occurs.
If t = t_max, 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.

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 pipeline trace ray instructions which can invoke a closest hit shader, 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 the pipeline trace ray instruction 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_KHR.

For rays traced with a pipeline trace ray instruction where no hit result was identified, the miss shader identified by the Miss Index parameter of the instruction is invoked. Control returns to the shader that executed the pipeline trace ray instruction once this shader returns. This shader is skipped if the miss shader identified is VK_SHADER_UNUSED_KHR.

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.