path: root/chapters/raytraversal.adoc

// Copyright 2018-2024 The Khronos Group Inc.
//
// SPDX-License-Identifier: CC-BY-4.0

[[ray-traversal]]
= 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
ifdef::VK_KHR_ray_query[code:OpRayQueryProceedKHR]
ifdef::VK_KHR_ray_query+VK_KHR_ray_tracing_pipeline[or]
ifdef::VK_KHR_ray_tracing_pipeline[a <<glossary-pipeline-trace-ray, pipeline trace ray>> instruction]
.
ifdef::VK_KHR_ray_tracing_pipeline[]
When the <<features-rayTracingPipeline, pname:rayTracingPipeline>> feature
is enabled, code:OpTraceRayKHR can: be used for <<ray-tracing, ray tracing>>
in a <<pipelines-ray-tracing, ray tracing pipeline>>.
endif::VK_KHR_ray_tracing_pipeline[]
ifdef::VK_KHR_ray_query[]
When the <<features-rayQuery, pname:rayQuery>> feature is enabled,
code:OpRayQueryProceedKHR can: be used in any shader stage.
endif::VK_KHR_ray_query[]


[[ray-intersection-candidate-determination]]
== 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.

[NOTE]
.Note
====
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 an 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:

[latexmath]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
\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)
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

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

[latexmath]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
\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}
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

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

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

latexmath:[\mathbf{k}] is the unit vector:

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

latexmath:[\mathbf{o}] and latexmath:[\mathbf{d}] are the ray origin and
unnormalized direction, respectively; the vector described by [eq]#x~as~#,
[eq]#y~as~#, and [eq]#z~as~# is any position in acceleration structure
space; and the vector described by [eq]#x~r~#, [eq]#y~r~#, and [eq]#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
latexmath:[\mathbf{xyz_{as}}] such that

[latexmath]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
\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 latexmath:[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.

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

[[raytraversal-ray-intersection-candidate-diagram]]
image::{images}/ray_intersection_candidate.svg[align="center",title="Ray intersection candidate",opts="{imageopts}"]

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]
.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 [eq]#t~min~# is zero or near zero;
an intersection may be detected on the emitting surface.
This case can usually be mitigated by offsetting [eq]#t~min~# slightly.
====

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

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 [eq]#b# and [eq]#c#
<<primsrast-polygon-barycentrics,barycentric coordinates>> on the triangle
where the above condition is met are made available to future shading.
ifdef::VK_KHR_ray_tracing_pipeline[]
If the ray was traced with a <<glossary-pipeline-trace-ray, pipeline trace
ray>> instruction, these values are available as a vector of 2 32-bit
floating point values in the code:HitAttributeKHR storage class.
endif::VK_KHR_ray_tracing_pipeline[]

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

    . <<ray-intersection-culling>>
    . <<ray-intersection-confirmation>>
    . <<ray-closest-hit-determination>>
    . <<ray-result-determination>>

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


[[ray-traversal-watertight]]
=== 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 <<drawing-triangle-lists, 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]]
== 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-traversal-culling-primitive]]
=== Ray Primitive Culling

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

If code: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 ename:VK_PIPELINE_CREATE_RAY_TRACING_SKIP_TRIANGLES_BIT_KHR was included
in the pipeline, traversal with <<glossary-pipeline-trace-ray, pipeline
trace ray>> instructions will all behave as if code:SkipTrianglesKHR was
included in their `Ray Flags` operand.

If code: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 ename:VK_PIPELINE_CREATE_RAY_TRACING_SKIP_AABBS_BIT_KHR was included in
the pipeline, traversal with <<glossary-pipeline-trace-ray, pipeline trace
ray>> instructions will all behave as if code:SkipAABBsKHR was included in
their `Ray Flags` operand.


=== Ray Mask Culling

Instances can: be made invisible to particular rays based on the value of
slink:VkAccelerationStructureInstanceKHR::pname: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 [eq]#pname:mask & `Cull Mask` ==
0#, the intersection is dropped, and no further processing occurs.


[[ray-traversal-culling-face]]
=== Ray Face Culling

As in <<primsrast-polygons-basic,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
<<primsrast-polygons-basic,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:

[latexmath]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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 latexmath:[x_r^i] and latexmath:[y_r^i] are the [eq]#x# and [eq]#y#
<<ray-intersection-candidate-determination,ray space coordinates>> of the
[eq]##i##th vertex of the [eq]#n#-vertex polygon (vertices are numbered
starting at zero for the purposes of this computation) and [eq]#i {oplus} 1#
is [eq]#(i {plus} 1) mod n#.

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

[NOTE]
.Note
====
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.
ename:VK_GEOMETRY_INSTANCE_TRIANGLE_FLIP_FACING_BIT_KHR was previously
annotated as
ename:VK_GEOMETRY_INSTANCE_TRIANGLE_FRONT_COUNTERCLOCKWISE_BIT_KHR because
of this.
====

ifdef::VK_KHR_ray_tracing_pipeline[]
If the ray was traced with a <<glossary-pipeline-trace-ray, pipeline trace
ray>> instruction, the code:HitKindKHR built-in is set to
code:HitKindFrontFacingTriangleKHR if the intersection is with front-facing
geometry, and code:HitKindBackFacingTriangleKHR if the intersection is with
back-facing geometry, for shader stages considering this intersection.
endif::VK_KHR_ray_tracing_pipeline[]

ifdef::VK_KHR_ray_query[]
If the ray was traced with code:OpRayQueryProceedKHR,
code:OpRayQueryGetIntersectionFrontFaceKHR will return true for intersection
candidates with front faces, or false for back faces.
endif::VK_KHR_ray_query[]

.Culling

If code: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 code: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
ename:VK_GEOMETRY_INSTANCE_TRIANGLE_FACING_CULL_DISABLE_BIT_KHR was included
in slink:VkAccelerationStructureInstanceKHR::pname:flags for the instance
which the intersected geometry belongs to.

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

The code:CullBackFacingTrianglesKHR and code: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
ename:VK_GEOMETRY_OPAQUE_BIT_KHR was included in the
slink:VkAccelerationStructureGeometryKHR::pname:flags when the geometry it
intersected with was built, otherwise it is considered non-opaque.

ifdef::VK_EXT_opacity_micromap[]
If the geometry includes an opacity micromap, the opacity of the
intersection at this point is instead derived as described in
<<ray-opacity-micromap,Ray Opacity Micromap>>.
endif::VK_EXT_opacity_micromap[]

ifdef::VK_KHR_ray_tracing_pipeline[]
If the intersection candidate was generated by an <<shaders-intersection,
intersection shader>>, the intersection is initially considered to have
opacity matching the AABB candidate that it was generated from.
endif::VK_KHR_ray_tracing_pipeline[]

However, this opacity can be overridden when it is built into an instance.
Setting ename:VK_GEOMETRY_INSTANCE_FORCE_OPAQUE_BIT_KHR in
slink:VkAccelerationStructureInstanceKHR::pname:flags will force all
geometries in the instance to be considered opaque.
Similarly, setting ename: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 code:OpaqueKHR or code:NoOpaqueKHR
in the `Ray Flags` parameter when tracing a ray.
code:OpaqueKHR forces all geometries to behave as if they are opaque,
regardless of their build parameters.
Similarly, code:NoOpaqueKHR forces all geometries to behave as if they are
non-opaque.

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

.Culling

If code: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 code: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 code:OpaqueKHR, code:NoOpaqueKHR, code:CullOpaqueKHR, and
code:CullNoOpaqueKHR `Ray Flags` are mutually exclusive.

ifdef::VK_EXT_opacity_micromap[]
[[ray-opacity-micromap]]
=== Ray Opacity Micromap

A ename: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 ename: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 ename:VK_GEOMETRY_OPAQUE_BIT_KHR flag in the geometry.

The opacity information in the micromap object is accessed using the
candidate intersection [eq]#u# and [eq]#v# coordinates.
The integer [eq]#u# and [eq]#v# are computed from [eq]#{lfloor}u{rfloor}
{plus} {lfloor}v{rfloor}#, clamping [eq]#{lfloor}u{rfloor}# as needed to
keep the sum less than or equal to [eq]#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.

image::{images}/micromap-subd.svg[align="center",title="Example ordering for micromap data",align="center",opts="{imageopts}"]

[NOTE]
.Note
====
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
ename:VK_GEOMETRY_INSTANCE_FORCE_OPACITY_MICROMAP_2_STATE_EXT in the
instance of the candidate intersection or
ename: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
ename: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
ename:VK_OPACITY_MICROMAP_FORMAT_4_STATE_EXT, each element is represented by
a two bit value at the index derived above.


[options="header"]
|====
| 4 State value | 2 State value | Special index value | 2 State override | Result
| 0 | 0 | ename:VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_TRANSPARENT_EXT         | Y | Ignored
| 0 | 0 | ename:VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_TRANSPARENT_EXT         | N | Ignored
| 1 | 1 | ename:VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_OPAQUE_EXT              | Y | Opaque
| 1 | 1 | ename:VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_OPAQUE_EXT              | N | Opaque
| 2 |   | ename:VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_TRANSPARENT_EXT | Y | Ignored
| 2 |   | ename:VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_TRANSPARENT_EXT | N | Non-opaque
| 3 |   | ename:VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_OPAQUE_EXT      | Y | Opaque
| 3 |   | ename:VK_OPACITY_MICROMAP_SPECIAL_INDEX_FULLY_UNKNOWN_OPAQUE_EXT      | N | Non-opaque
|====


endif::VK_EXT_opacity_micromap[]


[[ray-intersection-confirmation]]
== 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.

ifdef::VK_KHR_ray_tracing_pipeline[]
If the ray was traced with a <<glossary-pipeline-trace-ray, pipeline trace
ray>> instruction, an <<shaders-intersection, intersection shader>> is
invoked from the <<shader-binding-table>> according to the
<<shader-binding-table-indexing-rules, specified indexing>> for the
intersected geometry.
If this shader calls code:OpReportIntersectionKHR, a new intersection
candidate is generated as described
<<aabb-intersection-candidate-generation, below>>.
If the intersection shader is ename:VK_SHADER_UNUSED_KHR (which is only
allowed for a zero shader group) then no further processing of the
intersection candidate occurs.
endif::VK_KHR_ray_tracing_pipeline[]

[[aabb-intersection-candidate-generation]]
ifdef::VK_KHR_ray_tracing_pipeline[]
Each new candidate generated as a result of this processing is a generated
intersection candidate that intersects the AABB geometry, with a [eq]#t#
value equal to the `Hit` parameter of the code:OpReportIntersectionKHR
instruction.
The new generated candidate is then independently run through
<<ray-intersection-confirmation>> as a
<<ray-triangle-and-generated-intersection-candidates, generated
intersection>>.
endif::VK_KHR_ray_tracing_pipeline[]

ifdef::VK_KHR_ray_query[]
If the ray was traced with code:OpRayQueryProceedKHR, control is returned to
the shader which executed code:OpRayQueryProceedKHR, returning code:true.
The resulting ray query has a candidate intersection type of
code:RayQueryCandidateIntersectionAABBKHR.
code:OpRayQueryGenerateIntersectionKHR can: be called to commit a new
intersection candidate with committed intersection type of
code:RayQueryCommittedIntersectionGeneratedKHR.
Further ray query processing can: be continued by executing
code:OpRayQueryProceedKHR with the same ray query, or intersection can: be
terminated with code:OpRayQueryTerminateKHR.
endif::VK_KHR_ray_query[]
ifdef::VK_KHR_ray_tracing_pipeline+VK_KHR_ray_query[]
Unlike rays traced with a <<glossary-pipeline-trace-ray, pipeline trace
ray>> instruction, candidates generated in this way skip generated
intersection candidate confirmation; applications should: make this
determination before generating the intersection.
endif::VK_KHR_ray_tracing_pipeline+VK_KHR_ray_query[]

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


[[ray-triangle-and-generated-intersection-candidates]]
=== Triangle and Generated Intersection Candidates

For triangle and <<aabb-intersection-candidate-generation, 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.

ifdef::VK_KHR_ray_tracing_pipeline[]
If the ray was traced with a <<glossary-pipeline-trace-ray, pipeline trace
ray>> instruction, an <<shaders-any-hit, any-hit shader>> is invoked from
the <<shader-binding-table>> according to the specified indexing.
If this shader calls code:OpIgnoreIntersectionKHR, the candidate is dropped
and no further processing of the candidate occurs.
If the <<shaders-any-hit, any-hit shader>> identified is
ename:VK_SHADER_UNUSED_KHR, the candidate is immediately confirmed as a
valid hit and passes to the next stage of processing.
endif::VK_KHR_ray_tracing_pipeline[]

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

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


[[ray-closest-hit-determination]]
== Ray Closest Hit Determination

Unless the ray was traced with the code: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 [eq]#t# value is compared
to [eq]#t~max~# to determine which intersection is closer, where [eq]#t# is
the parametric distance along the ray at which the intersection occurred.

  * If [eq]#t < t~max~#, [eq]#t~max~# is set to [eq]#t# and the candidate is
    set as the current closest hit.
  * If [eq]#t > t~max~#, the candidate is dropped and no further processing
    of that candidate occurs.
  * If [eq]#t = t~max~#, the candidate may: be set as the current closest
    hit or dropped.

If code: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]]
== 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.

ifdef::VK_KHR_ray_tracing_pipeline[]
For rays traced with <<glossary-pipeline-trace-ray, pipeline trace ray>>
instructions which can: invoke a closest hit shader, if a closest hit result
was identified, a <<shaders-closest-hit, closest hit shader>> is invoked
from the <<shader-binding-table>> according to the
<<shader-binding-table-indexing-rules, specified indexing>> for the
intersected geometry.
Control returns to the shader that executed the
<<glossary-pipeline-trace-ray, pipeline trace ray>> instruction once this
shader returns.
This shader is skipped if either the ray flags included
code:SkipClosestHitShaderKHR, or if the <<shaders-closest-hit, closest hit
shader>> identified is ename:VK_SHADER_UNUSED_KHR.

For rays traced with a <<glossary-pipeline-trace-ray, pipeline trace ray>>
instruction where no hit result was identified, the <<shaders-miss, 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
ename:VK_SHADER_UNUSED_KHR.
endif::VK_KHR_ray_tracing_pipeline[]

ifdef::VK_KHR_ray_query[]
If the ray was traced with code:OpRayQueryProceedKHR, control is returned to
the shader which executed code:OpRayQueryProceedKHR, returning code:false.
If a closest hit was identified by <<ray-closest-hit-determination>>, the
ray query will now have a committed intersection type of
code:RayQueryCommittedIntersectionGeneratedKHR or
code:RayQueryCommittedIntersectionTriangleKHR.
If no closest hit was identified, the committed intersection type will be
code:RayQueryCommittedIntersectionNoneKHR.

No further processing of a ray query occurs after this result is determined.
endif::VK_KHR_ray_query[]