ImplicitBVH.jl

High-Performance Parallel Bounding Volume Hierarchy for Collision Detection

It uses an implicit bounding volume hierarchy constructed from an iterable of some geometric primitives' (e.g. triangles in a mesh) bounding volumes forming the ImplicitTree leaves. The leaves and merged nodes above them can have different types - e.g. BSphere{Float64} leaves merged into larger BBox{Float64}.

The initial geometric primitives are sorted according to their Morton-encoded coordinates; the unsigned integer type used for the Morton encoding can be chosen between UInt16, UInt32 and UInt64.

Finally, the tree can be incompletely-built up to a given built_level and later start contact detection downwards from this level.

Examples

Simple usage with bounding spheres and default 64-bit types:

using ImplicitBVH
using ImplicitBVH: BBox, BSphere

# Generate some simple bounding spheres
bounding_spheres = [
    BSphere([0., 0., 0.], 0.5),
    BSphere([0., 0., 1.], 0.6),
    BSphere([0., 0., 2.], 0.5),
    BSphere([0., 0., 3.], 0.4),
    BSphere([0., 0., 4.], 0.6),
]

# Build BVH
bvh = BVH(bounding_spheres)

# Traverse BVH for contact detection
traversal = traverse(bvh)
@show traversal.contacts

# output
traversal.contacts = [(1, 2), (2, 3), (4, 5)]

Using Float32 bounding spheres for leaves, Float32 bounding boxes for nodes above, and UInt32 Morton codes:

using ImplicitBVH
using ImplicitBVH: BBox, BSphere

# Generate some simple bounding spheres
bounding_spheres = [
    BSphere{Float32}([0., 0., 0.], 0.5),
    BSphere{Float32}([0., 0., 1.], 0.6),
    BSphere{Float32}([0., 0., 2.], 0.5),
    BSphere{Float32}([0., 0., 3.], 0.4),
    BSphere{Float32}([0., 0., 4.], 0.6),
]

# Build BVH
bvh = BVH(bounding_spheres, BBox{Float32}, UInt32)

# Traverse BVH for contact detection
traversal = traverse(bvh)
@show traversal.contacts

# output
traversal.contacts = [(1, 2), (2, 3), (4, 5)]

Build BVH up to level 2 and start traversing down from level 3, reusing the previous traversal cache:

bvh = BVH(bounding_spheres, BBox{Float32}, UInt32, 2)
traversal = traverse(bvh, 3, traversal)

Compute contacts between two different BVH trees (e.g. two different robotic parts):

using ImplicitBVH
using ImplicitBVH: BBox, BSphere

# Generate some simple bounding spheres (will be BVH leaves)
bounding_spheres1 = [
    BSphere{Float32}([0., 0., 0.], 0.5),
    BSphere{Float32}([0., 0., 3.], 0.4),
]

bounding_spheres2 = [
    BSphere{Float32}([0., 0., 1.], 0.6),
    BSphere{Float32}([0., 0., 2.], 0.5),
    BSphere{Float32}([0., 0., 4.], 0.6),
]

# Build BVHs using bounding boxes for nodes
bvh1 = BVH(bounding_spheres1, BBox{Float32}, UInt32)
bvh2 = BVH(bounding_spheres2, BBox{Float32}, UInt32)

# Traverse BVH for contact detection
traversal = traverse(
    bvh1,
    bvh2,
    default_start_level(bvh1),
    default_start_level(bvh2),
    # previous_traversal_cache,
    # num_threads=4,
)

Check out the benchmark folder for an example traversing an STL model.

Implicit Bounding Volume Hierarchy

The main idea behind the ImplicitBVH is the use of an implicit perfect binary tree constructed from some bounding volumes. If we had, say, 5 objects to construct the BVH from, it would form an incomplete binary tree as below:

Implicit tree from 5 bounding volumes - i.e. the real leaves:

Tree Level          Nodes & Leaves               Build Up    Traverse Down
    1                     1                         Ʌ              |
    2             2               3                 |              |
    3         4       5       6        7v           |              |
    4       8   9   10 11   12 13v  14v  15v        |              V
            -------Real------- ---Virtual---

We do not need to store the "virtual" nodes in memory; rather, we can compute the number of virtual nodes we need to skip to get to a given node index, following the fantastic ideas from [1].

Performance

As contact detection is one of the most computationally-intensive parts of physical simulation and computer vision applications, this implementation has been optimised for maximum performance and scalability:

• Computing bounding volumes is optimised for triangles, e.g. constructing 249,882 BSphere{Float64} on a single thread takes 4.47 ms on my Mac M1. The construction itself has zero allocations; all computation can be done in parallel in user code.
• Building a complete bounding volume hierarchy from the 249,882 triangles of xyzrgb_dragon.obj takes 11.83 ms single-threaded. The sorting step is the bottleneck, so multi-threading the Morton encoding and BVH up-building does not significantly improve the runtime; waiting on a multi-threaded sorter.
• Traversing the same 249,882 BSphere{Float64} for the triangles (aggregated into BBox{Float64} parents) takes 136.38 ms single-threaded and 43.16 ms with 4 threads, at 79% strong scaling.

Only fundamental Julia types are used - e.g. struct, Tuple, UInt, Float64 - which can be straightforwardly inlined, unrolled and fused by the compiler. These types are also straightforward to transpile to accelerators via KernelAbstractions.jl such as CUDA, AMDGPU, oneAPI, Apple Metal.

Roadmap

• Use KernelAbstractions.jl kernels to build and traverse the BVH; I think we just need a performant KA sort! function, the rest is straightforward.

References

The implicit tree formulation (genius idea!) which forms the core of the BVH structure originally appeared in the following paper:

[1] Chitalu FM, Dubach C, Komura T. Binary Ostensibly‐Implicit Trees for Fast Collision Detection. InComputer Graphics Forum 2020 May (Vol. 39, No. 2, pp. 509-521).

License

ImplicitBVH.jl is MIT-licensed. Enjoy.