nv_cluster_builder

nv_cluster_builder is a small generic spatial clustering C++ library, created to cluster triangle meshes for ray tracing. It is less than 2k lines of code and very similar to a recursive node splitting algorithm to create a bounding volume hierarchy (BVH). It is limited to axis aligned splits but also produces clusters with desirable attributes for raytracing.

Input

• Spatial locality
  • ${\color{red}\text{Bounding\ boxes}}$
  • ${\color{blue}\text{Centroids}}$
• ${\color{green}\text{Connectivity}}$ (Optional)
  • Adjacency lists
  • Weights

Output

Cluster items (membership)

• Ranges: { { ${\color{blue}0,4}$ } , { ${\color{red}4,4}$ } }
• Items: { ${\color{blue}3}$, ${\color{blue}4}$, ${\color{blue}6}$, ${\color{blue}1}$, ${\color{red}2}$, ${\color{red}7}$, ${\color{red}0}$, ${\color{red}1}$ }

Notable features:

• Primarily spatial, making clusters from bounding boxes
• Optional user-defined weighted adjacency
• Generic, not just triangles
• Customizable [min–max] cluster sizes
• Parallel, using std::execution
• Segmented API for clustering multiple subsets at once
• Knobs to balance optimizations

For a complete usage example, see https://github.com/nvpro-samples/vk_animated_clusters.

Usage Example

For more details, refer to nvcluster.h (and optionally nvcluster_storage.hpp).

...

// Create bounding boxes for each item to be clustered
std::vector<nvcluster::AABB> boundingBoxes{
    nvcluster::AABB{{0, 0, 0}, {1, 1, 1}}, // for example
    ...
};

// Generate centroids
std::vector<glm::vec3> centroids(boundingBoxes.size());
for(size_t i = 0; i < boundingBoxes.size(); i++)
{
  centroids[i] = 0.5f * (glm::vec3(boundingBoxes[i].bboxMin) + glm::vec3(boundingBoxes[i].bboxMax));
}

// Input structs
nvcluster::SpatialElements spatialElements{.boundingBoxes = boundingBoxes.data(),
                                           .centroids     = reinterpret_cast<const float*>(centroids.data()),
                                           .elementCount  = static_cast<uint32_t>(boundingBoxes.size())};
nvcluster::Input           input{
    .config =
        {
            .minClusterSize    = 128,
            .maxClusterSize    = 128,
            .costUnderfill     = 0.0f,  // zero to one (exclusive)
            .costOverlap       = 0.0f,  // zero to one (exclusive)
            .preSplitThreshold = 0,     // median-split bigger nodes (0=disable)
        },
    .spatialElements = &spatialElements,
    .graph           = nullptr, // optional adjacency - see test_clusterizer.cpp for examples of this in use
};

// Create context
nvcluster::ContextCreateInfo info{};
nvcluster::Context context;
nvclusterCreateContext(&info, &context);

// Create clusters
// This is a thin wrapper with std::vector output storage for nvcluster::nvclusterCreate(...)
nvcluster::ClusterStorage clustering;
nvcluster::generateClusters(context, input, clustering);

// Do something with the result
for(size_t clusterIndex = 0; clusterIndex < clustering.clusterRanges.size(); ++clusterIndex)
{
  const nvcluster::Range& range = clustering.clusterRanges[clusterIndex];
  for(uint32_t clusterItemIndex = 0; clusterItemIndex < range.count; ++clusterItemIndex)
  {
    uint32_t clusterItem = clustering.clusterItems[range.offset + clusterItemIndex];
    ...
  }
}

Build Integration

This library uses CMake and requires C++20. It is currently a static library, designed with C compatibility in mind with data passed as a structure of arrays and output allocated by the user. Integration has been verified by directly including it with add_subdirectory:

add_subdirectory(nv_cluster_builder)
...
target_link_libraries(my_target PUBLIC nv_cluster_builder)

If there is interest, please reach out for CMake config files (for find_package()) or any other features. GitHub issues are welcome.

Dependencies

None.

If tests are enabled (set the CMake BUILD_TESTING variable to ON), nv_cluster_builder will use FetchContent to download GoogleTest.

How it works

Authors and contact:

• Pyarelal Knowles (pknowles 'at' nvidia.com), NVIDIA
• Karthik Vaidyanathan, NVIDIA

Cluster goals:

• Consistent size for batch processing
• Spatially adjacent
• Well connected
• Small bounding box (low SAH cost)
• Low overlap
• Useful for ray tracing

The algorithm is basic recursive bisection:

1. Sorts inputs by centroids on each axis
2. Initialize with a root node containing everything
3. Recursively split until the desired leaf size is reached
   • Compute candidate split costs for all positions in all axes
   • Split at the lowest cost, maintaining sorted centroids by partitioning
4. Leaves become clusters

Novel additions:

• Limit split candidates to guarantee fixed cluster sizes
• Optimize for full clusters
• Optimize for less bounding box overlap
• Optimize for minimum ratio cut cost if adjacency exists

The optimizations are implemented by converting and summing additional costs with the surface area heuristic (SAH) cost and choosing a split position on any axis with minimum cost.

Fixed Size Clusters

Only split at $i \bmod C = 0$, with $i$ items to the left of a candidate split, to make clusters of size $C$. There will be at most one undersized cluster. This rule alone will largely break SAH, as shown for the clustering along just one axis. In reality, split candidates would be chosen for any axis

Relax the fixed $C$ constraint to allow a range, $[C_A, C_B]$ where $(1 \le C_A \le C_B)$. Only split if the target range cluster sizes could be formed on both sides. For example, the figure below shows forming clusters of size 127 or 128 items. Choosing splits in grey regions will produce clusters in the left node (top) and right node (bottom) of the desired size range. Limit split candidates to the intersection of the grey regions. The equivalent conditions are described the equations below, where $n$ is the number of items in the node being split.

$$𝑖 \bmod 𝐶_𝐴 \le (𝐶_𝐵 − 𝐶_𝐴) \lfloor \frac{𝑖}{𝐶_𝐴} \rfloor$$ $$(n - 𝑖) \bmod 𝐶_𝐴 \le (𝐶_𝐵 − 𝐶_𝐴) \lfloor \frac{n - 𝑖}{𝐶_𝐴} \rfloor$$

For small inputs it is possible that there is no overlap in valid ranges, in which case the algorithm falls back to choosing just one. Similarly to the fixed $C_A = C_B$ case, there will be at most one undersized cluster.

Maximize Cluster Sizes

A cluster "underfill" cost is introdued to encourage bigger clusters. For example, in the figure below a split position is being considered for clusters in the range [1, 4]. The split candidate would produce a node of 2.75 clusters on the left and 1.25 on the right. This results in $p$ missing cluster items. This value is converted to SAH units and summed. This library currently uses a linear cost with a tunable costUnderfill constant, but a transfer function to model the true cost, of e.g. perf or memory, would be ideal.

$$p_{\text{left}} = C_B \lceil \frac{i}{C_B} \rceil - i$$ $$p_{\text{right}} = C_B \lceil \frac{n - i}{C_B} \rceil - (n - i)$$ $$p = C_B ( \lceil \frac{i}{C_B} \rceil + \lceil \frac{n - i}{C_B} \rceil ) - n$$

Minimize Bounding Box Overlap

Bounding box overlap is bad for ray tracing because rays must enter both while in the overlap volume. A cost is added for overlapping bounding boxes, ver much like SAH it is just $n$ multiplied by the surface area of the bounding box intersection's box and balanced with a tunable costOverlap constant.

Minimize Adjacency Cut Cost

If provided, adjacency is integrated by adding the cut cost - the sum of weights of all item connections broken by the split - to each candidate split position. Ratio cut [Wei and Cheng, 1989] is used to avoid degenerate solutions. The cut cost is arbitrarily scaled by the number of items in the node to be SAH relative and added to the other costs above.

To compute the cut cost, the adjacency data is rebuilt to reference node items before each iteration of recursive node splitting. This allows cut costs to be computed with a prefix sum scan of summed starting and ending connection weights.

To explain, there are initially three arrays, sorted by item centroids in X, Y and Z respectively. After splitting, these arrays are partitioned, maintaining sorted order within nodes. These hold original item indices and in fact trivially hold the clustering result after splitting. The input adjacency arrays index original items, but we instead need the index in those initially sorted arrays. This is done by duplicating the adjacency arrays and scatter writing their node-sorted indices. The image below shows this for one axis.

When computing cut costs for a node, an array of summed weights is created. The image below shows an example with unit weights. The array is initialized with the sum of connecting item weights - positive for connections to the right and negative for connections to the left. Connections to other nodes are ignored. The reindexed adjacency arrays trivially give this information, comparing the connection index with the current item's index and the node boundaries. The weights array is then prefix summed to obtain the cut cost for each position in the node.

Citation

The BibTex entry to cite nv_cluster_builder is

@online{nv_cluster_builder,
   title   = {{{NVIDIA}}\textregistered{} {nv_cluster_builder}},
   author  = {{NVIDIA}},
   year    = 2025,
   url     = {https://github.com/nvpro-samples/nv_cluster_builder},
   urldate = {2025-01-30},
}

Limitations

Clusters are created by making recursive axis aligned splits. This is useful as it greatly reduces the search space and improves performance when clusters are used in ray tracing. However, more general clustering solutions than axis aligned splits are not considered.

Recursively splitting is done greedily, picking the lowest cost split which may not be a global optimum.

The algorithm is primarily spatial due to splitting in order of centroids, but solutions can be skewed by adjusting the costs in nvcluster::Config and adjacency weights in nvcluster::Graph::connectionWeights. For example, choosing adjacency weights to represent connected triangles or number of shared vertices can result in more vertex reuse within clusters. Weights may also represent face normal similarity or a balance of multiple attributes.

Badly chosen weights can result in degenerate solutions where recursive bisection splits off single leaves. This is both slow and rarely desirable.