heterogenous KNN

Summary: Interface and working implementation of ragged KNN. Benchmarks (which aren't ragged) haven't slowed. New benchmark shows that ragged is faster than non-ragged of the same shape.

Reviewed By: jcjohnson

Differential Revision: D20696507

fbshipit-source-id: 21b80f71343a3475c8d3ee0ce2680f92f0fae4de

pytorch3d/csrc/knn/knn.cu

@@ -8,10 +8,20 @@
 #include "dispatch.cuh"
 #include "mink.cuh"
 
+// A chunk of work is blocksize-many points of P1.
+// The number of potential chunks to do is N*(1+(P1-1)/blocksize)
+// call (1+(P1-1)/blocksize) chunks_per_cloud
+// These chunks are divided among the gridSize-many blocks.
+// In block b, we work on chunks b, b+gridSize, b+2*gridSize etc .
+// In chunk i, we work on cloud i/chunks_per_cloud on points starting from
+// blocksize*(i%chunks_per_cloud).
+
 template <typename scalar_t>
 __global__ void KNearestNeighborKernelV0(
     const scalar_t* __restrict__ points1,
     const scalar_t* __restrict__ points2,
+    const int64_t* __restrict__ lengths1,
+    const int64_t* __restrict__ lengths2,
     scalar_t* __restrict__ dists,
     int64_t* __restrict__ idxs,
     const size_t N,
@@ -19,18 +29,19 @@ __global__ void KNearestNeighborKernelV0(
     const size_t P2,
     const size_t D,
     const size_t K) {
-  // Stupid version: Make each thread handle one query point and loop over
-  // all P2 target points. There are N * P1 input points to handle, so
-  // do a trivial parallelization over threads.
   // Store both dists and indices for knn in global memory.
-  const int tid = threadIdx.x + blockIdx.x * blockDim.x;
-  const int num_threads = blockDim.x * gridDim.x;
-  for (int np = tid; np < N * P1; np += num_threads) {
-    int n = np / P1;
-    int p1 = np % P1;
+  const int64_t chunks_per_cloud = (1 + (P1 - 1) / blockDim.x);
+  const int64_t chunks_to_do = N * chunks_per_cloud;
+  for (int64_t chunk = blockIdx.x; chunk < chunks_to_do; chunk += gridDim.x) {
+    const int64_t n = chunk / chunks_per_cloud;
+    const int64_t start_point = blockDim.x * (chunk % chunks_per_cloud);
+    int64_t p1 = start_point + threadIdx.x;
+    if (p1 >= lengths1[n])
+      continue;
     int offset = n * P1 * K + p1 * K;
+    int64_t length2 = lengths2[n];
     MinK<scalar_t, int64_t> mink(dists + offset, idxs + offset, K);
-    for (int p2 = 0; p2 < P2; ++p2) {
+    for (int p2 = 0; p2 < length2; ++p2) {
       // Find the distance between points1[n, p1] and points[n, p2]
       scalar_t dist = 0;
       for (int d = 0; d < D; ++d) {
@@ -48,6 +59,8 @@ template <typename scalar_t, int64_t D>
 __global__ void KNearestNeighborKernelV1(
     const scalar_t* __restrict__ points1,
     const scalar_t* __restrict__ points2,
+    const int64_t* __restrict__ lengths1,
+    const int64_t* __restrict__ lengths2,
     scalar_t* __restrict__ dists,
     int64_t* __restrict__ idxs,
     const size_t N,
@@ -58,18 +71,22 @@ __global__ void KNearestNeighborKernelV1(
   // so we can cache the current point in a thread-local array. We still store
   // the current best K dists and indices in global memory, so this should work
   // for very large K and fairly large D.
-  const int tid = threadIdx.x + blockIdx.x * blockDim.x;
-  const int num_threads = blockDim.x * gridDim.x;
   scalar_t cur_point[D];
-  for (int np = tid; np < N * P1; np += num_threads) {
-    int n = np / P1;
-    int p1 = np % P1;
+  const int64_t chunks_per_cloud = (1 + (P1 - 1) / blockDim.x);
+  const int64_t chunks_to_do = N * chunks_per_cloud;
+  for (int64_t chunk = blockIdx.x; chunk < chunks_to_do; chunk += gridDim.x) {
+    const int64_t n = chunk / chunks_per_cloud;
+    const int64_t start_point = blockDim.x * (chunk % chunks_per_cloud);
+    int64_t p1 = start_point + threadIdx.x;
+    if (p1 >= lengths1[n])
+      continue;
     for (int d = 0; d < D; ++d) {
       cur_point[d] = points1[n * P1 * D + p1 * D + d];
     }
     int offset = n * P1 * K + p1 * K;
+    int64_t length2 = lengths2[n];
     MinK<scalar_t, int64_t> mink(dists + offset, idxs + offset, K);
-    for (int p2 = 0; p2 < P2; ++p2) {
+    for (int p2 = 0; p2 < length2; ++p2) {
       // Find the distance between cur_point and points[n, p2]
       scalar_t dist = 0;
       for (int d = 0; d < D; ++d) {
@@ -89,14 +106,16 @@ struct KNearestNeighborV1Functor {
       size_t threads,
       const scalar_t* __restrict__ points1,
       const scalar_t* __restrict__ points2,
+      const int64_t* __restrict__ lengths1,
+      const int64_t* __restrict__ lengths2,
       scalar_t* __restrict__ dists,
       int64_t* __restrict__ idxs,
       const size_t N,
       const size_t P1,
       const size_t P2,
       const size_t K) {
-    KNearestNeighborKernelV1<scalar_t, D>
-        <<<blocks, threads>>>(points1, points2, dists, idxs, N, P1, P2, K);
+    KNearestNeighborKernelV1<scalar_t, D><<<blocks, threads>>>(
+        points1, points2, lengths1, lengths2, dists, idxs, N, P1, P2, K);
   }
 };
 
@@ -104,25 +123,31 @@ template <typename scalar_t, int64_t D, int64_t K>
 __global__ void KNearestNeighborKernelV2(
     const scalar_t* __restrict__ points1,
     const scalar_t* __restrict__ points2,
+    const int64_t* __restrict__ lengths1,
+    const int64_t* __restrict__ lengths2,
     scalar_t* __restrict__ dists,
     int64_t* __restrict__ idxs,
     const int64_t N,
     const int64_t P1,
     const int64_t P2) {
   // Same general implementation as V2, but also hoist K into a template arg.
-  const int tid = threadIdx.x + blockIdx.x * blockDim.x;
-  const int num_threads = blockDim.x * gridDim.x;
   scalar_t cur_point[D];
   scalar_t min_dists[K];
   int min_idxs[K];
-  for (int np = tid; np < N * P1; np += num_threads) {
-    int n = np / P1;
-    int p1 = np % P1;
+  const int64_t chunks_per_cloud = (1 + (P1 - 1) / blockDim.x);
+  const int64_t chunks_to_do = N * chunks_per_cloud;
+  for (int64_t chunk = blockIdx.x; chunk < chunks_to_do; chunk += gridDim.x) {
+    const int64_t n = chunk / chunks_per_cloud;
+    const int64_t start_point = blockDim.x * (chunk % chunks_per_cloud);
+    int64_t p1 = start_point + threadIdx.x;
+    if (p1 >= lengths1[n])
+      continue;
     for (int d = 0; d < D; ++d) {
       cur_point[d] = points1[n * P1 * D + p1 * D + d];
     }
+    int64_t length2 = lengths2[n];
     MinK<scalar_t, int> mink(min_dists, min_idxs, K);
-    for (int p2 = 0; p2 < P2; ++p2) {
+    for (int p2 = 0; p2 < length2; ++p2) {
       scalar_t dist = 0;
       for (int d = 0; d < D; ++d) {
         int offset = n * P2 * D + p2 * D + d;
@@ -146,13 +171,15 @@ struct KNearestNeighborKernelV2Functor {
       size_t threads,
       const scalar_t* __restrict__ points1,
       const scalar_t* __restrict__ points2,
+      const int64_t* __restrict__ lengths1,
+      const int64_t* __restrict__ lengths2,
       scalar_t* __restrict__ dists,
       int64_t* __restrict__ idxs,
       const int64_t N,
       const int64_t P1,
       const int64_t P2) {
-    KNearestNeighborKernelV2<scalar_t, D, K>
-        <<<blocks, threads>>>(points1, points2, dists, idxs, N, P1, P2);
+    KNearestNeighborKernelV2<scalar_t, D, K><<<blocks, threads>>>(
+        points1, points2, lengths1, lengths2, dists, idxs, N, P1, P2);
   }
 };
 
@@ -160,6 +187,8 @@ template <typename scalar_t, int D, int K>
 __global__ void KNearestNeighborKernelV3(
     const scalar_t* __restrict__ points1,
     const scalar_t* __restrict__ points2,
+    const int64_t* __restrict__ lengths1,
+    const int64_t* __restrict__ lengths2,
     scalar_t* __restrict__ dists,
     int64_t* __restrict__ idxs,
     const size_t N,
@@ -169,19 +198,23 @@ __global__ void KNearestNeighborKernelV3(
   // Enabling sorting for this version leads to huge slowdowns; I suspect
   // that it forces min_dists into local memory rather than registers.
   // As a result this version is always unsorted.
-  const int tid = threadIdx.x + blockIdx.x * blockDim.x;
-  const int num_threads = blockDim.x * gridDim.x;
   scalar_t cur_point[D];
   scalar_t min_dists[K];
   int min_idxs[K];
-  for (int np = tid; np < N * P1; np += num_threads) {
-    int n = np / P1;
-    int p1 = np % P1;
+  const int64_t chunks_per_cloud = (1 + (P1 - 1) / blockDim.x);
+  const int64_t chunks_to_do = N * chunks_per_cloud;
+  for (int64_t chunk = blockIdx.x; chunk < chunks_to_do; chunk += gridDim.x) {
+    const int64_t n = chunk / chunks_per_cloud;
+    const int64_t start_point = blockDim.x * (chunk % chunks_per_cloud);
+    int64_t p1 = start_point + threadIdx.x;
+    if (p1 >= lengths1[n])
+      continue;
     for (int d = 0; d < D; ++d) {
       cur_point[d] = points1[n * P1 * D + p1 * D + d];
     }
+    int64_t length2 = lengths2[n];
     RegisterMinK<scalar_t, int, K> mink(min_dists, min_idxs);
-    for (int p2 = 0; p2 < P2; ++p2) {
+    for (int p2 = 0; p2 < length2; ++p2) {
       scalar_t dist = 0;
       for (int d = 0; d < D; ++d) {
         int offset = n * P2 * D + p2 * D + d;
@@ -205,13 +238,15 @@ struct KNearestNeighborKernelV3Functor {
       size_t threads,
       const scalar_t* __restrict__ points1,
       const scalar_t* __restrict__ points2,
+      const int64_t* __restrict__ lengths1,
+      const int64_t* __restrict__ lengths2,
       scalar_t* __restrict__ dists,
       int64_t* __restrict__ idxs,
       const size_t N,
       const size_t P1,
       const size_t P2) {
-    KNearestNeighborKernelV3<scalar_t, D, K>
-        <<<blocks, threads>>>(points1, points2, dists, idxs, N, P1, P2);
+    KNearestNeighborKernelV3<scalar_t, D, K><<<blocks, threads>>>(
+        points1, points2, lengths1, lengths2, dists, idxs, N, P1, P2);
   }
 };
 
@@ -257,6 +292,8 @@ int ChooseVersion(const int64_t D, const int64_t K) {
 std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdxCuda(
     const at::Tensor& p1,
     const at::Tensor& p2,
+    const at::Tensor& lengths1,
+    const at::Tensor& lengths2,
     int K,
     int version) {
   const auto N = p1.size(0);
@@ -267,8 +304,8 @@ std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdxCuda(
 
   AT_ASSERTM(p2.size(2) == D, "Point sets must have the same last dimension");
   auto long_dtype = p1.options().dtype(at::kLong);
-  auto idxs = at::full({N, P1, K}, -1, long_dtype);
-  auto dists = at::full({N, P1, K}, -1, p1.options());
+  auto idxs = at::zeros({N, P1, K}, long_dtype);
+  auto dists = at::zeros({N, P1, K}, p1.options());
 
   if (version < 0) {
     version = ChooseVersion(D, K);
@@ -294,6 +331,8 @@ std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdxCuda(
                                      <<<blocks, threads>>>(
                                          p1.data_ptr<scalar_t>(),
                                          p2.data_ptr<scalar_t>(),
+                                         lengths1.data_ptr<int64_t>(),
+                                         lengths2.data_ptr<int64_t>(),
                                          dists.data_ptr<scalar_t>(),
                                          idxs.data_ptr<int64_t>(),
                                          N,
@@ -314,6 +353,8 @@ std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdxCuda(
                                      threads,
                                      p1.data_ptr<scalar_t>(),
                                      p2.data_ptr<scalar_t>(),
+                                     lengths1.data_ptr<int64_t>(),
+                                     lengths2.data_ptr<int64_t>(),
                                      dists.data_ptr<scalar_t>(),
                                      idxs.data_ptr<int64_t>(),
                                      N,
@@ -336,6 +377,8 @@ std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdxCuda(
                                      threads,
                                      p1.data_ptr<scalar_t>(),
                                      p2.data_ptr<scalar_t>(),
+                                     lengths1.data_ptr<int64_t>(),
+                                     lengths2.data_ptr<int64_t>(),
                                      dists.data_ptr<scalar_t>(),
                                      idxs.data_ptr<int64_t>(),
                                      N,
@@ -357,6 +400,8 @@ std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdxCuda(
                                      threads,
                                      p1.data_ptr<scalar_t>(),
                                      p2.data_ptr<scalar_t>(),
+                                     lengths1.data_ptr<int64_t>(),
+                                     lengths2.data_ptr<int64_t>(),
                                      dists.data_ptr<scalar_t>(),
                                      idxs.data_ptr<int64_t>(),
                                      N,

pytorch3d/csrc/knn/knn.h

@@ -13,25 +13,38 @@
 //        containing P1 points of dimension D.
 //    p2: FloatTensor of shape (N, P2, D) giving a batch of pointclouds each
 //        containing P2 points of dimension D.
+//    lengths1: LongTensor, shape (N,), giving actual length of each P1 cloud.
+//    lengths2: LongTensor, shape (N,), giving actual length of each P2 cloud.
 //    K: int giving the number of nearest points to return.
 //    sorted: bool telling whether to sort the K returned points by their
 //        distance.
 //    version: Integer telling which implementation to use.
-//        TODO(jcjohns): Document this more, or maybe remove it before landing.
 //
 // Returns:
 //    p1_neighbor_idx: LongTensor of shape (N, P1, K), where
-//                     p1_neighbor_idx[n, i, k] = j means that the kth nearest
-//                     neighbor to p1[n, i] in the cloud p2[n] is p2[n, j].
+//        p1_neighbor_idx[n, i, k] = j means that the kth nearest
+//        neighbor to p1[n, i] in the cloud p2[n] is p2[n, j].
+//        It is padded with zeros so that it can be used easily in a later
+//        gather() operation.
+//
+//    p1_neighbor_dists: FloatTensor of shape (N, P1, K) containing the squared
+//        distance from each point p1[n, p, :] to its K neighbors
+//        p2[n, p1_neighbor_idx[n, p, k], :].
 
 // CPU implementation.
-std::tuple<at::Tensor, at::Tensor>
-KNearestNeighborIdxCpu(const at::Tensor& p1, const at::Tensor& p2, int K);
+std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdxCpu(
+    const at::Tensor& p1,
+    const at::Tensor& p2,
+    const at::Tensor& lengths1,
+    const at::Tensor& lengths2,
+    int K);
 
 // CUDA implementation
 std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdxCuda(
     const at::Tensor& p1,
     const at::Tensor& p2,
+    const at::Tensor& lengths1,
+    const at::Tensor& lengths2,
     int K,
     int version);
 
@@ -39,16 +52,18 @@ std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdxCuda(
 std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdx(
     const at::Tensor& p1,
     const at::Tensor& p2,
+    const at::Tensor& lengths1,
+    const at::Tensor& lengths2,
     int K,
     int version) {
   if (p1.is_cuda() || p2.is_cuda()) {
 #ifdef WITH_CUDA
     CHECK_CONTIGUOUS_CUDA(p1);
     CHECK_CONTIGUOUS_CUDA(p2);
-    return KNearestNeighborIdxCuda(p1, p2, K, version);
+    return KNearestNeighborIdxCuda(p1, p2, lengths1, lengths2, K, version);
 #else
     AT_ERROR("Not compiled with GPU support.");
 #endif
   }
-  return KNearestNeighborIdxCpu(p1, p2, K);
+  return KNearestNeighborIdxCpu(p1, p2, lengths1, lengths2, K);
 }

pytorch3d/csrc/knn/knn_cpu.cpp

@@ -4,27 +4,35 @@
 #include <queue>
 #include <tuple>
 
-std::tuple<at::Tensor, at::Tensor>
-KNearestNeighborIdxCpu(const at::Tensor& p1, const at::Tensor& p2, int K) {
+std::tuple<at::Tensor, at::Tensor> KNearestNeighborIdxCpu(
+    const at::Tensor& p1,
+    const at::Tensor& p2,
+    const at::Tensor& lengths1,
+    const at::Tensor& lengths2,
+    int K) {
   const int N = p1.size(0);
   const int P1 = p1.size(1);
   const int D = p1.size(2);
   const int P2 = p2.size(1);
 
   auto long_opts = p1.options().dtype(torch::kInt64);
-  torch::Tensor idxs = torch::full({N, P1, K}, -1, long_opts);
+  torch::Tensor idxs = torch::full({N, P1, K}, 0, long_opts);
   torch::Tensor dists = torch::full({N, P1, K}, 0, p1.options());
 
   auto p1_a = p1.accessor<float, 3>();
   auto p2_a = p2.accessor<float, 3>();
+  auto lengths1_a = lengths1.accessor<int64_t, 1>();
+  auto lengths2_a = lengths2.accessor<int64_t, 1>();
   auto idxs_a = idxs.accessor<int64_t, 3>();
   auto dists_a = dists.accessor<float, 3>();
 
   for (int n = 0; n < N; ++n) {
-    for (int i1 = 0; i1 < P1; ++i1) {
+    const int64_t length1 = lengths1_a[n];
+    const int64_t length2 = lengths2_a[n];
+    for (int64_t i1 = 0; i1 < length1; ++i1) {
       // Use a priority queue to store (distance, index) tuples.
       std::priority_queue<std::tuple<float, int>> q;
-      for (int i2 = 0; i2 < P2; ++i2) {
+      for (int64_t i2 = 0; i2 < length2; ++i2) {
         float dist = 0;
         for (int d = 0; d < D; ++d) {
           float diff = p1_a[n][i1][d] - p2_a[n][i2][d];

pytorch3d/ops/knn.py

@@ -4,37 +4,80 @@ import torch
 from pytorch3d import _C
 
 
-def knn_points_idx(p1, p2, K, sorted=False, version=-1):
+def knn_points_idx(
+    p1,
+    p2,
+    K: int,
+    lengths1=None,
+    lengths2=None,
+    sorted: bool = False,
+    version: int = -1,
+):
     """
     K-Nearest neighbors on point clouds.
 
     Args:
         p1: Tensor of shape (N, P1, D) giving a batch of point clouds, each
-            containing P1 points of dimension D.
+            containing up to P1 points of dimension D.
         p2: Tensor of shape (N, P2, D) giving a batch of point clouds, each
-            containing P2 points of dimension D.
-        K: Integer giving the number of nearest neighbors to return
+            containing up to P2 points of dimension D.
+        K: Integer giving the number of nearest neighbors to return.
+        lengths1: LongTensor of shape (N,) of values in the range [0, P1], giving the
+            length of each pointcloud in p1. Or None to indicate that every cloud has
+            length P1.
+        lengths2: LongTensor of shape (N,) of values in the range [0, P2], giving the
+            length of each pointcloud in p2. Or None to indicate that every cloud has
+            length P2.
         sorted: Whether to sort the resulting points.
         version: Which KNN implementation to use in the backend. If version=-1,
-                 the correct implementation is selected based on the shapes of
-                 the inputs.
+            the correct implementation is selected based on the shapes of the inputs.
 
     Returns:
-        idx: LongTensor of shape (N, P1, K) giving the indices of the
-             K nearest neighbors from points in p1 to points in p2.
-             Concretely, if idx[n, i, k] = j then p2[n, j] is one of the K
-             nearest neighbor to p1[n, i] in p2[n]. If sorted=True, then
-             p2[n, j] is the kth nearest neighbor to p1[n, i].
+        p1_neighbor_idx: LongTensor of shape (N, P1, K) giving the indices of the
+            K nearest neighbors from points in p1 to points in p2.
+            Concretely, if idx[n, i, k] = j then p2[n, j] is one of the K nearest
+            neighbors to p1[n, i] in p2[n]. If sorted=True, then p2[n, j] is the kth
+            nearest neighbor to p1[n, i]. This is padded with zeros both where a cloud
+            in p2 has fewer than K points and where a cloud in p1 has fewer than P1
+            points.
+            If you want an (N, P1, K, D) tensor of the actual points, you can get it
+            using
+                p2[:, :, None].expand(-1, -1, K, -1).gather(1,
+                    x_idx[:, :, :, None].expand(-1, -1, -1, D)
+                )
+            If K=1 and you want an (N, P1, D) tensor of the actual points, use
+                p2.gather(1, x_idx.expand(-1, -1, D))
+
+        p1_neighbor_dists: Tensor of shape (N, P1, K) giving the squared distances to
+            the nearest neighbors. This is padded with zeros both where a cloud in p2
+            has fewer than K points and where a cloud in p1 has fewer than P1 points.
+            Warning: this is calculated outside of the autograd framework.
     """
-    idx, dists = _C.knn_points_idx(p1, p2, K, version)
+    P1 = p1.shape[1]
+    P2 = p2.shape[1]
+    if lengths1 is None:
+        lengths1 = torch.full((p1.shape[0],), P1, dtype=torch.int64, device=p1.device)
+    if lengths2 is None:
+        lengths2 = torch.full((p1.shape[0],), P2, dtype=torch.int64, device=p1.device)
+    idx, dists = _C.knn_points_idx(p1, p2, lengths1, lengths2, K, version)
     if sorted:
-        dists, sort_idx = dists.sort(dim=2)
+        if lengths2.min() < K:
+            device = dists.device
+            mask1 = lengths2[:, None] <= torch.arange(K, device=device)[None]
+            # mask1 has shape [N, K], true where dists irrelevant
+            mask2 = mask1[:, None].expand(-1, P1, -1)
+            # mask2 has shape [N, P1, K], true where dists irrelevant
+            dists[mask2] = float("inf")
+            dists, sort_idx = dists.sort(dim=2)
+            dists[mask2] = 0
+        else:
+            dists, sort_idx = dists.sort(dim=2)
         idx = idx.gather(2, sort_idx)
     return idx, dists
 
 
 @torch.no_grad()
-def _knn_points_idx_naive(p1, p2, K, sorted=False) -> torch.Tensor:
+def _knn_points_idx_naive(p1, p2, K: int, lengths1, lengths2) -> torch.Tensor:
     """
     Naive PyTorch implementation of K-Nearest Neighbors.
 
@@ -43,25 +86,67 @@ def _knn_points_idx_naive(p1, p2, K, sorted=False) -> torch.Tensor:
 
     Args:
         p1: Tensor of shape (N, P1, D) giving a batch of point clouds, each
-            containing P1 points of dimension D.
+            containing up to P1 points of dimension D.
         p2: Tensor of shape (N, P2, D) giving a batch of point clouds, each
-            containing P2 points of dimension D.
-        K: Integer giving the number of nearest neighbors to return
-        sorted: Whether to sort the resulting points.
+            containing up to P2 points of dimension D.
+        K: Integer giving the number of nearest neighbors to return.
+        lengths1: LongTensor of shape (N,) of values in the range [0, P1], giving the
+            length of each pointcloud in p1. Or None to indicate that every cloud has
+            length P1.
+        lengths2: LongTensor of shape (N,) of values in the range [0, P2], giving the
+            length of each pointcloud in p2. Or None to indicate that every cloud has
+            length P2.
 
     Returns:
         idx: LongTensor of shape (N, P1, K) giving the indices of the
-             K nearest neighbors from points in p1 to points in p2.
-             Concretely, if idx[n, i, k] = j then p2[n, j] is one of the K
-             nearest neighbor to p1[n, i] in p2[n]. If sorted=True, then
-             p2[n, j] is the kth nearest neighbor to p1[n, i].
-        dists: Tensor of shape (N, P1, K) giving the distances to the nearest
-               neighbors.
+            K nearest neighbors from points in p1 to points in p2.
+            Concretely, if idx[n, i, k] = j then p2[n, j] is the kth nearest neighbor
+            to p1[n, i]. This is padded with zeros both where a cloud in p2 has fewer
+            than K points and where a cloud in p1 has fewer than P1 points.
+        dists: Tensor of shape (N, P1, K) giving the squared distances to the nearest
+            neighbors. This is padded with zeros both where a cloud in p2 has fewer than
+            K points and where a cloud in p1 has fewer than P1 points.
     """
     N, P1, D = p1.shape
     _N, P2, _D = p2.shape
+
     assert N == _N and D == _D
-    diffs = p1.view(N, P1, 1, D) - p2.view(N, 1, P2, D)
+
+    if lengths1 is None:
+        lengths1 = torch.full((N,), P1, dtype=torch.int64, device=p1.device)
+    if lengths2 is None:
+        lengths2 = torch.full((N,), P2, dtype=torch.int64, device=p1.device)
+
+    p1_copy = p1.clone()
+    p2_copy = p2.clone()
+
+    # We pad the values with infinities so that the smallest differences are
+    # among actual points.
+    inf = float("inf")
+    p1_mask = torch.arange(P1, device=p1.device)[None] >= lengths1[:, None]
+    p1_copy[p1_mask] = inf
+    p2_copy[torch.arange(P2, device=p1.device)[None] >= lengths2[:, None]] = -inf
+
+    # view is safe here: we are merely adding extra dimensions of length 1
+    diffs = p1_copy.view(N, P1, 1, D) - p2_copy.view(N, 1, P2, D)
     dists2 = (diffs * diffs).sum(dim=3)
-    out = dists2.topk(K, dim=2, largest=False, sorted=sorted)
-    return out.indices, out.values
+
+    # We always sort, because this works well with padding.
+    out = dists2.topk(min(K, P2), dim=2, largest=False, sorted=True)
+
+    out_indices = out.indices
+    out_values = out.values
+
+    if P2 < K:
+        # Need to add padding
+        pad_shape = (N, P1, K - P2)
+        out_indices = torch.cat([out_indices, out_indices.new_zeros(pad_shape)], 2)
+        out_values = torch.cat([out_values, out_values.new_zeros(pad_shape)], 2)
+
+    K_mask = torch.arange(K, device=p1.device)[None] >= lengths2[:, None]
+    # Create a combined mask for where the points in p1 are padded
+    # or the corresponding p2 has fewer than K points.
+    p1_K_mask = p1_mask[:, :, None] | K_mask[:, None, :]
+    out_indices[p1_K_mask] = 0
+    out_values[p1_K_mask] = 0
+    return out_indices, out_values

tests/bm_knn.py

@@ -13,6 +13,7 @@ def bm_knn() -> None:
     benchmark_knn_cpu()
     benchmark_knn_cuda_vs_naive()
     benchmark_knn_cuda_versions()
+    benchmark_knn_cuda_versions_ragged()
 
 
 def benchmark_knn_cuda_versions() -> None:
@@ -36,6 +37,25 @@ def benchmark_knn_cuda_versions() -> None:
     benchmark(nn_cuda_with_init, "NN_CUDA", nn_kwargs, warmup_iters=1)
 
 
+def benchmark_knn_cuda_versions_ragged() -> None:
+    # Compare our different KNN implementations,
+    # and also compare against our existing 1-NN
+    Ns = [8]
+    Ps = [4096, 16384]
+    Ds = [3]
+    Ks = [1, 4, 16, 64]
+    versions = [0, 1, 2, 3]
+    knn_kwargs = []
+    for N, P, D, K, version in product(Ns, Ps, Ds, Ks, versions):
+        if version == 2 and K > 32:
+            continue
+        if version == 3 and K > 4:
+            continue
+        knn_kwargs.append({"N": N, "D": D, "P": P, "K": K, "v": version})
+    benchmark(knn_cuda_with_init, "KNN_CUDA_COMPARISON", knn_kwargs, warmup_iters=1)
+    benchmark(knn_cuda_ragged, "KNN_CUDA_RAGGED", knn_kwargs, warmup_iters=1)
+
+
 def benchmark_knn_cuda_vs_naive() -> None:
     # Compare against naive pytorch version of KNN
     Ns = [1, 2, 4]
@@ -72,10 +92,27 @@ def knn_cuda_with_init(N, D, P, K, v=-1):
     device = torch.device("cuda:0")
     x = torch.randn(N, P, D, device=device)
     y = torch.randn(N, P, D, device=device)
+    lengths = torch.full((N,), P, dtype=torch.int64, device=device)
+
     torch.cuda.synchronize()
 
     def knn():
-        _C.knn_points_idx(x, y, K, v)
+        _C.knn_points_idx(x, y, lengths, lengths, K, v)
+        torch.cuda.synchronize()
+
+    return knn
+
+
+def knn_cuda_ragged(N, D, P, K, v=-1):
+    device = torch.device("cuda:0")
+    x = torch.randn(N, P, D, device=device)
+    y = torch.randn(N, P, D, device=device)
+    lengths1 = torch.randint(P, size=(N,), device=device, dtype=torch.int64)
+    lengths2 = torch.randint(P, size=(N,), device=device, dtype=torch.int64)
+    torch.cuda.synchronize()
+
+    def knn():
+        _C.knn_points_idx(x, y, lengths1, lengths2, K, v)
         torch.cuda.synchronize()
 
     return knn
@@ -85,9 +122,10 @@ def knn_cpu_with_init(N, D, P, K):
     device = torch.device("cpu")
     x = torch.randn(N, P, D, device=device)
     y = torch.randn(N, P, D, device=device)
+    lengths = torch.full((N,), P, dtype=torch.int64, device=device)
 
     def knn():
-        _C.knn_points_idx(x, y, K, 0)
+        _C.knn_points_idx(x, y, lengths, lengths, K, -1)
 
     return knn
 
@@ -96,10 +134,12 @@ def knn_python_cuda_with_init(N, D, P, K):
     device = torch.device("cuda")
     x = torch.randn(N, P, D, device=device)
     y = torch.randn(N, P, D, device=device)
+    lengths = torch.full((N,), P, dtype=torch.int64, device=device)
+
     torch.cuda.synchronize()
 
     def knn():
-        _knn_points_idx_naive(x, y, K)
+        _knn_points_idx_naive(x, y, K=K, lengths1=lengths, lengths2=lengths)
         torch.cuda.synchronize()
 
     return knn
@@ -109,9 +149,10 @@ def knn_python_cpu_with_init(N, D, P, K):
     device = torch.device("cpu")
     x = torch.randn(N, P, D, device=device)
     y = torch.randn(N, P, D, device=device)
+    lengths = torch.full((N,), P, dtype=torch.int64, device=device)
 
     def knn():
-        _knn_points_idx_naive(x, y, K)
+        _knn_points_idx_naive(x, y, K=K, lengths1=lengths, lengths2=lengths)
 
     return knn
 

tests/test_knn.py

@@ -8,6 +8,10 @@ from pytorch3d.ops.knn import _knn_points_idx_naive, knn_points_idx
 
 
 class TestKNN(unittest.TestCase):
+    def setUp(self) -> None:
+        super().setUp()
+        torch.manual_seed(1)
+
     def _check_knn_result(self, out1, out2, sorted):
         # When sorted=True, points should be sorted by distance and should
         # match between implementations. When sorted=False we we only want to
@@ -26,7 +30,7 @@ class TestKNN(unittest.TestCase):
         self.assertTrue(torch.all(idx1 == idx2))
         self.assertTrue(torch.allclose(dist1, dist2))
 
-    def test_knn_vs_python_cpu(self):
+    def test_knn_vs_python_cpu_square(self):
         """ Test CPU output vs PyTorch implementation """
         device = torch.device("cpu")
         Ns = [1, 4]
@@ -37,13 +41,19 @@ class TestKNN(unittest.TestCase):
         sorts = [True, False]
         factors = [Ns, Ds, P1s, P2s, Ks, sorts]
         for N, D, P1, P2, K, sort in product(*factors):
+            lengths1 = torch.full((N,), P1, dtype=torch.int64, device=device)
+            lengths2 = torch.full((N,), P2, dtype=torch.int64, device=device)
             x = torch.randn(N, P1, D, device=device)
             y = torch.randn(N, P2, D, device=device)
-            out1 = _knn_points_idx_naive(x, y, K, sort)
-            out2 = knn_points_idx(x, y, K, sort)
+            out1 = _knn_points_idx_naive(
+                x, y, lengths1=lengths1, lengths2=lengths2, K=K
+            )
+            out2 = knn_points_idx(
+                x, y, K=K, lengths1=lengths1, lengths2=lengths2, sorted=sort
+            )
             self._check_knn_result(out1, out2, sort)
 
-    def test_knn_vs_python_cuda(self):
+    def test_knn_vs_python_cuda_square(self):
         """ Test CUDA output vs PyTorch implementation """
         device = torch.device("cuda")
         Ns = [1, 4]
@@ -57,9 +67,53 @@ class TestKNN(unittest.TestCase):
         for N, D, P1, P2, K, sort in product(*factors):
             x = torch.randn(N, P1, D, device=device)
             y = torch.randn(N, P2, D, device=device)
-            out1 = _knn_points_idx_naive(x, y, K, sorted=sort)
+            out1 = _knn_points_idx_naive(x, y, lengths1=None, lengths2=None, K=K)
             for version in versions:
                 if version == 3 and K > 4:
                     continue
-                out2 = knn_points_idx(x, y, K, sort, version)
+                out2 = knn_points_idx(x, y, K=K, sorted=sort, version=version)
+                self._check_knn_result(out1, out2, sort)
+
+    def test_knn_vs_python_cpu_ragged(self):
+        device = torch.device("cpu")
+        lengths1 = torch.tensor([10, 100, 10, 100], device=device, dtype=torch.int64)
+        lengths2 = torch.tensor([10, 10, 100, 100], device=device, dtype=torch.int64)
+        N = 4
+        D = 3
+        Ks = [1, 9, 10, 11, 101]
+        sorts = [False, True]
+        factors = [Ks, sorts]
+        for K, sort in product(*factors):
+            x = torch.randn(N, lengths1.max(), D, device=device)
+            y = torch.randn(N, lengths2.max(), D, device=device)
+            out1 = _knn_points_idx_naive(
+                x, y, lengths1=lengths1, lengths2=lengths2, K=K
+            )
+            out2 = knn_points_idx(
+                x, y, lengths1=lengths1, lengths2=lengths2, K=K, sorted=sort
+            )
+            self._check_knn_result(out1, out2, sort)
+
+    def test_knn_vs_python_cuda_ragged(self):
+        device = torch.device("cuda")
+        lengths1 = torch.tensor([10, 100, 10, 100], device=device, dtype=torch.int64)
+        lengths2 = torch.tensor([10, 10, 100, 100], device=device, dtype=torch.int64)
+        N = 4
+        D = 3
+        Ks = [1, 9, 10, 11, 101]
+        sorts = [True, False]
+        versions = [0, 1, 2, 3]
+        factors = [Ks, sorts]
+        for K, sort in product(*factors):
+            x = torch.randn(N, lengths1.max(), D, device=device)
+            y = torch.randn(N, lengths2.max(), D, device=device)
+            out1 = _knn_points_idx_naive(
+                x, y, lengths1=lengths1, lengths2=lengths2, K=K
+            )
+            for version in versions:
+                if version == 3 and K > 4:
+                    continue
+                out2 = knn_points_idx(
+                    x, y, lengths1=lengths1, lengths2=lengths2, K=K, sorted=sort
+                )
                 self._check_knn_result(out1, out2, sort)