Add CPU implementation for nearest neighbor

Summary:
Adds a CPU implementation for `pytorch3d.ops.nn_points_idx`.

Also renames the associated C++ and CUDA functions to use `AllCaps` names used in other C++ / CUDA code.

Reviewed By: gkioxari

Differential Revision: D19670491

fbshipit-source-id: 1b6409404025bf05e6a93f5d847e35afc9062f05

pytorch3d/csrc/ext.cpp

@@ -11,7 +11,7 @@
 PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
   m.def("face_areas_normals", &face_areas_normals);
   m.def("packed_to_padded_tensor", &packed_to_padded_tensor);
-  m.def("nn_points_idx", &nn_points_idx);
+  m.def("nn_points_idx", &NearestNeighborIdx);
   m.def("gather_scatter", &gather_scatter);
   m.def("rasterize_points", &RasterizePoints);
   m.def("rasterize_points_backward", &RasterizePointsBackward);

pytorch3d/csrc/nearest_neighbor_points/nearest_neighbor_points.cu

@@ -4,7 +4,7 @@
 #include <float.h>
 
 template <typename scalar_t>
-__device__ void warp_reduce(
+__device__ void WarpReduce(
     volatile scalar_t* min_dists,
     volatile long* min_idxs,
     const size_t tid) {
@@ -54,7 +54,7 @@ __device__ void warp_reduce(
 //         is aligned.
 //
 template <typename scalar_t>
-__global__ void nearest_neighbor_kernel(
+__global__ void NearestNeighborKernel(
     const scalar_t* __restrict__ points1,
     const scalar_t* __restrict__ points2,
     long* __restrict__ idx,
@@ -123,7 +123,7 @@ __global__ void nearest_neighbor_kernel(
   // Unroll the last 6 iterations of the loop since they will happen
   // synchronized within a single warp.
   if (tid < 32)
-    warp_reduce<scalar_t>(min_dists, min_idxs, tid);
+    WarpReduce<scalar_t>(min_dists, min_idxs, tid);
 
   // Finally thread 0 writes the result to the output buffer.
   if (tid == 0) {
@@ -144,7 +144,7 @@ __global__ void nearest_neighbor_kernel(
 //    P2: Number of points in points2.
 //
 template <typename scalar_t>
-__global__ void nearest_neighbor_kernel_D3(
+__global__ void NearestNeighborKernelD3(
     const scalar_t* __restrict__ points1,
     const scalar_t* __restrict__ points2,
     long* __restrict__ idx,
@@ -204,7 +204,7 @@ __global__ void nearest_neighbor_kernel_D3(
   // Unroll the last 6 iterations of the loop since they will happen
   // synchronized within a single warp.
   if (tid < 32)
-    warp_reduce<scalar_t>(min_dists, min_idxs, tid);
+    WarpReduce<scalar_t>(min_dists, min_idxs, tid);
 
   // Finally thread 0 writes the result to the output buffer.
   if (tid == 0) {
@@ -212,7 +212,7 @@ __global__ void nearest_neighbor_kernel_D3(
   }
 }
 
-at::Tensor nn_points_idx_cuda(at::Tensor p1, at::Tensor p2) {
+at::Tensor NearestNeighborIdxCuda(at::Tensor p1, at::Tensor p2) {
   const auto N = p1.size(0);
   const auto P1 = p1.size(1);
   const auto P2 = p2.size(1);
@@ -231,7 +231,7 @@ at::Tensor nn_points_idx_cuda(at::Tensor p1, at::Tensor p2) {
     AT_DISPATCH_FLOATING_TYPES(p1.type(), "nearest_neighbor_v3_cuda", ([&] {
                                  size_t shared_size = threads * sizeof(size_t) +
                                      threads * sizeof(long);
-                                 nearest_neighbor_kernel_D3<scalar_t>
+                                 NearestNeighborKernelD3<scalar_t>
                                      <<<blocks, threads, shared_size>>>(
                                          p1.data_ptr<scalar_t>(),
                                          p2.data_ptr<scalar_t>(),
@@ -249,7 +249,7 @@ at::Tensor nn_points_idx_cuda(at::Tensor p1, at::Tensor p2) {
           size_t D_2 = D + (D % 2);
           size_t shared_size = (D_2 + threads) * sizeof(size_t);
           shared_size += threads * sizeof(long);
-          nearest_neighbor_kernel<scalar_t><<<blocks, threads, shared_size>>>(
+          NearestNeighborKernel<scalar_t><<<blocks, threads, shared_size>>>(
               p1.data_ptr<scalar_t>(),
               p2.data_ptr<scalar_t>(),
               idx.data_ptr<long>(),

pytorch3d/csrc/nearest_neighbor_points/nearest_neighbor_points.h

@@ -19,19 +19,22 @@
 //                     to p1[n, i] in the cloud p2[n] is p2[n, j].
 //
 
+// CPU implementation.
+at::Tensor NearestNeighborIdxCpu(at::Tensor p1, at::Tensor p2);
+
 // Cuda implementation.
-at::Tensor nn_points_idx_cuda(at::Tensor p1, at::Tensor p2);
+at::Tensor NearestNeighborIdxCuda(at::Tensor p1, at::Tensor p2);
 
 // Implementation which is exposed.
-at::Tensor nn_points_idx(at::Tensor p1, at::Tensor p2) {
+at::Tensor NearestNeighborIdx(at::Tensor p1, at::Tensor p2) {
   if (p1.type().is_cuda() && p2.type().is_cuda()) {
 #ifdef WITH_CUDA
     CHECK_CONTIGUOUS_CUDA(p1);
     CHECK_CONTIGUOUS_CUDA(p2);
-    return nn_points_idx_cuda(p1, p2);
+    return NearestNeighborIdxCuda(p1, p2);
 #else
     AT_ERROR("Not compiled with GPU support.");
 #endif
   }
-  AT_ERROR("Not implemented on the CPU.");
+  return NearestNeighborIdxCpu(p1, p2);
 };

pytorch3d/csrc/nearest_neighbor_points/nearest_neighbors_points_cpu.cpp

@@ -0,0 +1,38 @@
+// Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+
+#include <torch/extension.h>
+
+at::Tensor NearestNeighborIdxCpu(at::Tensor p1, at::Tensor p2) {
+    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 out = torch::empty({N, P1}, long_opts);
+
+    auto p1_a = p1.accessor<float, 3>();
+    auto p2_a = p2.accessor<float, 3>();
+    auto out_a = out.accessor<int64_t, 2>();
+
+    for (int n = 0; n < N; ++n) {
+        for (int i1 = 0; i1 < P1; ++i1) {
+            // TODO: support other floating-point types?
+            float min_dist = -1;
+            int64_t min_idx = -1;
+            for (int i2 = 0; i2 < P2; ++i2) {
+                float dist = 0;
+                for (int d = 0; d < D; ++d) {
+                    float diff = p1_a[n][i1][d] - p2_a[n][i2][d];
+                    dist += diff * diff;
+                }
+                if (min_dist == -1 || dist < min_dist) {
+                    min_dist = dist;
+                    min_idx = i2;
+                }
+            }
+            out_a[n][i1] = min_idx;
+        }
+    }
+    return out;
+}

tests/bm_nearest_neighbor_points.py

@@ -27,6 +27,13 @@ def bm_nn_points() -> None:
         warmup_iters=1,
     )
 
+    benchmark(
+        TestNearestNeighborPoints.bm_nn_points_cpu_with_init,
+        "NN_CPU",
+        kwargs_list,
+        warmup_iters=1,
+    )
+
     if torch.cuda.is_available():
         benchmark(
             TestNearestNeighborPoints.bm_nn_points_cuda_with_init,

tests/test_nearest_neighbor_points.py

@@ -21,11 +21,7 @@ class TestNearestNeighborPoints(unittest.TestCase):
         idx = dists2.argmin(2)
         return idx
 
-    def test_nn_cuda(self):
+    def _test_nn_helper(self, device):
-        """
-        Test cuda output vs naive python implementation.
-        """
-        device = torch.device("cuda:0")
         for D in [3, 4]:
             for N in [1, 4]:
                 for P1 in [1, 8, 64, 128]:
@@ -43,16 +39,32 @@ class TestNearestNeighborPoints(unittest.TestCase):
                         self.assertTrue(idx1.size(1) == P1)
                         self.assertTrue(torch.all(idx1 == idx2))
 
-    def test_nn_cuda_error(self):
+    def test_nn_cuda(self):
         """
-        Check that nn_points_idx throws an error if cpu tensors
+        Test cuda output vs naive python implementation.
-        are given as input.
         """
-        x = torch.randn(1, 1, 3)
+        device = torch.device('cuda:0')
-        y = torch.randn(1, 1, 3)
+        self._test_nn_helper(device)
-        with self.assertRaises(Exception) as err:
+
-            _C.nn_points_idx(x, y)
+    def test_nn_cpu(self):
-        self.assertTrue("Not implemented on the CPU" in str(err.exception))
+        """
+        Test cpu output vs naive python implementation
+        """
+        device = torch.device('cpu')
+        self._test_nn_helper(device)
+
+    @staticmethod
+    def bm_nn_points_cpu_with_init(
+        N: int = 4, D: int = 4, P1: int = 128, P2: int = 128
+    ):
+        device = torch.device('cpu')
+        x = torch.randn(N, P1, D, device=device)
+        y = torch.randn(N, P2, D, device=device)
+
+        def nn_cpu():
+            _C.nn_points_idx(x.contiguous(), y.contiguous())
+
+        return nn_cpu
 
     @staticmethod
     def bm_nn_points_cuda_with_init(