c++ marching cubes fix

Summary:
Fixes https://github.com/facebookresearch/pytorch3d/issues/1641. The bug was caused by the mistaken downcasting of an int64_t into int, causing issues only on inputs large enough to have hashes that escaped the bounds of an int32.

Also added a test case for this issue.

Reviewed By: bottler

Differential Revision: D53505370

fbshipit-source-id: 0fdd0efc6d259cc3b0263e7ff3a4ab2c648ec521

pytorch3d/csrc/marching_cubes/marching_cubes_cpu.cpp

@@ -71,8 +71,8 @@ std::tuple<at::Tensor, at::Tensor, at::Tensor> MarchingCubesCpu(
           if ((j + 1) % 3 == 0 && ps[0] != ps[1] && ps[1] != ps[2] &&
               ps[2] != ps[0]) {
             for (int k = 0; k < 3; k++) {
-              int v = tri[k];
-              edge_id_to_v[tri.at(k)] = ps.at(k);
+              int64_t v = tri.at(k);
+              edge_id_to_v[v] = ps.at(k);
               if (!uniq_edge_id.count(v)) {
                 uniq_edge_id[v] = verts.size();
                 verts.push_back(edge_id_to_v[v]);

tests/test_marching_cubes.py

@@ -854,6 +854,18 @@ class TestMarchingCubes(TestCaseMixin, unittest.TestCase):
             self.assertClose(verts2[0], expected_verts)
             self.assertClose(faces2[0], expected_faces)
 
+    def test_single_large_ellipsoid(self):
+        if USE_SCIKIT:
+            from skimage.draw import ellipsoid
+
+            ellip_base = ellipsoid(50, 60, 16, levelset=True)
+            volume = torch.Tensor(ellip_base).unsqueeze(0).cpu()
+            verts, faces = marching_cubes_naive(volume, 0)
+            verts2, faces2 = marching_cubes(volume, 0)
+
+            self.assertClose(verts[0], verts2[0], atol=1e-6)
+            self.assertClose(faces[0], faces2[0], atol=1e-6)
+
     def test_cube_surface_area(self):
         if USE_SCIKIT:
             from skimage.measure import marching_cubes_classic, mesh_surface_area