small fix for iou3d

Summary:
A small numerical fix for IoU for 3D boxes, fixes GH #992

* Adds a check for boxes with zero side areas (invalid boxes)
* Fixes numerical issue when two boxes have coplanar sides

Reviewed By: nikhilaravi

Differential Revision: D33195691

fbshipit-source-id: 8a34b4d1f1e5ec2edb6d54143930da44bdde0906

pytorch3d/csrc/iou_box3d/iou_box3d.cu

@@ -90,7 +90,8 @@ __global__ void IoUBox3DKernel(
         for (int b2 = 0; b2 < box2_count; ++b2) {
           const bool is_coplanar =
               IsCoplanarFace(box1_intersect[b1], box2_intersect[b2]);
-          if (is_coplanar) {
+          const float area = FaceArea(box1_intersect[b1]);
+          if ((is_coplanar) && (area > kEpsilon)) {
             tri2_keep[b2].keep = false;
           }
         }

pytorch3d/csrc/iou_box3d/iou_box3d_cpu.cpp

@@ -81,7 +81,8 @@ std::tuple<at::Tensor, at::Tensor> IoUBox3DCpu(
           for (int b2 = 0; b2 < box2_intersect.size(); ++b2) {
             const bool is_coplanar =
                 IsCoplanarFace(box1_intersect[b1], box2_intersect[b2]);
-            if (is_coplanar) {
+            const float area = FaceArea(box1_intersect[b1]);
+            if ((is_coplanar) && (area > kEpsilon)) {
               tri2_keep[b2] = 0;
             }
           }

pytorch3d/csrc/iou_box3d/iou_utils.cuh

@@ -138,6 +138,26 @@ FaceNormal(const float3 v0, const float3 v1, const float3 v2) {
   return n;
 }
 
+// The area of the face defined by vertices (v0, v1, v2)
+// Define e0 to be the edge connecting (v1, v0)
+// Define e1 to be the edge connecting (v2, v0)
+// Area is the norm of the cross product of e0, e1 divided by 2.0
+//
+// Args
+//    tri: FaceVerts of float3 coordinates of the vertices of the face
+//
+// Returns
+//    float: area for the face
+//
+__device__ inline float FaceArea(const FaceVerts& tri) {
+  // Get verts for face 1
+  const float3 v0 = tri.v0;
+  const float3 v1 = tri.v1;
+  const float3 v2 = tri.v2;
+  const float3 n = cross(v1 - v0, v2 - v0);
+  return norm(n) / 2.0;
+}
+
 // The normal of a box plane defined by the verts in `plane` with
 // the centroid of the box given by `center`.
 // Args

pytorch3d/csrc/iou_box3d/iou_utils.h

@@ -145,6 +145,26 @@ inline vec3<float> FaceNormal(vec3<float> v0, vec3<float> v1, vec3<float> v2) {
   return n;
 }
 
+// The area of the face defined by vertices (v0, v1, v2)
+// Define e0 to be the edge connecting (v1, v0)
+// Define e1 to be the edge connecting (v2, v0)
+// Area is the norm of the cross product of e0, e1 divided by 2.0
+//
+// Args
+//    tri: vec3 coordinates of the vertices of the face
+//
+// Returns
+//    float: area for the face
+//
+inline float FaceArea(const std::vector<vec3<float>>& tri) {
+  // Get verts for face
+  const vec3<float> v0 = tri[0];
+  const vec3<float> v1 = tri[1];
+  const vec3<float> v2 = tri[2];
+  const vec3<float> n = cross(v1 - v0, v2 - v0);
+  return norm(n) / 2.0;
+}
+
 // The normal of a box plane defined by the verts in `plane` with
 // the centroid of the box given by `center`.
 // Args