title: Why are Open3D normals incorrect in x-direction?
body: <p>I am computing normals for a pointcloud in Open3D</p>
  
  <p>Using:</p>
  
  <pre><code>points = np.random.uniform(-1, 1, (10000, 6))
  
  pointcloud = o3d.geometry.PointCloud()
  pointcloud.points = o3d.utility.Vector3dVector(points[:, [0, 1, 2]])
  pointcloud.colors = o3d.utility.Vector3dVector(points[:, [3, 4, 5]])
  pointcloud = o3d.geometry.voxel_down_sample(pointcloud, voxel_size=0.1)
  
  print("Recompute the normal of the downsampled point cloud ...")
  # Why are all the normals in the x direction positive?
  o3d.geometry.estimate_normals(
      pointcloud,
      #search_param=o3d.geometry.KDTreeSearchParamKNN(knn=250),
      search_param=o3d.geometry.KDTreeSearchParamHybrid(radius=1.0, max_nn=30)
  
  )
  
  print(np.round(np.asarray(pointcloud.normals).min(axis=0), 3))
  print(np.round(np.asarray(pointcloud.normals).max(axis=0), 3))
  </code></pre>
  
  <p>The result is:</p>
  
  <pre><code>[ 0. -1. -1.]
  [1. 1. 1.]
  </code></pre>
  
  <p>Why are all the x components of the normals positive?</p>