[Why the height of ground plane is computed like this? ](https://github.com/poodarchu/Det3D/blob/01258d8cb26656c5b950f8d41f9dcc1dd62a391e/det3d/datasets/nuscenes/nusc_common.py#L364) Your code: `zz = (-plane[0] * xx - plane[1] * yy - plane[3]) / plane[2]`