func createPlaneNode(center: vector_float3,
                   extent: vector_float3) -> SCNNode {
let plane = SCNPlane(width: CGFloat(extent.x),
height: CGFloat(extent.z))
let planeMaterial = SCNMaterial()
planeMaterial.diffuse.contents = UIColor.yellow.withAlphaComponent(0.4)
plane.materials = [planeMaterial]


let planeNode = SCNNode(geometry: plane)

  planeNode.position = SCNVector3Make(center.x, 0, center.z)

  planeNode.transform = SCNMatrix4MakeRotation(-Float.pi / 2, 1, 0, 0)

  return planeNode
}