override func viewWillAppear(_ animated: Bool) {
  super.viewWillAppear(animated)

  guard let referenceImages = ARReferenceImage.referenceImages(inGroupNamed: "Mona Lisa Room", bundle: nil) else {
    fatalError("Missing expected asset catalog resources.")
  }

  let configuration = ARWorldTrackingConfiguration()
  configuration.detectionImages = referenceImages

  sceneView.session.run(configuration, options: [.resetTracking, .removeExistingAnchors])
}