IOS11: How can I use Face Track to track images in a video?

Question

I can track an object by video, but I cannot track a face. when i use the face of the camera. code printing []

extension FaceTrackingViewController: AVCaptureVideoDataOutputSampleBufferDelegate {

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {

    let pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)

    let request = VNDetectFaceLandmarksRequest { [unowned self] request, error in
        if let error = error {
            self.presentAlertController(withTitle: self.title,
                                        message: error.localizedDescription)
        }
        else {
            print("\(request.results!)")
        }
    }


    do {
        try handler.perform([request], on: pixelBuffer!)
    }
    catch {
        print(error)
    }
}

}