1. A computer-implemented method of detecting lip activity, comprising: determining magnitude of optical flow in lip region of a detected face; determining magnitude of optical flow in at least one non-lip region of the detected face; comparing ratio of magnitude of optical flow in lip region of the detected face and magnitude of optical flow in at least one non-lip region of the detected face against a threshold; and if the ratio is greater than the threshold, recognizing lip activity of the detected face.