1. A computer-implemented method of detecting lip activity, comprising: capturing, by a camera of a computing device, an image of a user; detecting a face of the user on the captured image; determining, by a processor of the computing device, a magnitude of optical flow in a lip region of the detected face on the captured image; determining, by the processor, a magnitude of optical flow in at least one non-lip region of the detected face on the captured image; determining, by the processor, a ratio of the magnitude of optical flow in the lip region and the magnitude of optical flow in the at least one non-lip region of the detected face; comparing, by the processor, the ratio of the magnitude of optical flow in the lip region of the detected face and the magnitude of optical flow in the at least one non-lip region of the detected face against a threshold; and determining that a lip activity of the detected face has been detected when the ratio is greater than the threshold.