I. Introduction
The development of deep learning not only brings significant improvement to traditional visual tasks [1], [2], [3], [4] but gives birth to massive novel and heuristic vision applications [5], [6], [7], [8]. Deepfake, a new technique used to generate artificial media by deep neural networks [9], has raised public concerns about personal security and privacy. To fight against malicious facial deepfakes, many detection methods have been naturally proposed and extensively studied over recent years.