  Face recognition technology has been widely used in daily interactive
applications such as checking-in and mobile payment due to its convenience and
high accuracy. However, its vulnerability to presentation attacks (PAs) limits
its reliable use in ultra-secure applicational scenarios. A presentation attack
is first defined in ISO standard as: a presentation to the biometric data
capture subsystem with the goal of interfering with the operation of the
biometric system. Specifically, PAs range from simple 2D print, replay and more
sophisticated 3D masks and partial masks. To defend the face recognition
systems against PAs, both academia and industry have paid extensive attention
to developing face presentation attack detection (PAD) technology (or namely
`face anti-spoofing (FAS)').
