“…In most of these works, the device was not held naturally by users' but was fixed by using a stand [18,23,24,26,28,33,43,49], by using a headset [37], or by having another person hold it [51]. Even in works where the user was holding the phone, the authors reported that detection failures were often due to users not holding the phone in a way that would show their face and eyes in the camera's view [27,29,30,31,36].…”