“…In this respect, when the test-phase attacks are fundamentally different from those observed during the training phase, the performance of the classical two-class closed-set systems may be seriously compromised. Due to its significant practical importance, detection of unseen presentation attacks has received considerable attention both in face [6], [7], [8], [9], [10], [11], [12], [13] as well as in other biometric modalities [14], [15], [16], [17], [18].…”