I. Introduction
Face recognition is one of the most actively studied problems in computer vision and biometrics. Nowadays, video-based face recognition is an active research topic because of a wide range of applications including visual surveillance, access control, video content analysis, etc. Compared to still image-based face recognition, video-based face recognition is more challenging due to a much larger amount of data to be processed and significant intra/inter-class variations caused by motion blur, low video quality, occlusion, frequent scene changes, and unconstrained acquisition conditions.