I. Introduction
Object tracking is required by many vision applications, especially in video technology and visual interface systems. The ability to track facial motion is useful in applications such as face-based biometric person authentication, expression analysis [11], and human computer interaction [5], [34]. Visual tracking of faces can be utilized for recognizing basic activities from video sequences. A recent survey on video analysis of human dynamics is given by [33]. Detecting and tracking faces in video sequences is a challenging task because faces are nonrigid and their images have a high degree of variability. A huge research effort has already been devoted to detecting and tracking of facial features in 2–D and 3-D (e.g., [3], [8], [10], [17], [20], [35]).