1. Introduction
Head pose provides strong cues for human intention, motivation, and attention. Many applications rely on robust head pose estimation results, such as human behavior analysis, human computer interaction, gaze estimation etc. In the field of computer vision, a head pose is typically interpreted as an orientation of a person's head, which is represented by three angles: pitch, yaw and roll. The task of head pose estimation is challenging because of the large head appearance variation (expression, race and gender) and environmental factors (occlusion, noise and illumination). The techniques of head pose estimation are well summarized in [1].