1. INTRODUCTION
Hand gestures are commonly used for human-human communications. We do not even think about it when we use hand gestures because they feel so natural to us. It is a natural question whether the hand gesture could become one of the main modalities for human-computer interactions. Driven by this elusive goal, tremendous amount of research activities have been devoted to hand gesture recognition. So far, most of the research work has been focused on recognition from images and videos due to the popularity of digital cameras. The recent progress in depth sensors such as Microsoft's Kinect device has generated a new level of excitement in gesture recognition. With the depth information, a skeleton tracking system has been developed by Microsoft. Many game developers have leveraged the skeleton tracking system to design games with body gestures as the main interaction mode.