3D motion tracking is one key technology in computer animation, virtual reality, natural user interface, and so on. Over the last thirty years numerous projects with various approaches have developed motion tracking systems, including mechanical, ultrasonic, magnetic, and optical schemes. Recently, interest has also been growing in the motion tracking of small targets in complex environments and tracking subtle movements. For example, the behavior analysis of insects or other small creatures in the earth will contribute to the growth of biology or related sciences, and the precise analysis of dexterous finger motions will be useful for professional techniques or the skills required for traditional handcrafts or medical procedures. Unfortunately, common existing tracking systems cannot satisfy the requirements of these tasks because the targets are too small and their motions are complex and easily occluded.