Today, people live not only in the real world, but also in 3D virtual worlds. We spend time on virtual reality systems, graphics games, films of imaginary worlds and characters, Web-based 3D environments, and distributed interactive simulations. People represented by 3D avatars can travel in virtual worlds and meet with one another over the Internet. Advances in graphics software and hardware have enabled many new applications, and many virtual worlds are constructed with different models and activities.