I. Introduction
In the past years, there has been rapid development in 3-D applications, including 3-D TV and movie, virtual reality, 3-D city, and 3-D object retrieval. However, compared with the developments of 3-D applications, 3-D resources are still limited. Traditionally, 3-D models are either captured by 3-D cameras or designed by designers using computer-aided design tools. The former requires careful calibration and scans objects under strict conditions, and the latter is manually intensive.