1. Hitachi Researeh Laboratory, Hitachi, Ltd., 1-10mika-cho 7-chome, Hitachi, Ibaraki 319-12 Japan
2. University of Dortmund, Department of Computer Graphics, Otto-Hahn Str. 16, 44221 Dortmund, Germany
3. Georgia Institute of Technology, Graphics, Visualization, and Usability Center, and College of Computing, Atlanta, Georgia