Computer Graphics Laboratory, University of California San Francisco, 600 16th Street, San Francisco, CA 94143-2440, USA. meng@cgl.ucsf.edu|Computer Graphics Laboratory, University of California San Francisco, 600 16th Street, San Francisco, CA 94143-2440, USA. meng@cgl.ucsf.edu|Computer Graphics Laboratory, University of California San Francisco, 600 16th Street, San Francisco, CA 94143-2440, USA. meng@cgl.ucsf.edu|Computer Graphics Laboratory, University of California San Francisco, 600 16th Street, San Francisco, CA 94143-2440, USA. meng@cgl.ucsf.edu|Computer Graphics Laboratory, University of California San Francisco, 600 16th Street, San Francisco, CA 94143-2440, USA. meng@cgl.ucsf.edu