Franz Aurenhammer Johannes Wallner Martin Peternell Helmut Pottmann Voronoi Diagrams for Oriented Spheres. 33-37 2007 conf/isvd/2007 ISVD http://doi.ieeecomputersociety.org/10.1109/ISVD.2007.45 db/conf/isvd/isvd2007.html#AurenhammerWPP07