Prosenjit Bose Thomas C. Shermer Godfried T. Toussaint Binhai Zhu Guarding Polyhedral Terrains. 173-185 1997 7 Comput. Geom. db/journals/comgeo/comgeo7.html#BoseSTZ97 http://dx.doi.org/10.1016/0925-7721(95)00034-8