Mark de Berg Herman J. Haverkort Constantinos P. Tsirogiannis Visibility maps of realistic terrains have linear smoothed complexity. 163-168 2009 Symposium on Computational Geometry http://doi.acm.org/10.1145/1542362.1542397 conf/compgeom/2009 db/conf/compgeom/compgeom2009.html#BergHT09