John C. Whelan Mahes Visvalingam Formulated Silhouettes for Sketching Terrain. 90-96 2003 conf/tpcg/2003 TPCG http://csdl.computer.org/comp/proceedings/tpcg/2003/1942/00/19420090abs.htm db/conf/tpcg/tpcg2003.html#WhelanV03