Chenyi Xia David Hsu Anthony K. H. Tung A Fast Filter for Obstructed Nearest Neighbor Queries. 203-215 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3112&spage=203 2004 conf/bncod/2004 BNCOD db/conf/bncod/bncod2004.html#XiaHT04