Helwig Hauser Lukas Mroz Gian Italo Bischi Eduard Gröller Two-Level Volume Rendering. 242-252 2001 7 IEEE Trans. Vis. Comput. Graph. 3 http://www.computer.org/tvcg/tg2001/v0242abs.htm db/journals/tvcg/tvcg7.html#HauserMBG01