Thanks to Richard Coffey, Gordon Kindlmann, the members of the University of Utah Computer Graphics groups for help in the initial stages of the paper, and to Ramesh Raskar and Michael Cohen for discussing their work in progress with us. This work was supported in part by DARPA (F33615-96-C-5621) and the NSF Science and Technology Center for Computer Graphics and Scientific Visualization (ASC-89-20219). All opinions, findings, conclusions or recommendations expressed in this document are those of the author and do not necessarily reflect the views of the sponsoring agencies.