This research has been supported by the National Science Foundation under grants CCR-9205047 and DCA 9303181 and by the Department of Energy under the PICS grant. Special thanks to Rick Avila and Lisa Sobierajski for several enlightening discussions about PARC, volume rendering, and the implementation of VolVis. We are grateful to Juliana Freire for implementing the efficient Prolog algorithm described in Section 3 in the XSB system developed at Stony Brook by David Warren.