M. Levoy.
  <b>Efficient ray tracing of volume data </b>.
  <i>ACM Transactions of Computer Graphics</i>, 9(3):245-261, 1990.
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Levoy-1990-ERO.bib">(bibtex:
  Levoy-1990-ERO)</A>.<br />