E. B. Lum and K.-L. Ma.
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  In <i>Proceedings of the Non-Photorealistic Animation and Rendering
  Conference</i>, pages 67-74, 2002.
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Lum-2002-HPN.bib">(bibtex:
  Lum-2002-HPN)</A>.<br />