Ingmar Bitter, Arie E. Kaufman, and Mie Sato.
  <b>Penalized-distance volumetric skeleton algorithm </b>.
  <i>IEEE Transactions on Visualization and Computer Graphics</i>,
  7(3):195-206, 2001.
  published
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Bitter-2001-PVS.bib">(bibtex:
  Bitter-2001-PVS)</A>.<br />