Andrew Witkin and Michael Kass.
  <b>Spacetime Constraints</b>.
  <i>Computer Graphics</i>, 22(4):159-168, August 1988.
  <A HREF="http://vis.cs.brown.edu/docs/pdf/bib/g/Witkin-1988-SCO.pdf.html">(pdf)</A>
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Witkin-1988-SCO.bib">(bibtex:
  Witkin-1988-SCO)</A>.<br />