Martin Usoh, Kevin Arthur, Mary C. Whitton, Rui Bastos, Anthony Steed, Mel
  Slater, Frederick P. Brooks, and Jr.
 <b>Walking &gt; Walking-in-Place &gt; Flying, in Virtual
  Environments</b>.
 In <em>Proceedings of SIGGRAPH</em>, pages 359--364, 1999.
  <a href="http://vis.cs.brown.edu/docs/pdf/bib/g/Usoh-1999-WWF.pdf.html">(pdf)</a>
  <a href="http://vis.cs.brown.edu/results/bibtex/Usoh-1999-WWF.bib">(bibtex:
  Usoh-1999-WWF)</a>.