\begin{thebibliography}{1}

\bibitem{leejen01}
Y-C Lee and C-W Jen.
\newblock Improved quadratic normal vector interpolation for realistic shading.
\newblock {\em The Visual Computer}, 17(6):337--352, 2001.

\bibitem{ovwyv97}
C.~{van Overveld} and B.~Wyvill.
\newblock An algorithm for polygon subdivision based on vertex normals.
\newblock In {\em Computer Graphics International '97}, pages 3--12, 1997.

\bibitem{vpbm01}
A.~Vlachos, J.~Peters, C.~Boyd, and J.~Mitchell.
\newblock Curved {PN} triangles.
\newblock In {\em ACM Symposium on Interactive 3D Graphics 2001}, pages
  159--166, 2001.

\end{thebibliography}
