Much of the geometry code in Octave is based on the Qhull
library1.
Some of the documentation for Qhull, particularly for the options that
can be passed to delaunay, voronoi and convhull,
etc., is relevant to Octave users.
[1] Barber, C.B., Dobkin, D.P., and Huhdanpaa, H.T., "The Quickhull algorithm for convex hulls," ACM Trans. on Mathematical Software, 22(4):469–483, Dec 1996, http://www.qhull.org