ALGORITHM
v.hull calculates the planar convex hull of a sites list by
means of an algorithm like Graham scan, by computing
first the upper part of the convex hull, and the
the lower part. The algorighm has been converted to C
from the pseudo-code found in "Computational Geometry,
algorithms and applications", M. de Berg, M. van Kreveld,
M. Overmars, O. Schwarzkopf, Springer.
Some reference about the algoright can be found in the
online course "CMSC 754: Computational geometry" by Dave Mount.
Andrea Aime 9/2001
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chull.c
http://maven.smith.edu/~orourke/books/compgeom.html
http://maven.smith.edu/~orourke/books/ftp.html