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The following algorithm was sent to me by Romano Giannetti who got it
from the comp.graphics.algorithms FAQ where it was originally written
by Wm. Randolph Franklin. It works on convex or concave algorithms,
simple and complex.
It works by the ray tracing method. Starting at the
coordinates of the point in question draw a straight line in any
direction. If the number of times it intersects the polygon border is
odd the starting point is inside the polygon. If it is even the
starting point is outside the polygon.
Another good polygon algorithm page is Darel
Finley's. Check it out at