A flexible and highly robust polygon set operations library for use with c applications, as referenced in the comp.graphics.algorithms faq and the uiuc computational geometry pages.

features:
* difference, intersection, exclusive-or and union clip operations are
supported.
* polygons may be comprised of multiple disjoint contours.
* contour vertices may be given in any order - clockwise or
anticlockwise.
* contours may be convex, concave or self-intersecting.
* contours may be nested (i.e. polygons may have holes).
* output may take the form of either polygon contours or tristrips.
* hole and external contours are differentiated in the result.
* coincident edges and degenerate regions are handled correctly.
homepage: http://www.cs.man.ac.uk/~amurta/software/index.html#gpc