Library for polygon operations
I've recently encountered a need for a library or set of libraries to handle operations on 2D polygons. I need to be able to perform boolean/clipping operations (difference and union) and triangulation.
So far the libraries I've found are poly2tri, CGAL, and GPC. Poly2tri looks good for triangulation but I'm still left with boolean operations, and I'm unsure about its maturity.
CGAL and GPC are only free if my own project is free. My particular project isn't commercial, so I'm hesitant to pay or request for any licenses. But I may want to use my code for a future commercial project, so I'm hesitant about CGAL's open source licenses and GPC's freeware-only restriction. There doesn't seem to be any polygon clipping libraries with nice BSD-style licenses.
Oh, and C/C++ is preferred.
Answer
Clipper is an open source freeware polygon clipping library (written in Delphi and C++)^ that does exactly what you're asking (except for triangulation) - http://sourceforge.net/projects/polyclipping/
In my testing, Clipper is both significantly faster and far less prone to error than GPC (see more detailed comparisons here - http://www.angusj.com/delphi/clipper.php#features).
Re: Anti-grain Geometry (AGG) graphics library - it doesn't do polygon clipping, but simply uses GPC (which isn't free for commercial applications). However, Clipper does have AGG units to make clipping in AGG just as easy as GPC.
^ Edit: Clipper is now written in C# too (together with Perl, Ruby, Haskell and Flash modules written by third-parties).