Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!usna!dfr From: dfr@usna.NAVY.MIL (Prof. David F. Rogers) Newsgroups: comp.graphics Subject: Re: polygon clipping Message-ID: <382@usna.NAVY.MIL> Date: 8 May 91 13:34:12 GMT References: <348@usna.NAVY.MIL> <1991May7.181010.22047@pixar.com> Organization: U. S. Naval Academy, Annapolis, MD Lines: 21 In article <1991May7.181010.22047@pixar.com> markv@pixar.com (Mark VandeWettering) writes: !!!! Will clipping a triangle to a rectangular region always result !!!! in a convex polygon (or no polygon at all) ? ! !!!Yes, because the intersection of two convex sets is convex. ! !!No, it depends on whether you do an INTERIOR or an EXTERIOR clip. !!If you do and EXTERIOR clip you can get a concave polygon. ! !Well, we generally do mean interior clips when we speak about clipping !to a rectangular region. And anyway, the exterior of a rectangle is !not a convex set, so the original statement is correct as stands. ! !mark No, not necessarily, a simple example is any windowing system. The intersection of two convex sets is convex IN THE CONTEXT OF A BOOLIAN OPERATION. In the context of `clipping' this is not true since clipping CAN be either interior or exterior. Dave Rogers