Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!ccu.umanitoba.ca!ccu.umanitoba.ca!lambert From: lambert@silver.cs.umanitoba.ca (Tim Lambert) Newsgroups: comp.graphics Subject: Re: polygon clipping Message-ID: Date: 9 May 91 16:20:34 GMT References: <348@usna.NAVY.MIL> <1991May7.181010.22047@pixar.com> <382@usna.NAVY.MIL> Sender: news@ccu.umanitoba.ca Organization: Department of Computer Science, University of Manitoba Lines: 35 In-Reply-To: dfr@usna.NAVY.MIL's message of 8 May 91 13:34:12 GMT AN = neiderer@amsaa-seer.brl.mil (Andrew Neiderer) TL = lambert@cs.umanitoba.ca (Tim Lambert) DR = dfr@usna.NAVY.MIL (Prof. David F. Rogers) MV = markv@pixar.com (Mark VandeWettering) AN> Will clipping a triangle to a rectangular region always result AN> a convex polygon (or no polygon at all) ? TL> Yes, because the intersection of two convex sets is convex. DR> No, it depends on whether you do an INTERIOR or an EXTERIOR clip. DR> If you do and EXTERIOR clip you can get a concave polygon. MV> Well, we generally do mean interior clips when we speak about clipping MV> to a rectangular region. And anyway, the exterior of a rectangle is MV> not a convex set, so the original statement is correct as stands. DR> No, not necessarily, a simple example is any windowing system. OK. Consider X. Clipping regions are lists of (non-intersecting) rectangles. No exterior clipping. (Of course, when you have one window partly obscuring another, you are really doing an exterior clip against the obscuring window, but that is not the way X looks at it.) I understand "clipping" to mean interior clipping and "exterior clipping to R" to mean clipping to the exterior of R. DR> The intersection of two convex sets is convex IN THE CONTEXT OF DR> A BOOLIAN OPERATION. In the context of `clipping' this is not DR> true since clipping CAN be either interior or exterior. I don't understand. Are you saying that in the context of clipping `intersection' sometimes means `difference'? Tim