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- Newsgroups: comp.os.msdos.programmer
- Path: sparky!uunet!orca!javelin.sim.es.com!gclapp
- From: gclapp@javelin.sim.es.com (Glenn Clapp)
- Subject: Re: Polygon Question...
- Message-ID: <1992Jul21.221038.7051@javelin.sim.es.com>
- Organization: Evans & Sutherland Computer Corporation
- References: <1992Jul21.094844.1@camins.camosun.bc.ca>
- Date: Tue, 21 Jul 1992 22:10:38 GMT
- Lines: 25
-
- In article <1992Jul21.094844.1@camins.camosun.bc.ca> morley@camins.camosun.bc.ca writes:
- >Hi!
- >
- >Ok, this should be an easy one... Given 3 X,Y points that form a triangle,
- >what is the simplest (fastest) way to determine if they are clock-wise or
- >counter-clock-wise? I'm not interested in general polygons of N points,
- >just plain old triangles. Yes, this is for use in a graphics app.
- >
- >Thanks...
- >
- >MARK
- >morley@camins.camosun.bc.ca
- >
-
- Traverse the polygon, calculating 3 cross products (v1->v2 X v2->v3,
- v2->v3 X v3->v1), and (v3->v1 X v1->v2), and if the cross products are
- positive, the polygon is counterclockwise, otherwise they will be negetive.
- Also, if you have a sign flip between the different cross products, then
- the polygon is not convex.
-
- --
- | The real democratic American idea is not that everyone shall be on a |
- | level with every other, but that everyone shall have liberty, without |
- | hinderances, to be what God made him. -- Henry Ward Bleecher |
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