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Re: Is a point in a 4 vertex polygon? [message #14356 is a reply to message #14295]

Wed, 17 February 1999 00:00

David Kastrup
Messages: 33
Registered: February 1998

Member

Nando Iavarone <f.iavarone@acsys.it> writes:

> Thanks all.
> I found another one algorithm.
> It uses the vectorial product properties to check
> if a point is above or below a line.

Presming that you just do the croos product (which is just a
determinant, really) of vector a-M and b-a to check whether M lies to
the left of a-b, you should be aware that this algorithm, though fast,
works only with convex polygons.

If you draw a quadrilateral that is like an arrow tip, or like a delta
plane (namely two sharp triangles pointing backwards), then a point in
one of the tips will fail the criterion.

--
David Kastrup Phone: +49-234-700-5570
Email: dak@neuroinformatik.ruhr-uni-bochum.de Fax: +49-234-709-4209
Institut fï¿½r Neuroinformatik, Universitï¿½tsstr. 150, 44780 Bochum, Germany

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[Message index]

		Is a point in a 4 vertex polygon? By: Nando Iavarone on Fri, 12 February 1999 00:00
		Re: Is a point in a 4 vertex polygon? By: David Kastrup on Wed, 17 February 1999 00:00
		Re: Is a point in a 4 vertex polygon? By: wmc on Wed, 17 February 1999 00:00
		Re: Is a point in a 4 vertex polygon? By: Nando Iavarone on Wed, 17 February 1999 00:00
		Re: Is a point in a 4 vertex polygon? By: David Foster on Tue, 16 February 1999 00:00

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