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Re: some geometry questions. [message #48174 is a reply to message #48170]

Wed, 29 March 2006 15:56

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

qian writes:

> If I have 4 random points (x0,y0) (x1,y1), (x3,y3), (x4,y4), is there a
> simple way to decide whether one of them is inside of the triangle
> formed by the other three points?
>
> If none of them is in the triangle by others, how can I connect them in
> order to form a 4 edges polygon, instead of two head on triangles, when
> using order 1-2-3-4-1?
>
> like this:
> 1----2
> \ \
> 4\__\3
>
> not like this:
> 1-----2
> \ /
> /\
> / \
> 3-----4

You are looking for a "complex hull algorithm", such as this
one:

http://nms.csail.mit.edu/~aklmiu/6.838/convexhull/index.html

In IDL you can find the convex hull of a set of points
with the TRIANGULATE command:

http://www.dfanning.com/tips/convex_hull.html

Cheers,

David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/

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[Message index]

		Re: some geometry questions. By: Craig Markwardt on Fri, 31 March 2006 08:50
		Re: some geometry questions. By: Karl[1] on Wed, 29 March 2006 21:29
		Re: some geometry questions. By: David Fanning on Wed, 29 March 2006 15:56

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