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From: "GAURAV " <gsharda@engineering.uiowa.edu>
Newsgroups: comp.soft-sys.matlab
Subject: Re: Cut a plane through 3D surface
Date: Tue, 23 Nov 2010 06:44:04 +0000 (UTC)
Organization: University of Iowa
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Thanks a lot for your response. Appreciate it. :)
Walter Roberson <roberson@hushmail.com> wrote in message <8dIGo.30961$E64.12340@newsfe11.iad>...
> On 22/11/10 10:47 PM, GAURAV wrote:
> 
> > Suppose I have a bunch of 3D points. I want to cut a plane which passes
> > through 4 coplanar points.
> 
> You only need 3 coplanar points to establish a plane. If you are given 4 
> points instead, then you have to decide whether to _assume_ they are 
> co-planar or whether to _check_ whether they are co-planar and decide 
> what to do if they are not (or suppose they are very very close and it 
> might plausibly be round-off error?)
> 
> > I want to get rid of the points below the
> > plane and get the 3D surface above the plane. How can I do that?
> 
> There is more than one way to do that.
> 
> - once you have the plane, you can rotate all of the points so that the 
> plane aligns with the coordinate axes; the points "below" the plane 
> would all be on one side of the axes and the points "above" the plane 
> would all be on the other side.
> 
> - there are calculations you can do using dot products to determine the 
> angles. I remember how do this for 2D but I have forgotten at the moment 
> how to do it for 3D.
> 
> I'm sure you could do a google search on how to decide which side of a 
> plane a point is on.