From: <HIDDEN>
Newsgroups: comp.soft-sys.matlab
Subject: Re: plane equation with a radius
Date: Mon, 15 Mar 2010 18:38:05 +0000 (UTC)
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"Junghyun " <> wrote in message <hnl3gl$2lm$>...
> Hello,
> I need to find the  plane equation if 2 eigenvectors at a given point in a 3D space and a radius are given.
> How can I formulate it in Matlab?
> Thanks,
> Junghyun

  Your question is not at all clear to me Junghyun and I can only guess at what you mean.  My speculation is that you wish to define the circle which lies in a plane parallel to two given vectors a and b, and contains a given point c, with the circle's center positioned at this c and with its radius equal to a given value r.  If this is correct, do the following.

  If a and b are unit vectors and orthogonal, this circle can be expressed as follows:

 p = c + r*(cos(t)*a + sin(t)*b); % Point p traces out the circle in 3D

  More generally if a and b are any two non-parallel vectors, the circle can be expressed as:

 u = a/norm(a); % u is normalization of vector a
 v = cross(cross(a,b),a); % v is parallel to the plane and orthog. to u
 v = v/norm(v); % Normalize vector v
 p = c + r*(cos(t)*u + sin(t)*v); % Point p traces out the circle in 3D

  In either case, as parameter t varies from 0 to 2*pi, p starts at a, rotates past b, and on around again in a full circle to a at t = 2*pi.  

  The assumption is made that a and b are not parallel.  Otherwise the circle is not uniquely determined.

Roger Stafford