Hi Phil
The rotation axis is the locus of points which the transformation leaves
unmoved, i.e. the eigenvector of the transformation matrix which has a
unit eigenvalue. So writing the transformation in homogeneous form for
convenience: x' = Sx you need to solve x' = x, or Sx = x, either
analytically or just plug the matrix S into a canned eigenvector
routine.
Cheers
-- Ian
> -----Original Message-----
> From: [log in to unmask]
> [mailto:[log in to unmask]] On Behalf Of Phil Evans
> Sent: 29 July 2008 09:11
> To: [log in to unmask]
> Subject: Rotation axis
>
> If I've go a superposition transformation (x' = Rx + t), as
> it happens
> from a superposition in ccp4mg, how do I get the position &
> direction
> of the rotation axis (to draw in a picture)?
> I know that any (orthonormal) transformation can be represented as a
> rotation about an axis + a screw translation along that axis
>
> I'm sure I've done this before ...
>
> thanks
> Phil
>
>
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