screw

 Calculation of the screw axis
 function [n,point,phi,t]=screw(T)
 Input:    T   matrix containing the rotation matrix and transl. vector
               [R;[t1,t2,t3]'; 0 0 0 1]
        intersect    location of the screw axis where it intersects either the x=0 (intersect=1),
                       the y=0 (intersect=2), or the z=0 (intersect=3) plane.
            default: intersect=3
 Output:   n       unit vector with direction of helical axis
           point   point on helical axis
           phi     rotation angle (in deg)
           t       amount of translation along screw axis

 Comments:     Note that phi is b/w 0 and 180 deg. Right handed screw
               axis system. The "sign" of phi can be checked with direction
               of the unit vector (n).
 References:   (1) Spoor and Veldpaus (1980) Rigid body motion calculated
                   from spatial co-ordinates of markers. 
                   J Biomech 13: 391-393
               (2) Berme, Cappozzo, and Meglan. Rigid body mechanics
                   as applied to human movement studies. In Berme and 
                   Cappozzo: Biomechanics of human movement.

 Author:     Christoph Reinschmidt, HPL, UofCalgary
 Date:          Oct. 03, 1994
 Last Changes:  Nov-20-96