Index Page raxisa_c
A B C D E F G H I J K L M N O P Q R S T U V W X
Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries
#### Procedure
       void raxisa_c ( ConstSpiceDouble     matrix[3][3],
                       SpiceDouble          axis  [3],
                       SpiceDouble        * angle       )
#### Abstract
       Compute the axis of the rotation given by an input matrix
       and the angle of the rotation about that axis.
#### Required_Reading
       ROTATION
#### Keywords
       ANGLE,  MATRIX,  ROTATION
#### Brief_I/O
       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       matrix     I   3x3 rotation matrix in double precision.
       axis       O   Axis of the rotation.
       angle      O   Angle through which the rotation is performed.
#### Detailed_Input
       matrix     is a 3x3 rotation matrix in double precision.
#### Detailed_Output
       axis       is a unit vector pointing along the axis of the rotation.
                  In other words, `axis' is a unit eigenvector of the input
                  matrix, corresponding to the eigenvalue 1. If the input
                  matrix is the identity matrix, `axis' will be the vector
                  (0, 0, 1). If the input rotation is a rotation by pi
                  radians, both `axis' and -axis may be regarded as the
                  axis of the rotation.

       angle      is the angle between `v' and matrix*v for any non-zero
                  vector `v' orthogonal to `axis'.  `angle' is given in
                  radians.  The angle returned will be in the range from 0
                  to pi radians.
#### Parameters
       None.
#### Exceptions
       1) If the input matrix is not a rotation matrix (a fairly
          loose tolerance is used to check this) a routine in the
          call tree of this routine will signal an error indicating
          the problem.

       2) If the input matrix is the identity matrix, this routine
          returns an angle of 0.0, and an axis of ( 0.0, 0.0, 1.0 ).
#### Files
       None.
#### Particulars
       Every rotation matrix has an axis `a' such any vector `v'
       parallel to that axis satisfies the equation

          v = matrix * v

       This routine returns a unit vector `axis' parallel to the axis of
       the input rotation matrix.  Moreover for any vector `w' orthogonal
       to the axis of the rotation, the two vectors

           axis,
           w x (matrix*w)

          (where "x" denotes the cross product operation)

       will be positive scalar multiples of one another (at least
       to within the ability to make such computations with double
       precision arithmetic, and under the assumption that `matrix'
       does not represent a rotation by zero or pi radians).

       The angle returned will be the angle between `w' and matrix*w
       for any vector orthogonal to `axis'.

       If the input matrix is a rotation by 0 or pi radians some
       choice must be made for the axis returned.  In the case of
       a rotation by 0 radians, `axis' is along the positive z-axis.
       In the case of a rotation by 180 degrees, two choices are
       possible.  The choice made this routine is unspecified.
#### Examples
        This routine can be used to numerically approximate the
        instantaneous angular velocity vector of a rotating object.

        Suppose that r(t) is the rotation matrix whose columns
        represent the inertial pointing vectors of the bodyfixed
        axes of an object at time t.

        Then the angular velocity vector points along the vector
        given by:
                               T
           limit  axis( r(t+h)r )
           h-->0

        And the magnitude of the angular velocity at time t is given by:

                               T
           d angle ( r(t+h)r(t) )
           ----------------------   at   h = 0
           dh

        Thus to approximate the angular velocity vector the following
        code fragment will do

           [ Load t      into the double precision variable t
             Load h      into the double precision variable h
             Load r(t+h) into the 3 by 3 double precision array rth
             Load r(t)   into the 3 by 3 double precision array rt
              .
              .
              .
           ]

           /.
                                                        T
           Compute the infinitesimal rotation r(t+h)r(t)
           ./
           mxmt_c ( rth, rt, infrot );

           /.
           Compute the axis and angle of the infinitesimal rotation.
           /.
           raxisa_c ( infrot, axis, &angle );

           /.
           Scale axis to get the angular velocity vector.
           ./
           vscl_c ( angle/h, axis, angvel );
#### Restrictions
        1) If the input matrix is not a rotation matrix but is close enough
           to pass the tests this routine performs on it, no error will be
           signaled, but the results may have poor accuracy.

        2) The input matrix is taken to be an object that acts on (rotates)
           vectors---it is not regarded as a coordinate transformation.  To
           find the axis and angle of a coordinate transformation, input
           the transpose of that matrix to this routine.
#### Literature_References
        None.
#### Author_and_Institution
        N.J. Bachman    (JPL)
        W.L. Taber      (JPL)
        F.S. Turner     (JPL)
#### Version
       -CSPICE Version 1.0.1, 05-JAN-2005 (NJB) (WLT) (FST)

           Various header updates were made to reflect changes
           made to the underlying SPICELIB Fortran code.
           Miscellaneous header corrections were made as well.

       -CSPICE Version 1.0.0, 31-MAY-1999 (WLT) (NJB)
#### Index_Entries
       rotation axis of a matrix
#### Link to routine raxisa_c source file raxisa_c.c

    Wed Jun  9 13:05:28 2010