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slalib/m2av.f

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+      SUBROUTINE sla_M2AV (RMAT, AXVEC)
+*+
+*     - - - - -
+*      M 2 A V
+*     - - - - -
+*
+*  From a rotation matrix, determine the corresponding axial vector
+*  (single precision)
+*
+*  A rotation matrix describes a rotation about some arbitrary axis,
+*  called the Euler axis.  The "axial vector" returned by this routine
+*  has the same direction as the Euler axis, and its magnitude is the
+*  amount of rotation in radians.  (The magnitude and direction can be
+*  separated by means of the routine sla_VN.)
+*
+*  Given:
+*    RMAT   r(3,3)   rotation matrix
+*
+*  Returned:
+*    AXVEC  r(3)     axial vector (radians)
+*
+*  The reference frame rotates clockwise as seen looking along
+*  the axial vector from the origin.
+*
+*  If RMAT is null, so is the result.
+*
+*  Last revision:   26 November 2005
+*
+*  Copyright P.T.Wallace.  All rights reserved.
+*
+*  License:
+*    This program is free software; you can redistribute it and/or modify
+*    it under the terms of the GNU General Public License as published by
+*    the Free Software Foundation; either version 2 of the License, or
+*    (at your option) any later version.
+*
+*    This program is distributed in the hope that it will be useful,
+*    but WITHOUT ANY WARRANTY; without even the implied warranty of
+*    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+*    GNU General Public License for more details.
+*
+*    You should have received a copy of the GNU General Public License
+*    along with this program (see SLA_CONDITIONS); if not, write to the 
+*    Free Software Foundation, Inc., 59 Temple Place, Suite 330, 
+*    Boston, MA  02111-1307  USA
+*
+*-
+
+      IMPLICIT NONE
+
+      REAL RMAT(3,3),AXVEC(3)
+
+      REAL X,Y,Z,S2,C2,PHI,F
+
+
+
+      X = RMAT(2,3)-RMAT(3,2)
+      Y = RMAT(3,1)-RMAT(1,3)
+      Z = RMAT(1,2)-RMAT(2,1)
+      S2 = SQRT(X*X+Y*Y+Z*Z)
+      IF (S2.NE.0.0) THEN
+         C2 = (RMAT(1,1)+RMAT(2,2)+RMAT(3,3)-1.0)
+         PHI = ATAN2(S2/2.0,C2/2.0)
+         F = PHI/S2
+         AXVEC(1) = X*F
+         AXVEC(2) = Y*F
+         AXVEC(3) = Z*F
+      ELSE
+         AXVEC(1) = 0.0
+         AXVEC(2) = 0.0
+         AXVEC(3) = 0.0
+      END IF
+
+      END