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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
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