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DOUBLE PRECISION FUNCTION slDSEPV (V1, V2)
*+
* - - - - - -
* D S E P V
* - - - - - -
*
* Angle between two vectors.
*
* (double precision)
*
* Given:
* V1 d(3) first vector
* V2 d(3) second vector
*
* The result is the angle, in radians, between the two vectors. It
* is always positive.
*
* Notes:
*
* 1 There is no requirement for the vectors to be unit length.
*
* 2 If either vector is null, zero is returned.
*
* 3 The simplest formulation would use dot product alone. However,
* this would reduce the accuracy for angles near zero and pi. The
* algorithm uses both cross product and dot product, which maintains
* accuracy for all sizes of angle.
*
* Called: slDVXV, slDVN, slDVDV
*
* Last revision: 14 June 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., 51 Franklin Street, Fifth Floor,
* Boston, MA 02110-1301 USA
*
* Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
*-
IMPLICIT NONE
DOUBLE PRECISION V1(3),V2(3)
DOUBLE PRECISION V1XV2(3),WV(3),S,C
DOUBLE PRECISION slDVDV
* Modulus of cross product = sine multiplied by the two moduli.
CALL slDVXV(V1,V2,V1XV2)
CALL slDVN(V1XV2,WV,S)
* Dot product = cosine multiplied by the two moduli.
C = slDVDV(V1,V2)
* Angle between the vectors.
IF ( S.NE.0D0 .OR. C.NE.0D0 ) THEN
slDSEPV = ATAN2(S,C)
ELSE
slDSEPV = 0D0
END IF
END
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