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77 lines
2.1 KiB
Fortran
77 lines
2.1 KiB
Fortran
DOUBLE PRECISION FUNCTION sla_DSEPV (V1, V2)
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*+
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* - - - - - -
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* D S E P V
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* - - - - - -
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*
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* Angle between two vectors.
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*
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* (double precision)
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*
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* Given:
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* V1 d(3) first vector
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* V2 d(3) second vector
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*
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* The result is the angle, in radians, between the two vectors. It
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* is always positive.
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*
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* Notes:
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*
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* 1 There is no requirement for the vectors to be unit length.
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*
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* 2 If either vector is null, zero is returned.
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*
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* 3 The simplest formulation would use dot product alone. However,
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* this would reduce the accuracy for angles near zero and pi. The
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* algorithm uses both cross product and dot product, which maintains
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* accuracy for all sizes of angle.
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*
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* Called: sla_DVXV, sla_DVN, sla_DVDV
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*
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* Last revision: 14 June 2005
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*
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* Copyright P.T.Wallace. All rights reserved.
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*
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* License:
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program (see SLA_CONDITIONS); if not, write to the
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* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
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* Boston, MA 02111-1307 USA
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*
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*-
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IMPLICIT NONE
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DOUBLE PRECISION V1(3),V2(3)
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DOUBLE PRECISION V1XV2(3),WV(3),S,C
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DOUBLE PRECISION sla_DVDV
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* Modulus of cross product = sine multiplied by the two moduli.
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CALL sla_DVXV(V1,V2,V1XV2)
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CALL sla_DVN(V1XV2,WV,S)
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* Dot product = cosine multiplied by the two moduli.
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C = sla_DVDV(V1,V2)
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* Angle between the vectors.
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IF ( S.NE.0D0 .OR. C.NE.0D0 ) THEN
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sla_DSEPV = ATAN2(S,C)
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ELSE
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sla_DSEPV = 0D0
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END IF
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END
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