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106 lines
2.6 KiB
Fortran
106 lines
2.6 KiB
Fortran
SUBROUTINE sla_INVF (FWDS,BKWDS,J)
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*+
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* - - - - -
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* I N V F
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* - - - - -
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*
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* Invert a linear model of the type produced by the sla_FITXY routine.
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*
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* Given:
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* FWDS d(6) model coefficients
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*
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* Returned:
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* BKWDS d(6) inverse model
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* J i status: 0 = OK, -1 = no inverse
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*
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* The models relate two sets of [X,Y] coordinates as follows.
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* Naming the elements of FWDS:
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*
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* FWDS(1) = A
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* FWDS(2) = B
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* FWDS(3) = C
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* FWDS(4) = D
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* FWDS(5) = E
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* FWDS(6) = F
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*
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* where two sets of coordinates [X1,Y1] and [X2,Y1] are related
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* thus:
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*
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* X2 = A + B*X1 + C*Y1
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* Y2 = D + E*X1 + F*Y1
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*
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* the present routine generates a new set of coefficients:
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*
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* BKWDS(1) = P
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* BKWDS(2) = Q
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* BKWDS(3) = R
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* BKWDS(4) = S
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* BKWDS(5) = T
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* BKWDS(6) = U
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*
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* such that:
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*
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* X1 = P + Q*X2 + R*Y2
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* Y1 = S + T*X2 + U*Y2
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*
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* Two successive calls to sla_INVF will thus deliver a set
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* of coefficients equal to the starting values.
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*
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* To comply with the ANSI Fortran standard, FWDS and BKWDS must
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* not be the same array, even though the routine is coded to
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* work on many platforms even if this rule is violated.
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*
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* See also sla_FITXY, sla_PXY, sla_XY2XY, sla_DCMPF
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*
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* Last revision: 26 December 2004
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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 FWDS(6),BKWDS(6)
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INTEGER J
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DOUBLE PRECISION A,B,C,D,E,F,DET
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A=FWDS(1)
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B=FWDS(2)
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C=FWDS(3)
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D=FWDS(4)
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E=FWDS(5)
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F=FWDS(6)
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DET=B*F-C*E
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IF (DET.NE.0D0) THEN
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BKWDS(1)=(C*D-A*F)/DET
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BKWDS(2)=F/DET
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BKWDS(3)=-C/DET
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BKWDS(4)=(A*E-B*D)/DET
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BKWDS(5)=-E/DET
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BKWDS(6)=B/DET
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J=0
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ELSE
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J=-1
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END IF
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END
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