slaInvf -

SYNOPSYS

PARAMETERS

double *fwds

double *bkwds

int *j

DESCRIPTION

  Invert a linear model of the type produced by the slaFitxy routine.

Given

     fwds    double[6]      model coefficients

Returned

     bkwds   double[6]      inverse model
     *j      int            status:  0 = OK, -1 = no inverse

  The models relate two sets of [x,y] coordinates as follows.

Naming the elements of fwds

     fwds[0] = a
     fwds[1] = b
     fwds[2] = c
     fwds[3] = d
     fwds[4] = e
     fwds[5] = f

  Where two sets of coordinates [x1,y1] and [x2,y1] are related

thus

     x2 = a + b*x1 + c*y1
     y2 = d + e*x1 + f*y1

The present routine generates a new set of coefficients

     bkwds[0] = p
     bkwds[1] = q
     bkwds[2] = r
     bkwds[3] = s
     bkwds[4] = t
     bkwds[5] = u

Such that

     x1 = p + q*x2 + r*y2
     y1 = s + t*x2 + u*y2

  Two successive calls to slaInvf will thus deliver a set
  of coefficients equal to the starting values.

  See also slaFitxy, slaPxy, slaXy2xy, slaDcmpf

  P.T.Wallace   Starlink   30 October 1993

SEE ALSO

• slalib.3