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,y2] are related thus:

  x2 = A $+$ B $\ast$ x1 $+$ C $\ast$ y1
  y2 = D $+$ E $\ast$ x1 $+$ F $\ast$ 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 $\ast$ x2 $+$ R $\ast$ y2
  y1 = S $+$ T $\ast$ x2 $+$ U $\ast$ y2

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

palInvf ( double fwds[6], double bkwds[6], int $\ast$j )

fwds = double[6] (Given)

bkwds = double[6] (Returned)

j = int (Returned)

palFitxy, palPxy, palXy2xy and palDcmpf