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SUBROUTINE sla_UNPCD ( DISCO, X, Y )
*+
* - - - - - -
* U N P C D
* - - - - - -
*
* Remove pincushion/barrel distortion from a distorted [x,y] to give
* tangent-plane [x,y].
*
* Given:
* DISCO d pincushion/barrel distortion coefficient
* X,Y d distorted coordinates
*
* Returned:
* X,Y d tangent-plane coordinates
*
* Notes:
*
* 1) The distortion is of the form RP = R*(1+C*R^2), where R is
* the radial distance from the tangent point, C is the DISCO
* argument, and RP is the radial distance in the presence of
* the distortion.
*
* 2) For pincushion distortion, C is +ve; for barrel distortion,
* C is -ve.
*
* 3) For X,Y in "radians" - units of one projection radius,
* which in the case of a photograph is the focal length of
* the camera - the following DISCO values apply:
*
* Geometry DISCO
*
* astrograph 0.0
* Schmidt -0.3333
* AAT PF doublet +147.069
* AAT PF triplet +178.585
* AAT f/8 +21.20
* JKT f/8 +13.32
*
* 4) The present routine is a rigorous inverse of the companion
* routine sla_PCD. The expression for RP in Note 1 is rewritten
* in the form x^3+a*x+b=0 and solved by standard techniques.
*
* 5) Cases where the cubic has multiple real roots can sometimes
* occur, corresponding to extreme instances of barrel distortion
* where up to three different undistorted [X,Y]s all produce the
* same distorted [X,Y]. However, only one solution is returned,
* the one that produces the smallest change in [X,Y].
*
* P.T.Wallace Starlink 3 September 2000
*
* Copyright (C) 2000 Rutherford Appleton Laboratory
*
* 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., 59 Temple Place, Suite 330,
* Boston, MA 02111-1307 USA
*
*-
IMPLICIT NONE
DOUBLE PRECISION DISCO,X,Y
DOUBLE PRECISION THIRD
PARAMETER (THIRD=1D0/3D0)
DOUBLE PRECISION D2PI
PARAMETER (D2PI=6.283185307179586476925286766559D0)
DOUBLE PRECISION RP,Q,R,D,W,S,T,F,C,T3,F1,F2,F3,W1,W2,W3
* Distance of the point from the origin.
RP = SQRT(X*X+Y*Y)
* If zero, or if no distortion, no action is necessary.
IF (RP.NE.0D0.AND.DISCO.NE.0D0) THEN
* Begin algebraic solution.
Q = 1D0/(3D0*DISCO)
R = RP/(2D0*DISCO)
W = Q*Q*Q+R*R
* Continue if one real root, or three of which only one is positive.
IF (W.GE.0D0) THEN
D = SQRT(W)
W = R+D
S = SIGN(ABS(W)**THIRD,W)
W = R-D
T = SIGN((ABS(W))**THIRD,W)
F = S+T
ELSE
* Three different real roots: use geometrical method instead.
W = 2D0/SQRT(-3D0*DISCO)
C = 4D0*RP/(DISCO*W*W*W)
S = SQRT(1D0-MIN(C*C,1D0))
T3 = ATAN2(S,C)
* The three solutions.
F1 = W*COS((D2PI-T3)/3D0)
F2 = W*COS((T3)/3D0)
F3 = W*COS((D2PI+T3)/3D0)
* Pick the one that moves [X,Y] least.
W1 = ABS(F1-RP)
W2 = ABS(F2-RP)
W3 = ABS(F3-RP)
IF (W1.LT.W2) THEN
IF (W1.LT.W3) THEN
F = F1
ELSE
F = F3
END IF
ELSE
IF (W2.LT.W3) THEN
F = F2
ELSE
F = F3
END IF
END IF
END IF
* Remove the distortion.
F = F/RP
X = F*X
Y = F*Y
END IF
END
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