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    Compute distorted coordinates given TNX (Tangent + Iraf tnx
    distortion polynomial) coefficients.


    See http://fits.gsfc.nasa.gov/registry/tnx.html for the TNX convention
    This distortion convention is used by IRAF.
    The coefficients and information are passed via common block. This is because this
    routine is called by the intrinisc BROYDEN() function in AD2XY, and
    common blocks are the only way to pass parameters to the user supplied
    function in BROYDEN().

Calling Sequence

    res = TNX_EVAL(xy)


    xy - 2 elements vector giving the undistorted X,Y position


    res - 2 element vector giving the distorted position

Common Blocks

      common broyden_coeff,pv1,pv2
      pv1, pv2 are both structures giving the TNX coefficients. The
      pv1/pv2 naming convention is a hangover from tpv_eval.pro on
      which this approach is heavily based.
      pv1.functype gives the TNX function type. Only type 3
        (polynominal) is supported.
      pv1.xterms gives the type of cross-terms (1: full, 2: half, 0: none)
      pv1.etaorder gives the order in eta
      pv1.xiorder gives the order in xi
      pv1.coeff gives the actual coefficients.

Revision History

    Written M. Sullivan Mar 2014

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