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      Converts X, Y position on the image display to the the X,Y position
      on the corresponding data array. (These positions are identical
      only for an unroamed, unzoomed image with with pixel [0,0] of the
      image placed at position [0,0] on the image display.)

Calling Sequence

      UNZoom_XY, Xtv,Ytv,Xim,Yim, [ OFFSET =, ZOOM = ]


      XTV - Scalar or vector giving X position(s) as read on the image
            display (e.g. with Cursor, Xtv, Ytv,/DEVICE)
      YTV - Scalar or vector giving Y position(s) on the image display.
      If only 2 parameters are supplied then XTV and YTV will be modified
      on output to contain the image array coordinates.

Optional Keyword Input

      OFFSET - 2 element vector giving the location of the image pixel [0,0]
              on the window display. OFFSET can be positive (e.g if the
              image is centered in a larger window) or negative (e.g. if the
              only the central region of an image much larger than the window
              is being displayed.
              Default value is [0,0], or no offset.
      ZOOM - scalar giving the ratio of the size on the image display to the
            original data size. There is no capability for separate X
            and Y zoom. Default = 1.


      XIM,YIM - X and Y coordinates of the image corresponding to the
            cursor position on the image display.

Common Blocks

      If present, ZOOM_XY will use the TV and IMAGE common blocks which are
      defined in the MOUSSE software system (see
        http://archive.stsci.edu/uit/analysis.html) If the user is not using
      the MOUSSE software (which keeps track of the offset and zoom in each
      window) then the common blocks are ignored.


      The integer value of a pixel is assumed to refer to the *center*
      of a pixel.

Revison History

      Adapted from MOUSSE procedure W. Landsman March 1996
      Proper handling of offset option S. Ott/W. Landsman May 2000
      Put back common blocks for MOUSSE compatibility September 2004
      Fix algorithm for non-unity ZOOM values Aug. 2013

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