CORREL_IMAGES

       Compute the 2-D cross-correlation function of two images

       Computes the 2-D cross-correlation function of two images for
       a range of (x,y) shifting by pixels of one image relative to the other.

       Result = CORREL_IMAGES( image_A, image_B,
                        [XSHIFT=, YSHIFT=, XOFFSET_B=, YOFFSET_B=, REDUCTION=,
                        MAGNIFICATION=, /NUMPIX, /MONITOR  )

       image_A, image_B = the two images of interest.

       XSHIFT = the + & - shift to be applied in X direction, default=7.
       YSHIFT = the Y direction + & - shifting, default=7.
       XOFFSET_B = initial X pixel offset of image_B relative to image_A.
       YOFFSET_B = Y pixel offset, defaults are (0,0).
       REDUCTION = optional reduction factor causes computation of
                       Low resolution correlation of bin averaged images,
                       thus faster. Can be used to get approximate optimal
                       (x,y) offset of images, and then called for successive
                       lower reductions in conjunction with CorrMat_Analyze
                       until REDUCTION=1, getting offset up to single pixel.
       MAGNIFICATION = option causes computation of high resolution correlation
                       of magnified images, thus much slower.
                       Shifting distance is automatically = 2 + Magnification,
                       and optimal pixel offset should be known and specified.
                       Optimal offset can then be found to fractional pixels
                       using CorrMat_Analyze( correl_images( ) ).
       /NUMPIX - if set, causes the number of pixels for each correlation
                       to be saved in a second image, concatenated to the
                       correlation image, so Result is fltarr( Nx, Ny, 2 ).
       /MONITOR causes the progress of computation to be briefly printed.

       Result is the cross-correlation function, given as a matrix.

       Loop over all possible (x,y) shifts, compute overlap and correlation
       for each shift. Correlation set to zero when there is no overlap.

       Written, July,1991, Frank Varosi, STX @ NASA/GSFC
       Use ROUND instead of NINT, June 1995, Wayne Landsman HSTX
       Avoid divide by zero errors, W. Landsman HSTX April 1996
       Remove use of !DEBUG    W. Landsman   June 1997
       Subtract mean of entire image before computing correlation, not just
          mean of overlap region   H. Ebeling/W. Landsman   June 1998

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