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The cross correlation between two images can be evaluated by cross-correlation function (XCF) or cross-correlation coefficient (XCC).

The Cross Correlation Function (XCF) between any two images C_x(r) and C_y(r)  can be defined in terms of their Fourier transforms, given by,

           --------------------- [3717a]

The Cross Correlation Coefficient between the two images can be defined as,
           --------------------- [3717b]
where,
           and -- The mean values of the image I_A and I_B, respectively.
          x and y -- The coordinates in the two images (e.g. the coordinates of a pixel if the images are in digital format).

From Equation 3717b, we can know the coefficient XCC(A,B) is equal to 1 if the two images are absolutely identical. In the case of which Gatan DM is used, XCC(A,B) based on Equation 3717b can be computed by scripts.