TWOFILE
By Vinzenz Unger and Anchi Cheng
This is a general utility program to add or multiply/divide two (complex) images. As for the autocorrelation the cross-correlation between two images can be most easily calculated in Fourier space because the Fourier transform of the cross-correlation is simply the product of the transform (A) of one image and the complex conjugate transform (B*) of the other image. However, the order of the two transforms is not arbitrary in this multiplication. To obtain the Fourier transform of the cross-correlation that defines the lattice distortions in the image with respect to a given reference area one needs to multiply the loosely masked transform of the image with the complex conjugate of the reference transform obtained from the tightly filtered image. Furthermore, the origin of the reference-transform needs to be shifted to the location of the reference area to give a meaningful result. The program input cards are all preset and only the centre-coordinates of the reference need to be changed in the header section of the protocol.