FFTRANS
By Vinzenz Unger and Anchi Cheng
This program calculates Fourier transforms in either direction (i.e. image –> transform, transform –>image). For a forward transform the real space (image) origin (1,1) is shifted to (1, NY/2+1) and vice versa. The Fourier transform of an image with dimensions NX,NY is (NX/2+1), NY complex values. Each transform pixel therefore contains two parts of information representing the amplitude and phase of the component. Only the in- and output files need to be assigned (setenv IN <inputfile>, setenv OUT <outputfile>) to run the program. A brief description on how the FFT (Fast Fourier Transform) algorithm works can be found in the CCP4 package brochure. The FFT algorithm places certain constraints on the dimensions of an input array, i.e. it is necessary that the size of the image can be represented by a product of numbers that must not involve prime numbers larger than 19, otherwise the program will fail. For instance, arrays of 760*760 pixel (2*4*5*19), 770*770 (2*5*7*11), 780×780 (2*2*3*5*13) will work while an area of 790×790 pixel (2*5*79) cannot be transformed using the FFT algorithm because 79 is a prime number larger than 19.