Fourier Transform, Optical Computing, Free Space Optics.
perform again the FT of each sub-matrix… … divide by nx… … and do again the permutation of the first step: The four lines of the resulting matrix, ​(4,0,0,0), ​(3,-i,1,i)​, ​​(2,1-i,0,1+i), and ​(1,1,1,1) are the FTs of the original ones ​(1,1,1,1), (1,1,1,0)​, (1,1,0,0)​, and ​(1,0,0,0). Of