Fourier Transform, Optical Computing, Free Space Optics.
2-dimensional FTs of size 2 by 2—exactly what a (very small) optical device could do! It is easy to verify that it also works for larger arrays. For instance, changing the values of nx and ny to 3, we can compute the FTs of the arrays ​(1,1,1,1,1,1,1,1,1), (2,2,2,2,2,2,2,2,2)​, ...,