Today I found a program, which does fourier transforms on pictures and tried it on some basic patterns. One of those was a lattice of dots and I have attached this and its fourier transform to the thread. I would very much like if someone in basic details could explain what is going on. Why the fourier transform of the picture looks as it does. Mathematically I understand what there is to know about the fourier transform and how we represent a function in its frequency domain by writing it as a superposition of sinusoidal functions with different frequencies. In terms of the mathematical idea behind the FT what is it then, that I observe on the picture? What is my original function (the brightness?) and what does the frequency represent? Please don't hesitate to write a long answer.