Correlation is a method of comparing two sets of data. By definition, a two-dimensional correlation between two real objects, an input, s(x,y), and a reference, r(x,y), is defined as the Fourier Transform of the product of the two original function, which have themselves been Fourier transformed:

The Fourier Transform is one of the fundamental building blocks of frequency analysis and signal processing. It is the decomposition of a signal or image into its constituent frequency parts. The Fast Fourier Transform (FFT) algorithm and its many variants have equipped electronic processors with the tools to perform 1-D Fourier transforms at speed, largely responsible for the field of Digital Signal Processing.

However, 2-D data processing using the same techniques is highly processor intensive and process time is badly affected by increases in resolution. The Fourier Transform is by nature a parallel process, where each element of the transform is derived from a calculation involving every element in the input signal. By contrast, electronic systems are by their nature serial and therefore less well suited to processes involving large Fourier Transforms.