xcorr2_fft(a,b)

xcorr2_fft(A,B) computes the cross-correlation of matrices A and B using the FFT algorithm.
When matrices A and B are real, xcorr2_fft is numerically equivalent to xcorr2, but much faster.
For big matrices, the speed improvement can be up to 100 times!
The correlation is simply evaluated padding matrices a and b with zeros according to the size of the correlation map. In this way, the periodicity of the circular cross-correlation evaluated with the FFT becomes equivalent to the linear cross-correlation evaluated with conv2.
% Example:
a = rand(122); b=rand(332);
a = a-mean(a(:));
b = b-mean(b(:));

tic,cl = xcorr2(a,b);toc
Elapsed time is 0.223502 seconds.
tic,cf = xcorr2_fft(a,b);toc
Elapsed time is 0.030935 seconds.

max(abs(cf(:)-cl(:)))
ans = 4.1922e-13