Comparison of Matlab/Octave conv2() with FFT multiplication

fft_mult_vs_conv2.m

a = rand(7);
b = rand(7);

% -------------------------------------------------------------------------

tic();

% CONV
c1p = conv2(a,b);

% UNPAD
c1 = c1p(1:round(size(c1p,1)/2), 1:round(size(c1p,2)/2));

toc()

% -------------------------------------------------------------------------

tic();

% PAD
ap = zeros(size(a)*2);
ap(1:size(a,1), 1:size(a,2)) = a;
bp = zeros(size(b)*2);
bp(1:size(b,1), 1:size(b,2)) = b;

% FFT MULT
apf = fft2(ap);
bpf = fft2(bp);
c2f = apf .* bpf;
c2p = ifft2(c2f);

% UNPAD
c2 = c2p(1:round(size(c2p,1)/2), 1:round(size(c2p,2)/2));

toc()

% -------------------------------------------------------------------------

% Observations:
% - the results match (c1 == c2)
% - time with fft_mult is consistently faster than using conv2.