correlation between two images

i need to find the corrolation between 2 greyscale images and see if the signals are similar

RGB = imread('tree.jpg');%read first image
imshow(RGB)

RGB2 = imread('white horse.jpg');%read second image
imshow(RGB2)

I1 = rgb2gray(RGB);% we convert images to greyscale 
figure
imshow(I1)

I2 = rgb2gray(RGB2);% we convert images to greyscale
figure
imshow(I2)

subplot(2, 1, 2);
F=fft2(I1);
S1=fftshift(log(1+abs(F)));% we convert first image to frequency domain
imshow(S,[]);
%we found the centred specturm and applied the log transform in the same
%step
%this could also be done in 2 steps as follows:
%F1 = fft(I1);
% centered spectrum
%S= fftshift(F!);
% log transform
%S1 = log(1+abs(S));

subplot(2, 1, 2);
F2=fft2(I2);
S2=fftshift(log(1+abs(F2)));%convert second image to frequency domain
imshow(S,[]); 

% now we check the lengh of the signals and we compare them
len=length(I1);
len2 = length(I2);
if len>len2
    S4=resample(S1,len1,len2);
end
if len<len2
    S5=resample(S2,len1,len2);
end
% we make the images of the same size to be able to compare them using
% correlation


[acorrX,lagX]=xcorr(S4,S5);
figure;
plot(lagX,corrX);title('Correlation between 2 signals');
figure;

plot(lagX2,acorrX2);title('Correlation between signal and itself');
p=abs(acorrX-acorrX2);

matlab keeps telling me variable s4 not recognised, does anyone know the problem?

Answer 1

[Image Analyst]

You never assigned S4 because you never went into the if block, which means len is not greater than len2.

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