How can I find out Correlation between two image?

clc;

clear all;

close all;

a=imread('1.jpg');

b=imread('2.jpg');

corr2(a,b);

____________::Error Occured::_____________

Error using corr2

Error using corr2

Expected input number 1, A, to be two-dimensional.

Error in corr2>ParseInputs (line 35)

validateattributes(A, {'logical' 'numeric'}, {'real','2d'}, mfilename, 'A', 1);

Error in corr2 (line 21)

[a,b] = ParseInputs(varargin{:});

Error in featureExt (line 9)

corr2(a,b);

Answer 1

[Image Analyst]

They're probably color. Do it on one color channel at a time, or use rgb2gray() to convert them to grayscale. Untested code:

grayImage1 = rgb2gray(a);
grayImage2 = rgb2gray(b);
out = corr2(grayImage1, grayImage2);

[Syed Zenith Rhyhan]

Thanks a lot #Image Analyst

[Sampath Gamage]

@Image Analyst, I have a similar question. Assume the two images are 50x50 pixels each. Then does the correlation matrix have to be of the same size (ie. 50x50)? or is it a scaler (single number)?

[Image Analyst]

@Sampath Gamage what do you mean by the correlation matrix? If you use corr2() that basically says how much does each pixel's value move in tandem between the two images? It's a single scalar. If you use xcorr2() you will get an image that is the sum of the two image widths and it is the cross correlation of the two images.

help corr2

 CORR2 2-D correlation coefficient.
    R = CORR2(A,B) computes the correlation coefficient between A
    and B, where A and B are matrices or vectors of the same size.
 
    Class Support
    -------------
    A and B can be numeric or logical.
    R is a scalar double.
 
    Example
    -------
    I = imread('pout.tif');
    J = medfilt2(I);
    R = corr2(I,J)
 
    See also CORRCOEF, STD2.

    Documentation for corr2
       doc corr2

    Other uses of corr2

       gpuArray/corr2

help xcorr2

 XCORR2 Two-dimensional cross-correlation.
    XCORR2(A,B) computes the crosscorrelation of matrices A and B.
    XCORR2(A) is the autocorrelation function.
 
    % Example:
    %   Find the cross-correlation of two matrices
    a = [2 1 5; 3 1 3; 5 2 2];    
    b = [1 4 3; 2 5 6];
    xcorr2(a,b)
 
    See also CONV2, XCORR and FILTER2.

    Documentation for xcorr2
       doc xcorr2

[Sampath Gamage]

@Image Analyst, Thanks for the reply!

I think this is not just coding anymore. :) I'm trying to understand how to find the correlation of two images and how to present the result.

If we have two images of same size, (say 50x50 px) can we create a correlation map of 50x50 that gives the correlation of each pixel?

Or is the correlation between two images simply given by a single number?