how to use conditional entropy thresholding on 2D gray scal image?

Hi, I want to segment an image with spatial-Pal automatic thresholding method "conditional entropy", I found this code but I confused about the inputs, I suppose that "x" mean the input image, but I couldn't find what "y" is?

function z = conditionalEntropy (x, y)
% Compute conditional entropy H(x|y) of two discrete variables x and y.
% Written by Mo Chen ([e-mail]).

    assert(numel(x) == numel(y));
    n = numel(x);
    x = reshape(x,1,n);
    y = reshape(y,1,n);

    l = min(min(x),min(y));
    x = x-l+1;
    y = y-l+1;
    k = max(max(x),max(y));

    idx = 1:n;
    Mx = sparse(idx,x,1,n,k,n);
    My = sparse(idx,y,1,n,k,n);
    Pxy = nonzeros(Mx'*My/n); %joint distribution of x and y
    Hxy = -dot(Pxy,log2(Pxy+eps));

    Py = mean(My,1);
    Hy = -dot(Py,log2(Py+eps));

    % conditional entropy H(x|y)
    z = Hxy-Hy;
end

thanks

Answer 1

[Stalin Samuel]

refer the following http://www.cs.unm.edu/~williams/cs530/mutual2.pdf

[Rasha]

 stalin samuel, thanks so much for your answer.
but I still don't understand how can I input the image into the program through variables X and Y?