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Dear SPM experts,

We are trying to obtain the value of the Normalised Mutual Information(nmi) between two images and we have not achieved it yet.

We've found a script in the forum which was coded to do so but we have found some differences between it and the spm_coreg function and we don't know which to use since they give us different outputs.

cg_hist2d_img.m - Script from the forum

H  = H/(sum(H(:))+eps);
s1 = sum(Hmi,1);
s2 = sum(Hmi,2);
H  = H.*log2((H+eps)./(s2*s1+eps));
mi = sum(H(:));


spm_coreg

H  = H+eps;
   sh = sum(H(:));
   H  = H/sh;
   s1 = sum(H,1);
   s2 = sum(H,2);
H   = H.*log2(H./(s2*s1));
        mi  = sum(H(:));

nmi = (sum(s1.*log2(s1))+sum(s2.*log2(s2)))/sum(sum(H.*log2(H)));

When we apply it using as input the same image twice, we get a value above 1,
while we believe it should be 1 (or 0.998).

Are the values of NMI supposed to be between 0 and 1?

Any insight given will be gratefull


Thanks


Aran