CSNR function in MATLAB

function s=csnr(A,B,row,col)

[n,m,ch]=size(A);

if ch==1
e=A-B;
e=e(row+1:n-row,col+1:m-col);
me=mean(mean(e.^2));
s=10*log10(255^2/me);
else
e=A-B;
e=e(row+1:n-row,col+1:m-col,:);
e1=e(:,:,1);e2=e(:,:,2);e3=e(:,:,3);
me1=mean(mean(e1.^2));
me2=mean(mean(e2.^2));
me3=mean(mean(e3.^2));
mse=(me1+me2+me3)/3;
s = 10*log10(255^2/mse);
% s(1)=10*log10(255^2/me1);
% s(2)=10*log10(255^2/me2);
% s(3)=10*log10(255^2/me3);
end

return;