怎么用Matlab计算两幅图像的协方差

%frm，frame为两不同的灰度图像，他们的大小相同即分辨率，求他们的协方差
meanfrm = mean(mean(frm));%图像像素均值
meanframe = mean(mean(frame));
[row,col]=size(frm);
sum = double(0);
for i=1:row
for j=1:col
sum = sum+(frm(i,j)-meanfrm)*(frame(i,j)-meanframe);
end
end
cov = sum/(row*col);%协方差