How to find the 2-D correlation coefficient between the two images, each having same pixels?

I have to calculate the 2-D correlation coefficient between the images img1 and img2, each having same pixels. I am doing the following:

img1 = zeros(256);

img2 = ones(256); or you can take all white image

Corr_Coeff = corr2(img1, img2)

Output: NaN (??)

Answer 1

[KSSV]

Read about ssim.

You can try:

R = regression(Img1(:),Img2(:)) ; 

[Abdul Gaffar]

Error!

[KSSV]

What error?

[Abdul Gaffar]

Error occured while running: R = regression(Img1(:),Img2(:)) ;

[KSSV]

What error? You have to specify the error to help you.