MATLAB: How to replace certain gray level values in image to find information loss in both

entropyMATLAB

I'm finding information loss in Img1 and its replica Img2 when only a certain gray level values are allowed.

Entropy_Info_Loss=Entropy(Img1)-Entropy(img2);

where img2 must not have gray values other than eg.

only_include=[20, 30, 50].
img2(img1(:)~=only_include)=0;

However my result is all zeros image of img2. Can anyone assist me..

mask = ismember(img1, only_include);
img2 = img1;
img2(~mask) = 0;

this gives an error with imsubstract because the images are not of the same size.

Do the same for considering two images and show mag1+phase1 and so on, here I have shown how you calculate the mag and phase for one image

fft_result=fft2(im);                         
mag=abs(fft_result);                           
phase=angle(fft_result);