NPCR Error

For Image Quality Parameters I need to find NPCR for 2 images after encryption whose original image have 1 pixel difference. The code is:

% For Grayscale Jpeg Images

C=Cipher Image
C2c=Cipher Image after 1 Pixel difference in Original Image.

    siz=M*M;

     NPCR=0;
     SSS=0;
    for i=1:M
        for j=1:M

           if (C2(i,j)==C2c(i,j))
               SSS=SSS;
           else
               SSS=SSS+1;
           end
        end
    end
    SSS=SSS/siz;
    NPCR=SSS*100;

I get NPCR value=0.0015. Is my code not OK or the method for Cipher image. I should get a value around 99.67%

Suggest any solutions or error

[Renjith V Ravi]

I think the NPCR equation is wrong in your code

[majed samir]

SSS=SSS/siz;

error

true is

 SSS=siz/(siz+1)

[LOKESH]

This will always give 99.99% result..SSS=siz/(siz+1) will never need the loop..please check

[Parveiz Lone]

%% 3. NCPR score and p_value

npcr_score = sum( double( img_a(:) ~= img_b(:) ) ) / num_of_pix;

npcr_mu = ( largest_allowed_val ) / ( largest_allowed_val+ 1 );

npcr_var = ( ( largest_allowed_val) / ( largest_allowed_val+ 1 )^2 ) / num_of_pix;

npcr_pVal = normcdf(npcr_score, npcr_mu, sqrt( npcr_var ) );

npcr_dist = [ npcr_mu, npcr_var ];

Answer 1

[Walter Roberson]

The best encryption schemes would show no correlation between the encrypted versions of two original images that were nearly the same but not identical.