NPCR Error

Question

LOKESH 2012-4-25

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/36535-npcr-error

评论： Parveiz Lone 2020-5-12

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

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LOKESH 2018-6-6

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

Parveiz Lone 2020-5-12

%% 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 ];

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Answer 1

Walter Roberson 2012-4-25

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/36535-npcr-error#answer_45705

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