How to calculate UACI and NPCR between to cipher images whose plain images have only a one-pixel difference?

I write this code to explain my problem, this cod is simple encrypt algorithm to encrypt image by Xoring image with key

firstly I implement this code to incrypt image then save the cipher image under name 'enc1.bmp:

clc
clear all
tic
load rkey
a=imread('lena_gray.jpg');
[n1 , n2]=size(a);
epk=rkey( 1:n1 ,1:n2 );
enc=bitxor(a,epk);
dec=bitxor(enc,epk);
figure;
imshow(enc);
figure;
imshow(dec);
imwrite(enc,'enc1.jpg');

then i edit one pixel plain image by this step

a=imread('lena_gray.jpg');
a(1,3)=0; %
%the rest of code
.
.
. 
imwrite(enc,'enc2.jpg');

and implemet the rest code in step1 once again withe save the encrypted image under name 'enc2.bmp' .

Then i calculate NPCR and UACI between enc1 and enc2 cipher images by add function bellow:

The result are :

NPCR =

0.0468

UACI_value =

5.7267e-04

Why these result with thes value ? it is correct?

NPCR between images very small, this mean that no differents between enc1 and enc2 if i change on pixel only in plain image. But why the value not ideal as 99%. also UACI it is not ideal value.

I implement this code to anather complex encryption algorithm and i change on pixel in pllain image but i diddn't get differences between cipher images. Where is the mistack im my code?

My way for change on pixel in plain image correct?

please tell me what should i do to get ideal value for NPCR and UACI?

% calc NPCR
function [ NPCR ] = Cal_NPCR( imge,enc)


[rows,columns]=size(imge);
step=0;
for i=1:rows
    for j=1:columns
        if imge(i,j)~= enc(i,j)
           step=step+1;

        else 
             step=step+0;
        end
    end
end
NPCR =(step/(rows*columns))*100;
end

% calc UACI
function [UACI_value] = UACI( after_change,befor_change )
[row, col]=size(befor_change);
AB=[];
for i=1:row
    for j=1:col

AB(i,j)=abs(befor_change(i,j)-after_change(i,j));
    end
 end
    UACI_value = sum(AB(:))/(255*row*col)*100
end

Answer 1

[noor sattar]

% calc NPCR

function [ NPCR ] = Cal_NPCR( imge,enc)

[rows,columns]=size(imge);

step=0;

for i=1:rows

for j=1:columns

if imge(i,j)~= enc(i,j)

step=step+1;

else

step=step+0;

end

end

end

NPCR =(step/(rows*columns))*100;

end

% calc UACI

function [UACI_value] = UACI( after_change,befor_change )

[row, col]=size(befor_change);

AB=[];

for i=1:row

for j=1:col

AB(i,j)=abs(befor_change(i,j)-after_change(i,j));

end

end

UACI_value = sum(AB(:))/(255*row*col)*100

end

Answer 2

[yahia]

How to calculate UACI and NPCR for sbox AES