uaci score of 2 images

I need to find the uaci score of two images, one the plain image and other image which is 1 pixel value changed in the plain image. I used the below code and always gets zero. Please help me with the code.

 uaci_sum=0; var = 0;
   for i=1:256
       for j=1:256
        var1 = o(i,j);
       var2 = oc(i,j);
        dif = minus(var1,var2);
        diff = abs(dif);

        div = diff/255; 
         uaci_sum = uaci_sum+div;
       end

   end
 uaciscore=(uaci_sum/65536)*100;

Here o and oc are two images and the pixel value change in oc is at (2,8). And also whether there is any problem that the image matrices are of uint8?

Answer 1

[Walter Roberson]

Yes, the problem is that they are uint8. Subtracting a larger uint8 from a smaller one always gives 0, not a negative number.

You should replace

        dif = minus(var1,var2);
        diff = abs(dif);

        div = diff/255;

with

     div = (max(var1,var2) - min(var1,var2)) / 255;

or with

    div = abs(double(var1) - double(var2)) / 255;

[SNEHA P S]

No change for the result while i use the above statement

[Walter Roberson]

Your uaci_sum can be calculated without loops as

uaci_sum = sum(sum(abs(double(o(1:256, 1:256) - double(oc(1:256, 1:256))))) / 255;

Or, more simply if o and oc are 256 x 256,

uaci_sum = sum( abs(double(o(:)) - double(oc(:))) ) / 255;

I tested your code with

   o = randi([0 255], 256, 256, 'uint8');
   oc = o; oc(16142) = 132;   %change one pixel

   uaci_sum=0; var = 0;
   for i=1:256
       for j=1:256
         var1 = o(i,j);
         var2 = oc(i,j);
         div = abs(double(var1) - double(var2)) / 255;
         uaci_sum = uaci_sum+div;
       end
   end
   uaciscore=(uaci_sum/65536)*100;

the result was non-zero, and the result exactly agreed with

uaci_sum = sum( abs(double(o(:)) - double(oc(:))) ) / 255;
uaciscore=(uaci_sum/65536)*100;

[aaru sri]

Here the UACI is 2.333697150735294e-04 so its value means normallyUACI is written as 0.30% then how can we write it in this format

[Walter Roberson]

sprintf('%.2f%%', uaciscore * 100)

[aaru sri]

Can i use sprintf('%.2f%%', uaciscore * 10000) in my code as my UACIscore is coming as 1.149195374426213e-05

[Walter Roberson]

sprintf('%.2f%c', uaciscore * 10000, 8241)

This use the character ‱ https://www.fileformat.info/info/unicode/char/2031/index.htm which is the per-10000 symbol.