security analysis of image using UACI and NPCR for encrypted image

i am trying to find the UACI value for two encrypted images, having one pixel value changed in original plain image.i get value greater than 0.4, on the other hand in most of the papers i read having 0.334 or 0.34 maximum value for UACI. CAN ANYONE EXPLAIN THE UACI and NPCR ?what is the maximum achievable value of UACI?

npcr_score = sum( double( img_a(:) ~= img_b(:) ) ) / num_of_pix;
uaci_score = sum( abs( img_a(:) - img_b(:) ) ) / num_of_pix / largest_allowed_val;

i am using the above matlab code for finding NPCR and UACI. img_a and img_b both are encrypted image having one pixel value changed in original image.

Answer 1

[Walter Roberson]

uaci_score = sum( abs( double(img_a(:)) - double(img_b(:)) ) ) / num_of_pix / largest_allowed_val;

[faiz]

i tried but i didn't noticed any changes, can you share anything about the highest UACI value>i get 0.49, which is very high than the state-of -art work.

[Walter Roberson]

https://www.mathworks.com/matlabcentral/fileexchange/43603-npcr-and-uaci-measurements-with-statistical-tests

[yahia]

How to calculate UACI and NPCR for sbox aes