how to make image unique

i have alot of image files for each letter or a word. i do a hough transform, then detect lines using hough lines and then hough do hough peaks to calculate the peak points and filter only theta values which have significant voting.

what method do i use so that the distance calculated will be different for a distance calculated for the other image. i decided to use the peak values square each peak point and then sum and then square root. which is quite effective but not effective enough.

any other effective methodi can use to make it more effective so when matching other images that the original image gets first in similarity then the others follow.

current distance calculation the original images gets 3rd similarity.

thanks