For every pixel in a image, we will need to test it against every other pixel. '''This makes its complexity O(n^2)'''. For example a 256x256 has 65536 65,536 pixel. Each pixel would have to be tested against 65536 pixels to find out the nearest corresponding pixel. '''So it needs 65536 65,536 * 65536 65,536 = 4,294,967,296 array element look ups!'''