It can be normalised to reflect the probability that a randomly chosen pixel will have
colour ci and co-occur with another ci colour pixel in k distance by dividing to (8k h(ci))
which is the total number of pixels in distance k times h(ci) the probability of observing
colour ci
. The 8k is due to the distance calculation with the city block measure. For
example, a centre pixel has 8 related pixels in distance k = 1, 16 related pixels in k = 2.