Example

    If we consider three  images shwon in Fig. 3.3 and their absolute differences depicted in Fig. 3.4. The Euclidean distance between any two of the three patterns is the same, namely . However, we can see that image A is more similar to image B than it is to image C. This illustrates that the Euclidean distance may not be appropriate to characterize the similarity among images since it does not take into account the spatial information.