1 Introduction
The similarity concept is, in a general sense, an ambiguous and relative concept for humans. This is due to the fact that men have not got an extrinsic similarity measure. Intuitively, we say that two objects are similar through the external appearance or some local/global features, but we can not measure how much the first object looks like the second one. Nevertheless, we can always maintain whether or not they are similar. Even for this case, different results depending on the perception of each person would be obtained. In this field, Santini and Jain [1] have carried out a study on similarity measures for selecting images based on characteristics of human similarity measurement. Anyway, the similarity is usually determined as a distance between some extracted features or a vector that combines these [2].