In Figure 1, for example, the pairs {e 1 , e 2 } and {e 5 , e 6 } could easily be consecutively ranked, since the distance between e 1 and e 2 is roughly equal to that between e 5 and e 6 .
they would not naturally be clustered together by a human observer, because grouping of objects also needs to take into account the position of the surrounding elements.