A clusterization process in two-dimensional system of granular hard disks is investigated by two novel numerical methods: the nearest neighbourhood density and the anti-percolation function. The tendency of the band like structures in the clustered state is recognized to be driven by two factors: the stretching forces at the junction of two clusters (or two parts of one cluster) with different kinetic energies and the density fluctuations, which act as a seed for the empty ponds (voids free of particles). Moreover, the examples (and animations) of the collision of two clusters and the breakup of the granular band are presented.

PACS numbers: 45.70.--n, 45.70.Qj, 45.70.Vn