Illustration of a single cell iteration. Structures are built around density peaks by repeating this process iteratively. The initial peak is given in black. The other cells belonging to the structure are given in dark gray. The cells identified as part of the neighborhood are given in light gray. The neighbors discarded because of the shape constraints are given in red. The cell selected to be added to the structure is given in green. (a) Neighborhood definition. (b) Shape-constraint discarding. (c) Density-based selection. (d) Cell merging.