Representation of some 3D homogeneous and clumpy geometries studied in this paper. The star distribution is always localized in the centre of the shell, whereas the dust and the H i content are distributed around. The dust and H i distribution can be “homogeneous” (left), “clumpy” (middle) or “extremely clumpy” (right). In a “clumpy” distribution, the clumps and the interclump medium receive, respectively, high and low densities of dust and HI. In the case of an “extremely clumpy” distribution, all the dust and the H i content are distributed in clumps.