Representation of some 3D homogeneous and clumpy geometries. The stellar distribution is always localized at the center of the shell, while the dust and the Hi content are distributed around it. The dust and Hi distribution can be homogeneous (left), clumpy (middle), or extremely clumpy (right). In a clumpy distribution, the clumps and the interclump medium receive high and low densities of dust and HI, respectively. Nonetheless, the interclump medium remains optically thick for Lyα photons. For an extremely clumpy distribution, all the dust and the HI content are distributed in clumps, leaving the interclump medium optically thin for Lyα photons.