Scheme of our 3D geometry in which the distribution of HI and dust is clumpy within the galaxy disk and the outflowing halos. The observer is located on the right side, at infinity. We also add to this figure the most likely path followed by each group of Lyα photons in Fig. 11: while the Lyα photons “a” escape from the outflowing halo without undergoing any backscattering (they simply scatter on HI atoms toward the observer), the Lyα photons “b” experience one backscattering before escaping from the outflowing halo.