Calculated ionization fronts for volumes of gas with inhomogeneous, fractal density structures. As the gas exposed to the ionizing radiation does not exhibit an intrinsic symmetry, a three-dimensional treatment of the radiative transfer is indispensable (for computational details see Sect. 3). In image a) the deviations from a homogeneous density distribution are much smaller than in image b), and consequently the shape of the ionized volume is less spherical in b) than in a). (The calculation of the fractal density distributions has been based on the algorithms presented by Elmegreen & Falgarone 1996; Wood et al. 2005.)