Synthetic images indicating the distribution of the emission line strengths of Hα, [N ii] 6584 Å + 6548 Å, [O ii] 3726 Å + 3729 Å, and [O iii] 5007 Å + 4959 Å for two different values of the metallicity (0.1 Z_⊙ and 1.0 Z_⊙). Compared are cases where the gas is spherically symmetric with a homogeneous density structure (n_H = 10 cm^-3) and cases where the gas is fractally structured with an inhomogeneous density structure of the same mean density. The source of ionization is located in the center of the simulated volume ([40 pc] ³) and is represented by the respective 40 000 K dwarf star (D-40) at the same metallicity as the gas (Sect. 2.2).