Calculations of the Balmer decrement as a function of metallicity for various models. The thick light gray line is a metallicity sequence representing the homogeneous pure photoionization case with U = 0.01 (same sequence as in Fig. 2). The (short vertical) black line shows a U-sequence at a fixed metallicity Z = 0.01   Z_⊙ (see cyan dotted line in Fig. 2). Three metallicity sequences representing the κ-distribution case are shown. The corresponding κ value appears in the line color legend. The light green line represents a metallicity sequence for a 50 km   s^-1 shock combined with photoionization. Two sequences (labeled 7 and 8 in the legend) are shown that represent models with metallicity inhomogeneities. The values of Z_A (in Z_⊙ units) and θ (in dex) appear in the line color legend. An open circle indicates the position where w_i = 0.5. Yellow dashed lines represent the low-density recombination case-B Balmer decrement at three different temperatures (Osterbrock 1989).