False-colour frequency shift maps of the surface emission from opaque tori around different black holes. The torus parameters are n = 0.232 and r_k = 12  r_g, where r_g is the gravitational radius. The black-hole spin parameters are a = 0, 0.5 and 0.998 (panels from top to botom). The viewing inclinations of the tori are 45° (left column) and 85° (right column). For a = 0 the range of frequency shifts E/E₀ for i = 45° and i = 85° are (0.874,1.445) and (0.756,1.560), respectively. Similarly, for a = 0.5 the frequency shift ranges are (0.870,1.487) and (0.755,1.591). Finally, for a = 0.998 the corresponding frequency shift ranges are (0.864,1.535) and (0.767,1.616).