As in Fig. A.2, examples of line formation following Bertout & Magnan (1987). Left: a basic Schwarzschild mechanism produces a flat bottom profile captured by setting a = 1. Right: two accelerating spherical shells with acceleration a = 1.2: one expanding and the second falling, producing line doubling. The maximum contribution comes from the centre of the disc, the limbs being weighed down by the geometry of the problem, so the profile roughly behaves as if coming from 1D radiative transfer at disc centre. The red profiles correspond to the linear polarisation profiles.