Configuration of inhomogeneities on stellar surface. Left: flux on latitude–longitude (θ − ϕ) domain colour-coded and ranging from 0 (in dark violet, corresponding to the presence spots) and 1 (in orange, corresponding to the photosphere), to 1.1 (in bright yellow, corresponding to the presence of faculae). A dashed red line marks the centre of the visible disc, while the two adjacent dashed black lines mark the boundary of the observed disc. Right: projection of left mask in the visible disc in which the colour of each surface element accounts for the (left) flux and the pixel area, but not for the limb darkening. White lines mark a low resolution spherical grid of 3.6°, which is ten times lower than the actual grid spacing.