Illustration of the visual approach (see Sect. 4.2). Left column: wavelength-averaged squared visibilities obtained with PIONIER (upper panel) and AMBER (lower panel) for HD 150193 against baseline length. Each data point is plotted as a bar indicating the position angle of the observation. The date of the observation is indicated by the color. Middle and right columns: squared visibilities for synthetic observations based on the models M₁ (upper row) and M₂ (bottom row). Middle column: underlying brightness distribution of the synthetic data is time independent. Right column: we varied the brightness distribution by decreasing the star-to-disk flux ratio (M₁ (t)) or by changing the position of the disk feature (M₂(t), see Sect. 3.2 for details). The UV coverage as well as the error bars and the corresponding noise are adapted from the PIONIER observations of HD 150193.