We then constructed six synthetic spectral-line scans with idealised HMI filters, and used the MDI-like algorithm [@2012SoPh..278..217C] to reconstruct the spectral line and corresponding data products, including the estimate of the continuum intensity $I_{\rm c}$ from (\[eq:ic\]). This test is demonstrated in Fig. \[fig:spline\] in the upper row. It can be seen that in the case of the quiet Sun with an undisturbed spectral-line profile the reconstruction works well. However, when we introduced the artificial emission in the core of the line (other synthetic spectral scans were left unchanged), the reconstructed profile was shallower, with decreased line depth and also decreased continuum intensity. Thus the line-core emission itself cannot explain the apparent increase of the estimate of continuum intensity.

Having a simple model at our disposal, by using a trial-and-error approach we searched for the situation, which will lead in what we observe: the increase of $I_{\rm c}$ and decrease of $L_{\rm d}$. We found that for instance a strong line asymmetry together with line-core enhancement (as seen e.g. in Fig. \[fig:spline\] bottom panel) produces such a behaviour. Without the line-core enhancement, the asymmetry itself does not produce a required line-depth decrease. Again, our simple test only indicates that the 