Illustration of the second and third step of the procedure for estimating the QSO continuum. The highest values of f_r (blue curve) must tend to 1 (outside the bottom of the PDLA absorption and the LAB emission), when the profile for the DLA converges to a good solution; the blue curve diverges in the wavelength range corresponding to the bottom of the PDLA absorption and to the LAB emission, where it has to be interpolated. The red curve shows the spline interpolation performed over the highest points of the blue curve. The spline function has then to be multiplied by the previous estimate of the continuum to create a better estimate.