Left panel: sample of a ratio intensity image ( component of the K channel), the blue continuous line shows a transverse cut; right panel: the profile (blue line) along the cut is reported in comparison with the six thresholds (red continuous lines) derived by the Otsu transform. The lowest threshold is the 1st while the highest is the 6th. The abscissa range of the plot is in pixels and corresponds to 900 pixels, i.e., the solar disc.