Single-scale skeletons (Sect. 2.4). Red and yellow pixels belong to a clean reconstructed filament, whereas the blue pixels have been set to zero during the cleaning. The filament is measured at each pixel in the four main directions to determine the direction where the filament has the smallest half-maximum width. Along those directions (indicated by short straight lines), the brightest pixels (marked by yellow color) belong to the filament’s crest and define the skeleton.