Sketches of two types of overlapped filaments due to the projection effects. Deep and shallow regions are the source and local background regions, respectively. In the first case (left), the two filaments have different orientations, and the emission in the overlapping region will be accounted for by local background subtraction. In the second case (right), the two filaments have similar orientations and will lead to overestimated stacked results, evaluated in Sect. 3.2.