Data processing steps implemented to identify filaments from the Planck data. We start with the PlanckD₃₅₃ map (upper left panel) smoothed at 15′ resolution. The bandpass-filtered map (upper right panel) is produced using the spline wavelet decomposition, retaining only the scales between ℓ = 30 and 300. The lower eigenvalue map of the Hessian matrix, λ₋, is shown in the lower left panel. Structures identified in the high-latitude sky λ₋ map are shown in the lower right panel. The superimposed graticule is plotted in each image and labelled only on the lower right panel. It shows lines of constant longitude separated by 60° and lines of constant latitude separated by 30°. The same graticule is used in all plots of the paper.