Phase spiral detected by the edge detection. Top left: output of the algorithm applied to the same selection of stars as in Fig. 1, superposed to the counts from which it was computed. We used σ = 3. Bottom left: same but superposed to the projection coloured according to V_ϕ. Top right: comparison with the Gaussian filter (colour, see text). Bottom right: comparison with the Gaussian filter and the WT, with the respective values scaled to fit in the same plot, as indicated in the legend, in a one-dimensional case (see text).