2D histogram plots showing the classification of pixels. Panels (a), (b): 2D histograms of the proxy vs. the synthetic Hα intensity at Doppler velocity = 37 km/s (a) and −37 km/s (b). Panels (c), (d): The row normalised distribution of the same pixels corresponding to panels (a) and (b), respectively. We mark the dynamic threshold for the synthetic intensity with vertical dashed black lines. The static threshold for the proxy for feature identification is marked by the horizontal dashed black lines. The four pixel categories: true positive (TP), true negative (TN), false positive (FP), and false negative (FN) are also indicated on each of the panels.