Critical height above polarity inversion line pixels. Red and blue contours outline the pixels used for totalling the positive and negative radial magnetic flux, respectively. Two orange circles indicate the flux-weighted centres of the contoured positive and negative magnetic flux. The green rectangle shows the user-defined region of interest in which polarity inversion lines are detected, and inversion line pixels are coloured according to the local critical height. Pixels represent approximately 2.18 Mm in each spatial dimension.