Evolution of the C2.9 precursor flare towards the onset of the M5.6 flare. Each ChroTel Hα image has been individually scaled to its maximum intensity. The green and blue contours outline the filament and the flare brightenings, respectively, as observed at Hα. The white and grey contours correspond to the sunspot umbra-penumbra and penumbra-quiet Sun boundary, respectively. The yellow contour shows the mask used in deriving the amount of flux emerging near the neutral line, plotted in Fig. 8. The black and white crosses indicate the position of the leading edge of the brightness front in the current and previous frames, respectively. The dashed grey squares labelled 1 and 2 are small regions selected for analysing magnetic cancellation and/or emergence events.