Printing apparatus (20) includes a continuous blanket (24) and a set of motorized rollers (31), which advance the blanket at a constant speed through an image area. One or more print bars (38) eject droplets of ink at respective locations onto the blanket in the image area. One or more monitoring rollers (42), in proximity to the locations of the print bars, contact the blanket so as to be rotated by advancement of the blanket. Each monitoring roller includes an encoder (44), which outputs a signal indicative of a rotation angle of the monitoring roller. A control unit (40) collects, during a calibration phase, the signal from the encoders over multiple rotations of the monitoring rollers and computes runout correction factors. During an operational phase, the control unit synchronizes ejection of the droplets from the print bars using the computed runout correction factors.