a bag machine fabricates plastic bags by transversely cutting and sealing , at regularly spaced preselected locations , an elongate plastic web having graphic material repetitively printed thereon . in operation , the bag machine advances the web by a calculated draw length , and an optical sensor , sensitive to the passage of eyemarks printed in regularly spaced locations on the web , functions to ensure that the web is cut and sealed only at the desired locations . to avoid inaccuracies resulting from a gradual variance between the nominal draw length and the actual distance between eyemarks , a control system monitors the actual distance between eyemarks and sets the nominal draw length to the average actual spacing between eyemarks as measured over a preselectd number of successive eyemarks .