An agricultural harvester for harvesting a crop on a field. The harvester includes a chopping assembly configured to chop a crop residue of the harvested crop. The chopping assembly includes an inlet for receiving the crop residue, a rotor provided with one or more cutting tools and configured to chop the crop residue, a drive assembly configured to drive the rotor, and an outlet for outputting the chopped crop residue onto the field. The harvester further includes an image based sensor configured to generate a signal comprising an image of the chopped crop residue and a processing unit configured to receive the signal from the image based sensor, process the signal to derive a geometric parameter indicative of a geometry of the chopped crop residue, and determine a control signal for the drive assembly based on the geometric parameter.