In some examples, a printing system includes a printhead comprising a nozzle array including plural columns of nozzles to deliver printing fluid to a print medium, wherein nozzles in a first column of the columns are offset from nozzles in a second column of the columns. A controller is to cause introduction of a grain pattern into an image printed using the printhead onto the print medium by, for each respective pass of a plurality of passes of the printhead over the print medium, controlling an advance of the print medium by an uneven advance step for a print mask pattern that controls firing of the nozzles of the printhead, where the advance of the print medium by the uneven advance step causes an irregular pattern of firing of the nozzles of the printhead in successive passes of the plurality of passes.