Achieving system stability in anamorphic printer defect pre-compensation

Abstract

Every printing system has characteristic defects which detract from high quality printing. Xerographic printing systems show defects such as banding, mottled colors in large fill areas, trail-edge deletion and starvation where toner concentrations deplete at certain color edges, misregistration, and so on. Ink jet printing systems can show ink bleeding, streaking in the direction of head movement, and so on. If a printer defect occurs predictably, it is possible to pre-compensate for the defect by modifying the digital signal in such a way that the modifications cancel out or hide the expected defect. However, most printing defects, while statistically predictable, vary over time in the severity and extent of the defect. Some defects, such as misregistration, may vary in severity and direction with each print, because the defect is caused in part by paper feeding and shifting during printing. Other defects, such as trail-edge deletion, starvation, halo, tenting, etc. may change more slowly because they are controlled in part by environmental conditions such as humidity or definable printing conditions like paper type. The invention proposes a method whereby defects that vary sufficiently slowly in a particular printing system are measured for extent and severity at appropriate intervals, and the measurements are used to modify correction functions applied to the digital data to pre-compensate for these defects. In this way, the corrections applied will track more precisely the current extent and direction of the defects they are correcting.