Irrational Halftoning for Electronic Registration

Abstract

As xerography moves to intercept offset printing, image quality becomes a key ingredient of success. Classic halftoning methods, which generally deliver good, low noise halftone dots, have fixed positions in the scan field that hinder several possible improvements to these printing systems.First, exact halftone frequencies and angles would result if dot positions could be adjusted with arbitrary precision. This would improve the design of screen-sets that limit or reduce multiseparation moiré, or allow screen-sets that exhibit the classic rosette structure associated with offset printing.Second, electronic registration systems could emerge if the halftone dot positions could be adjusted in response to actuation commands from the printer. Such systems would automatically compensate for mechanical distortions caused by bent mirrors, elliptical rollers, and tandem color print stations, for instance, and thus save manufacturing costs for the mechanical system.Normally, the dot positions are fixed to small integer offsets (the angle corresponds to a “rational tangent”) in the scan field, thus preventing the occurrence of single separation moiré. When fractional dot positions are allowed (irrational tangent), moiré can result. Thus, if the moiré problem can be eliminated for irrational halftoning, the frequency and angle restrictions associated with rational tangent halftoning disappear.I will present one solution to this problem that subsamples a halftone cluster function stored in a look-up table to produce reduced moiré separations while printing. Halftone dot locations are computed by hardware, and dot cluster shapes typically do not repeat. I will show a simulation of a three-separation image printed on a 600 spi digital printer that uses irrational offsets (m30°, c75°, and k45°) designed to produce a classic rosette structure. I will also show a simulation of these same dots being electronically registered, or warped.