Electronic graphic arts screener that suppresses Moire patterns using pseudo-random font selection

摘要

Apparatus for a digital screener (30) and methods for use therein are disclosed for producing relatively smooth halftone dots at substantially any arbitrary screen angle and for suppressing a substantial source of screener induced Moire patterns that would otherwise appear in a reproduced color image generated by a raster based marking engine (20). Specifically, relatively smooth dots are generated and this Moire is suppressed by randomly varying the dot fonts that are used to write successive halftone dots in the screened image. A pseudo- random value is used to select one of a number, illustratively sixteen, of available halftone dot fonts. Each dot font can take on nearly any pattern (shape) as long as it is somewhat different from that of the other fonts, provided the patterns of all the dot fonts are relatively smooth, apart from normal dot boundary irregularities due to quantization. Once a particular font is chosen to produce any halftone dot in the screened image, no random variations are added to the font pattern. The random variation inherent in the selection of any particular dot font coupled with the dissimilarity among the patterns for the individual dot fonts, for any given contone value, substantially suppresses Moire patterns that might otherwise appear in the screened image. Moreover, writing a selected dot font without imparting any random variations to the shape of the font itself avoids any excessively jagged halftone dot edges other than dot boundary irregularities which normally occur from quantization. The use of sheared halftone dot fonts and a halftone dot font selection process that relies on constrained randomization are also disclosed herein.