The Lattice-Based Screen Set: A Square N-Color All-Orders Moire-Free Screen Set

摘要

Periodic clustered-dot screens are widely used for electrophotographic printers due to their print stability. However, moire is a ubiquitous problem that arises in color printing due to the beating together of the clustered-dot, periodic halftone patterns that are used to represent different colorants. This beating or interference phenomenon introduces spurious low frequency (large period) patterns in the printed output that are very objectionable to the viewer. The traditional solution in the graphic arts and printing industry is to rotate identical square screens to angles that are maximally separated from each other. For example, the classic three-color screen set rotates three identical square screens to the angles 15°, 45°, and 75°, respectively. However, the effectiveness of this approach is limited when printing with more than four colorants, i.e. N-color printing, where N > 4. Moreover, accurately achieving the angles that have maximum angular separation requires a very high resolution plate writer, as is used in commercial offset printing.In this paper, we propose a systematic way to design color screen sets for periodic, clustered-dot screens that offers more explicit control of the moire properties of the resulting screens when used in color printing. We find a general concept for moire-free screen design that is called lattice-based screen design. The basic concept behind our approach is the creation of the screen set on a 2-dimensional lattice in the frequency domain and then picking each fundamental frequency vector of the individual colorant planes in the created spectral lattice according to the desired properties. The halftone geometry of a screen set is the set of angles and frequencies in units of lines per inch (LPI) of each screen plane. The lattice-based screen design offers more flexibility in designing JV-color screen sets with different halftone geometries, and all of them are guaranteed to be all-orders moire-free. For example, by creating a square lattice in the frequency domain, square JV-color moire-free screen sets that consist of JV rotated square screens can be achieved. The proposed approach maintains the advantage of square clustered-dot screen design and is based on low addressability of digital printing. We also propose several symmetry measures, and use them to compare the proposed 4-color square screen set and the screen sets based on a previous moire-free N-color non-orthogonal approach. The proposed screen set is shown to have better symmetry properties.