Nonorthogonal Halftone Screens

Abstract

In color reproduction, the most troublesome moiré pattern is the second-order moiré, or the three-color moiré, usually produced by mixing of cyan, magenta and black halftone outputs. A classical 3-color zero-moiré solution is using three identical cluster halftone screens with different rotations: 15°, 45° and 75°, respectively. However, for most digital printing devices, the size and shape of halftone screens are constrained by the “digital grid”, which defines the locations of printed dots, and therefore, an exact 15° or 75° rotation of a cluster screen is impossible. Although there are many alternative approaches for moiré-free color halftoning, most of them only provide approximate solutions and/or have a tendency to generate additional artifacts associated with halftone outputs. The difficulty to achieve moiré-free color halftoning is greatly relieved by using nonorthogonal halftone screens, i.e., screens in general parallelogram shapes. As a matter of fact, there exist many practical solutions by combining three simple nonorthogonal halftone screens. In this paper, a general condition for 3-color zero-moiré solutions is derived. A procedure using integer equations to search practical solutions for different applications is also described.