Frequency Domain Design of Cluster Dot Screens

ABSTRACT Clustered dots screens are widely used in digital printing. Our research focuses on irregular cluster-dot screens. Irregular screens are appealing since there are many more irre gular screens than regular ones. As a result, they provide a larger set of multi-separation screen combinations for avoidi ng interaction between screenin g frequencies of concurrent screens. Their down side is that they often have harmonic artifacts. Our design challenge is to reduce these artifacts visual impact. We present frequency-domain based methods to reduce these artifacts visual impact. State-of-the-art screens incorporate many, predominantly spatial domain, design considerations which we cannot ignore. Accordingly, the proposed methods are designed to introduce minimal modifications to given screens. The proposed methods eliminate, or reduce the few artifacts visibl e in a set of irregular screens. This wo rk can be generalized to other printing technologies, and to screen design. 1. Introduction Digital Halftoning is the process of rendering digital continuous tone images in two tones (e.g. black and white). The goal is to create a dot pattern such that when viewed through the Human Visual Systems (HVS) low pass, it would look like the original continuous tone image. Various printing technologies limit the halftone dot pattern. For example Electro Photography (EP) rendering requires the dot pattern to be clustered, such as in screens, other considerations motivated screen designers for Liquid Electro Photography (LEP) technology to use also irregular screens. Irregular screens are repetitive only in large groups of cells and consequently may contain low frequency harmonic artifacts, more than regular screens. Harmonic artifacts are addressed in [4,9,10] where they are reduced using random dith er and spatial aver aging. In [11] a spatial cost function is used to minimize artifacts, and the cell centers are stochastically distributed. In [2] screen parameters (lpi and angle) are analyzed fo r artifacts. It provides for parameter sel ection identifying screen sets with least disturbing potential artifacts. Cluster shape is addressed in [5] by slightly and randomly distorting the halftone in each halftone cluster. Other methods venture out of the standard screening conventions, for example [7] removes tones with artifacts from the screen, and [3] uses green noise rather than grid based screens. A method for designing re gular monochrome screens is described in [8]. It is based on an optimization method known as Direct Binary Search (DBS). Resulting slight modifications in cluster seed locations create blue-noise type highlight and shadow patterns and regular cluster dot mi d-tone patterns, all in the same screen. Screens can be used for color images by applying a different screen to each color separation independently. Thus, single separation screens have to fit to each other. Specifically, good single separation screens can be used together in print only if overlaying two or more screens does not produce a low frequency artifact [1]. The scarcity of regular screens that can be combined to compatible multi-separation screen sets motivates using irregular screens, which require special design due to prevalence of harmonic artifacts. Our work is motivated by the fact that both single-separation harmonic-artifacts and multi-separation beating are a result of the dot pattern. As such they may be reduced by slight modifications of the dot pattern. Given a screen, we optimize cluster shapes to reduce the over all artifact level. The goal was to intro duce minimal changes so as to preserve as much of the spatial characteristics of the input screen; however the same methodology could be used for screen design. In the next section we detail our screening artifact model, in Section 3 we present artifact reduction methods, and in Section 4 we briefly discuss the results. 2. The model We look for the causes of visible harmonic artifacts in the screens dot patterns. Harmonic artifacts have harmonic causes, and thus we analyze the do t patterns in the frequency domain. 2.1. Single-separation artifacts For single separations we model artifacts for every tone leve l separately. The model for artifacts at a given tone level t is based on the 2D DFT of the corresponding dot pattern S