Computer and Robot Vision Vol. 1, by R.M. Haralick and Linda G. Shapiro, Addison-Wesley, 1992, ISBN 0-201-10887-1.

Computer and Robot Vision

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. 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. 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. Commercially available high-end digital printers cannot achieve this resolution. In this paper, we propose a systematic way to design color screen sets for periodic, clustered-dot screens that offer more explicit control of the moire properties of the resulting screens when used in color printing. We develop a principled approach for the 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 2D 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 lattice-based screen design offers more flexibility in designing ${N}$ -color screen sets with different halftone geometries, and all of them are guaranteed to be all-orders moire-free. We demonstrate the efficacy of our proposed method by introducing several new screen designs, and a comparison with published screen designs.

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

Aperiodic, clustered-dot, halftone patterns have recently become popular for commercial printing of continuous-tone images with laser, electrophotographic presses, because of their inherent stability and resistance to moiré artifacts. Halftone screens designed using the multistage, multipass, clustered direct binary search (MS-MP-CLU-DBS) algorithm can yield halftone patterns with very high visual quality. But the characteristics of these halftone patterns depend on three input parameters for which there are no known formulas to choose their values to yield halftone patterns of a certain quality level and scale. Using machine learning methods, two predictors are developed that take as input these three parameters. One predicts the quality level of the halftone pattern. The other one predicts the scale of the halftone pattern. To provide ground truth information for training these predictors, human subjects viewed a large number of halftone patches generated from MS-MP-CLU-DBS-designed screens and assigned each patch to one of four quality levels. For each patch, the location of the peak in the radially averaged power spectrum (RAPS) is calculated as a measure of the scale or effective line frequency of the pattern. Experimental results demonstrate the accuracy of the two predictors and the effectiveness of screen design procedures based on these predictors to generate both monochrome and color high quality halftone images.

A Machine Learning Approach to Design of Aperiodic, Clustered-Dot Halftone Screens via Direct Binary Search

Digital halftoning is an essential part of the process for printing color, continuous-tone content. Traditionally, the highest quality has been achieved with analog, offset lithographic presses, using color screen sets that yield periodic, clustered-dot halftone patterns. Increasingly, these systems are being supplanted by digital presses that are based on either electrophotographic or inkjet marking processes. Due to the inherent instability of the electrophotographic marking process, periodic, clustered-dot halftone patterns are also widely used with such presses. However, digital presses have much lower resolution than their analog counterparts. Simply mimicking the traditional screen designs used with commercial, offset presses will result in halftone patterns that are more susceptible to moire due to the interaction between the periodic patterns used to render the different color channels. This causes instability in the printed colors. The moire can be reduced by increasing the frequency of the halftone patterns. But this may make the print appear grainier than its analog counterpart. In this paper, we introduce a principled design procedure that allows one to design color screen sets that generate periodic, clustered-dot halftone patterns that improve color stability without increasing graininess. We present experimental results to support the benefits of our new color screen set design framework.

New Design for Compact Color Screen Sets for High-End Digital Color Press

Aperiodic, clustered-dot, halftone patterns have recently become popular for commercial printing of continuous-tone images with laser, electrophotographic presses, because of their inherent stability and resistance to moir&#x00E9; artifacts. Halftone screens designed using the multistage, multipass, clustered direct binary search (MS-MP-CLU-DBS) algorithm can yield halftone patterns with very high visual quality. But the characteristics of these halftone patterns depend on three input parameters for which there are no known formulas to choose their values to yield halftone patterns of a certain quality level and scale. Using machine learning methods, two predictors are developed that take as input these three parameters. One predicts the quality level of the halftone pattern. The other one predicts the scale of the halftone pattern. To provide ground truth information for training these predictors, human subjects viewed a large number of halftone patches generated from MS-MP-CLU-DBS-designed screens and assigned each patch to one of four quality levels. For each patch, the location of the peak in the radially averaged power spectrum (RAPS) is calculated as a measure of the scale or effective line frequency of the pattern. Experimental results demonstrate the accuracy of the two predictors and the effectiveness of screen design procedures based on these predictors to generate both monochrome and color high quality halftone images. 

With the emergence of high-end digital printing technologies, it is of interest to analyze the nature and causes of image graininess in order to understand the factors that prevent high-end digital presses from achieving the same print quality as commercial offset presses. In this paper, we report on a study to understand the relationship between image graininess and halftone technology. With high-end digital printing technology, irregular screens can be considered since they can achieve a better approximation to the screen sets used for commercial offset presses. This is due to the fact that the elements of the periodicity matrix of an irregular screen are rational numbers, rather than integers, which would be the case for a regular screen. To understand how image graininess relates to the halftoning technology, we recently performed a Fourier-based analysis of regular and irregular periodic, clustered-dot halftone textures. From the analytical results, we showed that irregular halftone textures generate new frequency components near the spectrum origin; and that these frequency components are low enough to be visible to the human viewer, and to be perceived as a lack of smoothness. In this paper, given a set of target irrational screen periodicity matrices, we describe a process, based on this Fourier analysis, for finding the best realizable screen set. We demonstrate the efficacy of our method with a number of experimental results.

Design of irregular screen sets that generate maximally smooth halftone patterns

The goal of this paper is to investigate the effect of choosing a certain halftone screen on image graininess development and to establish the metrics for image graininess in high-end digital printing technologies. With the obtained knowledge, our model helps us choose the optimal periodicity matrices for designing regular or irregular clustered dot halftones. The main advantage of the proposed model lies in predicting the graininess ratios based solely on the periodicity matrix. We conduct an in-depth Fourier analysis, which exposes to us all the details regarding the spectrum of a desired halftone screen. Hence, at this stage of the design process, there is no need to generate the halftone screens themselves.

Single separation analysis for clustered-dot halftones

HVS-Based Model for Superposition of Two Color Halftones.

In our previous work, we have presented an HVS-based model for the superposition of two clustered-dot color halftones, which are widely used for electrophotographic printers due to their relatively poor print stability. The model helps us to decide what are the best color assignments for the two regular or irregular halftones that will minimize the perceived error [1]. After applying our model to the superposition of three and four clustered-dot color halftones, it was concluded that this color assignment plays a significant role in image quality. Moreover, for different combinations of colorant absorptance values, their corresponding best color assignments turn out to be different. Hence, in this paper we propose to apply different color assignments within the image depending on the local color and content of the image. If the image content locally has a high variance of color and texture, the artifacts due to halftoning will not be as visible as the artifacts in smooth areas of the image. Therefore, the focus of this paper is to detect smooth areas of the image and apply the best color assigments in those areas. In order to detect smooth areas of the image, it was decided to segment the image based on the color of the content. We used the well-known K-means clustering algorithm along with an edge detection algorithm in order to segment an image into clusters. We then used our spatiochromatic HVS-based model for the superposition of four halftones in order to search for the best color assignment in a particular cluster. This approach is primarily directed towards good quality rendering of large smooth areas, especially areas containing important memory colors, such as flesh tones. We believe that content-color-dependent screening can play an important role for developing high quality printed color images.

Altyngul Jumabayeva

Papers

Design of irregular screen sets that generate maximally smooth halftone patterns

New Design for Compact Color Screen Sets for High-End Digital Color Press

Single separation analysis for clustered-dot halftones

HVS-Based Model for Superposition of Two Color Halftones.

Content-color-dependent screening (CCDS) using regular or irregular clustered-dot halftones