Omri Shacham

Clustered-Dot Halftoning With Direct Binary Search

In this paper, we present a new algorithm for aperiodic clustered-dot halftoning based on direct binary search (DBS). The DBS optimization framework has been modified for designing clustered-dot texture, by using filters with different sizes in the initialization and update steps of the algorithm. Following an intuitive explanation of how the clustered-dot texture results from this modified framework, we derive a closed-form cost metric which, when minimized, equivalently generates stochastic clustered-dot texture. An analysis of the cost metric and its influence on the texture quality is presented, which is followed by a modification to the cost metric to reduce computational cost and to make it more suitable for screen design.

Cost function analysis for stochastic clustered-dot halftoning based on direct binary search

Most electrophotographic printers use periodic, clustered-dot screening for rendering smooth and stable prints. However, periodic, clustered-dot screening suffers from the problem of periodic moir´e resulting from interference between the component periodic screens superposed for color printing. There has been proposed an approach, called CLU-DBS for stochastic, clustered-dot halftoning and screen design based on direct binary search. This method deviates from conventional DBS in its use of different filters in different phases of the algorithm. In this paper, we derive a closed-form expression for the cost metric which is minimized in CLU-DBS. The closed-form expression provides us with a clearer insight on the relationship between input parameters and processes, and the output texture, thus enabling us generate better quality texture. One of the limitations of the CLU-DBS algorithm proposed earlier is the inversion in the distribution of clusters and voids in the final halftone with respect to the initial halftone. In this paper, we also present a technique for avoiding the inversion by negating the sign of one of the error terms in the newly derived cost metric, which is responsible for clustering. This not only simplifies the CLU-DBS screen design process, but also significantly reduces the number of iterations required for optimization.

Electro-photographic model based stochastic clustered-dot halftoning with direct binary search

Most electrophotographic printers use periodic, clustered-dot screening for rendering smooth and stable prints. However, when used for color printing, this approach suffers from the problem of periodic moire´ resulting from interference between the periodic halftones of individual color planes. There has been proposed an approach, called CLU-DBS for stochastic, clustered-dot halftoning and screen design based on direct binary search. We propose a methodology to embed a printer model within this halftoning algorithm to account for dot-gain and dot-loss effects. Without accounting for these effects, the printed image will not have the appearance predicted by the halftoning algorithm. We incorporate a measurement-based stochastic model for dot interactions of an electro-photographic printer within the iterative CLU-DBS binary halftoning algorithm. The stochastic model developed is based on microscopic absorptance and variance measurements. The experimental results show that electrophotography-model based stochastic clustered dot halftoning improves the homogeneity and reduces the graini-ness of printed halftone images.

Design of color screen tile vector sets

For electrophotographic printers, periodic clustered screens are preferable due to their homogeneous halftone texture and their robustness to dot gain. In traditional periodic clustered-dot color halftoning, each color plane is independently rendered with a different screen at a different angle. However, depending on the screen angle and screen frequency, the final halftone may have strong visible moiré due to the interaction of the periodic structures, associated with the different color planes. This paper addresses issues on finding optimal color screen sets which produce the minimal visible moiré and homogeneous halftone texture. To achieve these goals, we propose new features including halftone microtexture spectrum analysis, common periodicity, and twist factor. The halftone microtexture spectrum is shown to predict the visible moiré more accurately than the conventional moiré-free conditions. Common periodicity and twist factor are used to determine whether the halftone texture is homogeneous. Our results demonstrate significant improvements to clustered-dot screens in minimizing visible moiré and having smooth halftone texture.

Electro-photographic-model-based halftoning

Most halftoning algorithms assume there is no interaction between neighboring dots or if there is, it is additive. Without accounting for dot-gain effect, the printed image will not have the appearance predicted by the halftoning algorithm. Thus, there is need to embed a printer model in the halftoning algorithm which can predict such deviations and develop a halftone accordingly. The direct binary search (DBS) algorithm employs a search heuristic to minimize the mean squared perceptually filtered error between the halftone and continuous-tone original images. We incorporate a measurementbased stochastic model for dot interactions of an electro-photographic printer within the iterative DBS binary halftoning algorithm. The stochastic model developed is based on microscopic absorptance and variance measurements. We present an efficient strategy to estimate the impact of 5×5 neighborhood pixels on the central pixel absorptance. By including the impact of 5×5 neighborhood pixels, the average relative error between the predicted tone and tone observed is reduced from around 21% to 4%. Also, the experimental results show that electrophotography-model based halftoning reduces the mottle and banding artifacts.

Design of color screen sets for robustness to color plane misregistration

Periodic clustered-dot screens are widely used for electrophotographic printers due to their homogeneous halftone texture and their robustness to dot gain. However, when applied to color printing, there are two important phenomena that limit the quality of printed color halftones generated using a screening technology: (1) moire´ due to the superposition halftone patterns corresponding to different periodicity matrices, and (2) appearance changes due to misregistration between different colorant planes. This paper focuses on analyzing the registration sensitivity of periodic, clustered-dot screens. To quantitatively measure the effect of registration errors, we introduce two new functions: (1) cost, and (2) risk of registration errors. We propose the notion of “visual equivalence”, and derive three propositions under which visual equivalence can be achieved, even when registration errors occur.

Clustered-dot halftoning with direct binary search

Printers employing electrophotographic technology typically use clustered-dot screening to avoid potential artifacts caused due to unstable dot rendering. Periodic clustered-dot screens are quite smooth, but also suffer from periodic moire artifacts due to interference with other color channels. Stochastic, clustered-dot screens provide an alternative solution. In this paper, we introduce a new approach for stochastic, clustered-dot halftoning and screen design based on direct binary search. The method deviates from conventional DBS in its use of different filters in different phases of the algorithm. We provide an intuitive explanation for the clustering achieved as a result of this modification, and also discuss ways to control the coarseness of clusters. Due to the non-applicability of conventional screen design procedures to the new halftoning method, we propose a new method for screen design also. Results include images of individually designed constant-tone halftones and halftones of screened folded ramps.

Stochastic clustered-dot screen design for improved smoothness

Printers employing electrophotographic technology typically use clustered-dot screening to avoid potential artifacts caused by unstable dot rendering. Periodic clustered-dot screens are quite smooth, but also suffer from periodic moir´e artifacts due to interference with other color channels. Stochastic, clustered-dot screens provide an alternative solution. In this paper, we introduce a new approach for stochastic, clustered-dot screen design based on Direct Binary Search (DBS). The method differs from the conventional DBS in its use of a modified cost metric which was derived in an earlier work from using different filters in the initialization and update phases of DBS. The objective of the chosen approach is to design screen for improved print smoothness by generating a homogeneous distribution of compact, uniformly-sized clusters. The results include halftone of a screened folded-ramp, compared against a screen designed with a previous method.