Improving printed halftone image quality

Seongjun Park, Purdue University

Abstract

Periodic, clustered-dot halftone patterns generated by screening with a threshold matrix are generally considered to be the preferred choice for printing with electrophotographic printers. This approach has the dual advantage of yielding more stable printed patterns than aperiodic, dispersed-dot halftoning methods, and being very computationally efficient, since the halftoning is performed on a pixel-by-pixel basis with simple comparison operations. However, there is a tradeoff with periodic, clustered-dot halftones between using a coarser screen to yield more stable halftone patterns and using a finer screen to yield better rendering of detail. Conventional way is to single halftone pattern in a document, but in this paper, we consider a dual-mode halftoning process that switches between periodic, clustered-dot screens of two different frequencies—a low frequency screen for smooth regions and a high frequency screen for detail regions. These regions are described by an object map that is extracted from a high-level representation of the page content to be rendered. This high-level representation is contained in the page description language (PDL) version of the page to be printed. This idea is beneficial in that we can reduce printing artifacts at smooth region and fine rendition at detail region. Nonetheless, one drawback of dual-mode halftoning approaches is appearance of boundary artifacts at the boundary between the smooth and detail halftones. When switching between periodic, clustered-dot halftones with two different frequencies, we may have an appearance similar to the "jaggies" that occur along an edge in a sampled image. To overcome the boundary artifact, our screens obey a harmonic relationship. In addition we implement a blending process based on a transition region. We propose a nonlinear blending process in which at each pixel, we choose the maximum of the two weighted halftones where weights vary according to the position in the transition region. Moreover, we describe an on-line tone-mapping for the boundary blending process, based on an off-line calibration procedure that effectively assures the desired tone values within the transition region. We will provide detail algorithm and experimental results for the dual-mode object oriented halftoning.

Degree

Ph.D.

Advisors

Allebach, Purdue University.

Subject Area

Electrical engineering

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