Halftone blending between smooth and detail screens to improve print quality with electrophotographic printers

摘要

In this paper, we consider a dual-mode halftoning process for the electrophotographic laser printer - a low frequency halftoning for smooth regions and a high frequency halftoning for detail regions. These regions are described by an object map that is extracted from the page description language (PDL) version of the document. This manner of switching screens depending on the local content provides a stable halftone without artifacts in smooth areas and preserves detail rendering in detail or texture areas. However, when switching between halftones with two different frequencies, jaggies may occur along the boundaries between areas halftoned with low and high frequency screens. To reduce the jaggies, 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 the 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.