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.
Proc. SPIE. 8652, Color Imaging XVIII: Displaying, Processing, Hardcopy, and Applications
KEYWORDS: Image processing algorithms and systems, Image segmentation, Image processing, Printing, Raster graphics, Halftones, Nonimpact printing, Binary data, Simulation of CCA and DLA aggregates, RGB color model
We describe a segmentation-based object map correction algorithm, which can be integrated in a new imaging
pipeline for laser electrophotographic (EP) printers. This new imaging pipeline incorporates the idea of
object-oriented halftoning, which applies different halftone screens to different regions of the page, to improve the
overall print quality. In particular, smooth areas are halftoned with a low-frequency screen to provide more
stable printing; whereas detail areas are halftoned with a high-frequency screen, since this will better reproduce
the object detail. In this case, the object detail also serves to mask any print defects that arise from the use of
a high frequency screen. These regions are defined by the initial object map, which is translated from the page
description language (PDL). However, the information of object type obtained from the PDL may be incorrect.
Some smooth areas may be labeled as raster causing them to be halftoned with a high frequency screen, rather
than being labeled as vector, which would result in them being rendered with a low frequency screen. To
correct the misclassification, we propose an object map correction algorithm that combines information from the
incorrect object map with information obtained by segmentation of the continuous-tone RGB rasterized page
image. Finally, the rendered image can be halftoned by the object-oriented halftoning approach, based on the
corrected object map. Preliminary experimental results indicate the benefits of our algorithm combined with
the new imaging pipeline, in terms of correction of misclassification errors.