Inherent to most multi-color printing systems is the inability to achieve perfect registration between the primary
separations. Because of this, dot-on-dot or dot-off-dot halftone screen sets are generally not used, due to the
significant color shift observed in the presence of even the slightest misregistration. Much previous work has
focused on characterizing these effects, and it is well known that dot-off-dot printed patterns result in a higher
chroma (C*) relative to dot-on-dot. Rotated dot sets are used instead for these systems, as they exhibit a much
greater robustness against misregistration. In this paper, we make the crucial observation that while previous
work has used color shifts caused by misregistration to design robust screens, we can infact exploit these color
shifts to obtain estimates of misregistration. In particular, we go on to demonstrate that even low resolution
macroscopic color measurements of a carefully designed test patch can yield misregistration estimates that are
accurate up-to the sub-pixel level. The contributions of our work are as follows: 1.) a simple methodology to
construct test patches that may be measured to obtain misregistration estimates, 2.) derivation of a reflectance
printer model for the test patch so that color deviations in the spectral or reflectance space can be mapped to
misregistration estimates, and 3.) a practical method to estimate misregistration via scanner RGB measurements.
Experimental results show that our method achieves accuracy comparable to the state-of-the art but expensive
geometric methods that are currently used by high-end color printing devices to estimate misregistration.
A new method for halftoning using high resolution pattern templates is described, that expands the low level rendering capabilities for engines that support this feature. This approach, denoted super resolution encoded halftoning (SREH) is an extension of the Holladay concept, and provides a compact way to specify high resolution dot growth patterns using a lower resolution Holladay brick. Fundamentally, this new halftoning method involves using the SRE patterns as building blocks for constructing clustered dot growth assemblies. Like the traditional Holladay dot description, the SRE halftone is characterized by a size, height, and shift, all of which are specified at the lower resolution. Each low resolution pixel position in the SRE halftone brick contains a pair of lists. The first of these is a list of digital thresholds at which a transition in SRE patterns occurs for that pixel position, and the second is the corresponding list of SRE codes. For normal cluster dot growth sequences, this provides a simple and compact mechanism for specifying higher resolution halftones. Techniques for emulating traditional high resolution Holladay dots using SREH are discussed, including mechanisms for choosing substitutions for patterns that do not exist among the available SRE patterns.
The 30-year history of the development of digital halftone technology within one company (Xerox) is followed from the viewpoint of the author’s involvement and participation. The history has an emphasis on the evolution in complexity from very simple threshold arrays through multi-center dots, high-addressability writing, non-orthogonal screens and other methods for avoidance of color moire. The paper is not meant to address all forms of digital halftoning, but concentrates on the requirements of laser-scanned xerography and clustered dots. Graphic examples of various halftone dot-growth sequences are provided. Key advances and lessons in the development of halftoning are summarized.
KEYWORDS: Image quality, Printing, Data conversion, Information operations, Visualization, Digital imaging, Data centers, Imaging technologies, Color imaging, Graphic arts
One of the image quality problems with printing images from the WEB is that the color information associated with those images is normally incomplete, or even incorrect. In order to generate a good printout of an arbitrary image from the WEB, certain assumptions have to be made. The major variant in image quality can be found in terms of the intended (gamma) the image was designed for. The example is a WEBsite, hosting an image collection. The site might allow you to browse and print images. However, browsing is normally done on a monitor with a (gamma) of roughly 2 and printing is done on a xerographic or ink-jet printer with a (gamma) of 1. The display and print images will therefore differ quite drastically, unless the (gamma) is corrected. Unfortunately, it is not known which (gamma) setting was used by the WEBsite, and therefore a fixed conversion (e.g.: always assuming the data is intended for monitor (gamma) ) is often faulty. This talk describes a way around this problem by trying to automatically identify the correct (gamma) based on the image data.
Halftone calibration of a black and white printer is a known process that involves printing and measuring patches for many different halftone levels. It is a tedious process that has to be repeated for every halftone dot or algorithm to be used. A new calibration procedure is described that uses a halftone-independent characterization of the printer and a pixel overlap model to predict the tone response of any halftone algorithm. This enables all halftone dots and algorithms to be calibrated with only one set of printer measurements.
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