Browse Prior Art Database

IMPROVED HIGHLIGHT AND SHADOW HALFTONE SCREENS

IP.com Disclosure Number: IPCOM000027785D
Original Publication Date: 1999-Aug-31
Included in the Prior Art Database: 2004-Apr-09
Document File: 2 page(s) / 105K

Publishing Venue

Xerox Disclosure Journal

Abstract

Disclosed is an improved halftone design with improved tone range and stability in the highlight and shadow regions. For highlight, the minimum dot size is set to the size that can be reliably printed. This size will be adjusted for each printing process and can be bigger than a full pixel. For shadow, the minimum hole size is determined the same way. A multi-centered screen design, with suitable randomization, and Pulse Width Position Modulation (PWPM), are used to increase the tone range and the number of gray levels.

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XEROX DISCLOSURE JOURNAL

IMPROVED HIGHLIGHT AND SHADOW HALFTONE SCREENS
Katherine Loj
Ying-wei Lin

Proposed Classification
U. S. C1. 399/181 Int. C1. G03g 15/04

Disclosed is an improved halftone design with improved tone range and stability in the highlight and shadow regions. For highlight, the minimum dot size is set to the size that can be reliably printed. This size will be adjusted for each printing process and can be bigger than a full pixel. For shadow, the minimum hole size is determined the same way. A multi-centered screen design, with suitable randomization, and Pulse Width Position Modulation (PWPM), are used to increase the tone range and the number of gray levels.

In traditional, multi-centered PWPM halftone design, dots first appear as partial pixels. The dots then grow until they touch to form holes. The holes thereafter shrink to partial pixels before disappearing. When a continuous gray wedge is screened with this kind of halftone and printed on a 600 Spots Per Inch (SPI), write black printer a portion of the highlight is lost due to the inability of the marking process to reproduce small dots. Thus, the minimum area coverage obtained with this type of machine is about seven percent.

An approach using a relatively low resolution (i.e., 600 SPI) to generate halftones at a reasonably high frequency (e.g., 150 cycles/inch) extends the highlights up to two percent area coverage without distracting contour lines. The basic concept is that the first dot to appear is to be a full pixel. Multi-centered dot design is next used to extend the tone range and increase the number of gray levels. Thereafter, PWPM is used to increase the number of gray levels further, and randomize the screen values at the highlights to eliminate contour lines when the dot structure changes. For write white systems, the same procedure is used for the shadow region.

In an actual implementation it was f...