Browse Prior Art Database

METHOD FOR LAYING OUT HALFTONE SCREENS IN COLOR PRINTING USING SHIFTED SCREENS

IP.com Disclosure Number: IPCOM000026789D
Original Publication Date: 1993-Aug-31
Included in the Prior Art Database: 2004-Apr-06
Document File: 4 page(s) / 224K

Publishing Venue

Xerox Disclosure Journal

Abstract

In general, the printing industry uses a technique called halftoning for printing continuous tone images with binary printers. For example, a black and white photograph on the front page of a newspaper actually consists of white, black and many intermediate grays. In order to print represent colors with black ink, the dark gray area is printed with many larger black dots positioned close to each other while the light gray area is printed with less number of smaller black dots positioned further apart from each other. Thus, when viewing papers from a normal distance, the different patterned dot densities are interpreted by a viewers eyes as different grays. An equivalent technique is adapted for electronic printing where the size of a halftone dot is controlled by varying the numbers of pixels in a halftone cell.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 47% of the total text.

Page 1 of 4

XEROX DISCLOSURE JOURNAL

METHOD FOR LAYING OUT HALFTONE SCREENS IN COLOR PRINTING USING SHIFTED SCREENS
Roger Yea-Dong Yang

Proposed Classification
U.S. C1.358/454 Int. C1. H04n OO/OO

FIG. I

9 8

5 6

10

FIG. 2

XEROX DISCLOSURE JOURNAL - \'()I. 18, NO. 3 Jtlly/August 1993 447

[This page contains 1 picture or other non-text object]

Page 2 of 4

METHOD FOR LAYING OUT HALFTONE SCREENS IN COLOR PRINTING USING SHIFTED SCREENS (Cont'd)

In general, the printing industry uses a technique called halftoning for printing continuous tone images with binary printers. For example, a black and white photograph on the front page of a newspaper actually consists of white, black and many intermediate grays. In order to print represent colors with black ink, the dark gray area is printed with many larger black dots positioned close to each other while the light gray area is printed with less number of smaller black dots positioned further apart from each other. Thus, when viewing papers from a normal distance, the different patterned dot densities are interpreted by a viewers eyes as different grays. An equivalent technique is adapted for electronic printing where the size of a halftone dot is controlled by varying the numbers of pixels in a halftone cell.

Additional problems are manifested when expanding from black and white printing to color printing. Instead of one layer of black halftone dots, three layers of cyan, magenta and yellow halftone dots (or sometimes a fourth layer of black dots) are used to reproduce an original. For a set of perfectly transparent inks, these three clusters of cyan, magenta and yellow halftone dots can be laid on top of each other to reproduce a desired color. In reality, however, these inks interfere with each other by absorbing light in regions of other inks, consequently generating incorrect colors if the inks are on top of each other. To solve this problem, one conventional method in electronic printing applies rotated screens which lays three layers of color dots at three different angles. Figure 1 shows an example where three screens (clear dots, patterned dots and black dots) are rotated by 0, 30, 60 degrees or 0, 25, -25 degrees (represented on the figure by lines 2,3 and 4). Consequently, most dots in any given area on the paper form three clusters of color dots near or partially overlapped to each other, but not exactly on top of each other.

Rotated screens, however, introduce complications by generating repeated patterns that are otherwise known as moire patterns. When printing with rotated screens, depending on the angles of the screens and the distance between the color dots, dots will overlap each other at the same relative positions after a certain number of pixels in both horizontal and vertical directions. This repetition of dots or moire patterns extend in both directions, and when viewed from a normal viewing distance, appears to be a pattern of small circles or small crosses as shown in Figure 1. Usu...