Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1998-01-07
2001-07-17
Coles, Edward (Department: 2622)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S296000, C358S451000, C358S451000, C358S534000, C358S535000, C358S536000, C347S240000, C382S270000
Reexamination Certificate
active
06262811
ABSTRACT:
BACKGROUND OF THE INVENTION
A circuit for storing halftone screen arrays which can be clocked and arranged so that a number of dots can be generated from a single Holladay brick of threshold values, the arrangement allowing a change of phase, shift and counting direction between scans.
The simplest halftone screen can be thought of as a set of threshold values stored in a memory of a printer. As a stream of multi-bit pixels is received, each is compared to its associated value in the screen, and printed either as a ONE or a ZERO. A number of types (single centered, multi-centered and stochastic) and sizes of halftone screens are required to optimally print various kinds of graphics, pictures and text. Larger screens can provide a greater number of color levels, at the expense of needing more storage. Also, in a four (or more) color printer, each separation is normally oriented at a different angle, and therefore must have its own screen. The storage of the resultant number of screens becomes a problem, and a means for reducing the memory requirement would be a significant improvement.
U.S. Pat. No. 4,185,304, Electronic Halftone Screening by Thomas M. Holladay, describes an efficient method of storing and using a screen which requires the minimum number of locations for each halftone dot, and is incorporated by reference herein. It was determined that if the minimum number of values were arranged into rows and columns, the result is a rectangle, or “brick”, of numbers, each row of which of which is easily accessible to form each succeeding scan line of a halftoned image.
A large improvement would be to reduce the memory requirement by modifying the system so that one brick could be used to generate more than one halftone screen.
SUMMARY OF THE INVENTION
A normal halftoning circuit is conceptually arranged in the form of several circular shift registers, and continually compares each current pixel to an appropriate threshold to generate an output bit. To the extent that the Holladay brick has several lines, each successive scan line of video is compared to the next line in the brick. Along with this cycling to the next line, to the extent that the dots are not exactly vertical, there must also be a shift of one or more thresholds in the horizontal (fast scan) direction from one row of bricks to the next.
This invention is based on the realization that a single brick can be used to generate two or more halftone dots by varying the direction of readout from the right or left, by using a variable number of pixel shifts between scan lines and by varying the starting pixel for each scan line. In this way, a number of screens which can have complementary alignment angles and/or differing phase relationships can be made from one brick.
REFERENCES:
patent: 4736254 (1988-04-01), Kotera et al.
patent: 5166809 (1992-11-01), Surbrook
patent: 5898505 (1997-06-01), Lin et al.
patent: 6101002 (2000-08-01), Urasawa
Chang Chan
Hains Charles M.
Hembrock Gwendolyn L.
Coles Edward
Cunha Robert E.
Lamb Twyler
Xerox Corporation
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