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Browse Prior Art Database

HALFTONING AT LOW SCAN RATES

IP.com Disclosure Number: IPCOM000024227D
Original Publication Date: 1979-Oct-31
Included in the Prior Art Database: 2004-Apr-02
Document File: 2 page(s) / 190K

Publishing Venue

Xerox Disclosure Journal

Abstract

At low scan rates, e.g., 70 to 100 pixels per inch, it is difficult to obtain enough gray scale levels to minimize contouring without using an objectionably low screen frequency, e.g., 25 dots per inch.

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XEROX DISCLOSURE JOURNAL

HALFTONING AT LOW SCAN RATES Proposed Classification

Keith T. Knox U.S. Cl. 358/283 Louis 0. Mailloux mt. Cl. H04n 1/40 Paul G. Roetling

At low scan rates, e.g., 70 to 100 pixels per inch, it is difficult to obtain enough gray scale levels to minimize contouring without using an objectionably low screen frequency, e.g., 25 dots per inch.

If the horizontal bit rate is increased (for example, doubled) an improvement is realized. The image is expanded horizontally (by replicating pixels) and is haiftoned with an asymmetric halftone cell. An image containing 200 bits per inch

horizontally and 100 bits per inch vertically is now in the proper proportion for use with a higher screen frequency, e.g., 35 dots per inch, for the same number of gray scales.

It is also advantageous to asymmetrically shape the optical spot consistent with the bit period. The width of the spot along the direction of scan can be many times narrower than the bit period and preferably should be less than the reciprocal of the bit period. Perpendicular to the scan, the edges of the spot should be approximately gaussion to provide uniform exposure in a write white system.

Steeper perpendicular edges on the spot provide increased sharpness for text but place more stringent demands on scan placement accuracy. In a write black system (particularly for text) background is not a problem, and the spot size can be considerably reduced with more relaxed requirements on scan accurac...