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REDUCTION OF FALSE CONTOURING THROUGH PIXEL STRETCHING

IP.com Disclosure Number: IPCOM000026541D
Original Publication Date: 1992-Aug-31
Included in the Prior Art Database: 2004-Apr-06
Document File: 2 page(s) / 138K

Publishing Venue

Xerox Disclosure Journal

Abstract

A common imaging defect found in digital halftones is known as false contouring. It occurs when there are too few gray levels available in a given halftone thereby causing visible gray steps to appear as slowly varying shades of gray. This problem is especially severe in lower cost systems that image at lower scan resolutions.

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XEROX DISCLOSURE JOURNAL

REDUCTION OF FALSE Proposed Classification CONTOURING THROUGH PIXEL U.S. C1.3581456 STRETCHING Int. C1. H04n 1/40 Jerome A. Rosenthal

A common imaging defect found in digital halftones is known as false contouring. It occurs when there are too few gray levels available in a given halftone thereby causing visible gray steps to appear as slowly varying shades of gray. This problem is especially severe in lower cost systems that image at lower scan resolutions.

In the conventional approach used to construct digital halftones, the number of gray levels that can be produced by a halftone cell is equal to the number of pixels contained in the cell. For example, a 300 spots per inch imaging system can construct a 100 cells to the inch halftone with 9 pixels per cell. Since the cell can only contain an integral number of pixels, it will produce 9 shades of gray in addition to the color white. If imaging systems were capable of producing fractional pixel values such as 1.5 pixels or 2.5 pixels, then the number of producible gray levels would be increased and false contouring would be reduced.

The present invention is directed toward the use of two halftoning algorithms for an input scanner, one of them operating at twice the scan frequency of the rastor output scanner. By comparing the results of the two algorithms, the ROS can be directed to print sufficient fractional pixels to extend the gray scale for the purpose of avoiding false contouring.

The implementation of this approach consists of the following steps:

   1. Input Scanning: The input document is scanned once and is subjected to two halftoning algorithms. In a 300 spots per inch system, one of the halftoning algorithms would be at 300 spi consisting possibly of 100 cells per inch and 9 levels of gray. The second halftoning algorithm would be at a higher scan frequency and consistent with more levels of gray. The second algorithm could be based on a 600 spots per inch system, resulting in 36 levels of gray for a 100 cells per inch halftone. The number of pixels actuated by the two algorithms would be compared and the amount of fractional pixels required in the 300 spi system, to achieve the gray value of the 600 spi algorithm, would be identified. Since it is the purpose of this example to increase the number of gray levels by a factor of four, two extra...