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HALFTONE RANGE EXTENSION THROUGH INTENSITY CONTROL

IP.com Disclosure Number: IPCOM000026540D
Original Publication Date: 1992-Aug-31
Included in the Prior Art Database: 2004-Apr-06
Document File: 2 page(s) / 120K

Publishing Venue

Xerox Disclosure Journal

Abstract

The art of digital halftoning consists of representing an input as an array of cells wherein each cell contains a fixed number of pixels. Variation in tone for each cell is achieved by controlling the number of pixels that are black or white within the cell. For example, at 600 spots or pixels to the inch, a screen can be constructed with 36 levels at 100 cells to the inch. If the number of halftone levels is to be increased. the corresponding cell frequencies must be reduced. Likewise, if the number of cell frequencies is to be increased the number of tonal levels must be sacrificed.

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XEROX DISCLOSURE JOURNAL

HALFTONE RANGE EXTENSION Proposed Classification THROUGH INTENSITY CONTROL US. C1.358/456 Jerome A. Rosenthal Int. C1. H04n 1/40

The art of digital halftoning consists of representing an input as an array of cells wherein each cell contains a fixed number of pixels. Variation in tone for each cell is achieved by controlling the number of pixels that are black or white within the cell. For example, at 600 spots or pixels to the inch, a screen can be constructed with 36 levels at 100 cells to the inch. If the number of halftone levels is to be increased. the corresponding cell frequencies must be reduced. Likewise, if the number of cell frequencies is to be increased the number of tonal levels must be sacrificed.

There is an inherent image quality trade off between coarseness of cell structure and the number of gray levels when using this approach. The most severe image quality defect resulting from an insufficient number of gray levels is false contouring, wherein areas of slowly changing gray levels are represented as visible steps in the gray level on a halftone copy. This problem becomes increasingly severe as the fundamental pixel rate of the printer is decreased. For example, at 300 spots per inch, a 100 cell screen can only support 9 levels of gray.

The present invention is directed toward increasing the effective number of gray steps and halftone imaging by controlling the ROS intensity on a cell by cell basis. Accordingly, implementation with regard to document scanning is the same as in current practice. However, the input image processing is extended so that the image will be halftoned at 100 cells to the inch and at both 600 and 300 spots per inch. Each cell is halftoned according to two algorithms, one for 600 spots per inch producing 36 levels of gray, and the other at 300 spots per inch producing 9 levels of gray. An IOT receives the output of the 300 spi algorithm, plus a two bit correction for each halftone cell. When the 600 spi algorithm...