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Optical Engineering

Moire Factors And Visibility In Scanned And Printed Halftone Images
Author(s): Joseph Shou-Pyn Shu; Chia Lung Yeh
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Paper Abstract

Unacceptable moire distortion may result when images that include periodic structures such as halftone dots are scanned. In the frequency domain, moire patterns correspond to visible aliased frequencies. In the spatial domain, moire patterns are evident as cyclic changes in the size of halftone dots, producing visible periodic "beat" patterns. Moire pattern formation depends on the following factors: (1) the halftone screen frequency, (2) the scan frequency, (3) the angle between the scan direction and the halftone screen, (4) the scanner aperture size and shape, (5) quantization errors from the thresholding operation, (6) scanner and printer noise, and (7) the ink flow in the paper during printing. This paper analyzes the visibility of moire patterns in terms of these factors. In addition, the paper describes an approach to reducing the visibility of moire patterns by directly manipulating the moire formation factors. With an appropriate selection of the scan frequency and screen angle for a given screened image, moire beat frequencies in the scanned image can be reduced to a subvisible level. This approach thereby achieves moire reduction without the need for scanning at extreme scan frequencies or postscan image processing. The strengths of this ap-proach are (1) no data volume increase due to high frequency scan and (2) no need for time-consuming postscan processing. Computer-simulated and actual scan images are presented to illustrate the approach.

Paper Details

Date Published: 1 July 1989
PDF: 8 pages
Opt. Eng. 28(7) 287805 doi: 10.1117/12.7977037
Published in: Optical Engineering Volume 28, Issue 7
Show Author Affiliations
Joseph Shou-Pyn Shu, NYNEX Corporation (United States)
Chia Lung Yeh, Apple Computer Inc. (United States)

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