Prediction of screener-induced moire in digital halftone pattern generation

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Title: Prediction of screener-induced moire in digital halftone pattern generation
Author: Comeau, Richard
Abstract: In the graphic arts, objectionable moire patterns are often observed on films or printed products due to the interaction of various periodic structures of halftone images. A particular type of moire pattern that results from digital halftoning at arbitrary angles and frequencies using a virtual screen function has been studied. A computer program was developed that produces uniform digital halftone patterns using a virtual screen approach and that calculates the corresponding amplitude spectra. It was found that aliasing due to the sampling of the virtual screen causes low frequency components in the amplitude spectrum. Moire patterns with fundamental vector frequencies equal to those of the strong aliased components were observed in halftone images reconstructed on a film recorder. Moire was also observed at frequencies not represented or under represented in the amplitude spectrum. It is shown that this moire effect is due to the additive beating of two or more higher frequency components that differ by the frequency of the observed moire. It is suggested that the non-linearities of the film recording process amplify this effect . The effects on the resulting moire patterns of varying the halftone parameters of dot size, dot shape, screen angle, and screen frequency were examined. In general, the amplitude spectra are complex, indicating many overlapping patterns. Screener induced moire was found to behave in a nearly identical manner to that induced by digital scanning of an existing halftone .
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Date: 1990-08

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