V. Ostromoukhov, N. Rudaz, I. Amidror, P. Emmel, R.D. Hersch
Int. Symposium on Advanced Imaging and Network Technologies, Conf. Holographic and Diffractive Techniques, Oct. 96, Berlin, SPIE Vol. 2951, 126-133
In a recent publication [Ostro95], a new image reproduction technique, Artistic Screening, was presented. It incorporates freely created artistic screen elements for generating halftones. Fixed prede fined dot contours associated with given intensity levels determine the screen dot shape's growing behav iour. Screen dot contours associated with each intensity level are obtained by interpolation between the fixed predefined dot contours. A user-defined mapping transforms screen elements from screen element definition space to screen element rendition space. This mapping can be tuned to produce various effects such as dilatations, contractions and non-linear deformations of the screen element grid.
Although Artistic Screening has been designed mainly for performing the creation of graphic designs of high artistic quality, it also incorporates several important anti-counterfeiting features. For example, bank notes or other valuable printed matters produced with Artistic Screening may incorporate both full size and microscopic letters of varying shape into the image halftoning process.
Furthermore, Artistic Screening can be used for generating screen dots at varying frequencies and orien tations, which are well known for inducing strong moiré effects when scanned by a digital colour copier or a desktop scanner. Moiré effects due to scanning of frequency modulated dots and lines have been dis cussed by Spannenburg.
However, it is less known that frequency-modulated screen dots have at each screen element size a differ ent reproduction behaviour (dot gain). When trying to reproduce an original by analog means, such as a photocopier, the variations in dot gain induce strong intensity variations at the same original intensity lev els. In this paper, we present a method for compensating such variations for the target printer, on which the original security document is to be printed. Potential counterfeiters who would like to repro duce the original with a photocopying device may only be able to adjust the dot gain for the whole image and will therefore be unable to eliminate the undesired intensity variations produced by variable fre quency screen elements
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