Method and Apparatus for Generating Digital Half-tone Images Using a Rotated Dispersed Dither Matrix.

Victor Ostromoukhov

U.S. Patent No. 5,438,431

Method and apparatus enabling color or black/white bi-level or multi-level halftone images to be displayed or printed on various raster output devices, in particular on display devices (cathode ray tubes, plasma displays, liquid crystal displays) or on printing devices (printers, telefaxes). The originality of the present invention is found in the dither matrix R^^c*n which is obtained by applying on a c-fold replication of a well-dispersed threshold matrix D^ⁿ a one-to-one discrete rotation of Pythagorean angle alpha = arctan(b/a) or beta = arctan(a/b) around an arbitrary point, where {a,b,c} are Pythagorean triplets satisfying Diophantine equation a^²+b^²=c^² and n is an integer. The images produced using the disclosed method are visually pleasing due to the absence of visible artifacts and to the small typical structure size. In addition, this method offers very smooth intensity gradations throughout the dynamic range. The disclosed method can be applied to different one-to-one discrete rotations of a c-fold replication of a well-dispersed threshold matrix D^ⁿ but it has proved especially effective in the case where a 5-fold replication of well-dispersed threshold matrix D^ⁿ is rotated by angle alpha = arctan(3/4) or beta = arctan(4/3). The most interesting cases are those where n= 8, 16, and where the dither matrices can faithfully reproduce respectively 65 and 257 distinct intensity levels.

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