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Electronic Halftoning for Color Reproduction

IP.com Disclosure Number: IPCOM000089602D
Original Publication Date: 1977-Nov-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 60K

Publishing Venue

IBM

Related People

Stucki, P: AUTHOR

Abstract

Color reproductions are made by printing yellow, magenta, cyan and eventually black inks in that sequence. In the case of lithographic printing, where the ink film thickness is necessarily uniform, the strength of the colors is varied by printing size-modulated dots in the same way as in ordinary halftone monochrome printing. The orientation of the screen used in making the color plates is very important. In order to keep the formation of interference or Moire patterns as unnoticeable as possible, the angular separation has to be large. In four-color reproduction the strong colors cyan and magenta, as well as black, are printed under screening angles of 15 Degrees, 75 Degrees and 45 Degrees, allowing a 30 Degrees separation between them. Yellow is printed under 90 Degrees.

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Electronic Halftoning for Color Reproduction

Color reproductions are made by printing yellow, magenta, cyan and eventually black inks in that sequence. In the case of lithographic printing, where the ink film thickness is necessarily uniform, the strength of the colors is varied by printing size-modulated dots in the same way as in ordinary halftone monochrome printing. The orientation of the screen used in making the color plates is very important. In order to keep the formation of interference or Moire patterns as unnoticeable as possible, the angular separation has to be large. In four-color reproduction the strong colors cyan and magenta, as well as black, are printed under screening angles of 15 Degrees, 75 Degrees and 45 Degrees, allowing a 30 Degrees separation between them. Yellow is printed under 90 Degrees.

While it is relatively easy to conceive digital screens running under 45 Degrees and 90 Degrees, it is much more difficult to design intermediate tilts. An approach to solve this problem (Fig. 1) is to approximate a given tilt-angle Alpha by Alpha' = arc tg a/c, where a and c are integer values only. Points A and C (Fig. 2) correspond to the centers of, e.g., "black" clusters, points B and D are the centers of "white" clusters, respectively. Digital halftone screen values 1 < i(b) </= Q(max)/2, where Q(max) represents the maximum number of amplitude quantization levels, are then placed around the cluster centers A and C. Similarly, values Q(max)/ 2 +...