Keywords: dithering, halftoning, threshold matrices, genetic algorithms, ink jet marking devices.

Digital halftoning is a method used to convert continuous tone images into images with a limited number of tones, usually only two: black and white. In this work digital halftoning methods were applied to very low-resolution ink jet marking machines. The main problem is to do the halftoning, so that the bi-level result image does not contain artefacts, such as moiré, lines or clusters, caused by dot placement. The main optimisation problem in this work is that the average density of the dot pattern should interpolate as precisely the original image pixel values as possible. Also the dot pattern spectrum should be skewed towards high frequencies (blue noise) or middle frequencies (green noise). In our approach a hybrid of genetic algorithms with added local search and backtracking is applied to optimise digital halftoning threshold matrices. The main object of this paper is to construct the fitness function for genetic algorithm optimisation. Several fitness function components were tested and evaluated in order to achieve a satisfactory optimisation result. A threshold matrix optimisation program was programmed in C-language and the resulting threshold matrices were tested with the Khoros image processing system. After evaluation, the best threshold matrices were selected and tested in a real ink-jet marking device.

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