Journal of Electronic ImagingError diffusion algorithm with output position constraints for homogeneous highlight and shadow dot distribution
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In most images processed using error diffusion techniques, a nonhomogeneous arrangement of dots (‘‘worm’’ artifacts) may occur for highlight and shadow regions. In this article we propose a method to reduce the worm artifacts using spatial constraints for the dot arrangement. For a pixel in the shadow or highlight region, a dot is placed only if a minimum distance constraint between the current pixel and the placed dots is satisfied. If the distance constraint is not satisfied, the placement of the current dot is postponed and the quantizer error is diffused to the unprocessed pixels. The proposed method verifies the distance constraint without computing geometric distances. Instead, a roadmap enlarging from the current pixel location is used to check for placed dots. A dot is placed only if no other dot along the scanned roadmap is found. The scanned length of the roadmap depends on the input gray level and is accessed from a look-up table. The paper includes an example of halftone image processed with the proposed method.