In the bilevel halftoning technique using a stochastic screen, an input image value is thresholded by a corresponding screen value (threshold) to represent two printable output levels, i.e. black and white. This halftoning method can be easily extended to multilevel halftoning (multitoning) by scaling the threshold before thresholding. However, if a general transformation function is used for the scaling of the threshold, banding artifacts can appear due to similar dot distributions near the printable output levels, thus resulting in discontinuity in the smooth tone transition region. Accordingly, in order to reduce such banding artifacts, this paper proposes a multitoning method that arranges the gray-level distribution by controlling the blending proportion of the adjacent output pixels based on improved threshold scaling functions. The proposed method consists of two thresholding processes to print the majority pixels and the adjacent output pixels. For input pixels within the neighborhood of the printable output levels, the majority pixels are first placed according to the point-wise comparison result between the fixed input value and the corresponding threshold value that is scaled by a general threshold scaling function. This allows the proportion of majority pixels to be controlled using the fixed input value. The adjacent output pixels are introduced to preserve a gray-level balance near the printable output levels based on the comparison result between the input value and the corresponding threshold value that is scaled by an improved threshold scaling function. The gray-level distribution across the banding regions is affected by two control factors of the improved threshold scaling function that control the blending point of the adjacent output pixels. To prove that the improved threshold scaling function, with specific factor values, could achieve a smoother visual transition, several observers investigated the gray-level distribution around the printable output levels. The experimental results confirmed that the proposed method was able to reduce the banding artifacts in both gray and color images and a better color reproduction was produced.
Tae-Yong Park, In-Su Jang, Oh-Seol Kwon, Kee-Hyon Park, Yeong-Ho Ha, "Banding-Artifact Reduction Using an Improved Threshold Scaling Function in Multitoning with Stochastic Screen" in Journal of Imaging Science and Technology, 2007, pp 502 - 508, https://doi.org/10.2352/J.ImagingSci.Technol.(2007)51:6(502)