Text-enhanced error diffusion is proposed to sharpen text regions in complex documents, including natural images with a multifunctional printer. To enhance the sharpness of the text regions, the input image is segmented into text and background regions using the maximum gradient difference. Edge-enhanced error diffusion is then applied to the text regions to sharpen the text, while Floyd and Steinberg's error diffusion is applied to the background regions to obtain smooth dot patterns. However, this combination of algorithms can generate two kinds of artifact around the text regions: boundary and dot-elimination artifacts. Boundary artifacts are a series of dots distributed around text blocks, and this propagation error generated below a text line by the edge-enhanced error diffusion is largely diffused forward into the background region. Thus, to gradually decrease these propagation errors, a grayscale dilation operator is processed along the boundary of a text block, thereby creating a gradual dilated transition region. Edge-enhanced error diffusion using different multiplicative parameters is then applied to these regions. Meanwhile, dot-elimination artifacts are dot-disappearing phenomena occurring around high-frequency regions due to the characteristic of the edge-enhanced error diffusion to sharpen edge regions more. Thus, an error scaling factor is inserted in front of the error filter in the architecture of the edge-enhanced error diffusion to scale down the propagation errors. Experiments demonstrate that text readability is improved by increasing the sharpness of the text regions, and a less grainy appearance is simultaneously achieved compared with conventional edge-enhanced error diffusion in the background regions.
Jae-Hyun Kwon, Chang-Hwan Son, Yang-Ho Cho, Yeong-Ho Ha, "Text-Enhanced Error Diffusion Using Multiplicative Parameters and Error Scaling Factor" in Journal of Imaging Science and Technology, 2006, pp 437 - 447, https://doi.org/10.2352/J.ImagingSci.Technol.(2006)50:5(437)