18 January 2010 Descreening of scanned images
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Abstract
Screen or halftone pattern appears on the majority of images printed on electrophotographic and ink-jet printers as well as offset machines. When such halftoned image is scanned, a noisy effect called a Moiré pattern often appears on the image. There are plenty of methods proposed for descreening of images. Common way is adaptive smoothing of scanned images. However the descreening techniques face the following dilemma: deep screen reduction and restoration of contone images leads to blurring of sharp edges of text and other graphics primitives, on the other hand insufficient smoothing keeps screen in halftoned areas. We propose novel descreening algorithm that is primarily intended for preservation of sharpness and contrast of text edges and for restoration contone images from halftone ones accurately. Proposed technique for descreening of scanned images comprises five steps. The first step is decrease of edge transition slope length via local tone mapping with ordering; it is carried out before adaptive smoothing, and it allows better preservation of edges. Adaptive low-pass filter applies simplified idea of Non-Local Means filter for area classification; similarity is calculated between central block of window and different adjacent block that is selected randomly. If similarity is high then current pixel relates to flat region, otherwise pixel relates to edge region. For prevention of edges blurring, flat regions are smoothed stronger than edge regions. By random selection of blocks we avoid the computational overhead related to excessive directional edge detection. Final three stages include additional decrease of edge transition slope length using local tone mapping, increase of local contrast via modified unsharp mask filter, that uses bilateral filter with special edge-stop function for modest smoothing of edges, and global contrast stretching. These stages are intended to compensate decreasing of sharpness and contrast due to low-pass filtering, it allows to enhance visual quality of scanned image. For parameters adjusting for different scanning resolutions and comparison with existing techniques test target and criteria were proposed. Also the quality of proposed approach is evaluated by surveying observer's opinions. According to obtained outcomes the proposed algorithm demonstrates good descreening capabilities.
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Ilya V. Kurilin, Ilya V. Kurilin, Ilia V. Safonov, Ilia V. Safonov, HoKeun Lee, HoKeun Lee, Sang Ho Kim, Sang Ho Kim, } "Descreening of scanned images", Proc. SPIE 7528, Color Imaging XV: Displaying, Processing, Hardcopy, and Applications, 75280D (18 January 2010); doi: 10.1117/12.838444; https://doi.org/10.1117/12.838444
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