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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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