Most commercially printed images are halftoned using a screening process. In order to reproduce printed documents containing images, one usually performs descreening or inverse halftoning to avoid possible moire patterns. There exists a variety of gray scale halftone descreening techniques in the literature. However, color halftone descreening is still an ongoing research topic. In this paper, we present two descreening approaches: suboptimal FIR filter and a two-stage color sigma filter. The suboptimal FIR descreening filter offers an efficient descreening approach for gray scale halftoned images. In the mean time, a color halftone descreening technique based on the color sigma filter does not assume any a priori knowledge about the halftoning process, making it applicable to any color halftone image. Similar to the anisotropic diffusion algorithm and total variation minimization techniques designed for gray scale images, the color sigma filter is an O(N) algorithm which can smooth out variation within each region and preserve edge information in the RGB color space. When combined with halftone segmentation techniques, a complete document processing algorithm for gray-scale and color documents can be created.
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