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Removing Halftone Patterns From Scanned Images

Using illustrations from published technical articles is part of many lectures and presentation citations, and given the availability of flat bed scanners, should be easy to accomplish. The problem that remains is removing halftone patterns and other periodic noise that result from printing and scanning technology. Practically all magazines and newspapers are printed using a regular halftone pattern that uses an array of dots varied in size to produce the visual illusion of continuous gray scale. Color images are usually printed with three, four or even more such patterns using different colored inks and different pattern orientations. Scanning such images into a computer can introduce a further pattern due to the moire interference between the printed pattern and the spacing of the sensors in the scanner. Such patterns are also characteristic of images obtained from single-chip video cameras because of the color filters present in front of the light sensors on the chip.