descreening

Descreening is a digital image processing technique used to remove or reduce the moiré patterns that appear when scanning a printed image. These patterns are caused by the interference between the regular dot pattern of the halftone screen used in printing and the grid of pixels in the scanner. When a scanner captures a printed image, it essentially samples this halftone pattern, and if the angles or frequencies of the two patterns are not perfectly aligned, a swirling or wavy moiré effect can occur.

The goal of descreening is to smooth out these moiré artifacts, restoring a more natural appearance to the scanned image. Various algorithms exist for descreening, often employing frequency-domain or spatial-domain filtering techniques. Frequency-domain methods might involve identifying and suppressing the dominant frequencies associated with the moiré pattern. Spatial-domain methods might use blurring or averaging techniques in localized areas to break up the repetitive moiré structure. The effectiveness of a descreening method can depend on the original print's characteristics, such as the screen ruling (lines per inch) and the type of ink used. Over-processing can lead to a loss of detail or excessive softness in the image.