6. Descreening of Scanned Images

Screen or halftone patterns are inherent to most of the images printed by electrophotographic, ink-jet printers and offset machines. When a halftoned image is scanned, a noisy effect called a Moiré pattern often appears on the scanned image. So, to avoid this, the screen should be suppressed before scanning or copying. There are plenty of methods for descreening or inverse halftoning of images. However, the algorithms each face the following dilemma: screen reduction and restoration of contone areas leads to blurring of sharp edges of text and graphics primitives, on the other hand insufficient smoothing keeps the screen and consequently the Moiré effect in halftoned areas. In this chapter, we depict two descreening algorithms intended for preservation of sharpness and contrast of sharp edges and for accurate restoration of continuous-tone areas. The quality of the approaches are evaluated by surveying observer’s opinions and numerical metrics. We consider referenced and blind quality measures. To be possible to apply the reference measure we propose an approach to simulate halftoned images.