Screening Pictorial Images with Gray Level Reproducibility

Abstract

This paper describes an improved digital screening technique to convert a continuous-tone image into a halftone (binary) image for pictorial image reproduction. The technique first generates a digital screen in gray scale (e.g., from 0 to 255) based on a set of mathematical equations; and then it adaptively thresholds the screen at each pixel using threshold value equal to the pixel gray intensity of input image to produce the output image which contains standard halftone dots. The screened image holds as many gray shades (e.g., 255) as in the input image by yielding different halftone dot sizes so that the binary image looks as if it has gray scales. The screen can be generated for either linear or nonlinear transformations from image gray tones into spatial sizes of halftone dots in order to preserve or to enhance low frequency information (i.e., average gray tone within the area of a halftone cell), respectively. In addition, the technique preserves sharp edges in the input image by producing asymmetric halftone dots. This technique works on a wide range of screen frequency (i.e., halftone dots per inch) and scan frequency (i.e., pixels per inch). Scanned and processed images are illustrated.