A method has been found for separating control of high and low spatial frequencies in a continuous-tone image during conversion to a halftone (binary) form for printing. This method allows edge enhancement, while simultaneously limiting creation of spurious Moiré patterns due to image frequencies near the halftone screen frequency. Macroscopic grey scale control is achieved by determination of average grey level by low-pass filtering of the image. This information is used to set halftone dot size. Microscopic or detail contrast is controlled as in the ordinary halftone process, that is, by screen shape and screen modulation, which therefore affects partial dot structure. In the absence of detail, the results of this process match ordinary halftones. In detailed areas, however, the low-frequency control limits generation of spurious low-frequency Moiré patterns, while free choice of screen modulation allows enhancement of edges up to the common limit where image noise becomes dominant.
© 1976 Optical Society of AmericaPDF Article