Leon C. Williams

Halftone Processing In The Xerox 7650 Image Scanner

A review of the halftone processing techniques used in the Xerox 7650 Pro Imager document scanner is presented. The halftone processing techniques which are necessary for high quality, continuous tone document scanners to function in lithographic, xerographic and similar graphic arts environments are described. These processing techniques include image histogram manipulation, electronic screening, halftone removal and image enhancement. A method of automatic contrast adjustment which compensates for low contrast continuous tone or colored background images is shown. The need for programmable halftone generation to adapt to a wide variety of printer reproduction characteristics is discussed. A programmable halftone removal technique, and its application to halftoned pictorials, is addressed from a practical perspective. This discussion focuses particularly on the problem of the moire encountered when applying halftone patterns to scanned images which contain halftone pictorials themselves. Utilizing several example images, it is shown that by combining the described halftone image processing techniques available in the 7650 scanner, high quality, hardcopy images can be obtained on a variety of high contrast printers from a variety of continuous tone and halftone input documents.