A digital circuit for detecting changes in the gray scale of scanned images is disclosed. Each line of video information produced by scanning the image is digitized. Each digital sample is compared to digital samples delayed a predetermined amount to generate a difference signal which is indicative of...http://www.google.com.hk/patents/US4143401?utm_source=gb-gplus-share專利 US4143401 - System for generating line drawing of a scanned image
