DIGITAL CIRCUIT FOR ACCURATELY DETECTING CHANGES IN THE GRAY SCALE OF A SCANNED IMAGE

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 a change in the gray scale having a component perpendicular to the direction of scan. The digitized samples of the video signal are also stored in a memory having a capacity sufficient to store data representing a plurality of scan lines with a typical memory having a capacity for storing digital information representing eight scan lines. Selected digital samples from the stored data are compared to samples of the line being scanned to detect changes in the gray scale having a component parallel to the direction of scan. These changes in the gray scale are combined to generate a signal indicative of the central portion of the change in the gray scale of the image being scanned to drive a printer to generate a line drawing with the position of the lines corresponding to the central portion of the changes in gray scale of the scanned image.