DETECTING LINE SEGMENTS AND PREDETERMINED PATTERNS IN AN OPTICALLY SCANNED DOCUMENT

Line segments in stored picture data derived from an optically scanned document are detected by detecting line elements in the picture data, determining if the length of a line element is greater than a threshold length l.sub.th, determining a connecting relationship of one line element of greater than threshold length to another line element of greater than threshold length, and identifying a line segment in accordance with the connecting relationship of the line elements. Predetermined patterns formed of the detected line segments are detected by sensing if two line segments adjoin each other, determining the adjoining relationship of those two line segments, and identifying the pattern which includes those two line segments as a function of their adjoining relationship.