1. A method of extracting features from a gray-scale image, said method comprising the steps of: a) generating polygons which approximate a curved surface represented by said gray-scale image; b) extracting features with respect to common edges of adjoining polygons; and c) generating boundaries of document elements based on said features, wherein said step b) comprises the steps of b1) cutting said surface by a horizontal plane to generate intersection lines between said horizontal plane and said polygons around each one of said common edges, b2) extracting directional features with respect to each one of said common edges based on said intersection lines, and b3) extracting convex/concave features with respect to each one of said common edges based on said intersection lines.