Where 0 S i S 7 and z' is a non-negative integer, El- means
the decomposed edge plane, and 0(27, y) is the edge angle in
radian at the position (x, y) of the original edge image. Each
decomposed edge plane (E1) is used in both removing text
region and the Hough transform to find line features.
The characters on the document makes edges with various
edge angles as shown in Fig.4. Fig.4 shows the decomposed
edge planes from Fig.2(a). Edges from characters are located
across some decomposed edge planes compared with line fea-
tures, although the captured picture has perspective distortions.
However, line features from the boundary of a paper appear
mainly in one edge plane as shown in Fig.4(c), 4(d), 4(g), and
4(h).