When the image is a human face as illustrated in FIG. 4A, in a dark area such as eyes and a mouth, the gradient vectors K calculated as described above are directed to the center of each of the eyes and the mouth, as illustrated in FIG. 4B. In a bright area such as a nose, the gradient vectors K are directed outside from the position of the nose, as illustrated in FIG. 4B. Further, since the densi