Abstract: A given picture is scanned to store picture cell data thereof into memory positions of a first memory device. Non-boundary signs are written into a second memory device at its memory positions corresponding to the memory positions, filled by the picture cell data, of the first memory device. Coordinate values of the boundary picture cell data in the first memory device are produced. Then, discrimination signs are obtained by comparing the coordinate values, and are written into the second memory device at the memory positions corresponding to the picture cells on the closed boundary. The discrimination signs stored in the second memory device are checked to determine whether each of the non-boundary signs corresponds to picture cell data inside or outside of the closed boundary.