PURPOSE: To generate such a dot pattern that a formed image is not deformed by calculating contour control points by using only a part which showing the features of the image among basic data of the image.

CONSTITUTION: The contour control points of, for example, a character 'H', parts of contour lines based upon them, and all the contour control points 201 - 202 of the character 'H' are as shown in figures. Then the contour control points 201 - 212 showing the contours of the image are calculated and only parts showing the features among the contour control points 201 - 212 are utilized when necessary according to information on the calculated contour control points to form a reduced image of the image. Consequently, the dot pattern of the reduced image with which the reduced image close to the form of a basic image can be generated.