A symmetrical image is generated from an entire object image having an approximately symmetrical shape. The object image has a plurality of control points. The object image may be a polygon having vertices as the control points. In a first step, an operator inputs image data indicative of the positions of the control points. In a second step, an axis of symmetry of the object image is determined automatically or manually. In a third step, a first control point is paired with a second control point having a symmetrical relationship with the first control point. In a fourth step, the image data is modified so that the first and second control points have symmetrical positions about the axis of symmetry. In this modification, the first point is moved to the symmetrical position, whereas the second point is fixed.