Abstract: The characteristic curve is moved only along the first control curve, and the resultant curve is subjected to a rotation and a homothetic deformation keeping the similarity, thereby generating an intermediate curve having the ends located on the first and second control curves. A plurality of such intermediate curves can be generated by executing this operation with various values of distances by which the characteristic curve is moved. Data associated with the surface to be generated is obtained from the group of the intermediate curves and is then subjected to a perspective transformation for projecting data onto a desired two-dimensional screen; as a result, the generated data is displayed on a display unit.