Abstract: An apparatus for converting a three dimensional data inputted in one system to a data for another system so that the converted data is used for displaying graphical models representing a real object includes a unit for separating a three dimensional data into a Gregory patch data and a non-Gregory patch data, a unit for converting the Gregory patch data to a general patch data and a unit coupled to both the separating unit and to the converting unit for transforming the general patch data and the non-Gregory Patch data to a data used for another system. Furthermore, the converting unit is used for receiving information on control points of the Gregory patch, and for transforming the information into another information on control points and weights of the general patch data, and the general patch data is a rational Bezier patch. The converting unit is further used for transforming the information into another information on a non-uniform rational B-Spline patch.