Shape data processing method

A method of interactively preparing a mold for a three-dimensional object, the object having a surface represented by predetermined shape data, said method comprising the steps of:

• displaying to an operator an image of the three-dimensional object;
• receiving from the operator data designating at least a portion of the three-dimensional object;
• forming a virtual projection surface of the designated portion of said object based upon said predetermined shape data;
• dividing said surface represented by said predetermined shape data into small regions;
• detecting the center of gravity of a column portion extending from each of said small regions toward said projection surface;
• detecting a normal to each of said small regions;
• and
• determining the center of gravity of the designated portion of said object by accumulating said center of gravity while concurrently weighting it with the volume of the current small region, said accumulating and said weighting being performed in accordance with the orientation of the normal from each small region to said projection surface, said orientation being in one of two directions, one direction being toward and the other being away from said projection surface;
• and
• displaying to the operator data representative of the center of gravity of said object and in response to operator input generating manufacturing data from the predetermined shape data;
• and
• milling raw material based upon the manufacturing data to form a mold corresponding to the three-dimensional object.