To constitute an operation method capable of calculating a shape factor highly accurately, at high speed and at low cost, concerning calculation of the shape factor required in calculation of energy between the solid surfaces or light energy in a computer graphic based on radiosity in an optical three- dimensional structure.

In a shape factor operation by a projection method, summit coordinate values of a projected graphic are delivered to a three-dimensional image processing part mounted on an operating system of a computer, and the operating system and the three-dimensional image processing part are allowed to execute automatic processing of area calculation of the projected graphic and interference processing between each projection surface. When a quadrangular element is larger compared with the distance between each quadrangular element for calculating the shape factor, the element is subdivided so that an angle formed by the center point of one element and two summits of the other element becomes a fixed value or less, to thereby realize speed increase, cost reduction and precision improvement of the shape factor operation.