A method for generating bump map data substantially in real time for use in a 3-dimensional computer graphics system. Each first female retainer defines a single open-end retaining groove for receiving one first male retainer to limit the angle of rotation of the cover member on the base member. The system is configured to group the heating elements based on load characteristics of the heating elements and turn the groups on and off sequentially in a matter that promote efficient and smooth operations. The RFDC convertor part 100 is formed by two transistors QRD, QDD working as a diode, and transistors QR1\u02dcR3, QD1\u02dcD3 and resistances RR1\u02dcR3 for forming high resistances at anode sides and cathode sides of these diodes, respectively.