To provide an improved fog effect in an interactive 3D graphic system.

Improved fog simulation is provided by providing usage of an inverse exponential fog density function and an inverse exponential-squared fog density function in fog calculation. An improved exponential fog density function and exponential-squared fog density function are provided providing an ability of programming a fog starting point value. A range error in proportion to movement of a line of sight in a direction separating away from a Z-axis is prevented by using a range adjusting function for adjusting fog on the basis of an X position of a rendered picture element. A fog calculation unit example, a fog control function example and a fog related register example are disclosed also.