To provide an image generation system, a program and an information storage medium by which a normal vector of an object is calculated with a small processing load.

Constitutional points CP1 to 15 (apexes) of the object WS are calculated in real time based on sampling points SP1 to SP15 and the normal vector of the object is calculated based on positions of the sampling points and positions of the constitutional points. A shading processing, an environmental texture mapping processing and a hit processing are performed based on the calculated normal vector and an image of the object is generated based on the calculated constitutional points. The normal vector is calculated by correcting the direction of the vector to face the direction to connect the sampling points with the constitutional points (converting the direction to be line symmetrical to a Y axis). Coordinates of the constitutional points of the object are calculated based on a function to use the coordinates of the sampling points and time as an argument. The image of the object to be expressed on a free curved surface by using control points of the free curved surface as the sampling points.