The distance between a moving character and a fixed character which are
displayed in a pseudo-three-dimensional space on a display monitor screen
is calculated by identifying one of polygons which make up the fixed
character and which contains a point of planar coordinates representing
the position of the moving character, and calculating the distance between
the moving character and the polygon which has been identified. A polygon
representing a mark associated with the moving character is applied to a
plane of a polygon of the fixed character by determining a normal vector
which is normal to the plane of the polygon at a predetermined position
thereon, determining a first vector contained in the plane of the polygon
perpendicularly to the normal vector, determining a second vector
contained in the plane of the polygon perpendicularly to both the normal
vector and the first vector, and determining the rotation matrix from the
normal matrix, the first matrix, and the second matrix.