google.ca/patents/US7774155
Acceleration data which is output from an acceleration sensor is obtained. A rotation motion of an input device around a predetermined direction as a rotation axis is determined by comparing a start point in a two-dimensional coordinate system which is represented by the first acceleration data obtained in a predetermined period, and an end point in the two-dimensional coordinate system which is represented by the last acceleration data obtained in the predetermined period. Coordinate axes of the two-dimensional coordinate system are defined based on components of the two axial directions o...