In optical image processing methods for determining the occupation of a vehicle seat, it is advantageous to know the position of the vehicle seat (14), even in the case when said seat is occupied. The above is achieved, whereby the side surface of the backrest (11), facing the camera (30) is part of a boundary plane. A section (T, B), between a separating plane, cutting vertically through the vehicle seat and the boundary plane (B) is at least partly not covered by a seat occupant in the direction of the camera. By means of an image processing method the position of the seat may be determined.