Conventionally, various techniques have been proposed for detecting three-dimensional objects. A fitting technique may be a leading one. In the fitting technique, a position, a size and the like of the object are estimated from three dimensionally-measured coordinates. For example, an elliptic sphere is expressed by equation (1).
                              [                      Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                                                          (                                  x                  -                                      X                    0                                                  )                            2                                      a              2                                +                                                    (                                  y                  -                                      Y                    0                                                  )                            2                                      b              2                                +                                                    (                                  z                  -                                      Z                    0                                                  )                            2                                      c              2                                      =        1                            (        1        )            
In the equation, X0, Y0, and Z0 indicate center coordinates of the elliptic sphere, and a, b, and c are parameters determining a size of the elliptic sphere. In order to estimate a position and a size of the elliptic sphere, it is necessary to identify the center coordinates and the other parameters. Furthermore, in the case where the elliptic sphere is rotated, it is necessary to identify nine parameters including a three-dimensional rotation angle. For example, least squares, extended Hough transform, or a Kalmnan filter method is used in the identification.
Various techniques for detecting a human face have also been proposed. For example, a pair of stereographic color images is obtained with a pair of color image capture devices, and a distance image is generated from the pair of stereographic color images. A model expressing an outline of a face is generated from the distance image. On the other hand, a flesh-colored image region and an edge are extracted from one of the pair of stereographic color images. A correlation between the edge and flesh-colored image region and the outline model is correlated to detect a face region (see Patent Document 1).
There is also proposed a technique for detecting an occupant in a vehicle. For example, an ellipse whose shape approximates the occupant head is previously stored as a reference head image. An area-image sensor is provided in the vehicle to obtain an image including the occupant head. Many boundaries are extracted from brightness values of the image, and the outline of the substantially elliptic shape is detected from the boundaries. The occupant is identified by matching between the detected outline of the substantially elliptic shape and the reference head image (see Patent Document 2).    Patent Document 1: Japanese Patent Publication Laid-Open No. 2002-216129    Patent Document 2: Japanese Patent Publication Laid-Open No. 2004-53324