(1) Field of the Invention
The present invention relates to a technology for improving speed and accuracy of face detection function used, for example, when photographing a person with digital cameras, and more particularly to a face image detection device provided in an image recording apparatus such as a digital camera and a personal computer.
(2) Description of the Related Art
The technology for detecting a location of human face has been used not only for surveillance systems, but also for commercial digital still cameras. In recent years, digital cameras have gained widespread use; the processing speed and the technology for high-image quality have dramatically improved. For example, the technology for detecting a face location has been in practical use for automatic focus function, automatic exposure correction function, and individual authentication by face identification for a person (especially for face) in digital cameras, along with the improvement on the performance of digital still camera, for example. In addition, the face location detecting technology is useful and expected to be widely applied as a technology for extracting a person from a moving image or still image when searching an image, indexing, or organizing an image album.
Patent Reference: Japanese Unexamined Patent Application Publication No. 2006-72770 has been proposed as a conventional technology for detecting the face location which enables improvement in accuracy of location detection regardless of the direction of face. In Patent Reference, when detecting the location of a face, a front face image is divided into the left half and the right half, and the location of the face image is detected using two sets of parameters, one is for the left half and the other is for the right half, without using parameters for the front face image. This increases a location detection rate for a face, not facing front but turning sideways.
However, in Patent Reference, it is necessary to perform reference and comparison processes for each set of parameters in order to detect the location of face image from one piece of input image since the location of the face image is detected using the two sets of parameters as described above. To put it differently, it is necessary to perform the reference comparison process twice. This causes problems in the time required for detecting the location of face image, and increase in power consumption necessary for detecting the location of face image.
Furthermore, in Patent Reference, the location of face image is detected with high accuracy when a person is turning sideways, however, there is a problem that the face image is not detected with high accuracy when the face image is rotated around the optical axis of a digital camera as the central axis.