说明：  aleix@ecn. Semantic queries to a database of images are more desirable than low-level feature queries, because they facilitate the user s task. One such approach is the object-related image retrieval. In the context of face images, it is of interest to retrieve images based on people s names and facial expressions. However, when images of the database are allowed to appear at dif- ferent facial expressions, the face recognition approach encounters the expression-invariant problem, i.e. how to robustly identify a person s face for which its learn-