face detection research confronts the full range of challenges found in general purpose, object class recognition.
However, the class of faces also has very apparent regularities that are exploited by many heuristic or model-based methods or are readily “learned” in data-driven methods.
One expects some regularities when defining classes in general, but they may not be so apparent.
Finally, though faces have tremendous within-class variability, face detection remains a two class recognition problem (face versus nonface).