OpenCV provides the Haar Feature-based Cascade Classifiers for face detection, this model was presented by Paul Viola and Michael Jones in 2001.
This method apply series of classifiers to every subwindow of input picture, the first one classifier eliminates a large number of non-faces examples with very little processing. The other classifiers eliminate additional negatives but require additional computation. After several stages of processing the number of sub-windows have been reduced radically.
Thanks to IEEE Zewail City Student Branch.