Haar

Haar Cascade is usually used in face detection, in fact, it can be used to detect any object.
There are many trained Haar classifier in OpenCV source code:

frank@frank-GL753VD:~$ ls /home/frank/1T/back0529/opencv/data/haarcascades/
haarcascade_eye_tree_eyeglasses.xml
haarcascade_eye.xml
haarcascade_frontalcatface_extended.xml
haarcascade_frontalcatface.xml
haarcascade_frontalface_alt2.xml
haarcascade_frontalface_alt_tree.xml
haarcascade_frontalface_alt.xml
haarcascade_frontalface_default.xml
haarcascade_fullbody.xml
haarcascade_lefteye_2splits.xml
haarcascade_licence_plate_rus_16stages.xml
haarcascade_lowerbody.xml
haarcascade_profileface.xml
haarcascade_righteye_2splits.xml
haarcascade_russian_plate_number.xml
haarcascade_smile.xml
haarcascade_upperbody.xml




Ref: wiki
它們因為與哈爾小波轉換 極為相似而得名，是第一種即時的人臉檢測運算。