title: How to choose the cascade file for face detection?
body: <p>I am a freshman for face detection. These days I try to compile the OpenCV2.1 code for face detection. I found that there are about 4 cascade files for front face detection, which are "haarcascade_frontalface_alt.xml","haarcascade_frontalface_alt_tree.xml","haarcascade_frontalface_alt2.xml" and "haarcascade_frontalface_default.xml"</p>
  
  <p>I did not find any documents to describe the difference among them, which is prefer for face detection task?</p>