Hi. I want to detect a face and draw line exactly around the face to crop it.

I searched a lot and used the EmguCV face detector. but now my code just draw a "rectangle" around the face and if I crop it, I will have a rectangle with a face inside it and it's not useful for me. this is the core of my simple code:

Code:
CascadeClassifier _cascadeClassifier = new CascadeClassifier(Application.StartupPath + "/haarcascade_frontalface_alt_tree.xml");
...