Ref :  http://android-developers.blogspot.tw/2015/08/face-detection-in-google-play-services.html

Here are the method calls available to a face object:

• getEulerY() - Returns the rotation of the face around the vertical axis -- i.e. has the neck turned so that the face is looking left or right [The y degree in the above image]
• getEulerZ() - Returns the rotation of the face around the Z azis -- i.e. has the user tilted their neck to cock the head sideways [The r degree in the above image]

    z ->  r

     Y   -> y