how to classify normal faces and smiling faces. What Technique is involved behind this?
I think someone asked this earlier this summer. There are several patents covering this. Look them up and they will tell you the whole algorithm. That's what they have to do in order for their patent to be granted.
You can also try by looking around here: http://iris.usc.edu/Vision-Notes/bibliography/contentspeople.html#Face%20Recognition,%20Detection,%20Tracking,%20Gesture%20Recognition,%20Fingerprints,%20Biometrics It has sections like:
21.3.6 Facial Expressions and Emotion Analysis and Description
22.214.171.124 Face Expression Recognition and Analysis, Facial Expression Analysis, Facial Expressions 126.96.36.199.1 Three-Dimensional Face Expression Recognition and Analysis 188.8.131.52.2 Face Action Units for Expressions and Motion Analysis 184.108.40.206.3 Face Expression Recognition from a Single Image 220.127.116.11 Emotion Recognition from Face Images 18.104.22.168.1 Emotion Recognition, from Other Than Faces 22.214.171.124 Facial Expressions, Overviews, Surveys, Data