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
18.104.22.168 Face Expression Recognition and Analysis, Facial Expression Analysis, Facial Expressions 22.214.171.124.1 Three-Dimensional Face Expression Recognition and Analysis 126.96.36.199.2 Face Action Units for Expressions and Motion Analysis 188.8.131.52.3 Face Expression Recognition from a Single Image 184.108.40.206 Emotion Recognition from Face Images 220.127.116.11.1 Emotion Recognition, from Other Than Faces 18.104.22.168 Facial Expressions, Overviews, Surveys, Data