Nowadays some advanced cameras have the feature of "smile shot" without the photographer pressing the shutter. How do the cameras achieve this? How does it recognise the smile?
It is the same cameras that have face detection. Even though the face detection is shows as a box around the faces, it internally has a model for the eyes and mouth, so it already knows where the mouth is. The shape of the mouth is then used to detect the smile by computing a single "probability of a smile" parameter. You set a threshold for when it should be considered a smile. When the parameter is above this threshold, it will take a photo. The sensitivity and accuracy of this parameter varies from person to person, so you have to adjust the threshold accordingly.
I have worked on image processing and image recognition. Not for smile recognition on picture; but the process might be this one:
An algorithm analyzes the video stream in real time (the "live preview" display on the rear screen for a point and shot camera) to detect people face, and then to detect if there is a smile on the detected faces or not.
If you are interested in face detection, you can read this article on Wikipedia.