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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?

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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.

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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.

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