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In order to get more images, you can use GENKI-4k Face, Expression, and Pose Dataset [1]

The GENKI-4K dataset contains 4,000 face images spanning a wide range of subjects, facial appearance, illumination, geographical locations, imaging conditions, and camera models. All images are labeled for both Smile content (1=smile, 0=non-smile) and Head Pose (yaw, pitch, and roll parameters, in radians).

I have used this database to train a smile classifier using LBP + SVM approach (some years ago) and results were quite satisfactory. Steps:

• (1) Face detection
• (2) Get Local Binary Pattern from the face
• (3) SVM classification

Step (2) can be decomposed in:

• crop face region
• normalize face region based on eye coordinates
• histogram equalization or something similar
• Apply LBP to a NxM non-overlapping region

The public GENKI-4K dataset is available for download here [2].

[1] http://mplab.ucsd.edu/wordpress/?page_id=398

[2] http://mplab.ucsd.edu/wordpress/wp-content/uploads/genki4k.tar

Two sample images: