Developed by Abhinav Dhall at the Australian National University in Canberra and colleagues, the software analyses all the faces in a photo to give the shot an overall "mood score".
The team used face tracking software to analyse the smiles of the faces in a group by noting the positions of nine spots on the face such as the corners of the mouth and eyes. A machine learning algorithm, trained on photos that had been pre-labelled by humans, then used this data to give each face a smile intensity score.
The team also programmed the system to incorporate information from volunteers, who assessed how important the intensity of any individual's smile was to the overall mood score of a photo. Those who were standing near the centre of a picture were given a stronger weighting, for example, while partially obscured faces were less influential.