Figure 2.

Schematic representation of the performed experiments: (a–c) body–device distance experiment: While keeping the light and the body–device angle conditions constant, we modified the distance of acquisition; (d–f) body–device angle experiment: while keeping the light and the body–device distance conditions constant, we modified the angle of acquisition; (g–i) light influence experiment: while keeping the body–device distance and the body–device angle conditions constant, we modified the illumination conditions during the acquisitions.