Although remote gaze tracker setups are less invasive and more comfortable to the user, they have an undeniable disadvantage of the low tolerance for the user’s head movements. Compensating the noise produced by natural head movements in the gaze estimation process still represents the greatest limitation for most of the remote gaze trackers. For that reason, some researchers seek techniques to improve the gaze estimation so that the users can move their heads during an eye tracking session.
It's possible to use infrared light sources for generating reflection points (i.e., glints) in the user's cornea. These glints are formed by virtual images of the light emission sources, and they are created on frontal corneal surface in which acts as a convex mirror. In general, glints are used as reference points in relation to eye rotation movements. In practice, the alterations suffered by the pupil center position depend exclusively of eye rotation movements. On the other hand, the alterations suffered by the glints depend of user’s head movements.
Let's see how we can control the user’s head movements through the analysis of the relation between a glint and the pupil center. In the Figure 1, we show the pupil-glint vector instead of pupil center. Follow the same steps described in this tutorial and calibrate the system. Then move the center of the eyeball and see how much using the glint can increase the robustness of the gaze estimation against the head movements.