[tseid]

Its a tough feature to implement- the sun's position in the glass depends on your perspective as well, not just the sun's position in the sky.

Easier to do in a helmet than in a windshield, I suppose.

[eemil]

Perspective shouldn't be a problem. Some cars already do eye tracking, using a camera placed near the gauge cluster, as part of a driver monitoring system.

[LorenPechtel]

Eye tracking isn't enough--you need to know where the eyes are, not where they are looking.

[httpsterio]

that's the easier part of eye tracking. eye tracker sensors work by working out where your eyes are and determine where you're looking by calculating the angle of your pupils and approximating the direction.