Rotation invariant pupil detection (well, almost)

The advantage of first transforming the face so that is level and then performing eye corner and pupil detection is that we get a high degree of rotation invariance:

With that said, once the head tilt becomes large enough, the horizontal pupil movements will be too small compared to the noise. Considering a 90 degree tilt, the pupils will only move vertically when tracking the moving objects.

To fix this, one would probably have to adjust for the head tilt angle by combining the corresponding proportion of horizontal and vertical pupil movement.