Examples - Driver Drowsiness Detection: Systems and Solutions - page 45

Additionally, in cases where the eyes are occluded in the subsequent frame, e.g.
when the driver's head tilts forward during nodding, we use a skin color-based
localization method to confirm that the face is still where it is expected to be, even
though the eyes are occluded.
Once the eyes are successfully tracked in a given frame, the pixels containing
only the eyes' region are cropped, converted to grayscale, and fed to the SVM
classifier, which will determine the current state of the eyes: open or closed.

5.2.3 Warning Stage: Nod Analysis

The precondition for reaching the warning state is that either the driver keeps his or
her eyes closed for prolonged amount of time or that specific head movement that
can be classified as nodding started to happen. In our prototype, every time closed
eyes are detected at a given point of time, timers will be activated to determine
the duration of that state. Based on those timers, the system tries to determine if the
driver is blinking or something else is happening. If the timer exceeds the duration of
time considered to be safe to keep eyes closed while driving, the system switches to
the warning stage. Once in the warning stage, the eyes continue to be tracked in the
same manner as in the regular stage. The biggest difference is that the eye location
and eye state are monitored more closely in order to detect if prolonged state of
closed eyes continues or if a driver starts (or continues) to nod. If the behavior
persists, it would mean that it is time to switch to final stage and alert the driver of
the potential danger. If, on the other hand, the eyes become open at any time while
the system is in a warning stage, that is used as a signal to return system into regular
tracking stage and reset all the counters.

Head Position Monitoring

Sudden, sharp drop of the driver's head followed by slow recovery back up to the
normal position might indicate that the driver is nodding and could be showing signs
of sleepiness. In general, the position of a driver's head while driving feeling rested
is usually above the upper threshold that system extrapolated in the initialization
stage and does not change significantly over time (Fig. 5.5 ).
When the eyes start to move vertically down and their position crosses the
upper threshold, our system considers that to be the potential beginning of a
nodding sequence (Fig. 5.6 a). If the head continues to vertically drop, portrayed
with continuous downward movement of the eyes, eventually the lower threshold
will be crossed (Fig. 5.6 b). If true nodding occurrence is detected, crossing upper
and lower thresholds should happen rather quickly, while recovering back up is
usually a much slower process (Fig. 5.6 c, d).