The Understanding of Meaningful Events in Gesture-Based Interaction - Intelligent Video Event Analysis and Understanding - page 8

(the initialization stage), the tracking procedure begins and actions are being recog-
nized. Also, whenever tracking is lost, the initialization procedure must be repeated
with the fingers positioned in the same predefined regions.
The same initialization concept but for the entire hand this time is implemented
in the HandVU system of Kolsch et al. [23] which employs tracking of multiple
features (flock of birds). Figure 5 illustrates the general idea behind systems that
employ tracking algorithms: the tracker starts processing when the hand is present
in a given location, a necessary condition to compute the values of the features that
will be monitored and followed in the succeeding video frames.

Fig. 5 Trackers usually require initialization which consists in presenting the system with the
object to be tracked. An initialization event can be triggered whenever the hand is present at
some particular location. The features required for the particular tracker are computed when
the hand-detected event is triggered.

Malik and Laszlo [31] use location criteria in order to ease the segmentation pro-
cess when detecting the user's hands for their Visual Touchpad system. The location
information is employed indirectly this time and is implemented by using a black
colored pad on top of which the hands can be reliably detected due to the high
difference in contrast between skin and background colors.
Locations and regions of interest represent powerful event triggers, irrespectively
whether they are being used for initialization only or during the actual process of
gesture detection and acquisition. They are usually combined with posture events for
which predefined hand postures are being required in order for the location events
to be confirmed. Two different scenarios can thus be identified for such events that
employ location criteria: initialization-only and continuous tracking and monitoring.

2.2 Posture-Based Events

Posture events are triggered whenever a predefined posture is detected. They have
been used in order to mark the start and stop timestamps of a motion gesture