Human and Virtual Agent Expressive Gesture Quality Analysis and Synthesis - Coverbal Synchrony in Human-Machine Interaction - page 284

Figure 4. Bounding volume real-time computation: at time T 1 the user has a contracted
posture, thus the BV value is very low; at time T 2 the user reaches an expanded posture,
exhibiting a higher value for the BV.
(Color image of this fi gure appears in the color plate section at the end of the topic.)
where m i is the mass of the i- th user 's body segment (e.g., head,
right/left shoulder, right/left elbow and so on) and v i is the velocityof the
i- th segment, computed as the difference in the position of the segment
at the current Kinect frame and the position at the previous frame.
3.1.3 Mid-level features: Impulsivity index
Impulsive body movements are those performed quickly, with a high
energy and by suddenly moving spine/limbs in a straight direction.
We adopt this definition after reviewing some literature about human
movement analysis and synthesis. Wilson et al. (1996) found that stroke
gestures have a lower number of execution phases compared to other
conversational gestures, suggesting that their execution is shorter in
time. In Laban's theory, impulsive gestures have quick time and free
flow, that is, they are executed quickly with energy flowing through
body in a consciously uncontrolled way. Finally, Bishko (1991) defines
impulsive gestures as “an accent leading to decreasing intensity”.
The measure we propose for the Impulsivity Index mid-level
feature is a combination of the two low-level features: Kinetic Energy
(KE) and Bounding Volume (BV). KE is firstly used to identify the
gesture duration dt : for example, using an adaptive threshold, when KE
becomes greater than the threshold we identify the gesture beginning
time; when KE goes below the threshold we identify the ending time.
Then, if KE is higher than a fixed energy threshold and the gesture
length dt is lower than a fixed time threshold, then Impulsivity Index is
equal to the ratio between the variation of BV and the gesture length dt :
let time threshold = 0.45 s;
let energy threshold = 0.02;