  Across a plethora of social situations, we touch others in natural and
intuitive ways to share thoughts and emotions, such as tapping to get one's
attention or caressing to soothe one's anxiety. A deeper understanding of these
human-to-human interactions will require, in part, the precise measurement of
skin-to-skin physical contact. Among prior efforts, each measurement approach
exhibits certain constraints, e.g., motion trackers do not capture the precise
shape of skin surfaces, while pressure sensors impede direct skin contact. In
contrast, this work develops an interference-free 3D visual tracking system
using a depth camera to measure the contact attributes between the bare hand of
a toucher and the forearm of a receiver. The toucher's hand is tracked as a
posed and positioned mesh by fitting a hand model to detected 3D joints,
whereas the forearm is extracted as a detailed 3D surface. Based on a contact
model of point clouds, the spatiotemporal contact changes are decomposed as six
high-resolution time-series attributes, i.e., contact area, indentation depth,
absolute velocity, and three orthogonal velocity components, together with
contact duration. To examine the system's capabilities and limitations, two
experiments were performed. First, to evaluate its ability to discern human
touches, one person delivered cued social messages, e.g., happiness, anger,
sympathy, to another using their preferred gestures. The results indicated that
messages, gestures, and the individual touchers, were readily discerned from
their contact attributes. Second, the measurement accuracy was validated
against independent devices, including an electromagnetic tracker, pressure
mat, and laser sensor. While validated here in the context of social
communication, this system is extendable to human touch interactions such as
maternal care of infants and massage therapy.
