  Current Virtual Reality systems are designed for interaction under visual
control. Using built-in cameras, headsets track the user's hands or hand-held
controllers while they are inside the field of view. Current systems thus
ignore the user's interaction with off-screen content -- virtual objects that
the user could quickly access through proprioception without requiring
laborious head motions to bring them into focus. In this paper, we present
HOOV, a wrist-worn sensing method that allows VR users to interact with objects
outside their field of view. Based on the signals of a single wrist-worn
inertial sensor, HOOV continuously estimates the user's hand position in
3-space to complement the headset's tracking as the hands leave the tracking
range. Our novel data-driven method predicts hand positions and trajectories
from just the continuous estimation of hand orientation, which by itself is
stable based solely on inertial observations. Our inertial sensing
simultaneously detects finger pinching to register off-screen selection events,
confirms them using a haptic actuator inside our wrist device, and thus allows
users to select, grab, and drop virtual content. We compared HOOV's performance
with a camera-based optical motion capture system in two folds. In the first
evaluation, participants interacted based on tracking information from the
motion capture system to assess the accuracy of their proprioceptive input,
whereas in the second, they interacted based on HOOV's real-time estimations.
We found that HOOV's target-agnostic estimations had a mean tracking error of
7.7 cm, which allowed participants to reliably access virtual objects around
their body without first bringing them into focus. We demonstrate several
applications that leverage the larger input space HOOV opens up for quick
proprioceptive interaction, and conclude by discussing the potential of our
technique.
