Stepping-Driven Locomotion Interfaces - Human Walking in Virtual Environments - page 240

Fig. 11.2 Six-phase walking-in-place gait cycle. Stepping gestures can be quantified by detecting
the events, measuring how high the legs are raised, and noting the timing between the events. Note
that (1) the phase when both feet are on the floor is called double support , and (2) stepping frequency
can be calculated from the time stamps of any three successive events. (Reproduced from Wendt
[ 40 ])
Gaiter is a WIP system enabling locomotion in a virtual scene of unlimited size with
some limited real-space maneuvering [ 36 ]. Knee excursion in the horizontal plane,
measured by shin-worn trackers, differentiates virtual and real steps. In a virtual step,
i.e., stepping-in-place, the knee moves out (and up) and back again; in a real step
the knee moves out and stays out as the user takes the real step. Startup latency is
half a step since the system cannot tell if the step is real or virtual until the knee has
reached its maximum extent and either stopped or begun to travel back.
Yan et al. designed a system that set locomotion speed based on leg speed dur-
ing the period of high leg acceleration occurring just after foot-off [ 45 ]. Using re-
sults from the biomechanics literature and experimentally developed relationships
among leg-lift speed, step frequency, and forward velocity for natural walking and
for stepping-in-place, the team developed a user-specific linear function relating the
stepping-in-place leg-lift speed and forward velocity. Speed was set once per step us-
ing this function. Motion did not begin until a leg-lift speed threshold was exceeded,
resulting in a starting latency of approximately one-quarter of a step. The thresh-
old prevented false steps and allowed (slow) maneuvering steps. Per-step movement
was spread across frames and a Kalman filter was used to smooth forward movement
between leg-lifts.