Biomedical Engineering Reference
In-Depth Information
use CAVEs in the design process of creating automobiles, farm equipment, airplanes,
jet engines, and other devices.
On the other hand, CAVEs are not particularly ideal when a large amount of
ambulatory navigation is necessary. Navigation in these cases is conceptually similar
to operating a glass elevator floating in the VE—a user can walk a few steps back and
forth within the elevator, but ultimately must use some other controls to move the
elevator through space. CAVEs that incorporate a natural walking interface, such as
an omni-directional treadmill, are more suited to large-scale navigation tasks but are
relatively rare. Stepping in place in a traditional CAVE can also approximate natural
walking to a degree, but is a suboptimal solution relative to using a treadmill system
or walking in an HMD.
1.4.2.3 HMD-Based Systems
Classic HMD-Based System
HMD-based VEs allow users to wear the virtual display and carry it with them
during movement (Fig.
1.1
c). Unlike CAVEs or desktop VEs, which are in a fixed
location and require the user to remain in a relatively fixed space, HMDs can allow
users to roam freely around a larger area. From the perspective of providing or
simulating sensory information about space, this is an excellent way to simulate spa-
tial sensory information accurately. Naturalistic navigation through virtual spaces is
accomplished by physically walking, turning one's body, or looking around. All of
the spatial senses including vision, audition, proprioception, vestibular, and effer-
ence can access veridical information about one's movements and current state in
the simulated environment, with the exception that peripheral visual information is
often lacking or absent due to the decreased FOV. This is particularly true relative to
CAVEs, which can offer a FOV equal to that of natural viewing, minus any obstruc-
tion of shutter glasses. However with HMDs, the loss of visual information in the
periphery can vary widely depending on the specific device being used (e.g., FOVs
ranging from below 30
◦
to 150
◦
or more). Despite offering a view frustum similar to
that of a desktop VE display, HMDs can create a stronger sense of visual immersion
by allowing users to turn their heads and look around, giving the impression that the
virtual world truly surrounds them, and also by using blinders that occlude a user's
view of the surrounding physical environment. This prevents the visual experience
for users of desktop VEs in which perceived movement in the optic flow of the VE
conflicts with perceived stability of the world that surrounds the monitor.
Despite the potential of HMDs to provide natural walking and complete sensory
feedback for navigation, this benefit has been limited historically by a lack of space
available for navigation. While it is relatively easy to create a very large VE, the
HMD systems that portray them have typically been confined to a small physical
area due to (a) the size of one's VE facility, such as a small laboratory room, (b) the
range and capabilities of available motion tracking equipment, (c) the fixed-length
cable that tethers the HMD to a rendering computer, or often (d) a combination of
