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Tracking Systems
INPUT DEVICES
Cesar Martinez Internetworked Virtual Reality
COMP6461 September 2002
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INPUT DEVICES
Successful human-computer interaction requires
efficient transfer of information from human to
computers. Such communication is mediated via
input devices.
While most input devices are conceptually simple
a lack of standards, especially in VR
interaction, prevents input devices from being
interchangeable.
Applications are generally designed to work with
a fixed set of input devices.
While this allows to exploit such devices to the
fullest both in terms of function and
performance, it has several disadvantages The
application may present a limited choice of
input devices. Support for a new device may
require changes in the application, or the
application may not be able to fully support the
features of the new device.
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INPUT DEVICES
Traditionally, computers use input devices
ranging from switches and punched cards to
keyboards and mice. However, it is obvious that
such devices are insufficient for treading the
pathways of the virtual world. VR departs from
conventional Human-Computer Interaction and
naturally requires a different set of user input
tools.
Rough classification of input devices discrete,
continuous, combination of both.
Discrete Generate one event at a time based on
the user action e.g. Keyboard, Pinch Glove, etc.
Continuous Continuously generate events in
isolation or in response to user action e.g.
Trackers, Datagloves.
Combination/Hybrid Devices Devices have the
ability to generate both discrete and continuous
events Classic example Mouse others
Joysticks, Tablets.
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INPUT DEVICES
Sufficient graphics capabilities have become
available on standards PCs. Add-on high speed
graphics processors are inexpensive and give PC
rendering horsepower that rivals low-to-mid-range
graphics workstations.
X
z
Y
Computer mouse is the facto standard for
interacting with desktop computers.
Position
Orientation
Robust, professional applications have an
absolute requirement to coordinate free-moving
human motions with image control. In these cases,
you need to instantly know not just position but
orientational data.
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INPUT DEVICES
Tracking Systems
Human exploration in virtual environments
requires technology that can accurately measure
the location and the orientation of one or
several users as they move and interact in the
environment. This is referred to as tracking
users in the environment. The location and the
orientation of each user are measured with
respect to the virtual environment coordinate
system.
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INPUT DEVICES
Sensor
Control Box
- Process signal
- Communicates
Signal
Signal
Source
Computer
Source Position N
Tracking Systems
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INPUT DEVICES
External sensor (e.g. camera)
Artificial source (e.g. reflector)
On-body sensors
On-body sensor and source
Computer
Computer Interface controller
Artificial source Ultrasound generator
Tracking Systems
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INPUT DEVICES
Tracking Systems
The usefulness of tracking devices in VR
environments depends to a large degree on whether
the computer can track the movements of the
source fast enough to keep the virtual world
synchronized with the users actions.
Degrees of Freedom (DOF)
It is defined as the number of independent
dimensions one must use to define unambiguously
the state of the system
Morrison and Newell 1998
This ability is determined by the lag, or
latency, of the signal, and the sensors update
rate.
The signal lag is the delay between the change of
the position and orientation of the target being
tracked and the report of the change to the
computer.
The update rate is the rate at which measurements
are reported to the computer.
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INPUT DEVICES
Tracking Systems
Inside-in systems sensor(s) and source(s) are
both worn on the body
Inside-out systems sensor(s) on the body
sense(s) external artificial / natural source(s)

Outside-in systems external sensor(s) sense(s)
artificial / natural source(s) on the body
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INPUT DEVICES
Tracking Systems
POLHEMUS Fastrack
Magnetic Trackers
EM trackers have a transmitter that emits EM
field along three orthogonal axes that are
detected by sensors. The sensors report
information about their position and orientation
with respect to the source.
Advantages . Relatively inexpensive . Minimal
shadowing problems . Fairly accurate . Multiple
object tracking available . Detectors are small,
unobtrusive . Popular, resources are available
Disadvantages . Stated performance only occurs
in ideal situations . Susceptible to magnetic
interference and interference caused by nearby
metal objects . Accuracy dependent on distance
between emitter and detector . Workspace
limited by emitter strength
Stylus
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INPUT DEVICES
Tracking Systems
Mechanical Trackers
Have a rigid structure with several joints. One
end is attached to the object to be tracked and
the other end is fixed in place. The joint angles
are measured and reported to give the objects
position and orientation.
Advantages . No shadowing problem. . Not
affected by external fields or materials . If
attached well, can very accurately reflect the
users joint angles. . Potential for being less
expensive as the technology has been developed
elsewhere . Sensors are inherently fast, and
will not limit bandwidth or latency . Body
based systems are not limited to a confined
workspace . No emitter required.
Disadvantages . Mechanical device attached to
soft tissue tradeoff between comfort and
accuracy . Must be robust enough to fit multiple
users of different height, weight, and sex. .
Potential to be bulky and heavy, limiting user's
movement . Ground based systems limit user's
workspace . Need for calibration . A kinematic
mismatch can occur, causing relative motion
between the person and device
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INPUT DEVICES
Tracking Systems
Acoustic Trackers
Use ultrasonic sound. A source produces pulses
that are received by a set of microphones
usually arranged as a triangle. The time each
pulse reaches the different microphones gives
the source position and orientation.
Advantages . Inexpensive . Longer ranges than
magnetic trackers . Microphones are small,
unobtrusive
Disadvantages . Line of sight must be maintained
. Latency proportional to distance . Many
emitters/receivers required for multiple targets
. Phase coherent method must be calibrated,
drift problem . Ambient noise interference .
Interference cause by echoes from hard surfaces
. Atmospheric distortion, temperature effects
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INPUT DEVICES
Optical Trackers
Use a combination of LEDs, video cameras, and
image processing. The LEDs can be placed on the
object to be tracked while the cameras are fixed
or the cameras are placed on the object and there
is an array of LEDs in the ceiling. The
position and orientation are obtained from
signal processing techniques.
Advantages . Nothing attached to user. . Senses
person, not just joints or points thus, person's
body can be virtual icon, rather than being
artificially created from limited information
Disadvantages . Cameras limit workspace. . Not
accurate. . Not robust. . Slow. . Correspondence
problem. . Shadowing. . Expensive.
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INPUT DEVICES
Inertial Trackers
Use gyroscopes to measure the three orientation
angles. They are connected by a cable to the
computer system.
Advantages . No shadowing problems . No emitter
required . Not effected by external fields,
materials . Potentially small, unobtrusive
sensors
Disadvantages . Relatively new VR technology .
Drift problems from integration . Need for
calibration . Possible nonlinearity
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INPUT DEVICES
Evaluation Criteria
. Interference and noise . Mass, Inertia and
Encumbrance . Multiple Tracked Points . Price
. Registration . Resolution . Lag . Update Rate .
Range
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INPUT DEVICES
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INPUT DEVICES