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80 4. Building a Practical VR System
Figure 4.21. By placing markers (the letters A, C and D) on the objects to be tracked,
it is possible to determine their locations within the scene and track their movement.
Using multiple camera views allows the tracking to continue so long as each marker is
visible in at least one camera’s field of view. (Scene courtesy of Dr. B. M. Armstrong
and Mr. D. Moore.)
4.3.5 Mechanical Tracking
Mechanical tracking determines the position of a tracked point by connecting
it to a reference point via an articulated linkage of some kind. H aptic devices
used to give force feedback in a limited desktop working volume are going
to provide, as a side effect, accurate tracking of the end effector. Trackers of
this kind measure joint angles and distances between joints. Once this infor-
mation is known, it is relatively simple to determine the position of the end
point. Trackers of this type have been used to capture whole-body motions,
and body suits which measure the positions of all major joints have been con-
structed. Other mechanical devices fit around fingers and hands to acquire
dextrous and small-scale movement, for example in remote surgery and main-
tenance work in harsh environments. Figure 4.22 illustrates such mechanical
tracking systems.
Figure 4.22. The Metamotion Gypsy 5 is a whole-body motion-capture system using
articulated linkages to acquire orientation information. (The images are provided
courtesy of Anamazoo.com).
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4.3. Technology for Motion Tracking and Capture 81
4.3.6 Location Tracking
In Section 4.3.3, it was pointed out that the theory of acoustic tracking re-
lied on the time of flight of a sound pulse between an emitter and sensor.
The same concept applies to electromagnetic (EM) waves, but because these
travel at the speed of light, it is necessar y to determine the time of flight much
more accurately. However, using EM waves in the micro wave frequency band
allows tracking devices to operate over larger distances. Because these sys-
tems are a little more inaccurate than those based on hybrid inertial/acoustic
methods, we prefer to think of them as location trackers.
Perhaps one of the best-known radio-based tracking and navigation sys-
tems is the global positioning system, ubiquitously known as GPS. The GPS
(and Galileo, the European equivalent) is based on a system of Earth-orbiting
satellites [11] and may offer a useful alternative to the more limited range
tracking systems. It has the advantage of being relatively cheap and easy to
interface to handheld computers, but until the accuracy is reliably in the sub-
millimeter range, its use r emains a hypothetical question.
Back on the ground, Ubisense’s Smart Space [19] location system uses
short-pulse radio technology to locate people to an accuracy of 15 cm in
three dimensions and in real time. The system does not suffer from the draw-
backs of conventional radio-frequency trackers, which suffer from multipath
reflections that might lead to errors of several meters. In Smart Space, the
objects/people being tracked carry a UbiTag. This emits pulses and com-
municates with UbiSensors that detect the pulses and are placed around and
within the typical coverage area (usually 400 m
2
) being sensed. By using two
different algorithms—one to measure the difference in time of arrival of the
pulses at the sensors and the other to detect the angle of arrival of the pulses
at the sensor—it is possible to detect positions with only two sensors.
The a dvantage of this system is that the shor t pulse duration makes it
easier to determine which are the direct signals and which arrive as a result
of echoes. The fact that the signals pass readily through walls reduces the
infrastructure overhead.
Another ground-based location tracking system that works both inside
buildings and in the open air is ABATEC’s Local Position Measurement
(LPM) system [1]. This uses microwave radio frequencies (∼5–6 GHz) emit-
ted from a group of base stations that can determine the location of up to
∼16,000 small transponders at a rate of more than 1000 times per second
and with an accuracy of 5 cm. The base stations are connected via optical
fiber links to a hub that interfaces to a standard Linux PC.
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82 4. Building a Practical VR System
These location-tracking systems might not be the best option for tracking
delicate movement or orientation, e.g., for measuring coordination between
hand and eye or walking round a small room or laboratory. However, there
are many VR applications such as depicting the deployment of employees in
an industrial plant, health-care environment or military training exercises to
which this technology is ideally suited.
4.3.7 Hybrid Tracking
H ybrid tracking systems offer one of the best options for easy-to-use, simple-
to-configure, accurate and reliable motion-tracking systems. There are many
possible combinations, but one that works well involves combining inertial
and acoustic tracking. The inertial tracking component provides very rapid
motion sensing, whilst acoustic tracking provides an accurate mechanism
for initializing and calibrating the inertial system. If some of the acoustic
pulses are missed, the system can still continue sending position information
from the inertial sensor. In the system proposed by Foxlin et al. [7], a head-
mounted tracking system (HMTS) carries an inertial sensor calibrated from
an acoustic system. The acoustic system consists of a network of transponders
placed around the boundaries of the tracing volume (typically a large room).
The HMTS has a light emitter which sends a coded signal to trigger ultrasonic
pulses from the transponders one at a time. The time of flight for the sound
pulse from transponder to acoustic receptor (microphone) gives its distance.
With three or four received ultrasonic pulses, the position of the sensor can be
determined. Having eight or more transponders allows for missed activations,
due to the light emitter not being visible to the transponder or the response
not being heard by the sound sensor. The system has many other refinements,
such as using multiple groups of transponders and applying sophisticated sig-
nal processing (Kalman filtering) algorithms to the ultrasound responses to
eliminate noise and echoes. Figure 4.20(b) illustrates the general idea. The
advantages of this system are its robu stness due to transponder redundancy,
noise and error elimination due to transponders being activated by the sensor
itself and the electronic signal processing of the ultrasonic response.
4.3.8 Commercial Systems
Successful commercial systems developed in recent years have used optical,
magnetic or combined acoustic and inertial tracking. We have seen Ascension
Technology’s Flock of Birds magnetic tracking system [2] used in conjunction
with the human modeling and simulation package Jack [20] to make accurate
measurements of torque and stress forces on people operating equipment. Its
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4.4. Summary 83
scanning range is typically up to 3 m. laserBIRD [3], also from Ascension
Technology, is another successful product using an optical scanning approach.
It is immune to distortion, has a wide area coverage and is typically used for
head tracking up to a distance of 1.8 m. Using a combination of inertial
and acoustic techniques, InterSense’s IS-900 [10] and associated products can
operate over very large ranges, typically up to 18 m.
For motion-capture work, Ascension Technology’s MotionStar system [4]
uses the same magnetic approach as the Flock of Birds and can sense from up
to 108 points on 18 different performers. Meta Motion’s Gypsy [12] is a
mechanical tracking system that does not suffer from any occlusion problems
and has a moderate cost.
There are many other commercial products for motion tracking, and an
even greater number of custom systems in use in research labs. The few that
we highlight here are ones that we have seen in use or used ourselves and
know that they live up to the claims. For illustrative purposes, Figure 4.23
gives a sense of their physical form.
Figure 4.23. Some example commercial motion-tracking systems. (a) Ascension’s
Flock of Birds (magnetic tracking). (b) InterSense’s IS-900 acoustic tracking system
(Photographs courtesy of Dr. Cathy Craig, Queen’s University Belfast).
4.4 Summary
In this chapter, we examined the current state of the technology used to build
VR systems. It seems that VR on the desktop, for applications such as engi-
neering design and the creative arts, is flourishing and providing a valuable
and substantially complete service. On the other hand, any system that at-
tempts to free the virtual visitor from her seat or desktop still has a long way
to go, especially in the area of interacting with the sense of touch.
In any practical VR system, the ability to sense where things are and
what they are doing in the real world is a significant problem. It requires
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84 4. Building a Practical VR System
quite complex mechanical and electronic hardware and often sophisticated
software processing, too. F or example, to avoid delays in obtaining position
measurements, a system will often tr y to predict a real-world movement being
tracked. The software will in many circumstances also apply signal processing
techniques so that errors and missing data do not cause significant problems
to VR application programs making use of real-time, real-world tracking as
an input.
In the future, it is likely that display technology will continue to improve.
For example, new flexible LCD-like displays may allow us to cover the surface
of a cave with exceptionally bright displays that don’t need space-consuming
back projection or shadow-casting front projection, either. Indeed, there is
no reason why immersive VR has to rely on HMDs or cave-wall projections
alone. HMDs with a see-through facility cannot paint a background a round
the real elements in a scene, whilst in a cave, it is not possible to put 3D
objects right up close to the user. However, a system involving a wireless
lightweight see-through HMD worn in a stereoscopic cave is just around the
corner. Most of the human interface hardware and projection equipment is
already available, and any PC is power ful enough to run the software.
It is perhaps in the area of large-scale interaction with our senses of touch
and inertial movement where the significant challenges lie. Thus, technically
adequate hardware is still needed to realize the full potential of VR.
Bibliography
[1] ABATEC Electronic AG. Local Position Measurement. http://www.lpm-world.
com/.
[2] Ascension Technology. Flock of Birds. http://www.ascension-tech.com/
products/flockofbirds.php.
[3] Ascension Technology. laserBIRD. http://www.ascension-tech.com/products/
laserbird.php.
[4] Ascension Technology. MotionStar. http://www.ascension-tech.com/produ cts/
motionstar.php.
[5] ARToolKit. http://www.hitl.washington.edu/artoolkit/.
[6] D.A.Bowman,E.Kruijffm,J.J.LaViolaandI.Poupyrev.3D User Interfaces:
Theory and Practice. Boston, MA: Addison Wesley, 2005.
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