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4.3. Technology for Motion Tracking and Capture 75
well over short periods of time and can work at high frequencies (that is, it
can report position often and so can cope well in fast changing environments).
Inertial tracking is best used in conjunction with one of the other methods,
because of the problem of initialization and the increases in error over time
without recalibration. Other problems with this method are electrical noise
at low frequencies, which can give the illusion of mov ement where there is
none, and a m isalignment of the gravity correction vector. Inertial tracking
has the potential to work over very large volumes, inside buildings or in open
spaces.
4.3.2 Magnetic Tracking
Magnetic tracking gives the absolute position of one or more sensors. It can
be used to measure range and orientation. There are two types of m agnetic-
tracking technologies. They use either low-frequency AC fields or pulsed DC
Figure 4.18. (a) A tuning-fork gyro. (b) The fork is fabricated using MEMS tech-
nology in the sensing system used in the BEI Systron Donner Inertial Divisions
GyroChip technology.
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76 4. Building a Practical VR System
fields which get around some of the difficulties that A C fields have when the
environment contains conducting materials. Magnetic trackers consist of a
transmitter and a receiver that both consist of three coils arranged orthog-
onally. The transmitter excites each of its three coils in sequence, and the
induced currents in each of the receiving coils are measured continuously, so
any one measurement for position and orientation consists of nine values.
Figure 4.19. (a) Magnetic induction in one sensing coil depends on the distance d
or d
from the field generating coil and the angle it makes with the field θ.Three
coils can sense fields in mutually perpendicular directions. (b) Three emitter coils and
three sensing coils allow the six degrees of freedom (position (x, y, z), and orientation,
pitch, roll and yaw) to be determined.
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4.3. Technology for Motion Tracking and Capture 77
The signal strength at each receiv er coil falls off with the cube of the
distance from transmitter and the cosine of the angle between its axis and the
direction of the local magnetic field (see F igure 4.19).
The strength of the induced signal as measured in the three receiving
coils can be compared to the known strength of the transmitted signal to
calculate distance. By comparing the three induced signal strengths amongst
themselves, the orientation of the receiver may be determined.
Magnetic tracking sensors can feed their signals back to the data
acquisition hardware and computer interface via either a wired or a wire-
less link. A typical configuration will use one transmitter and up to
10 sensors.
The main disadvantages of magnetic motion tracking are the problems
caused by the high-strength magnetic field. Distortions can occur due to a
lot of ferromagnetic material in the environment, which will alter the field
and consequently result in inaccurate distances and orientations being de-
termined. The device also tends to give less accurate readings the further
the sensors are away from the transmitter, and for sub-millimeter accuracy, a
range of 2–3 m may be as big as can be practically used.
4.3.3 Acoustic Tracking
Acoustic tracking uses triangulation and ultrasonic sound waves, at a
frequency of about 40 kHz, to sense range. In an acoustic system, pulses of
sound from at least three, and often many more, emitters placed at different
locations in the tracked volume are picked up by microphones at the point
being tracked. The time it takes the sound wave to travel from the source
to microphone is measured and the distance calculated. By using several
pulses from several emitters, the position can be determined (see Fig-
ure 4.20). If three microphones are placed at the location being tracked then
the orientation can also be worked out. An alternative to this time of flight
range determination is to measure the phase difference between the signals ar-
riving from different sensors when they all emit the same ultrasonic pulse at
the same time. This gives an indication of the distance moved by the tar-
get during the pulse interval. Because the speed of sound in air depends on
temperature and air pressure, the equipment has to be calibrated before use
or else have a built-in mechanism to sense and account for temperature and
pressure.
Using the time of flight of sound waves to measure distance suffers from
the problem of echoes being falsely identified as signals. I t is also possible for
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78 4. Building a Practical VR System
Figure 4.20. (a) Acoustic tracking takes place by triangulation. Two emitters are
shown and give rise to a set of possible locations for the sensor lying on the circle of
intersection between the spheres whose radius depends on time of flight and speed
of sound in air. A third emitter will differentiate between these possibilities. (b)
A sophisticated system has three microphone sensors mounted together, and they
receive signals from an array of ultrasonic emitters or transponders.
the microphones to be obscured by objects between source and destination,
so the sound pulses might not be heard at all. However, the biggest drawback
with acoustic tracking is the slow speed of the sound pulse. At 0
◦
C, the
speed of sound is about 331 m/s, and thus in a typical working volume it
could take 10–20 ms for a single pulse to be sent and received. With the
need for multiple pulses to be checked before a valid position/orientation can
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4.3. Technology for Motion Tracking and Capture 79
be sent to the computer, it may only be possible to sample positions and
orientations 10–20 times a second. This may not be fast enough to track
rapid movements accurately enough.
4.3.4 Optical Tracking
One method of optical tracking works by recognizing highly visible markers
placed on the objects to be tracked. If three or more cameras are used, the
marker’s 3D position can be obtained by simple triangulation and the known
properties of the cameras. However, to achieve greater accuracy and more
robust position determination, techniques such as those discussed in Chap-
ter 8 will need to be employed. Optical tracking of point sources cannot
provide orientation information, and it suffers badly from the light sources
being obscured by other parts of the scene. Overcoming such difficulties
might require several cameras. To obtain orientation information is more dif-
ficult. Some methods that have been tried include: putting several differently
shaped markers at each tracked point, or arranging for a recognizable pattern
or lights to be generated around the tracked point.
Many novel optical tracking ideas have been proposed. For example, sen-
sors on the tracked point may be excited when a laser beam which is contin-
ually scanning the scene strikes them (like the electron beam on a TV or a
bar-code reader in the supermarket). Laser ranging can give the distance to
the sensor. Three scanners will determine a position either by triangulation or
simply distance measures. Three detectors per sensed point will allow orienta-
tion information to be obtained. Because the speed of light is so much faster
than the speed of sound in air, an optical tracking system does not suffer any
of the acoustic delay.
As we shall see in Chapter 8, the science of computer vision and pattern
recognition offers a reliable way of tracking visually recognizable markers in
video images, which can be acquired from a network of cameras overlooking
the working volume. This idea comes into its own when we wish to add
virtual objects to the real-world view so t hat they appear to be part of the
scene, as happens in augmented reality. This form of optical tracking can
be used in practice by extending the open source augmented reality toolkit,
the ARToolKit [5]. For example, the markers that are attached to the objects
shown in the three camera views of Figure 4.21 can be tracked and located
very accurately if the locations of the cameras are known and they have been
calibrated. Camera calibration is discussed in Section 8.2.2, and a tracking
project is described in Chapter 18.
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