Game Development Reference
In-Depth Information
electronic. The prices of these trackers are currently very high for consumer
applications;
Cables used for electric connection with the receiver can prove to be a hindrance in
the free movement of the solid observed. Systems with wire-free receiver-electronic
unit connections are also available. These systems are described in the chapter on
body locating interfaces, as they are mainly used to capture body movements
(motion capture);
and especially, the impossibility of using alternating magnetic field devices in the
presence of metal parts. Except if they are fixed; in which case, a pre-calibration
to offset the disturbances is possible. This calibration needs to be done tactfully
and is done only in special cases, for example in an airplane cockpit.
The latency time is relatively short and is comparable between the two types of trackers.
This time increases if multiple receivers are operating and especially if the measure-
ments are filtered. Let's not forget that the total response time also depends on the
time of data transmission (via serial or parallel link) between the control unit and the
computer. If the reader wants to know more about the latency time of different tracker
models, they were studied and measured by Adelstein et al. (1996).
6.3.4 Compass
The earth creates a huge magnetic field which can be used in location measurement.
Miniature electronic compasses work using the Hall effect. This effect is created by a
deflection of current in a thin metal plate when this plate is placed in a magnetic field
perpendicular to its plane. We can determine the earth's magnetic north using two Hall
effect sensors perpendicular to each other.
Combining these with inclinometers and gyroscopes, InterSense has designed
various orientation trackers providing three degrees of freedom, based on inertial
technology. This inertial device measures the angular rotational velocity using a gyro-
scope, earth's gravity with an inclinometer and earth's magnetic field using compasses.
Rotational velocities of three directions are integrated to obtain the three rotations.
Inclinometer and the compass are used to offset the drifts of the gyroscope. Amovement
prediction algorithm is provided with this sensor, which could be useful in decreas-
ing the delay time in the “action - perception'' loop of any virtual reality system. In
theory, the range of devices is unlimited (limited in practice by the length of cables)
and the blanking problem does not arise. However, the main drawback of this type of
tracker is that it can be disrupted due to the neighbouring metal parts and local varia-
tions of magnetic north. This problem is less crucial if only the relative measurements
are being used. InterSense offers various trackers based on this principle. The inertial
sensor InterTrax2 measures three degrees of freedom (three rotations in 360 ).
6.4 OPTICAL TRACKERS
6.4.1 Introduction
There is no standard tracker in this category. In fact, optical trackers have become the
front-runners in a few years' time, outdoing magnetic sensors. On the one hand, this
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