Game Development Reference
In-Depth Information
It should be noted that in case of a tracker the cursor movement should be
natural, i.e. with no significant learning effort. Unlike the 3D mouse and joysticks
with which the users, except those who are used to this type of interfaces, move the
cursor manually, thoughtfully and less naturally. Though a tracker can be used in 3D
cursors, joysticks and 3D mouse cannot be used in sensors that locate the user's hand.
These sensors are described in the chapter covering manual motor interfaces.
Various principles of physics are currently in use; each of them has at least one
advantage over the others. Some of the physical phenomena used are:
Mechanics;
Electromagnetism and
Optics.
6.2 MECHANICAL TRACKERS
6.2.1 Mechanical trackers measuring distances
In this type of trackers, there is a permanent mechanical connection between the object
detected and its environment while providing 6 DOF to the object or 3 DOF to a point.
In general, the mechanical connection consists of a chain of several joint rods having
a ball pivot at its end which gives a 3 DOF angular orientation. Relative movements
between the mechanical parts are rotational. They are measured using potentiome-
ters or incremental optical sensors. These simple measurements constitute the main
advantages of this type of device. The measurements are accurate, quick (up to 300
measurements per second) and their response time is very short (2 to 3ms). Though
the relatively low price is an additional advantage, its major inconvenience which is
inherent to this principle is the constraint of freedom of movement: the range of mea-
surement is limited and the sensor creates obstacles in the movements of the object
being observed. This type of tracker is mainly used to measure the precise movement
of the head in certain experiments.
6.2.2 Mechanical trackers determining an orientation,
speed or acceleration
Many types of mechanical trackers are covered in this section as they are based on
mechanical principles that provide, directly or indirectly, the measurement of only
one degree of freedom . It is therefore necessary to combine them either with each
other or with other “non-mechanical'' trackers to create 3 or 6 DOF location sen-
sors. Some of these trackers were developed for aircraft and missile guiding systems.
Miniature trackers with an affordable production cost were designed using similar
principles for the virtual reality market. Some of these can now be seen on low-cost
head-mounted displays. They are rarely sold separately, but are connected to pro-
vide 3 or 6 DOF trackers. They have the advantage of having less limitation in the
measuring range unlike most of the other trackers. The different types of mechani-
cal sensors used in virtual reality are inclinometers, gyroscopes and rate gyros and
accelerometers.
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