Game Development Reference
In-Depth Information
Location sensors
Philippe Fuchs and Hervé Mathieu
6.1.1 Spatial location
This chapter describes the sensors that capture the real-time location of any object to
know its position and (or) to follow its path. These motor interfaces are indispensable
in virtual reality techniques. In this field, the object to be located is often a body part,
the entire body or an object handled by the user. These devices, motor interfaces for
virtual reality, can also be used in other sectors like metrology, robotics, etc. When
they are used in virtual reality, we call them “trackers''. These sensors are designed to
measure only the spatial position of a moving reference point which is linked to the
object observed, with respect to a fixed reference point. In the following chapter, we
will describe special interfaces, such as data suits and data gloves, used to locate the
body or a body part. These special location interfaces can use trackers for operating.
The position of a reference point linked to an object can be determined perfectly if
the tracker measures the 6 parameters (3 angles and 3 distances) connected to the six
degrees of freedom (or 6 DOF) of the reference point. These 6 parameters are defined by
the matrix of geometric transformation between the reference point in the environment
and the reference point linked to the object. This transformation includes twomotions-
rotation and translation. The parameters can be defined in different ways and generally
the measuring systems let the user choose from various representations. The position
sensor is either connected to the object or is fixed with respect to the environment. In
the latter case, the sensor determines the location parameters by observing the object.
A perfect tracker (or “almost perfect'' not considering the price) does not exist. Every
tracker described in this topic has at least one operating constraint or performance
restrictions. This type of interface is however very much in demand in virtual reality
techniques. Nevertheless, designing and creating a reasonably-priced location sensor
that provides six extremely accurate measurements in real time (at a frequency higher
than 100Hz), with no operating constraints, is a very difficult technical challenge.
You will be surprised to note that even in our age of highly developed techniques,
giving six real-time measurements seems very complex. From the first edition of this
book (Fuchs, 1996) covering virtual reality interfaces up to now, there has been no
significant progress in trackers, which is contrary to what we had expected! As far as
the main trackers used in virtual reality are concerned, we haven't seen much of an
improvement except in the case of optical trackers.
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