Development of an Augmented Reality system using ARToolKit

Development of an Augmented Reality system using ARToolKit and user invisible markers
somewhat lower the experienced fun, but it may have disastrous results for a heart
patient during an operation supported by an AR system. It is not only important to
provide correct registration when the user is in a fixed position, but it also is to be
maintained during movements of the user. For establishing this, the position and
orientation of the user’s head need to be tracked.
This refers to the problem of head tracking. Another problem is related to the tracking
of used tools. This is required for the purpose of interaction within the augmented
scene. The physical instrument’s position and orientation need to be measured for
determining the target and context of interaction. Not for all AR environments this has
an equal importance. In an AR painting application for children the erroneous colouring
of the virtual frog instead of the virtual princess may give some surprise, but one may
think of more impact when something comparable occurs in the AR surgeon room.
Tracking can be divided into two types of tracking systems; outside-in and inside-out
systems. The distinction between those two is based upon the configuration of sensors
and emitters in relation to the tracked objects. Outside-in systems have their sensors
mounted at a fixed location in the scene. The objects to be tracked are equipped with
emitters or landmarks. On the other hand, inside-out systems use a direct way by having
their sensors directly attached to the tracked objects.
Tracking in AR have a set of stringent requirements that need to be met to be really
useful in practice [Pin02]. These are the following:
• High 6 Degree Of Freedom ( DOF) spatial accuracy in position and orientation
• No (very little) jitter, i.e. noise in the output of the tracking system
• High update rates (min. 30 Hz, better several 100Hz)
• No (very little) lag, i.e. delay from measurement to tracker output
• Full mobility of the users (no cables, no restricted volumes of operation)
For AR tracking several approaches have been tried. Each of those has their own
strengths and weaknesses; to combine strengths and/or to compensate for weaknesses
hybrid approaches are used. Now will be shortly discussed five different approaches
that have been proposed for tackling the tracking problem in AR [Bru03]. For each
approach one example is given. It needs to be said that these examples are not unique;
there are available more. To complete the discussion hybrid approaches, outdoor
tracking and collaborative AR are shortly mentioned.
2.3.1 Inertial tracking
Accelerometers and gyroscopes are applied for determining the acceleration for position
determination, and the orientation for position orientation. These can not be measured
directly, but are deduced by using secondary measures. Voltage values are related to the
displacement mass and the rate of change of direction. These provided voltage values
are a source of error; they are influenced by noise, bias errors and quantization error.
Integration is applied once for deriving the desired position, and twice for determining
orientation. This process is subject to drift which introduces the need for periodical
recalibration.
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