Development of an Augmented Reality system using ARToolKit

Development of an Augmented Reality system using ARToolKit and user invisible markers
fewer resources are available; power, computation and sensors are examples of those.
For indoor applications hybrid tracking offers possibilities but this holds even more for
outdoor AR. Also the Global Position System (GPS) could be taken advantage of,
although this is not a very accurate technique. Usually a combination of inertial and
optical tracking is applied.
In collaborative AR users are collaborating within one single environment. In such a
situation each user has its own view on the environment. Also each user separately can
interact with objects in the environment. This aspect adds one dimension to the tracking
problem; that is, head tracking and tool tracking needs to be done for multiple users.
Nevertheless, the before described tracking methods can be applied.
2.4 Registration
The previous section described the tracking problem, composed of both head tracking
and tool tracking. These both are meant for being used for another aspect that is present
in AR. This aspect is presented as the registration problem, and is concerned with the
alignment between virtual and real elements. This is a mostly important and
complicated matter for three reasons. First of all, the human visual system is very
sensitive to small misregistrations, even if they are in the order of few pixels. Second,
for AR applications tighter requirements are put upon on error tolerances that are
allowed. To make things even more complex, one also has to cope with system delay;
the time interval that lies in between tracking and the actual superimposing of the
corresponding images.
Figure 2.29 and Figure 2.30 show the result of an extreme misregistration compared to
the correct registration of a virtual car against the real background environment.
The accuracy of registration of virtual objects is determined by the alignment errors that
result. The possible alignment errors can be classified in two categories [Azu97]:
• Static errors, which are defined as the misalignment the user can perceive when
he is immobile.
• Dynamic errors, which are defined as the misalignment the user can perceive
when he moves.
Dynamic errors are by far the greatest contributors to registration errors perceived by
users. Although static errors have less impact they can not be ignored. There are various
sources of error that can be pointed out. For static errors the most important are optical
distortion, errors in the tracking system, mechanical misalignments and incorrect
viewing parameters. Dynamic errors are caused by unavoidable system latency.
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