Development of an Augmented Reality system using ARToolKit

Development of an Augmented Reality system using ARToolKit and user invisible markers
used for calculating a view matrix which specifies the relation between the real box and
the virtual coordinate system.
Figure 2.31 The wooden calibration box Figure 2.32 Calibration of a
virtual axis with the calibration
box
2.5.2 Calibration-free AR
Calibration-free AR, as proposed in [Kuk98], is based upon the observation that the
projection of all the points in a set can be expressed as a linear combination of just the
projections of four 3D points taken from that set. This results in an affine virtual object
representation relative to the reference frame defined by the four points in space. No
metric information about the objects in the camera’s view or the calibration parameters
of the camera is needed. Also dynamically changing camera parameters are
compensated for, by constantly updating the viewing matrix. Two images from different
camera positions are taken, in which four predefined points are tracked. This suffices to
construct the affine mapping of virtual objects. These points can be preselected fiducial
markers, or uniform-intensity regions in the real scene.
2.5.3 Autocalibration
This third manner of calibration makes use of redundant information for the calibration
process. In [Hof00] is presented an approach to calibrate an electronic compass in an
outdoor situation by using additional rate gyroscopes. Since the electronic field of the
earth is influenced by distortions, which also vary with the geographic position, it is not
sufficient to calibrate the electronic compass for a single location. For solving this, a
real time procedure is used for constantly recalibrating the compass. This is done by
taking into account measurements from the rate gyroscopes to adjust the estimates given
by the compass. It was shown that autocalibration converge to solutions obtained by
manual calibration using a magnetic turntable.
2.6 Interaction
The foregoing described the three basic aspects necessary for creating AR. But, to give
it real potential, the user must have the opportunity to interact with the AR environment.
In [Bow04] four interaction styles for AR, each offering different interaction levels are
mentioned. Those will be shortly discussed now, in order of increasing interaction. It
needs to be pointed out that interaction styles for AR in principle are related to the
display devices that are being used. The changing state of technology is expected to
expand applicability of interaction styles to other display devices as well.
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