Development of an Augmented Reality system using ARToolKit

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Development of an Augmented Reality system using ARToolKit and user invisible markers 2.3.3 Magnetic tracking Magnetic tracking tries to take advantage of magnetic fields for measuring range and orientation. Either low frequency AC fields or pulsed DC fields can be used. In the transmitter a set of three coils wound around a magnetic core is incorporated for generating magnetic fields. A receiver also has three coils in the same configuration. In one cycle three measurements of magnetic field strength are taken for each coil. These nine measurements are used for determining the position and orientation of the receive coils relative to the source coils. Magnetic tracking does not suffer from line of sight problems. A large range is supported by magnetic tracking, while it still provides accurate measurements. Disadvantages however take some glance off the great potential of this type of tracking. Electromagnetic interference and field distortion caused by radios and metal surfaces have their influence on the magnetic fields; magnetic tracking is prone to distortions. The high accuracy diminishes with increasing distance. High latency also is related to the use of magnetic tracking. The Polhemus Fastrak system estimates the position and oientation of a tiny receiver as it moves through space. One unit contains all the components needed for the tracker to carry out its function. Up to four receivers are supported simultaneously. Figure 2.27 shows the tracking unit with its connections and one receiver. 2.3.4 Mechanical tracking Figure 2.27 Polhemus Fastrak magnetic tracker Mechanical trackers measure joint angles and lengths between joints. One known position is sufficient to derive all the other absolute positions from relative joint measurements. Mechanical tracking systems either are ground-based or body-based. In the first approach one point of the tracker is attached to the ground at a known position. The second approach usually implements its function in an exoskeleton. Mechanical tracking is not influenced by magnetic interference and line of sight problems. It provides low lag, accurate measurements of position and orientation. It provides a good way for tracking small volumes. But, also the intrusiveness of mechanical trackers needs to be annotated. Physical range of motion can be limited and prohibited. Gypsy4 is a human motion tracking system, that applies 43 motion sensors placed around 17 human joints. Multiple users can be tracked by the system at the same 17
Development of an Augmented Reality system using ARToolKit and user invisible markers moment. They need to wear an exoskeleton which contains the motion sensors. Figure 2.28 shows a user wearing such an exoskeleton. 2.3.5 Optical tracking Figure 2.28 Gypsy4 exoskeleton For optical tracking light is used to measure angles. Each point in the image plane provides a ray from that pixel to the centre of projection. As the distance between the transmitter and receiver increases, the optical energy diminishes with the square of that distance. As detector can be used video and Charged Coupled Device (CCD) cameras, or lateral photodiodes. This depends on the type of targets that is used; passive or active. Passive targets, such as reflective materials or fiducial markers are not powered. On the other hand, active targets such as LEDs are powered. Optical tracking systems are also referred to as image-based systems, as recorded images from a target are used for determining position and orientation. Optical tracking provides high accuracy at a fast rate, which is not limited by a large area. Magnetic interference does not have any influence. However, line of sight problems are present. Intensity and coherence of light sources are a limiting factor for acquiring good quality images which are the input for optical tracking. In ARToolKit is implemented an optical tracking system that uses fiducial markers for calculating the position and orientation of a camera. This is discussed in more detail in the next chapter. 2.3.6 Hybrid tracking, outdoor tracking and collaborative AR tracking All of the before described tracking approaches have their own weaknesses and strengths. To compensate for this and to combine advantages, methods for tracking can be combined. This is the desired perspective but still some difficulties need to be overcome to integrate them. Increasing system complexity is a resulting fact from integration. But it results in a more accurate and more robust way to establish tracking. The described tracking technologies are shown to work in constrained indoor environments. Such environments can be completely prepared, are limited in size and can be controlled. Outdoor environments have opposing properties; they are unprepared, are principally unlimited in size and do not offer possibilities for modification. Also 18
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Development of an Augmented Reality system using ARToolKit

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