Development of an Augmented Reality system using ARToolKit

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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. 15
Development of an Augmented Reality system using ARToolKit and user invisible markers Inertial tracking does not suffer from magnetic interference problems and for it to function no line of sight is needed. Additional advantages are the fact that it senses orientation in a fast and direct way and that inertial tracking does not put a limit on the range over which it can be used. However, the method is not accurate for slow position changes. Also position and orientation are only measured in 3 DOF. Xsens Motion Technologies have developed an inertial tracker, named MTx, that provides 3D orientation and kinematic data; 3D acceleration, 3D rate of turn and 3D earth magnetic field. The tracker has a highly compact design, as shown in Figure 2.25. 2.3.2 Acoustic tracking Figure 2.25 Mtx inertial tracker This kind of tracking devices usually applies ultrasonic sound waves for carrying out their task. This is sound that has a frequency above 20 kHz, which lies above the frequency range of the human ear. Using only one transmitter/receiver pair provides a distance measurement from the target to a fixed point. For providing information on a 3D position either one receiver and three transmitters or three receivers and one transmitter are needed. Three transmitters and three receivers are employed for estimating position and orientation. The method employed for estimating distances is converting the time to receive a signal to a distance. Acoustic tracking offers a lightweight, inexpensive tracking solution that is robust against distortion by magnetic interference. Besides the advantages some disadvantages attached to the technology can be identified. Acoustic interference influences the ultrasonic receivers. Also echoing of waves is a source of distortion. Low accuracy is established since the speed of sound in air is not constant but varies with the environmental conditions. The constant need for an available line of sight reduces the reliability of tracking. Intersense have developed an acoustic tracking system that uses sonic emitters and discs for sending out and reflecting ultrasound. A processor is used for timing the return pulse. The tracking device is encompassed within a solid box, the Intersense IS-900, shown in Figure 2.26. Figure 2.26 Intersense IS-900 acoustic tracker 16
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