Development of an Augmented Reality system using ARToolKit

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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. 23
Development of an Augmented Reality system using ARToolKit and user invisible markers 2.6.1 3D Data browsing The primate idea for the application of AR was to use it for superimposing virtual objects on the real world. The overlaid information can be three-dimensional, but can also be textual, in voice form or video annotation. This allows a mechanic to have an augmented view showing repair instructions, or to guide users through an unknown environment showing virtual direction pointers. The interaction between the user and the AR environment is limited to navigation within the overlaid information space. There exist no means for manipulating, modifying or interaction with both the virtual and real world. Although 3D data browsing can be supported by several display devices, many of the AR applications that are guided by this interaction style use an HMD in combination with 6 DOF optical or magnetic tracking. 2.6.2 3D AR interfaces To offer the user possibilities for interaction within AR environments 6 DOF input devices can be used. These devices are able to support natural and familiar interaction metaphors. To have knowledge on the target and context of interaction it is necessary to track their position and orientation. This allows seamless spatial interaction, that is, a user can interact with virtual objects anywhere in the physical environment by using special-purpose input devices. But for interaction with physical objects real hands are used. These different input modalities lead to a violation of the natural workflow. Another shortcoming of this interaction style is the lack of sufficient tactile feedback, which is a stringent requirement for many real world applications. 2.6.3 Augmented surfaces This interaction style is related to the before describe spatial display technique. Virtual 3D objects are only registered to selected work surfaces that allow user to interact with them through familiar and traditional tools, such as a pen. But also specifically designed physical handles could be used. The interaction with physical objects is known as tangible interaction. One desirable property is that one single input modality is used for accessing both the virtual and the real world. Compared to the 3D AR interfaces interaction is significantly eased, but it introduces a spatial reduction in interaction. Full interaction in three dimensions is nearly impossible, and is limited to the 2D augmented surface. 2.6.4 Tangible AR Tangible AR tries to combine the advantages of 3D AR interfaces and augmented surfaces; it offers a way of undisturbed spatial interaction using only one input modality. Usually a video see-through HMD is used for implementing this interaction style, combined with optical registration. Virtual objects are registered to markers, physical objects that contain a known pattern. Interaction with the virtual objects is done through manipulating the physical, tangible interface elements. Also a paddle, a paper object with an attached marker could be used. This provides a transparent interface for two-handed 3D interaction with both the virtual and real objects in the environment. Also this interaction can take place anywhere in the physical environment. 24
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