Vision-based Augmented Reality Applications

Augmented Reality (AR) is a technique for overlaying virtual objects onto the real world. AR has recently been applied to many kinds of entertainment applications by using visionbased tracking technique, such as [Klein & Drummond, 2004, Henrysson et al., 2005, Haller et al. 2005, Schmalstieg & Wagner, 2007, Looser et al. 2007]. AR can provide users with immersive feeling by allowing the interaction between the real and virtual world. In the AR entertainment applications, virtual objects (world) generated with Computer Graphics are overlaid onto the real world. This means that the real 3D world is captured by a camera, and then the virtual objects are superimposed onto the captured images. By seeing the real world through some sort of displays, the users find that the virtual world is mixed with the real world. In such AR applications, the users carry a camera and move around the real world in order to change their view points. Therefore the pose and the position of the moving user’s camera should be obtained so that the virtual objects can be overlaid at correct position in the real world according to the camera motion. Such camera tracking should also be performed in real-time for interactive operations of the AR applications. Vision-based camera tracking for AR is one of the popular research areas because the visionbased method does not require any special device except cameras, in contrast with sensorbased approaches. And also, marker-based approach is a quite easy solution to make the vision-based tracking robust and running in real-time. This chapter focuses on marker-based approach. Especially, “AR-Toolkit” [H. Kato & M. Billinghurst, 1999] is a very popular tool for implementing simple on-line AR applications. ARToolkit is a kind of planar square marker for the camera tracking and estimates the camera position and pose with respect to the marker. By using the camera position and pose, virtual objects are overlaid onto the images as if the objects exist in the real world where the marker is placed. Since the user only has to place the marker, this kind of markerbase registration is very easy to implement AR systems. If only one marker is utilized, however, the camera's movable area is limited so that the camera (user) can see the marker. Moreover, when the marker cannot be recognized properly because of a change in its visibility, the registration of the virtual objects is getting unstable. In order to solve such problems, using multiple markers is a popular way. When multiple markers are utilized in a vision-based method, it is necessary to know the geometrical arrangement information of the marker such as their position and pose in advance. For example, the method in [Umlauf et al., 2002] requires the position and pose of a square marker. The method in [Kato et al., 2000] also needs the position of a point marker in advance. In [Genc et al., 2002], they proposed two-step approach; learning process and O pe n A cc es s D at ab as e w w w .ite ch on lin e. co m