Development of an Augmented Reality system using ARToolKit

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Development of an Augmented Reality system using ARToolKit and user invisible markers 2.2.3.1 Screen based video see-through displays For this category of display devices a video camera is used for capturing a scene. Videomixing augments the capture video with virtual objects. The resulting merged images are presented to the user by using a regular monitor. This approach is sometimes also referred to as a “window on the world” setup. It offers a very efficient way of implementing AR since only a standard PC and off the shelf components are needed. But this also introduces a number of disadvantages concerning the use of screen based AR [Bim04a]. Due to the relative small monitor sizes, a small FOV is supported. Also the resolution of the merged images is a limiting factor. Usually direct interaction with the real environment and the augmented images is not supported. Figure 2.17 shows a typical setup, and Figure 2.18 shows an augmented dinosaur’s movement simulation. Figure 2.17 Typical window on the Figure 2.18 Augmented simulation world setup 2.2.3.2 Spatial optical see-through displays This type of display makes use of spatial optical combiners that are physically positioned in the environment. Those are able to generate images that are aligned within the environment. Different kind of optical combiners exist that could be taken advantage of; mirror beam splitters, transparent screens and optical holograms. If a mirror beam splitter is used a choice exists between a curved and a planar one. Holograms capture a scene by recording amplitude, wavelength and phase information. This allows for reconstructing complete optical wavefronts. This results in the captured scenery having a three-dimensional appearance that can be observed from different perspectives [Bim04b]. The image of the real world and the overlaid image are visible at the same time. One property these optical combiners have in common is that they do not follow the movements of the user but rather support moving around them. Below are shown the three kinds of optical combiners. Disadvantages related to the described optical combiners are the following [Bim04a]. Since the optics and the display technology are spatially aligned mobile applications are not supported. Direct interaction with the real and virtual objects that are located behind the optics is prevented by the nature of the used optics. The limited size of the screens 13
Development of an Augmented Reality system using ARToolKit and user invisible markers and optical combiners causes unnatural cropping of virtual objects outside the display area. Figure 2.19 Beam splitter Figure 2.20 Transparent screen Figure 2.21 Optical hologram 2.2.3.3 Projection based spatial displays This approach is not using a plane or surface to project an image on, but instead, the images are directly projected on the surface of physical objects. Images can be projected using one single static projector, but for a greater display area also a steerable projector or even multiple projectors can be applied. Figures 2.22 through 2.24 show the use of a single static projector for augmentation of a real object with projected images [Ras02]. Figure 2.22 Hardware setup Figure 2.23 Real object Figure 2.24 Augmented object Some issues related to the use of head-attached displays are resolved by projection based AR [Bim04a]. The FOV is theoretically unlimited, and a scalable resolution is supported. Besides the usability factor is significantly improved; ergonomics are paid more attention to and eye accommodation is relieved. But also several new problems are introduced. The complexity of consistent geometric alignment and colour calibration increases with the number of applied projectors. Conventional projectors can only focus on one single focal plane. When non-planar surfaces are used for projection images blur is caused. The size, shape and colour of the physical objects´ surfaces limit the display area. Since front-projection is utilized interacting users and physical objects cause shadows. Usually projection based AR only supports one single user. 2.3 Tracking For an AR system to be able to give the user the idea of seeing the real and virtual images as fused it is required to correctly overlay virtual objects on the real world. That is, virtual objects must be accurately registered in all dimensions. The inability to establish this can result in negative effects ranging from distraction to a completely unusable AR system. For an AR entertainment system incorrect registration may 14
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Development of an Augmented Reality system using ARToolKit

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