Development of an Augmented Reality system using ARToolKit

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Development of an Augmented Reality system using ARToolKit and user invisible markers projection depths are used for the real world and the virtual objects; either the eyes have to switch between those two or perceive one depth as unsharp. Registration of virtual objects heavenly relies on precise head tracking. Every time optical see-through HMDs are used they need to be calibrated which is difficult and a time consuming task. 2.2.1.2 Head-mounted projector As HMDs can be divided into two kinds, this also holds for this type of displays. The two types that can be distinguished are head-mounted projective displays and projective head-mounted displays. The first type makes use of retro-reflective screens that are mounted in front of the eyes of the user [Hua04]. Such surfaces have the property to reflect light back along its incident direction. A projective lens in combination with a beam-splitter is used to redirect an image from a miniature LCD display onto these surfaces. The incoming light is retro-reflected back to the exit pupil to have the user observe the projected image. This concept is presented in Figure 2.6. Figure 2.6 Head-mounted projective Figure 2.7 Projective head-mounted display concept display concept The approach that uses projective head-mounted displays does not, as opposed to the earlier described one, use retro-reflective screens in front of the user. Instead, images are projected along the viewer’s line of sight by a ceiling attached projector [Kij97]. Two half mirrors are applied to combine the back directed projected images and the user’s view of the environment. This concept is shown in Figure 2.7. Compared to HMDs both kinds of head-mounted projectors offer a larger FOV. Headmounted projective also displays decrease the effect of inconsistency of accommodation and convergence. However, several shortcomings are related to the use of headmounted projectors [Bim04a]. The technology as developed up until now is not ergonomic in use and currently available prototypes are cumbersome. Limited resolution and brightness can be achieved by the used miniature LCD screens and projectors. For the projective head-mounted displays an extra requirement is the presence of a ceiling, which limits its use to indoor environments. Head-mounted projective displays require special retro-reflective surfaces for bright imagery. 9
Development of an Augmented Reality system using ARToolKit and user invisible markers 2.2.1.3 Retinal displays As the earlier described display devices use small displays in front of the eyes to project images, this category directly projects an image on the retina of the eye. This is done by aiming a small laser spot at the retina and sweeping it over using a raster pattern. To be used without causing any damage the lasers have very low power. Despite this bright images can be created, with a potential greater FOV compared to screen-based displays. This concept is shown in Figure 2.8. Figure 2.8 Retinal display concept As current retinal display technology is not yet as developed as other display technologies, additional disadvantages are introduced [Bim04a]. Only monochrome red images can be presented, since cheap blue or green lasers do not yet exist. Stereoscopy can also not be supported by existing retinal displays. 2.2.2 Hand-held displays Hand-held displays combine multiple components into one single device; processor, memory, display and interaction technology. Usually these devices are small, lightweight, wire-less and mobile. Images are generated at arm length. Such devices can be used as a window or a magnifying glass. The video see-through approach is most promising to be used with this kind of devices. A camera captures a real time video stream of the environment and before displaying such video material to the user graphical augmentations are overlaid. The position and orientation of the camera are tracked so that result correct overlays. There are available different kinds of hand-held displays. Among those are Personal Digital Assistant (PDA), mobile phone, hand-held mirror display and hand-held video projector. For the category of head-attached display the necessity existed to explain the underlying concept. For the now treated displays these are assumed to be known. Taking this into account, only examples of hand-held displays applied in AR are given. Thereby needs to be noted that the presented examples only serve as illustration; they are not unique in their kind. But first are discussed the difficulties to overcome when using hand-held displays for AR [Bim04a]. The image processing suffers from high delays and a low frame rate when PDAs or mobile phones are used, since limited processor capacity and rendering components are available. The cameras nowadays installed in mobile phones offer a fixed focus, which only makes them effective in a certain depth range. A comparable difficulty is related to hand held-hand video projectors; only projection at a certain depth range is supported. Additionally, all hand-held displays do not provide completely 10
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