Handbook of Multimedia for Digital Entertainment and Arts- P17 pptx

21 Projector-Camera Systems in Entertainment and Art 479 Physically Viewing Interaction By projecting images directly onto everyday surfaces, a projector-camera system may be used for creating augmentation effects, such as virtually painting the object surface with a new color, new texture, or even an animation. Users can interact directly with such projector-based augmentations. For example, they may observe the object from different sides, while simultaneously experiencing consistent occlusion effects and depth, or they can move nearer or further from the object, to see local details and global views. Thus, the intuitiveness of physical interaction and advantages of digital presentation are combined. This kind of physically interactive visualization ability is suitable for use in situations when virtual content is mapped as a texture on real object surfaces. View-dependent visual effects such as highlighting to simulate virtually shiny surfaces require tracking of the users’ view. Multi-user views can also be supported by time-multiplexing the projection for multiple users, with each user wearing a synchronized shutter glass allowing the selection of individual views. But this is only necessary for view-dependent augmentations. Furthermore, view tracking and stereoscopic presentation ability enables virtual objects to be displayed not only on the real surface, but also in front of or behind the surface. A general geometric framework to handle all these variants is described in [26]. The techniques described above, only simulate the desired appearance of an augmented object which is supposed to remain fixed in space. To make the projected content truly user-interactive, more information apart from viewpoint changes is required. After turning an ordinary surface into a display, it is further desirable to ex- tend it to become a user interface with an additional input channel. Thereby, cameras can be used for sensing. In contrast to other input technologies, such as embedded electronics for touch screens, tracked wand, or stylus and data gloves often used in virtual environments; vision-based sensing technology has the flexibility to support different types of inputting techniques without modifying the display surface or equipping the users with different devices for different tasks. Differing from interaction with special projection screens such as electronically enabled multi-touch or rear-projected screens, some of the primary issues associated with vision-based interaction with front-projected interfaces are the illuminations on the detected hand and object, as well as cast of shadows. In following subsections, two types of typical interaction approaches with spatial projector-camera systems will be introduced, namely near distance interaction and far distance interaction. Vision based interaction techniques will be the main focus and basic interaction operations such as pointing, selecting and manipulation will be considered. Near Distance Interaction In near-distance situations where the projection surface is within arm’s length of the user, finger touching or hand gestures are intuitive ways to select and manipulate the 480 O. Bimber and X. Yang interface. Apart from this, the manipulation of physical objects can also be detected and used for triggering interaction events. Vision-based techniques may apply a visible light or infrared light camera to capture the projected surface area. To detect finger touching on a projected surface a calibration process, similar to the geometric techniques presented in section “Geo- metric Image Correction”, is needed to map corresponding pixels between projector and camera. Next, fingers, hands and objects need to be categorized as part of the foreground in order to separate them from the projected surface background. When interactions take place on a front-projected surface, the hand is illuminated by the displayed images and thus the appearance of a moving hand changes quickly. This renders segmentation methods, based on skin color or region-growing methods as useless. Frequently, conventional background subtraction methods are also unreliable, since the skin color of a hand may become buried in the projected light. One possible solution to this problem is to expand the capacity of the background subtraction. Despite, its application to an ideal projection screen which assumes enough color differences from skin color as in [27], the background subtraction can also be used to take into account different background and foreground re- flectance factors. When the background changes significantly, a segmentation may fail. An image update can be applied to keep the segmentation robust, where an artificial background may be generated from the known input image for a projector with geometric and color distortions corrected between the projector and camera. Another feasible solution is to detect the changing pixel area between the frames of the captured video to obtain a basic shape of the moving hand or object. Noise can then be removed using image morphology. Following this, a fingertip can be detected by convolution with a fingertip-shaped template over the extracted image, as in [28]. To avoid the complex varying illumination problem for visible light, an infrared camera can be used instead, together with an infrared light source to produce in- visible shadow of a finger on the projected flat surface, as shown in [29]. The shadow of the finger can then be detected by the infrared camera and can thus be singularly used to detect the finger region and fingertip. To enable screen intera- tion by finger touching, the positioning of the finger, either touching the surface or hovering above it, can be further determined by detecting the occlusion ratio of the finger shadow. When the finger is touching the surface, its shadow is fully oc- cluded by the finger itself; while the finger is hovering over the surface, its shadow is larger. It is also possible to exclude the projected content from the captured video by interlacing the projecting images and captured camera frames using synchronized high-speed projectors and cameras, so that more general gesture recognition algorithms can be adopted as those reviewed in [30]. To obtain more robust detection results, specific vision hardware can also be utilized, such as real-time depth cameras that are based on the time-of-flight principle [31]. 21 Projector-Camera Systems in Entertainment and Art 481 Far Distance Interaction In a situation where the projection surface is beyond the user’s arm length, laser pointer interaction is an intuitive way to select and manipulate projected interface components. Recently, laser pointer interaction has used for interacting with large scale projection display or tiled display at a far distance [32]. To detect and track a laser dot on a projection surface in projector-camera systems, a calibrated camera covering the projecting area is often used. The location and movement of a laser dot can be detected simply by applying an intensity thresh- old to the captured image – assuming that the laser dot is much brighter than the projection. Since the camera and the projector are both geometrically calibrated, the location of the laser dot on the camera image can be mapped to corresponding pixels on projection image. The “on” and “off” status of the laser pointer can be mapped to mouse click events for selecting particular operations. One or more virtual objects that are supposed to be intersected with the laser dot or a corresponding laser ray can be further calculated from the virtual scene geometry. More events for laser pointer interaction can be triggered by temporal or spatial gestures, such as encircling, or simply by adding some hardware on laser pointers, such as buttons and embedded electronics for wireless communication. Multiple user laser pointer interaction can also be supported for large projection areas where each user’s laser pointer is distinguishable. This can be supported by time-multiplexing the laser or by using different laser colors or patterns. User studies have been carried out to provide optimized design parameters for laser pointer interaction [33]. Although laser pointing is an intuitive technique, it also suffers from issues such as hand-jittering, inaccuracy and slow interaction speeds. To overcome the hand-jittering problem, which is compounded at greater distances, filtering-based smoothing techniques can be applied, though may lead to discrepancy between the pointing laser dot and the estimated location. Infrared laser pointers may solve this problem, but according to user study results, visible laser lights are still found to be better for interaction. Apart from laser pointing, other tools such as a tracked stylus or specially designed passive vision wands [34] tracked by a camera have proven to be flexible and efficient when interacting with large scale projection displays over distances. Gesture recognition provides a natural way for interaction in greater distances without using specific tools. It is mainly based on gesture pattern recognition with or without hand model reconstruction. Evaluating body motions is also an intuitive way for large scale interaction, where the body pose and motion are estimated and behavior patterns may be further detected. When gesture and body motion are the dominant modes of interaction with projector-camera systems, shadows and varying illumination conditions are the main challenges, though shadows can also be utilized for detecting gesture or body motion. In gesture or body interaction, background subtraction is often used for detecting the moving body from the difference between the current frame and a reference background image. The background reference image must be regularly updated so 482 O. Bimber and X. Yang as to adapt to the varying luminance conditions and geometry settings. More complex models have extended the concept of background subtraction beyond its literal meaning. A thorough review of the background extraction methods is presented in [35]. Vision-based human action recognition approaches can be generally divided into four phases. The model initialization phase ensures that a system commences its operation with a correct interpretation of the current scene. The tracking phase seg- ments and tracks the human bodies in each camera frame. The pose estimation phase estimates the pose of the users in one or more frames. The recognition phase can recognize the identity of individuals as well as the actions, activities and behaviors performed by one or more user. Details about video based human action detection techniques are reviewed in [36]. Interaction with Handheld Projectors Hand-held projectors may display images on surfaces anywhere at anytime while they are being moved by the user. This is especially useful for mobile projector- based augmentation, which superimposes digital information in physical environments. Unlike other mobile displays such as provided by PDAs or mobile phones, hand-held projectors offer a consistent visual combination of real information gather from physical surfaces with virtual information. This is possible without context switching between information space and real space, thus seamlessly blurring the virtual and real world. They can be used, for instance, as interactive information flashlights [37] – displaying registered image content on surface portions that are illuminated by the projector. Although hand-held projectors provide great flexibility for ubiquitous computing and spontaneous interaction, there are fundamental issues to be addressed before a fluid interaction between the user and the projector is possible. When using a hand- held projector to display on various surfaces in a real environment, the projected image will be dynamically modulated and distorted by the surfaces as the user moves. When the user stops moving the projector, the presented image still suffers from shaking by the user’s unavoidable hand-jitter. Thus, a basic requirement for hand-held projector interaction is to produce stable projection. Image Stabilizing One often desired form of image stabilization is to produce a rectangular 2D image on a planar surface – independently of the projector’s actual pose and movement. In this case, the projected image must be continuously warped to keep the correct aspect ratio and to remain undistorted. The warping process is similar to the geometric correction techniques described earlier. The difference, however, is that the 21 Projector-Camera Systems in Entertainment and Art 483 target viewing perspective is usually pointing towards the projection surface along its normal direction, while the position of the hand-held projector may keep on changing. To find the geometric mapping between the projector and the target perspective, the projector’s six degrees of freedom may be obtained from an attached tracking device. The homography is an adequate method to represent this geometric mapping when the projection surface is planar. Instead of using the detected four vertices of the visible projection area to calculate the homography matrix, another practical technique is to identify laser spots displayed from laser-pointers that are attached to the projector-camera system. The laser spots are brighter and therefore easier to detect. In [38], hand-jittering was compensated together with the geometry correction, by continuously tracking the projector’s pose and warping the image at each time-step. A camera attached to the projector detects visual markers on the projection surface, that are used for warping the projected image accordingly. In [42]a similar stabilization approach is described. Here, the projector pose relative to the display surface is recovered up to an unknown translation in the display plane. Pointing Techniques After the stabilization of the projector images, several techniques can be adopted to interact with the displayed content. Controlling a cursor by laser pointing (e.g., with a projector-attached laser pointer) represents one possibility. In this case, common desktop mouse interaction techniques can be mapped directly to hand-held projectors. The projector’s center pixel ray can also be used instead of a laser pointer to control the mouse cursor. One of the biggest problems associated with these methods are size reductions and cropping of the display area, caused by the movement of the projector when controlling the cursor. Using a secondary device such as a tracked stylus or a separate laser pointer can overcome these limitations, however the user needs both hands for interaction. Mounting a touch pad or other input devices on the projector is also possible, but might not be as intuitive as a direct pointing with the projector itself. Selection and Manipulation Based on the display and direct pointing ability described above, mouse like interaction can be emulated such as selecting a menu or performing a cut-and-paste operation by pointing the cursors on the projected area and pressing buttons mounted on the projector. However, in this scenario, the hand jitter problem, similar to laser pointer interaction, also exists – making it difficult to locate the cursor in specific and small areas. The jitter problem is intensified when cursor pointing is combined with mouse button-pressing operations. Adopting specially designed interaction techniques rather than emulating common desktop GUI methods, can alleviate this problem. 484 O. Bimber and X. Yang One proven and efficient interaction technique for hand-held projectors is the crossing based widget technique [37]. Crossing based widget is operated by moving the cursor to cross the widget in a specific direction (e.g. from outside to inside, or from top to bottom), while holding the mouse button. This technique avoids pointing the cursor and pressing a button at the same time. Crossing widget can be used for hand-held projectors to support commonly used desktop GUI elements, such as menus and sliders. Crossing based menu items can be activated by crossing from one direction; and deactivated by crossing from the opposite direction. All actions are executed by releasing the mouse button. Different colors can be used to indicate the crossing directions. Hierarchical menus can also be supported. Similarly, the crossing based slider is activated by crossing the interface in one direction, deactivated by crossing it in the opposite direction, and adjusted according to the cursor movement parallel to the slider. Another specially designed interaction technique is called zoom-and-pick widget, proposed by [39]. It was designed to implement the simultaneous use of stable high-resolution visualization and pixel-accurate pointing for hand-held projectors. The widget is basically a square magnification area, located around the current pointing cursor position. A circular dead zone is defined within this area. The center of the dead zone is treated as an interaction hot-spot. The widget remains static when the pointing cursor is moving within the dead zone. To gain pixel-accurate pointing ability, a rim is defined around the dead zone. Each crossing of the cursor from the dead zone into the rim triggers a single pixel movement of the widget in the direction of the pointer movement. If the pointer is moving beyond the dead zone and the rim, the widget will be relocated to include the pointer in its dead zone again. Multi-user Interaction Hand-held projectors also pose new chances and challenges for multi-user interaction. In contrast to other multi-user devices such as tabletop displays, primarily used for sharing information with others, or other mobile devices such as personal mobile phones; hand-held projectors, due to their portability, and personal usage, are suitable both for shared and individual use. Multiple hand-held projectors combine the advantages of public and personal display systems. The main issues associated with multi-user interaction and hand-held projectors are primarily concerned with design for ownership, privacy control, sharing, and so on. The name of the owner of a displayed object can be represented by specially designed label widgets placed on the object and operated using crossing based operations. The overlap of two or more cursors can signify consent from multiple users to accomplish collaborative interactive task, such as coping a file or blending two images between the users. Snapping and docking actions can be performed by multiple users in order to quickly view or modify connected information between multiple objects. Multiple displayed images from more than one user can be blended directly or semantically. By displaying high resolution images when the user moves 21 Projector-Camera Systems in Entertainment and Art 485 closer to the display surface, a focus-and-context experience can be achieved by providing refined local details. More details can be found in [40]. Environment Awareness Due to their portability, hand-held projectors are mainly used spontaneously. There- fore, it is desirable to enhance the hand-held projectors with environment awareness abilities. Geometric and photometric measurement and object recognition and tracking capacities, would enable the projector to sense and respond to the environment accordingly. Geometric and photometric awareness can be implemented using, for example, structured light techniques, as described in section “Structured Light Scanning”. For object recognition and tracking, the use of a passive fiducial marker (e.g., supported with open source computer vision toolkits such as ARToolkit[41]) is a cheap solution. However, it is not visually attractive which may disturb the appearance of the object and may fail as a result of occlusion or low illumination. Unpowered passive RFID tags can be detected via a radio frequency reader without being visible. They represent another inexpensive solution for object identification. However, they do not support pose tracking. The combination of RFID tags with photo-sensors, called RFIG, has been developed in order to obtain both – object identification and object position. The detection of the object position is implemented by projecting Gray codes onto the photo-sensors. In this way the Gray code is sensed by each photo-sensor and allows computing the projection of the sensors to the projector image plane, and consequently enables projector registration. More details about RFIG are referred to [42]. Interaction Design and Paradigm In the sections above, techniques for human interaction with different configurations of projector-camera systems were presented. This subsection, however, will introduce higher level concepts and methods for interaction design and interaction paradigms for such devices. Alternative configurations such as steerable projector and moveable surfaces will also be discussed briefly. Projector-based systems for displaying virtual environments assume high quality, large field of view, and continuous display areas which often evoke feelings of immersion and presence, and provide continuous interaction spaces. In contrast, spatial projector-camera systems that display on everyday surfaces may produce blended and warped images with average quality and a cropped field of view. The cropped view occurs as a result of the constricted display area, discontinuous images on different depth levels, and surfaces with different modulation properties. Due to these discrepancies, it is not always possible to directly adopt interaction techniques from immersive virtual environments or from conventional augmented reality applications. 486 O. Bimber and X. Yang For example, moving a virtual object using the pointing-and-drag technique, which is often adopted in virtual environments, may not be the preferred method in a projector-based augmented environment, since the appearance of the virtual object may vary drastically as it is moved and displayed on discontinuous surfaces with different depths and material properties. Instead, grasp-and-drop techniques may be better suited to this situation, as discussed in [43]. Furthermore, the distance between the user and display surface is important for designing and selecting interaction techniques. It was expected that pointing interaction is more suitable for manipulating far distance objects, while touching is suitable for near distance objects. However, contradictory findings, derived from user studies for interaction with projector-camera systems aimed for implementing augmented workspace [43], have proven otherwise. Users were found unwilling to touch the physical surfaces even at close range distances after they learned distance gestures such as pointing. Instead, users frequently continued using the pointing method, even for surfaces located in close proximity to them. The reason for this behavior may be two-fold. Firstly, users may prefer to use a consistent technique for manipulation such as pointing. Secondly, it seems that the appearance and materials of the surfaces affect the user’s willingness to interact with them [44]. Several interaction paradigms have been introduced with or for projector-camera systems. Tangible user interfaces were developed to manipulate projected content using physical tangible objects [45]. Vision based implicit interaction techniques have also been applied to support subtle and persuasive display concepts derived from ubiquitous computing [46]. The peephole paradigm is discussed as a concept to describe the projected display as a peephole for the physical environment [47]. Varying bubble-like free-form shapes of the projected area based on the environment enables a new interface that moves beyond regular fixed display boundaries [48]. Besides hand-held projectors which enable ubiquitous display, steerable projectors also bring new interaction concepts, such as everywhere displays. Such systems enable projections on different surfaces in a room, and to turn them into an interaction interfaces. The best way to control a steerable projector during the interaction, however still needs to be determined. Body tracking can be combined with steerable projections to produce a paradigm called user-following display [49], where the user’s position and pose are tracked. Projection surfaces are then dynamically selected and modulated accordingly, based on a measured and maintained three- dimensional model of the surfaces in the room. Alternatively, laser pointers can be used and tracked by a pan/tilt/zoom camera to control and interact with a steerable projector unit [50]. Another issue for interaction with steerable projectors is the question of how to support a dynamic interfaces which can change form and location on the fly. A vision-based approach can solve this problem by decoupling interface specifications from its location in space and in the camera image [51]. Besides the projectors themselves, projection surfaces might also be moveable rather than remain static in the environment. They may be rigidly moveable flat screens, semi-rigidly foldable objects such as a fan or an umbrella, or deformable objects such as paper and cloth. Moveable projection surfaces can provide novel interfaces and enable unique interaction paradigms such as foldable displays or 21 Projector-Camera Systems in Entertainment and Art 487 organic user interfaces [52]. Tracking the pose or deformation of such surfaces, however, is an issue that still needs to be addressed. Cheap hardware trackers have been used recently to support semi-rigid surfaces [53]. Vision-based deformation detection algorithms may be useful in future for supporting deformable display surfaces. Application Examples The basic visualization and interaction techniques that have been presented in the sections above enable a variety of new applications in different domains. In general, projector-camera systems can be applied to interactive or non-interactive visual presentations in situations where the application of projection screens is not possible, or not desired. Several examples are outlined below. Embedded Multimedia Presentations Many historic sites, such as castles, caves, or churches, are open to public. Flat panel displays or projection screens are frequently being used for presenting visual information. These screens, however, are permanently installed features and unnecessarily cover a certain amount of space. They cannot be temporally disas- sembled to give the visitors an authentic impression of the environment’s ambience when required. Being able to project undistorted images onto arbitrary existing surfaces offers a potential solution to this problem. Projectors can display images that are much larger than the device itself. The images can be seamlessly embedded, and turned off any time to provide an unconstrained experience. For these reasons, projector- camera systems and image correction techniques are applied in several professional domains, such as historic sites, theater, festivals, museums, public screen presentations, advertisement displays, theme parks, and many others. Figure 2 illustrates two examples for a theater stage projection at the Karl-May Festival in Elspe (Germany), and an immersive panoramic projection onto the walls of the main tower of castle Osterburg in Weida (Germany). Both are used for displaying multimedia content which is alternately turned on and off during the main stage performance and the museum presentation respectively. Other examples of professional applications can be found at www.vioso.com. Superimposing Museum Artifacts Projector-camera systems can also be used for superimposing museum artifacts with pictorial content. This helps to communicate information about the displayed objects more efficiently than secondary screens. 488 O. Bimber and X. Yang Fig. 2 Projection onto physical stage setting (top), and 360 degree surround projection onto natural stone walls in castle tower (bottom). Image courtesy: VIOSO GmbH, www.vioso.com In this case, a precise registration of the projector-camera system is not only necessary to ensure an adequate image correction (e.g., geometrically, photometrically, and focus), but also for displaying visual content that is geometrically registered to the corresponding parts of the object. Figure 3 illustrates two examples for superimposing visual content, such as color, text and image labels, interactive visualizations of magnifications and un- derdrawings, and visual highlights on replicas of a fossil (primal horse displayed by Senckenberg Museum Frankfurt, Germany) and paintings (Michaelangelo’s Creation of Adam, sanguine and Pontormo’s Joseph and Jacob in Egypt, oil on wood) [22]. In addition to augmenting an arbitrary image content, it is also possible to boost the contrast of low contrast objects, such as paintings whose colors have faded after a long exposure to sun light. The principle techniques describing how this can be achieved are explained in [19]. Spatial Augmented Reality Projector-camera systems cannot only acquire parameters that are necessary for image correction, but also higher level information, such as the surrounding scene geometry. This, for instance, enables corrected projections of stereoscopic images [...]... Computer Vision and Image Understanding, Vol 104, No 2–3, pp 90–126, 2006 37 X Cao and R Balakrishnan, “Interacting with dynamically defined information spaces using a handheld projector and a pen,” Proceedings of the 19th annual ACM symposium on User interface software and technology (UIST ’06), pp 225–234, 2006 38 P Beardsley, J Van Baar, R Raskar, and C Forlines, “Interaction using a handheld projector,”... Department of Animation, Emily Carr University of Art and Design, Vancouver, BC, Canada e-mail: lbishko@ecuad.ca V Zammitto, M Nixon, and H Wei School of Interactive Arts and Technology, Simon Fraser University, Vancouver, BC, Canada e-mail: vzammitt@sfu.ca; mna32@sfu.ca; huaxinw@sfu.ca A.V Vasiliakos University of Peloponnese, Nauplion, Greece e-mail: vasilako@ath.forthnet.gr B Furht (ed.), Handbook of Multimedia. .. and Computer Graphics (TVCG), Vol 10, No 3, pp 290–301, 2004 494 O Bimber and X Yang 13 M S Brown, P Song, and T - J Cham, “Image Pre-Conditioning for Out -of- Focus Projector Blur,” Proceedings of IEEE Conference on Computer Vision and Pattern Recognition (CVPR), Vol II, pp 1956–1963, 2006 14 Y Oyamada and H Saito, “Focal Pre-Correction of Projected Image for Deblurring Screen Image,” Proceedings of. .. IEEE Computer Graphics and Applications, Vol 25, No 1, pp 39–43, 2005 39 C Forlines, R Balakrishnan, P Beardsley, J van Baar, and R Raskar, “Zoom -and- pick: facilitating visual zooming and precision pointing with interactive handheld projectors,” Proceedings of the 18th annual ACM symposium on User interface software and technology (UIST ’05), pp 73–82, 2005 40 X Cao, C Forlines, and R Balakrishnan, “Multi-user... integrate characters, narrative, and drama as part of their design One can see this pattern through the emergence of games like Assassin’s Creed (published by Ubisoft 2008), Hotel Dusk (published by Nintendo 2007), and Prince of Persia series (published by Ubisoft), which emphasized character and narrative as part of their design M.S El-Nasr ( ) School of Interactive Arts and Technology, Simon Fraser University,... in Entertainment and Art 495 33 B Myers, R Bhatnagar, J Nichols, C Peck, D Kong, R Miller, and A Long, “Interacting at a distance: measuring the performance of laser pointers and other devices,” Proceedings of the SIGCHI conference on Human factors in computing systems (CHI ’02), pp 33–40, 2002 34 X Cao and R Balakrishnan, “Visionwand: interaction techniques for large displays using a passive wand... simulations and interactive 3D environments for a wide variety of applications [4–11] Several great examples are displayed in the projects developed by Institute of Creative Technologies at University of Southern California, where they utilize 3D environments with rich characters to teach cultural norms and foreign language, among other subjects These applications provide a safe and comfortable environment for. .. ego, and superego The id is full of animal instincts and operates by prioritizing pleasure satisfaction, but is purely unconscious The ego mediates between the id, the superego, and the external world by evaluating the consequences of actions The superego is formed by the mandates that have been internalized, and the ideal image of oneself The ego and superego have unconscious, preconscious, and conscious... not fit into any of the three other categories Similar to Kretschmer, Sheldon and Stevens [27] developed a quantitative classification of personality along three dimensions of body types: Endomorphy (fatness), mesomorphy (muscularity), and ectomorphy (thinness), where each dimension was defined on a score from 1 to 7 For example, a person with 7 for endomorphy, 1 for mesomorphy, and 1 for ectomorphy represents... visualization of an architectural lighting simulation (left), and a stereoscopically projected spatial augmented reality game (right) Door, window, illumination and the car are projected Flexible Digital Video Composition Blue screens and chroma keying technology are essential for digital video composition Professional studios apply tracking technology to record the camera path for perspective augmentations of . huaxinw@sfu.ca A.V. Vasiliakos University of Peloponnese, Nauplion, Greece e-mail: vasilako@ath.forthnet.gr B. Furht (ed.), Handbook of Multimedia for Digital Entertainment and Arts, DOI 10.1007/978-0-387-89024-1. phones; hand-held projectors, due to their portability, and personal usage, are suitable both for shared and individual use. Multiple hand-held projectors combine the advantages of public and personal. the main tower of castle Osterburg in Weida (Germany). Both are used for displaying multimedia content which is alternately turned on and off during the main stage performance and the museum

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