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3 The Prototypes

The museum exhibit that was used is an iconic 12th century Scottish artefact known as the Lewis Queen chess. The artefact is displayed in the National Museum of Scotland behind a glass case. Both prototypes use the same visual illusion but employ different media, one digital and one non-digital. The visual illusion is borrowed from the theatre tradition and is called Peppers Ghost. A large sheet of glass is placed between the audience and the stage. A ghostly image of an actor below the stage is then projected onto the glass giving the illusion that a ghost is on stage with the actors. Using the Peppers Ghost illusion we employed two different media, a 3-D printed replica of the chess piece and a haptic device. Both used the glass of the museum case itself as a reflective surface thus ensuring that the focus was the real object or the haptic experience.

The replica was created by laser scanning the original artefact, mirroring it to the original (i.e. lateral inversion of the scan data before printing) so that the users experience would match the object in the case. The need to present mirror images as part of virtual reality and co-location interfaces is part of virtual reality issues [3] [18]. The replica was then painted black so that it would absorb light thereby reducing its reflection in the glass case. The replica is placed facing the chess piece at an equal distance from the display glass (Fig.1). When a user places her hands on the replica and concentrates her gaze at the original piece behind the glass, she can see her hands reflected in the glass apparently touching the real artefact in the display case (Fig.2). Because she sees the actual artefact (and her hands) and touches the replica she experiences the sensation that she is actually touching the artefact itself. The illusion is further strengthened by placing a cover over the replica to shield it from the users direct gaze. This cover also contains a light to illuminate the users hands so that their reflection is brighter.

Fig. 1. The Lewis Chess piece behind the glass and the mirrored 3-D printed replica

The second prototype uses the same illusion but employs a Sensable Omni 6DoF haptic device instead of the users hands. The haptic device is placed outside the display case and positioned towards the left of where the replica was so that the reflection of the pen-like stylus of the haptic device is positioned close to the artefact in the display glass. Instead of a replica, a haptic model created from the laser scan of the artefact is algorithmically positioned into the haptic devices workspace at an equal distance from the display case (Fig.3). The haptic version is invisible but the model can be traced and felt in the physical space by moving the stylus using the same combined visual and haptic feedback as with the replica prototype.

Fig. 2. Visitor interaction with the replica. Her gaze is concentrated at the original artefact behind the glass

This is a novel way of using a haptic device for immersing museum visitors into a deep understanding of the museum exhibits. In [19] museum visitors explore the surface of a digital daguerreotype case from the collection of the Natural History Museum of Los Angeles County. Similarly, the Haptic Museum, developed in the University of Southern California, is a haptic interface that allows museum visitors to examine virtual museum artefacts with a haptic device [21]. The 'Museum of Pure Form' is a virtual reality system that allows the user to interact with virtual models of 3-D art forms and sculptures using haptic devices and small-scale haptic exoskeletons [1,2]. The Senses in Touch II, which was installed in the Hunterian Museum in Glasgow, was designed to allow blind and partially-sighted museum visitors, particularly children, to feel virtual objects in the collection via a PC and Wingman haptic mouse [13]. The projects described above have used detailed virtual models of the museum artefacts and allowed the visitor to explore them with the haptic technology. Our goal was to diverge from the computer screen, and use the haptic technology in a way that evokes direct haptic interaction with the physical artefact without actually touching it providing the illusion of doing so.

Equally, the Peppers ghost technique has been used in the Virtual Showcase, a mirror-based interface for viewing real artefacts augmented with virtual geometry [3]. In Virtual Showcase no touch is used as the focus is on adding additional virtual objects and other elements onto the real object. ARToolkit-based optical tracking is used for tracking the users head movement in real time to ensure collocation with the virtual components. Head tracking was important in the Virtual Showcase because of the virtual geometry. In our prototype no tracking of the head is required. As long as the

Fig. 3. The haptic device prototype

replica or the invisible haptic model are placed in exactly the same orientation and same distance from the surface of the glass case, the illusion of co-location is preserved under all translations and rotations of the viewing angle which preserve a direct line of sight from the viewpoint through the case wall to the real artefact.

3.1 Embodiment and Sense-Making

Our interaction with the world around us is embodied and multi-modal and we make sense of the world by enacting in it. Enactive knowledge is direct, in the sense that it is natural and intuitive, based on the perceptual array of motor acts. The goal of both prototypes was to create an embodied and immersive experience for the visitors in order to provide a sense of authenticity for the ancient artifact. Embodiment and situated cognition places interaction at the center of meaning making and extends the concept of mind to include the body and environment [25], [7], [22].

The illusion of manipulation that the presented interaction technique creates can be explained as directly analogous to a classic experiment in situated cognition and perceptual plasticity [20]. A subject places their hand on a table hidden behind a partition. On the other side of the partition they can see a dummy hand. Their real hand and dummy hand are touched by fingers of a researcher which are synchronized to touch in the same place at the same time but the subject can only see the finger that touches the dummy hand not the one that touches their real hand. After a short time the subject perceives the dummy hand to be their own. In the two prototypes, the visual stimulus of the researchers visible finger is replaced by the reflection of the users own hands or haptic probe. The synchronized haptic stimulus is provided by the subjects fingers touching the replica or felt through the haptic device when it collides with the invisible virtual 3d model of the artefact. The haptic device enables haptic exploration and sense-making through multi-modal motor action which makes them an enactive interface [12]. The combination of haptic and visual feedback in both prototypes enriches the array of senses during the interactive experience and creates more dimensions of embodiment than having only visual cues.

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