## 4 Multimodal Interaction
### Interaction Based on the Visual Channel
In simple contexts such as objects control, either in situ or remotely, a graphical user interface (GUI) implemented on a smartphone could be appreciated, but it does not give the presentation of synthesized images directly to the human eye; differently head-worn display (of Head Mounted Display HMD) are expected to increase the users' attention [51]. These AR devices can be classified into optical and video see-through HMDs, depending on whether actual images are viewed directly by the user or via a video input. The video see-through head-mounted display (HMD) allows two image sources (the real world and the computer-generated world) by a dual-webcam module and an immersive HMD display whereas the optical see-through HMDs mix virtual objects, and the user can see through them (computer-generated world) [52]. These devices, together with sensors based on high-speed networking, can allow users to perceive natural AR experiences in the augmented reality environment with the communication method of AR datasets used for recognizing and tracking them anyplace and anytime [44]. Based on these considerations, the real environment can be added with virtual objects. The same approach is proposed by [53] adopting an RGB-D camera in conjunction with simultaneous localization and mapping (SLAM) to retrieve the user's movement trajectory and obtain information related to the real environment. The SLAM with monocular camera in AR context [54; 55], where only some virtual objects are placed within a scene of the real world, removes the limitation of VR and avoids simulating every entity within the scene. In [53], segmentation and shape fitting of objects are used to construct virtual objects to replace (within the virtual world) actual objects encountered (within the real world). Even if the authors claim that this system is suitable only for single-room venues and applicable mainly to static environments, it seems interesting for increasing the experience of visiting museums because the presence of objects can be enriched with others related to the same theme or with multimedia content. A more detailed study about the user interaction on Augmented Reality using Smart Glasses is proposed by [56] where the attention is focused on Epson Moverio [57] and Microsoft HoloLens [58]. It is observed that a narrow field of view (FOV) makes it difficult to visualize information of AR display; however, more intuitive interaction is achieved, including hand-gesture-based interaction. The see-through display is classified in (a) stereo rendering and direct augmentation, (b) non-stereo rendering and indirect augmentation with video background, (c) non-stereo rendering and indirect augmentation without video background. The first method superimposes virtual information directly onto its physical artifact through the see-through display of smart glasses; it requires a registration process. The second overlays virtual information indirectly onto the physical artifact in the image taken by the smart glasses' AR camera, whereas the former presents only the virtual information on the display. The hand-gesture interaction is given by a depth sensor attached to the smart AR glasses; even if it requires a calibration, in the context of cultural heritage the method (a) can be exploited to show as example an image before restoration and then by gesture the user can rotate it to observe different perspectives. For this purpose, HoloLens reveal more suitable due to the wider FOV. It should be also underlined that the accuracy the hand gesture and visual registration in wearable AR is expected to be further improved in the next future [56]. The role of ICT in tourism has been recognized as able to provide unique opportunities for innovative organizations to redesign tourism products to address individual needs and to satisfy consumer wants [59], its impact is leading to e-Tourism.