Interactive horizontal surfaces enjoy increasing popularity for all kinds of usages such as sharing and view- ing of media, planning trips, browsing, or gaming. The constant increase in terms of technical features and the decrease of the price for such surfaces will lead to their pervasive usage for example at home, in offices, in hotels, in lounges, or in public buildings such as schools, universities, or libraries within the next decade. Their large size and multi-touch capabilities support in particular co-located collaborative interactions. However, this also raises various privacy related questions when considering the information that could be displayed or stored on them. In contrast to mobile phones, interactive surfaces are public or semi-public devices and anyone nearby can see what is displayed. The use of interactive surfaces for displaying, dis- cussing and sharing private media (e.g., pictures) or information stored on the user's mobile phone (e.g., contacts, address information, or documents) is a frequently discussed scenario. Here, a mobile phone needs to first establish a connection to the inter- active surface and then, for instance, all pictures stored on the device or a thumbnail view of the pictures can directly be shown at the table. Another possibility is that the user remotely selects information in private on the mobile phone before it is shown on the surface. It is likely that most users store information on their mobile devices that they do not wish to show or share with others. This depends on the location in which the interactive surface is placed, the current situation, the relationship to the bystanders and the information to be shared. This might range from settings at home where one wants to share holiday pictures with close family members to public settings in a hotel lobby where one wants to share only pictures of recently visited sights. Therefore, as users decide depending on the current context which data are appropriate for sharing with the current audience, effective means are required for selecting which data is to be shared. In particular, smart phones and their camera feature allow users to create large numbers of photos in diverse contexts. Interaction techniques are required that allow users to select from a large number of photos what they wish to share within a specific context. 

To address this, we designed and developed Select&Place2Share and Select&Touch2Share. Both techniques enable pre-selection of information on the mobile phone before showing it on the table (initiated by touching or placing the phone on the table as shown in Figure 1(a) and 1(b)). In the third technique, Shield&Share, the user touches the surface with the side-edge of the phone so that the phone is placed like a viewing shield (see Figure 1(c) and 1(c)). On the phone's screen, the user can see a high-resolution preview of the selected file. At the same time, on the area right in front of the mobile phone facing the user, thumbnail views with navigation controls are displayed. For sharing a photo, the user simply drags the corresponding thumbnail from the menu bar at the bottom of the phone onto the public surface area. There, the photo is displayed visible for everyone around the surface. The main contributions of this work are the three novel interaction techniques Select&Touch2Share, Select&Place2Share, and Shield&Share that were influenced by previous work in this area, and the results gained from a comparative user study. The results indicate that users highly appreciate interaction techniques that support protecting their privacy. Further, users prefer interaction techniques that allow them to specify which items to share while the phone is held in the their hand.

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