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Figure 1. People performing mid-air gestures to select items on interactive public displays. The system was installed at three locations: (L1) on a large wall-projection in a coworking space, (L2) on a rear-projection screen in a students' cafeteria, (L3) on an LCD at a venue's opening event ABSTRACTMost of today's public displays only show predefined contents that passers-by are not able to change. We argue that interactive public displays would benefit from immediately usable mid-air techniques for choosing options, expressing opinions or more generally selecting one among several items. We propose a design space for hand-gesture based mid-air selection techniques on interactive public displays, along with four specific techniques that we evaluated at three different locations in the the field. Our findings include: 1) if no hint is provided, people successfully use Point+Dwell for selecting items, 2) the user representation could be switched from Mirror to Cursor after registration without causing confusion, 3) people tend to explore items before confirming one, 4) in a public context, people frequently interact inadvertently (without looking at the screen). We conclude by providing recommendations for designers of interactive public displays to support immediate usability for mid-air selection.

L arge interactive displays are becoming increasingly prevalent in urban public life. This is due largely to rapid advances in display and projection technologies, input methods that enable different types of interaction mechanisms, and our expanding understanding of the potential for interaction variations and scenarios. Large displays have moved out of research laboratories into public spaces such as museums, libraries, plazas, and architectural facades, where they present information and enhance experiences in a highly visual and often interactive way. Researchers from disciplines such as human-computer interaction, architecture, social sciences, design, art, and media theory have started exploring public-display installations' potential for educational, entertaining, participative, and evocative experiences.The design and development of these installations can be informed by previous research on, for instance, large interactive displays in collaborative and educational scenarios. However, public settings have unique characteristics and therefore impose unique challenges. Public spaces attract diverse audiences who differ in age, interests, and experience with technology and who will engage in spontaneous and often unpredictable activities, individually and in groups. In addition, public settings' spatial layouts, sizes, lighting conditions, and social connotations affect which display technologies and interaction techniques are adequate and how people will interact with and experience an installation.