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Concurrent interaction with physical and digital media is ubiquitous in knowledge work. Although tabletop systems increasingly support activities involving both physical and digital media, patterns of use have not been systematically assessed. This paper contributes the results of a study of spatial usage patterns when physical and digital items are grouped and sorted on a tabletop work surface. In addition, analysis reveals a dual character of occlusion, involving both inconvenient and desirable aspects. We conclude with design implications for hybrid tabletop systems.

a) Aluminum (b) Glass (c) MDF (d) Tile Figure 1: We explored the thermal reflectivity of different surfaces for interaction with projected surfaces. The thermal reflectivity allows to sense users that perform in-air gestures inside and outside the thermal camera's direct field-of-view. Four of the eight surfaces we analyzed and which can be found in normal office environments are presented above. ABSTRACTThermal cameras have recently drawn the attention of HCI researchers as a new sensory system enabling novel interactive systems. They are robust to illumination changes and make it easy to separate human bodies from the image background. Far-infrared radiation, however, has another characteristic that distinguishes thermal cameras from their RGB or depth counterparts, namely thermal reflection. Common surfaces reflect thermal radiation differently than visual light and can be perfect thermal mirrors. In this paper, we show that through thermal reflection, thermal cameras can sense the space beyond their direct field-of-view. A thermal camera can sense areas besides and even behind its field-of-view through thermal reflection. We investigate how thermal reflection can increase the interaction space of projected surfaces using camera-projection systems. We moreover discuss the reflection characteristics of common surfaces in our vicinity in both the visual and thermal radiation bands. Using a proof-of-concept prototype, we demonstrate the increased interaction space for hand-held camera-projection system. Furthermore, we depict a number of promising application examples that can benefit from the thermal reflection characteristics of surfaces.

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In this paper, we present a novel device concept that features double-sided displays which can be folded using predefined hinges. The device concept enables users to dynamically alter both size and shape of the display and also to access the backside using fold gestures. We explore the design of such devices by investigating different types and forms of folding. Furthermore, we propose a set of interaction principles and techniques. Following a usercentered design process, we evaluate our device concept in two sessions with low-fidelity and high-fidelity prototypes.