In this paper, we present a Human-Computer Interaction (HCI) design pattern language that bundles existing knowledge on tabletop design and offers solutions to recurring problems. Our patterns enable not only developers, designers, and domain experts to improve their existing systems and facilitate the design process of new systems, we also encourage novice users to comprehend the variety of tabletop research and commercial products in this domain. We consider our language as a starting point to create a sustainable body of knowledge that will be extended and refined by the community.

We present Silicone iLluminated Active Peripherals (SLAP), a system of tangible, translucent widgets for use on multitouch tabletops. SLAP Widgets are cast from silicone or made of acrylic, and include sliders, knobs, keyboards, and buttons. They add tactile feedback to multi-touch tables, improving input accuracy. Using rear projection, SLAP Widgets can be relabeled dynamically, providing inexpensive, battery-free, and untethered augmentations. Furthermore, SLAP combines the flexibility of virtual objects with physical affordances. We evaluate how SLAP Widgets influence the user experience on tabletops compared to virtual controls. Empirical studies show that SLAP Widgets are easy to use and outperform virtual controls significantly in terms of accuracy and overall interaction time.of on-screen controls have begun to show. For example, typing on a projected soft keyboard is difficult due to the lack of tactile feedback, but returning to physical input devices is not always an option. On a large table surface, a physical keyboard is either far away from the on-screen locus of attention, or it blocks part of the projection when put onto the table. On-screen buttons and scrollbars also lack tactile feedback, making it hard to operate them fluidly, but physical counterparts are not readily available.SLAP (Silicone ILluminated Active Peripherals) are transparent physical widgets made from flexible silicone and acrylic. As input devices, they combine the advantages of physical and virtual on-screen widgets. They provide a haptic operation experience with tactile feedback, supporting fluid and eyes-free operation. At the same time, thanks to their transparency, they support dynamic software-controlled labeling, using the rear projection of the interactive table they rest on. SLAP Widgets are also very simple hardware devices, without the need for tethering or any power, making them highly robust and affordable for research and prototyping. When made from silicone, they are even physically flexible and can literally be "slapped" on a table or tossed across the table from one user to another.After a review of related research, the remainder of this paper introduces the hardware and software architecture be-Figure 1. SLAP Widgets. a) Keypads with two and three buttons. b) Knob. c) Slider. d) Keyboard.

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We introduce dynamic physical properties as an additional degree of freedom for passive tabletop controls. Using electromagnetic actuation, we manipulate attributes of tangibles on the fly, such as perceived weight, spring resistance, friction, and latching. We describe our actuation concepts, prototypes, and measurements showing that magnetic fields can change physical effects in a linear way. Controlled experiments reveal that participants can tactually distinguish four rendered resistance levels of a button prototype and easily detect dynamic detents in a continuous slider. Finally, we describe how adjustable physical properties in tangibles can enhance tabletop interaction.