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Tangible objects on capacitive multi-touch surfaces are usually only detected while the user is touching them. When the user lets go of such a tangible, the system cannot distinguish whether the user just released the tangible, or picked it up and removed it from the surface. We introduce PERCs, persistent capacitive tangibles that "know" whether they are currently on a capacitive touch surface or not. This is achieved by adding a small field sensor to the tangible to detect the touch screen's own, weak electromagnetic touch detection probing signal. Thus, unlike previous designs, PERCs do not get filtered out over time by the adaptive signal filters of the touch screen. We provide a technical overview of the theory be- hind PERCs and our prototype construction, and we evaluate detection rates, timing performance, and positional and angular accuracy for PERCs on a variety of unmodified, commercially available multi-touch devices.Through their affordable circuitry and high accuracy, PERCs open up the potential for a variety of new applications that use tangibles on today's ubiquitous multi-touch devices.

Tangibles on interactive surfaces enable users to physically manipulate digital content by placing, manipulating, or removing a tangible object. However, the information whether and how a user grasps these objects has not been mapped out for tangibles on interactive surfaces so far. Based on Buxton's Three-State Model for graphical input, we present an interaction model that describes input on tangibles that are aware of the user's grasp. We present two examples showing how the user benefits from this extended interaction model. Furthermore, we show how the interaction with other existing tangibles for interactive tabletops can be modeled.

Capacitive multi-touch displays are designed to detect touches from fingers that often change the location. This is quite the opposite of our goal: detect passive objects placed on them. In fact, these systems usually contain filters to actively reject such inactive input data. We present a technical analysis of this problem and introduce Passive Untouched Capacitive Widgets (PUCs). Unlike previous approaches, PUCs do not require power, they can be made entirely transparent, and they do not require internal electrical or software modifications. Most importantly they are detected reliably even when no user is touching them.