Current touch devices are adept at tracking finger touches, but cannot distinguish if multiple touches are caused by different fingers on a single hand, by fingers from both hands of a single user, or by different users. This limitation significantly reduces the possibilities for interaction techniques in touch interfaces. We present GhostID, a capacitive sensor that can differentiate the origins of multiple simultaneous touches. Our approach analyzes the signal ghosting, already present as an artifact in a frequency-division touch controller, to differentiate touches from the same hand or different hands of a single user (77% reliability at 60 fps) or from two different users (95% reliability at 60 fps). In addition to GhostID, we also develop a framework of user-differentiation capabilities for touch input devices, and illustrate a set of interaction techniques enabled by GhostID.

We report on the Latency Hammer, a low-cost yet highaccuracy and high-precision automated tool that measures the interface latency of touchscreen devices. The Hammer directly measures latency by triggering a capacitive touch event on a device using an electrically actuated touch simulator, and a photo sensor to monitor the screen for a visual response. This allows us to measure the full end-toend latency of a touchscreen system exactly as it would be experienced by a user. The Hammer does not require human interaction to perform a measurement, enabling the acquisition of large datasets. We present the operating principles of the Hammer, and discuss its design and construction; full design documents are available online. We also present a series of tools and equipment that were built to assess and validate the performance of the Hammer, and demonstrate that it provides reliable latency measurements.

This paper reports on two experiments designed to further our understanding of users' perception of latency in touch- based systems. The first experiment extends previous efforts to measure latency perception by reporting on a unified study in which direct and indirect form-factors are compared for both tapping and dragging tasks. Our results show significant effects from both form-factor and task, and inform system designers as to what input latencies they should aim to achieve in a variety of system types. A follow-up experiment investigates peoples' ability to perceive small improvements to latency in direct and indirect form-factors for tapping and dragging tasks. Our results provide guidance to system designers of the relative value of making improvements in latency that reduce but do not fully eliminate lag from their systems.

Hotkeys are an efficient method of selecting commands on a keyboard. However, these shortcuts are often underused by users. We present Métamorphe, a novel keyboard with keys that can be individually raised and lowered to promote hotkeys usage. Métamorphe augments the output of traditional keyboards with haptic and visual feedback, and offers a novel design space for user input on raised keys (e.g., gestures such as squeezing or pushing the sides of a key). We detail the implementation of Métamorphe and discuss design factors. We also report two user studies. The first is a user-defined interface study that shows that the new input vocabulary is usable and useful, and provides insights into the mental models that users associate with raised keys. The second user study shows improved eyes-free selection performance for raised keys as well as the surrounding unraised keys.