Deformable interfaces offer new possibilities for gestures, some of which have been shown effective in controlled laboratory studies. Little work, however, has attempted to match deformable interfaces to a demanding domain and evaluate them out of the lab. We investigate how musicians use deformable interfaces to perform electronic music. We invited musicians to three workshops, where they explored 10 deformable objects and generated ideas on how to use these objects to perform music. Based on the results from the workshops, we implemented sensors in the five preferred objects and programmed them for controlling sounds. Next, we ran a performance study where six musicians performed music with these objects at their studios. Our results show that (1) musicians systematically map deformations to certain musical parameters, (2) musicians use deformable interfaces especially to filter and modulate sounds, and (3) musicians think that deformable interfaces embody the parameters that they control. We discuss what these results mean to research in deformable interfaces.

Elastic, deformable displays allow users to give input by pinching, pushing, folding, and twisting the display. However, little is known about what gestures users prefer or how they will use elasticity and deformability as input. We report a guessability study where 17 participants performed gestures to solve 29 tasks, including selection, navigation, and 3D modeling. Based on the resulting 493 gestures, we describe a user-defined gesture set for elastic, deformable displays. We show how participants used depth and elasticity of the display to simulate deformation, rotation, and displacement of objects. In addition, we show how the use of desktop computers as well as multi-touch interaction affected users' choice of gestures. Finally, we discuss some unique uses of elasticity and deformability in gestures.

This paper investigates users' ability to perform force-sensitive tapping and explores its potential as an input modality in touch-based systems. We study force-sensitive tapping using Expressive Touch, a tabletop interface that infers tapping force from the sound waves created by the users' finger upon impact. The first part of the paper describes the implementation details of Expressive Touch and shows how existing tabletop interfaces can be augmented to reliably detect tapping force across the entire surface. The second part of the paper reports on the results of three studies of force-sensitive tapping. First, we use a classic psychophysic task to gain insights into participants' perception of tapping force (Study 1). Results show that although participants tap with different absolute tapping forces, they have a similar perception of relative tapping force. Second, we investigate participants' ability to control tapping force (Study 2) and find that users can produce two force levels with 99% accuracy. For six levels of force, accuracy drops to 58%. Third, we investigate the usability of force tapping by studying participants' reactions to seven force-sensitive touch applications (Study 3).

Shape-changing handheld devices are emerging as research prototypes, but it is unclear how users perceive them and which experiences they engender. The little data we have on user experience is from single prototypes, only covering a small part of the possibilities in shape change. We produce 51 videos of a shape-changing handheld device by systematically varying seven parameters of shape change. In a crowd-sourced study, 187 participants watched the videos and described their experiences using rating scales and free text. We find significant and large differences among parameters of shape change. Shapes that have previously been used for notifications were rated the least urgent; the degree of shape change was found to impact experience more than type of shape change. The experience of shape change was surprisingly complex: hedonic quality were inversely related to urgency, and some shapes were perceived as ugly, yet useful. We discuss how to advance models of shape change and improve research on the experience of shape change.

We introduce the concept of tangibles that users can manipulate with their feet. We call them kickables. Unlike traditional tangibles, kickables allow for very large interaction surfaces as kickables reside on the ground. The main benefit of kickables over other foot-based modalities, such as foot touch, is their strong affordance, which we validate in two user studies. This affordance makes kickables well-suited for walk-up installations, such as tradeshows or museum exhibits.

Shape change is increasingly used in physical user interfaces, both as input and output. Yet, the progress made and the key research questions for shape-changing interfaces are rarely analyzed systematically. We review a sample of existing work on shape-changing interfaces to address these shortcomings. We identify eight types of shape that are transformed in various ways to serve both functional and hedonic design purposes. Interaction with shape-changing interfaces is simple and rarely merges input and output. Three questions are discussed based on the review: (a) which design purposes may shape-changing interfaces be used for, (b) which parts of the design space are not well understood, and (c) why studying user experience with shape-changing interfaces is important.

We present interaction techniques for tangible tabletop interfaces that use active, motorized tangibles, what we call Tangible Bots. Tangible Bots can reflect changes in the digital model and assist users by haptic feedback, by correcting errors, by multi-touch control, and by allowing efficient interaction with multiple tangibles. A first study shows that Tangible Bots are usable for fine-grained manipulation (e.g., rotating tangibles to a particular orientation); for coarse movements, Tangible Bots become useful only when several tangibles are controlled simultaneously. Participants prefer Tangible Bots and find them less taxing than passive, non-motorized tangibles. A second study focuses on usefulness by studying how electronic musicians use Tangible Bots to create music with a tangible tabletop application. We conclude by discussing the further potential of active tangibles, and their relative benefits over passive tangibles and multi-touch.

Tangible user interfaces for manipulating audio and music focus mostly on generating music on the spot, but rarely on how electronic musicians balance preparation and improvisation in staging live performances or on how the audience perceives the performances. We present mixiTUI, a tangible sequencer that allows electronic musicians to import and perform electronic music. mixiTUI is developed in collaboration with electronic musicians, with a focus on live arranging, on visualizations of music, on tokens that represent key elements in live performances, and on how the audience experiences the tangible interface. We present an evaluation of mixiTUI in a concert with 117 participants and argue that mixiTUI improves both the audience's and the musician's experience.