p115-han - Low-Cost Multi-Touch Sensing through Frustrated...

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Low-Cost Multi-Touch Sensing through Frustrated Total Internal Reflection Jefferson Y. Han Media Research Laboratory New York University 719 Broadway. New York, NY 10003 E-mail: [email protected] ABSTRACT This paper describes a simple, inexpensive, and scalable technique for enabling high-resolution multi-touch sensing on rear-projected interactive surfaces based on frustrated total internal reflection. We review previous applications of this phenomenon to sensing, provide implementation details, discuss results from our initial prototype, and outline future directions. ACM Classification: H.5.2 [ User Interfaces ]: Input Devices and Strategies General Terms: Human Factors Keywords: multi-touch, touch, tactile, frustrated total internal reflection INTRODUCTION While touch sensing is commonplace for single points of contact, it is still difficult and/or expensive to construct a touch sensor that can register multiple simultaneous points of contact. Multi-touch sensing enables a user to interact with a system with more than one finger at a time, as in chording and bi-manual operations. Such sensing devices are inherently also able to accommodate multiple users simultaneously, which is especially useful for larger shared-display systems such as interactive walls and tabletops. Initial investigations, though sparse due to the prohibitive availability of these devices, nonetheless reveal exciting potential for novel interaction techniques [1][2][11][12][19][23][26][27]. We present a simple technique for robust multi-touch sensing at a minimum of engineering effort and expense. It is based on frustrated total internal reflection (FTIR) , a phenomenon familiar to both the biometric and robot sensing communities. It acquires true touch image information at high spatial and temporal resolutions, is scalable to large installations, and is well suited for use with rear-projection. It is not the aim of this paper to explore the multi-touch interaction techniques that this system enables, but rather to make the technology readily available to those who wish to do so. RELATED WORK A straightforward approach to multi-touch sensing is to simply utilize a plurality of discrete sensors, making an individual connection to each sensor as in the Tactex MTC Express [20]. They can also be arranged in a matrix configuration with some active element (e.g. diode, transistor) at each node, as in the device featured in Lee et al.’s seminal work [11], and also in Westerman and Elias’s commercial FingerWorks iGesturePad [3][22]. Through careful driving techniques, it is possible to gather multi-touch information from a purely passive matrix of force-sensitive-resistors (FSRs) as developed by Hillis [6], or capacitive electrodes, such as in [18] and the recent SmartSkin [19], and thus achieve a great reduction in complexity. However, these devices still require very many connections, which keeps their resolution limited in practice (under 100×100). Furthermore, these systems are visually opaque, forcing systems to resort to top-projection for integration with a graphic display.
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