IPT & EGVE Workshop (2005) R. Blach, E. Kjems (Editors) Näprä — Affordable Fingertip Tracking with Ultrasound M. Reunanen 1 , K. Palovuori 2 , T. Ilmonen 1 and W. Mäkelä 3 1 Telecommunications Software and Multimedia Laboratory, Helsinki University of Technology, Finland 2 Institute of Electronics, Tampere University of Technology, Finland 3 Media Lab, Helsinki University of Art and Design, Finland Abstract In this paper we present Näprä, a novel tracking device suitable for fine motor interaction. The motivation for building the device was the need to track users’ fingertips in an immersive free-hand drawing environment. Such tracking offers significant benefits for fine-grained artwork. Out of the numerous tracking methods ultrasound was chosen because of its affordability and low computational requirements. The design and implementation of both the hardware and software are discussed in detail in their respective sections. The device was evaluated in practice by two user tests, the first involving ten professional artists and the latter seven ordinary users. The results obtained in the tests are presented to reader as well as some directions for future work. Categories and Subject Descriptors (according to ACM CCS): H.5.2 [User Interfaces]: Input devices and strategies H.5.1 [Multimedia Information Systems]: Artificial, augmented, and virtual realities J.5 [Arts and Humanities]: Fine arts 1. Introduction Immersive free-hand drawing as an art form has received some attention since the late 90’s. The best known examples of such research are conducted by Schkolne [SPS01] and Keefe [KFM * 01]. Both groups have used tangible interface elements in their drawing systems. The work presented in this paper belongs to the same frame of reference of immer- sive art: the underlying goal was to help artists better express themselves in a virtual environment using natural means of interaction. As an artists’ tool a stylus or a wand-like device allows for only limited fine motor interaction, greatly limiting the rich variety of gestures achievable with a human hand and fingers. Especially the fingertips are capable of performing fine-grained tasks. To track the exact position of the user’s fingertips in a virtual environment there are two possible ap- proaches. The first, direct method would be to track the fin- gertips using the default tracking system of the particular virtual environment. However, the fingertips differ from the usual tracking targets presenting challenges to the tracking system. A serious obstacle is the large number of fingertips that need to be tracked separately. The other approach is to track fingertips in another refer- ence frame using a secondary tracking system. This method has been often used with data gloves to track the palm of the hand with the primary — often magnetic — tracking system and from this tracked and therefore known reference frame the fingertips. A low-cost dataglove provides only rough in- formation about the bending of the fingers, which is less than satisfactory for artistic use. To be able to capture the full spectrum of the fine motor movements of the artist a better solution is required. High-end datagloves provide the needed functionality but on the other hand often cost several thou- sands of dollars thus making them unsuitable for small-scale projects. The two practical affordable techniques for accurate fin- gertip tracking are video and ultrasound. Both have been used in numerous virtual reality setups for large scale track- ing and to a certain extent they share the same characteris- tics: occlusion and environmental noise (light or sound) hurt the accuracy. Optical tracking has been succesfully used for fingertips [RB04][PCI03][VRC99] but the inherent prob- lems of the technique still remain partially unsolved. A great advantage for optical tracking is that the user only needs to wear simple markers or in the best case nothing extra in her c  The Eurographics Association 2005.