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Page 00000053 THE 3D AUGMENTED MIRROR: MOTION ANALYSIS FOR STRING PRACTICE TRAINING Kia Ng, Oliver Larkin,' Thijs Koerselman,' Bee Ong,1 Diemo Schwarz,2 Frederic Bevilacqua2 1ICSRiM - University of Leeds, School of Computing & School of Music, Leeds LS2 9JT, UK www.icsrim.org.uk 2 IRCAM - CNRS STMS, 1 place Igor-Stravinsky, 75004 Paris, France www.ircam.fr email@example.com, www.i-maestro.org ABSTRACT In this paper we present a system being developed in the context of the i-Maestro project , for the analysis of gesture and posture in string practice training using 3D motion capture technology. The system is designed to support the teaching and learning of bowing technique and posture, providing multimodal feedback based on real-time analysis of motion capture data. We describe the pedagogical motivation, technical implementation and features of our system. 1. INTRODUCTION String players often use mirrors to observe themselves practicing. Recently we have seen the increased use of video recording in music education as a practice and teaching tool . For the purpose of analysing a performance these tools are not ideal since they offer a limited 2D, fixed perspective view. Using 3D Motion Capture technology it is possible to overcome this limitation by visualising the instrument and the performer in a 3D environment . Furthermore, the motion capture data can be analysed to provide relevant feedback about the performance. Feedback can then be delivered to the users in a variety of ways in both real time (online) and delay-time (offline) situations. This can help teachers to illustrate different techniques and can also to identify a student's problems. It can help students to develop awareness of their playing and to practice effectively. This paper presents a system for the analysis and visualisation of 3D motion data with synchronised video and audio. It discusses the interface between Max/MSP and a motion capture system, a library of Jitter objects developed for analysis and visualisation, and preliminary work on a sonification module for realtime auditory feedback based on analyses of motion capture data. 2. RELATED WORK Although a great deal of research in the area of new musical interfaces looks at the violin family, the majority of this work lies in the context of contemporary composition and performance or labbased gesture analysis. There are few examples of these interfaces being developed specifically for music pedagogy applications. 3D motion capture has been used in  to assist in piano pedagogy by visualising the posture of a professional performer and comparing it to that of the student. Motion capture has also been featured in a double bass instruction DVD  to help illustrate elements of bowing technique. The use of auditory feedback to teach musical instrument skills has been discussed in . For related research on the sonification of motion data, see  and . For the mapping of motion data to music see . 3. ANALYSES AND FEEDBACK Through our own experience, discussion with teachers, and by studying existing string pedagogy literature we have identified areas of interest for analysis. These include: * Relationships between the bow and instrument body (e.g. the angle between the bow and the bridge, the part of the bow that is used and the point at which the bow meets the strings). * 3D trajectories/shapes traced by the bow and bowing arm (see Figure 1) * Body Posture (e.g. the angles of the neck, bowing arm and wrist) el....N.......I W.Figure 1. A screenshot of the 3D augmented mirror interface showing synchronised video and motion capture data with 3D bowing trajectories. 53
Page 00000054 4. 3D MOTION CAPTURE 5. 3D AUGMENTED MIRROR The details of the motion capture process have been covered sufficiently elsewhere    so here we briefly explain our setup. 4.1. System We use a VICON 8i Optical Motion capture system1 with twelve infra red cameras (200fps) and a passive marker system. The high frame rate allows us to capture rapid bowing motions in detail and slow them down significantly maintaining an adequate resolution. 4.2. Markers and Models We attach markers to the bow (see Figure 3), the instrument and the performer's body. Markers are placed strategically in order to: 1. maximise visibility to the cameras 2. minimise the risk of displacement 3. minimise interference with the performance 4. minimise damage to the instrument 5. provide the necessary information for the required data analyses. During the performance it is desirable to use as few markers as possible, to reduce the amount of marker data and to allow the performer as much freedom as possible. This may result in the loss of some information that may be necessary for data analyses. The solution is to create a static model of the instrument with a large number of markers. Once we have this model we can remove some of the markers and use the simplified model for the performance (see Figure 2). The system reconstructs the locations of the missing markers in real time by applying offsets based on the static model. Our system, which we call the "3D Augmented Mirror", is implemented in Max/MSP and Jitter using the MAV (Motion Analysis and Visualisation) framework. Rather than providing a fixed configuration, our system allows different analysis and visualisation modules to be scripted dynamically. This functionality is essential since we do not want to restrict users to certain types of analysis nor do we want to fix the user interface to any particular configuration. The 3D Augmented Mirror supports synchronised recording and variable rate playback of motion capture data, audio and video. VICON MOTION CAPTURE SYSTEM TCP/I F------------------------ VICONBRIDGE VIDEO AUDIO TCP/I SESSION | ANALYSIS MODULES MaxMSP Dynamic Jitter Scripting & MAV S VISUALISATION MODULES Visualisation Sonification Figure 4. 3D Augmented Mirror - System Overview. The "session" section deals with data management and transport controls. 5.1. Interface to the Motion Capture System We developed an application (called VICONbridge) which streams the data from the VICON system to Max using the TCP/IP protocol. The application parses incoming motion data and sends the filtered and reformatted data to the jit.net.recv object. 5.2. MAV Framework The MAV framework is designed as a collection of algorithms and Jitter objects based on a shared library. The objects perform various analysis and visualisation tasks using low level data processing and dynamic binding to realise a highly effective and flexible framework. Although we do not explicitly deal with video processing, the Jitter API offers a great amount of freedom over the standard Max API. Since Jitter objects are not "physically" bound to a Max patcher, it is possible to dynamically script the MAV objects at runtime2. We chose Lua3 as the scripting language to implement the dynamic environment within Max, using the jit.gl.lua external developed by Wesley Smith4. An 2 See Jitter API reference: http;//wwwxcycling74.com/ 3 http://www.lua.org 4 http://www. mat.ucsb.edu/~whsrnith/ Figure 2. Cello static model (top) and performance model (bottom) Figure 3. A cello bow with three markers. Markers are spaced unevenly in order to track the object's orientation. I http://www.vicon.corn 54
Page 00000055 advantage of using Lua is its ability to link custom C modules. By linking Lua to the same dynamic library as the MAV externals we are able to access and share any data type or function between the library and Lua scripts, keeping CPU intensive tasks in C. We use this technique to separate visualisation algorithms from data analysis and processing. 5.3. Data Representation Our system stores motion capture data using the Motion Analysis' TRC format, with audio and video as separate files. We are currently adding support for SDIF  which will allow us to store synchronised audio, motion and analysis data in one file. We use several SDIF data streams in order to combine multiple representations of data. The first stream contains standard time domain sampling (1TDS) matrices for the audio data. A second stream contains all motion capture data structured for every frame and stored in a custom matrix type with columns for ID, X, Y and Z data and a row for each marker. The third stream will contain multiple custom defined matrices for storing analysis data. SDIF data is represented by timetagged frames, which allows streams at arbitrary data rates to be synchronised. This approach is compatible with that described in  and would facilitate the correlation of data obtained with other types of gesture capture system e.g. IRCAM's augmented violin . Figure 5. Screenshot showing analyses during a cello Performance. The coordinates system is placed on the bridge of the cello and the angle of the bow is visualised. A graph shows the distance traveled by the bow over each stroke. 6. ANALYIS METHOD Using the MAV library, the 3D Augmented Mirror system can perform a number of different data analyses by processing the 3D motion data. These analyses can be visualised using graphs and other graphical representations based on OpenGL (see Figure 1 and Figure 5) 1 http://www.notionanalysis.com 6.1. Dynamic Coordinates System The data from the motion capture system gives us the absolute location of each marker. To perform several of the desired analyses, it is necessary to derive a local coordinates system based on specific markers rather than using arbitrary world co-ordinates. For example, to monitor bowing, the local coordinates system will typically be positioned on the instrument body in order to focus on the interactions between the bow and the instrument. 6.2. Bow Stroke Segmentation Using a local coordinates system we can extract the direction of the bowing motion by calculating the difference in distance (over consecutive frames) between the tip of the bow and the point at which the bow intersects the plane that is perpendicular to the fingerboard plane (see figure 5). A change of direction indicates a segmentation point (i.e. change of bow). This segmentation can reveal the connection between bow strokes and other characteristics of the performance. 6.3. Orientation By choosing two markers, the user may define a vector and monitor its orientation in relation to the local coordinates system. By setting the two markers on the bow and a local origin on the bridge, we can use this process to analyse the angle of the bow in relation to the bridge. 6.4. Bowing Gestures Our system provides the option to visualise the trajectories drawn by bowing movements (see Figure 1). Taking this further, we are segmenting and studying individual shapes. There are a number of interesting features that may be extracted from these shapes, for example the direction and orientation of the shape, its bounding volume and its "smoothness". We are particularly interested in studying the relationship of these shapes to the music being played and to different bowing techniques. For example, Figure 6 shows the passage being played in Figure 1, which creates a clear figure of eight shape. Figure 6. Excerpt from Bach's Partita No. 3 in B major. BMW 1006 (Preludio) 7. SONIFICATION OF DATA ANALYSES We are using sonification as an alternative means of representing the information gathered from analyses of 3D motion data. This has potential for a number of reasons: 55
Page 00000056 * Sound is the primary feedback mechanism in instrumental performance, therefore it is logical to assume that to a musician auditory feedback may be more relevant and useful than visual feedback. * When a student is practicing, they may not be able to use visual representations . Their eyes may be occupied reading the score. * Sonification allows better temporal resolution which is appropriate for monitoring rapid movements such as those involved with bowing technique. * Sonification may allow multiple streams of information to be communicated to the user simultaneously. 7.1. Implementation We have implemented a sonification tool in Max/MSP, initially using the angle of the bow in relation to the instrument as input data in order to monitor "parallel bowing". We provide different options including controlling synthesised sounds and also processing the instrument sound in order to create a sonification that is related to the performance. We believe this may offer a more appropriate and usable feedback than synthesised sounds. Sonification may be discreet or continuous depending on the user's preference. Discreet mode allows two separate settings to monitor (user defined) "correct" and "incorrect" angles of the bow. In continuous mode, the angle of the bow is sonified so the performer will be made aware of the angle as they play. The parameter mapping may be adjusted for both modes so that deviations in the input can provide a different range of sound modification. Since it may be undesirable to constantly hear the sonification, we provide a threshold control. This allows the sonification to interject only when necessary. 8. CONCLUSION AND FUTURE WORK We presented a system for the analysis of 3D motion capture data within the Max/MSP environment and multimodal delivery of analysis data. Although this system is firstly aimed at studying string performance, it is by no means limited to this functionality. Development is ongoing, and we plan to add more analysis, visualisation and sonification possibilities based on further discussion with teachers and students. This process will involve trials and validation in real pedagogical situations. We plan to integrate our system with other technologies related to the i-Maestro project such as MPEG SMR  and score/gesture following . Primarily we are interested in ways of annotating gesture analysis data onto the score and indexing motion capture recordings to musical phrases. 9. ACKNOWLEDGEMENTS The i-Maestro project is partially supported by the European Community under the Information Society Technologies (IST) priority of the 6th Framework Programme for R&D (IST-026883, www.imaestro.org). Thanks to all i-Maestro project partners and participants, for their interest, contributions and collaboration. 10. REFERENCES  I-MAESTRO EC IST project, http://www.i-maestro.org  Ong, B. Khan, A Ng, K Bellini, P Mitolo, N and Nesi, P. "Cooperative Multimedia Environments for Technology-Enhanced Music Playing and Learning with 3D Posture and Gesture Supports", in Proc. ICMC2006, Tulane University, USA, 2006  Mora, J., Won-Sook, L., Comeau, G., Shirmohammadi, S., El Saddik., A. "Assisted Piano Pedagogy through 3D Visualization of Piano Playing", in Proc. HAVE, Ottawa, Canada, 2006  Daniel, R. Self-assessment in performance. British Journal of Music Education, vol. 18, pp. 215-226 Cambridge University Press, 2001  Sturm, H. Rabbath F. The Art of the Bow, 2006 http://www.artofthebow.com/  Ferguson, S. "Learning Musical Instrument Skills Through Interactive Sonification", Proc. NIME06, Paris, France, 2006  Bevilacqua, F., Ridenour, J. and Cuccia. D. "3D Motion Capture Data: Motion Analysis and Mapping to Music", in Proc. SIMS. Santa Barbara, USA. 2002  Verfaille, V. Quek. O, Wanderley M.M. "Sonification of Musicians' Ancillary Gestures", in Proc. ICAD, London, UK, 2005  Kapur, A., Tzanetakis, G., Virji-Babul, N., Wang G. and Cook. P.R. "A Framework for Sonification of Vicon Motion Capture Data", in Proc DAFXO5, Madrid, Spain, 2005.  Bellini, P., Nesi, P., Zoia, G. "Symbolic music representation in MPEG", IEEE Multimedia, 12(4), pp. 42-49, 2005  Schwarz, D. and Wright, M. "Extensions and Applications of the SDIF Sound Description Interchange Format", in Proc ICMC2000, Berlin, Germany, 2000  Bevilacqua, F. Guedy F, Schnell, N., Flety, E., Leyroy, N. "Wireless sensor interface and gesturefollower for music pedagogy", to appear in Proc. NIMEO7.  Jensenius, A. R., Kvifte, T. and God0y R. I. "Towards a gesture description interchange format", in Proc NIME06, IRCAM, Paris, France 56