This article presents the development of the improvisation piece Transformation for electric violin and live electronics. The aim of the project was to develop an "invisible" technological setup that would allow the performer to move freely on stage while still being in full control of the electronics. The developed system consists of a video-based motion-tracking system, with a camera hanging in the ceiling above the stage. The performer's motion and position on stage is used to control the playback of sonic fragments from a database of violin sounds, using concatenative synthesis as the sound engine. The setup allows the performer to improvise freely together with the electronic sounds being played back as she moves around the "sonic space." The system has been stable in rehearsal and performance, and the simplicity of the approach has been inspiring to both the performer and the audience.