Filtering Motion Capture Data for Real-Time Applications

In this paper we present some custom designed filters for real-time motion capture applications. Our target applica-tion is motion controllers, i.e. systems that interpret hand motion for musical interaction. In earlier research we found e↵ective methods to design nearly optimal filters for real-time applications. However, to be able to design suitable filters for our target application, it is necessary to estab-lish the typical frequency content of the motion capture data we want to filter. This will again allow us to deter-mine a reasonable cuto ↵ frequency for the filters. We have therefore conducted an experiment in which we recorded the hand motion of 20 subjects. The frequency spectra of these data together with a method similar to the residual analy-sis method were then used to determine reasonable cuto↵ frequencies. Based on this experiment, we propose three cuto ↵ frequencies for di↵erent scenarios and filtering needs: 5, 10 and 15 Hz, which correspond to heavy, medium and light filtering, respectively. Finally, we propose a range of real-time filters applicable to motion controllers. In partic-ular, low-pass filters and low-pass di↵erentiators of degrees one and two, which in our experience are the most useful filters for our target application. 1