Spatial and Temporal Performance Characteristics in a Two-Dimensional Human Motion Analysis System Using Digital Video Capture
Digital Video Capture , Spatial , Temporal , Calibration , Human Motion Analysis
A testing framework was developed to address system spatial and temporal performance characteristics in a two-dimensional (2D) human motion analysis system using commercially available digital video capture. The first testing protocol involved developing a method to evaluate system spatial performance characteristics with respect to accuracy, precision, and resolution. A physical model comprising a calibration frame was constructed with phantom postures selected to represent joint angles and off-plane movement typical of the activities of interest. This provided reference angles to which angles measured from digitally captured images were compared using the Bland and Altman method. Validation experiments confirmed that the principal sources of error were due to off-plane motion and pixel resolution in the video capture and analysis systems. In these analyses, it was verified that simulated experimental conditions could be corrected using the direct linear transform (DLT); however, the removal of parallax still resulted in 2 degrees of error in measured joint angles. The main source of error was resolution of the data acquisition system verified through Monte Carlo simulations. The second testing protocol involved developing a simple method to determine the temporal accuracy of motion analysis systems incorporating digital video cameras and a pendulum. A planar column pendulum with a natural frequency of 0.872 Hz was used to analyse five systems incorporating commercially available cameras and a single codec. The frame rate for each camera was measured to be within 3% of the US National Television Systems Committee (NTSC) broadcasting digital video standard of 29.97 fps.; however some cameras produced a frame duplication artefact. Least squares curve-fitting using a sinusoidal function revealed RMS differences between 3-5% for angular position and 5-15% for angular speed compared to the captured motion data. It was shown that some digital-video cameras and computer playback software contain data compression technology that may produce substantial temporal frame inaccuracies in recovered video sequences and that temporal accuracy should be evaluated in digital-based human motion analysis systems prior to their use in experimentation.