Abstract
Missing information in motion capture data caused by occlusion or detachment of markers is a common problem that is difficult to avoid entirely. The aim of this study was to develop and test an algorithm for reconstruction of corrupted marker trajectories in datasets representing human gait. The reconstruction was facilitated using information of marker inter-correlations obtained from a principal component analysis, combined with a novel weighting procedure. The method was completely data-driven, and did not require any training data. We tested the algorithm on datasets with movement patterns that can be considered both well suited (healthy subject walking on a treadmill) and less suited (transitioning from walking to running and the gait of a subject with cerebral palsy) to reconstruct. Specifically, we created 50 copies of each dataset, and corrupted them with gaps in multiple markers at random temporal and spatial positions. Reconstruction errors, quantified by the average Euclidian distance between predicted and measured marker positions, was ≤ 3 mm for the well suited dataset, even when there were gaps in up to 70% of all time frames. For the less suited datasets, median reconstruction errors were in the range 5–6 mm. However, a few reconstructions had substantially larger errors (up to 29 mm). Our results suggest that the proposed algorithm is a viable alternative both to conventional gap-filling algorithms and state-of-the-art reconstruction algorithms developed for motion capture systems. The strengths of the proposed algorithm are that it can fill gaps anywhere in the dataset, and that the gaps can be considerably longer than when using conventional interpolation techniques. Limitations are that it does not enforce musculoskeletal constraints, and that the reconstruction accuracy declines if applied to datasets with less predictable movement patterns.
Highlights
Loss of marker-information due to, for example, occlusion or marker detachment [1] often imposes challenges in marker based motion analysis [2]
Several additional approaches for the “missing marker problem” have been proposed [4,5,6,7,8,9,10]. These methods utilize the high covariance between marker coordinates that is typical for human motion tracking data [11, 12] and reconstruct missing markers from the information provided by the available markers
For CP-gait, if we exclude the LWRAmarker, the mean reconstruction accuracy was less than 1.5 cm even if the last half of the trajectory was missing
Summary
Loss of marker-information due to, for example, occlusion or marker detachment [1] often imposes challenges in marker based motion analysis [2]. The standard methods for filling gaps in marker trajectories are linear or spline interpolation, or reconstructing the trajectory in a local coordinate frame [3]. These approaches are restricted to gaps of short duration or to rigid body segments carrying 4 or more markers. Several additional approaches for the “missing marker problem” have been proposed [4,5,6,7,8,9,10] These methods utilize the high covariance between marker coordinates that is typical for human motion tracking data [11, 12] and reconstruct missing markers from the information provided by the available markers. The method’s capability to successfully reconstruct datasets with less repetitive or predictive movements, for instance during a gait transition phase, needs investigation
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