Improved accuracy in 3D analysis using DLT after lens distortion correction.

This study aimed at assessing the applicability of a robust method to determine and correct lens distortion before using the direct linear transformation (DLT) algorithm in three-dimensional motion analysis. The known length of a rigid bar was reconstructed under different conditions of working volume (interpolation or extrapolation), number of cameras (2 or 4), position of the cameras (wide or narrow angle between optical axes), camera focal distance (4 or 8 mm) and number of control points (CPs; 8, 12, 18 or 162), through four different camera set-ups. The accuracy (percent root mean square error) of Set-up 2 (non-extrapolated working volume; two cameras; 4 mm focal distance; narrow optical axes angle) decreased with less CPs (162: 0.73%; 8: 2.78%). Set-up 1 (non-extrapolated working volume; two cameras; 8 mm focal distance; wide optical axes angle), Set-up 3 (Set-ups 1 and 2 used simultaneously) and Set-up 4 (extrapolated working volume; two cameras; 4 mm focal distance; wide optical axes angle) showed minor differences in accuracy across groups of CPs, with maximum values of 0.84%, 1.20% and 1.71%, respectively. Random errors were the main source of decreased accuracy of Set-ups 2 and 4.The proposed procedure enables accurate results with no modification in the DLT-based analysis system, even with smaller calibration frames, less CPs and wide field-of-view cameras.

Quantitative underwater 3D motion analysis using submerged video cameras: accuracy analysis and trajectory reconstruction.

In this study we aim at investigating the applicability of underwater 3D motion capture based on submerged video cameras in terms of 3D accuracy analysis and trajectory reconstruction. Static points with classical direct linear transform (DLT) solution, a moving wand with bundle adjustment and a moving 2D plate with Zhang's method were considered for camera calibration. As an example of the final application, we reconstructed the hand motion trajectories in different swimming styles and qualitatively compared this with Maglischo's model. Four highly trained male swimmers performed butterfly, breaststroke and freestyle tasks. The middle fingertip trajectories of both hands in the underwater phase were considered. The accuracy (mean absolute error) of the two calibration approaches (wand: 0.96 mm - 2D plate: 0.73 mm) was comparable to out of water results and highly superior to the classical DLT results (9.74 mm). Among all the swimmers, the hands' trajectories of the expert swimmer in the style were almost symmetric and in good agreement with Maglischo's model. The kinematic results highlight symmetry or asymmetry between the two hand sides, intra- and inter-subject variability in terms of the motion patterns and agreement or disagreement with the model. The two outcomes, calibration results and trajectory reconstruction, both move towards the quantitative 3D underwater motion analysis.