Three-dimensional videography of swimming with fanning periscopes

The purpose of this paper is to describe a three-dimensional videography method with panning periscopes for reconstructing swimming techniques. Two panning periscope systems, developed in our laboratory, were used for the data collection. A control object (1 x 1.2 x 2) m(3) with 72 markers was placed in seven consecutive locations along the (1.5 x 8.4 x 2) m(3) calibration space. The position of the control object was recorded while panning the periscope systems, and for each of the seven locations the DLT camera parameters were determined for eight fields equally spaced through the panning motion. Each DLT camera parameter was then expressed as a function of the camera orientation by using a least-squares, second-order, polynomial regression equation, so that the DLT camera parameters for any camera orientation could be predicted (Yu et al., J. Biomechanics 26, 741-751, 1993). The three-dimensional coordinates of desired body landmarks at an instant were determined from the digitized two-dimensional coordinates for two cameras and the predicted DLT camera parameters at that instant. The accuracy of the method was evaluated by (a) the resultant errors in computing three-dimensional coordinates of precalibrated static points, and (b) the errors in computing the length of a scale rod pulled through the calibration space. The mean resultant errors ranged from 8.34 to 16.44 mm for the above- and from 9.93 to 16.22 mm for the below-water control volumes. The mean error in computing lengths on the scale rod ranged from 3.32 to 5.83 mm for three 0.5 m lengths, and 9.97 mm for a 1.5 m length. The method produced acceptable results in the reconstruction of three-dimensional motions recorded from a large space above and below the water surface.