A PANNING DLT PROCEDURE FOR 3-DIMENSIONAL VIDEOGRAPHY

The direct linear transformation (DLT) method [Abdel-Aziz and Karara, APS Symposium on Photogrammetry. American Society of Photogrammetry, F alls Church, VA (1971)] is widely used in biomechanics to obtain three -dimensional space coordinates from film and video records. This metho d has some major shortcomings when used to analyze events which take p lace over large areas. To overcome these shortcomings, a three-dimensi onal data collection method based on the DLT method, and making use of panning cameras, was developed. Several small single control volumes were combined to construct a large total control volume. For each sing le control volume, a regression equation (calibration equation) is dev eloped to express each of the 1 1 DLT parameters as a function of came ra orientation, so that the DLT parameters can then be estimated from arbitrary camera orientations. Once the DLT parameters are known for a t least two cameras, and the associated two-dimensional film or video coordinates of the event are obtained, the desired three-dimensional s pace coordinates can be computed. In a laboratory test, five single co ntrol volumes (in a total control volume of 24.40 x 2.44 x 2.44 ml) we re used to test the effect of the position of the single control volum e on the accuracy of the computed three dimensional space coordinates. Linear and quadratic calibration equations were used to test the effe ct of the order of the equation on the accuracy of the computed three dimensional space coordinates. For four of the five single control vol umes tested, the mean resultant errors associated with the use of the linear calibration equation were significantly larger than those assoc iated with the use of the quadratic calibration equation. The position of the single control volume had no significant effect on the mean re sultant errors in computed three dimensional coordinates when the quad ratic calibration equation was used. Under the same data collection co nditions, the mean resultant errors in the computed three dimensional coordinates associated with the panning and stationary DLT methods wer e 17 and 22 mm, respectively. The major advantages of the panning DLT method lie in the large image sizes obtained and in the ease with whic h the data can be collected. The method also has potential for use in a wide variety of contexts. The major shortcoming of the method is the large amount of digitizing necessary to calibrate the total control v olume. Adaptations of the method to reduce the amount of digitizing re quired are being explored.