ESTIMATION OF 6 DEGREE-OF-FREEDOM RIGID-BODY SEGMENT MOTION FROM 2-DIMENSIONAL IMAGE DATA

The human body is often modelled as a set of linked-segments moving in space. Each segment may be considered rigid, and hence, will move wit h 6 degrees of freedom (df). Conventional photogrammetric kinematic me thods estimate Rigid Body (RB) motion through knowledge of the locatio ns of 3 or more non-collinear points on the RB. These points are measu red in two positions, one control, the other transformed. Methods of T hree Dimensional Reconstruction (3DR) are commonly used to obtain thes e measures. Within 3DR, each spatial marker requires coincident observ ation from at least two images. If two or more observations are unavai lable, 3DR cannot proceed; thus potentially valid marker observations from a single image cannot be used to assist in 6 df measurement of an RB. Insufficient 3DR may result in regions of incomplete 6 df data du ring time-series analyses of motion. This paper presents a method, TRA CK26, which estimates 6 df directly from all available two dimensional image data. Use of this method with 4 or more non-collinear markers o n an RB can help to eliminate regions of missing 6 df kinematic data. An experiment with a uniformly revolving eleven marker RB shows TRACK2 6 to be more accurate and more successful, in practice, than a typical conventional RE tracking method. Where success is judged as being the ability to provide 6 df estimates, the conventional method returned a success rate of 46.6% compared to 100% for TRACK26. If one considers the success rate of determining 3D points alone, 3DR was found to be i nferior to either of the RB tracking methods. It is also noted that TR ACK26 can be used to provide 6 df RB motion estimates with the use of a series of single images. Given these advantages it is concluded that TRACK26 can provide more accurate and complete data for use in linked -segment analyses of human motion.