SMART3D: Tracking of movements surveyed by a multiple set of TV cameras

Abstract

The use of passive markers in automatic motion analysis has the incomparable advantage of allowing the recording of a completely free movement. This is a prerequisite for collecting reliable quantitative data on sport gestures. However, the data processing required to reconstruct the three-dimensional trajectories of the markers is computationally demanding. For each frame each surveyed point must be assigned to the corresponding marker and the threedimensional positions of the markers computed. The complexity of the problem greatly increases when the trajectories become so close that it becomes difficult to distinguish between them, and when some of the markers are hidden during the movement; when more than two TV cameras are used, there is the additional problem of tracking the trajectory of a single marker over different pairs of TV cameras. In this paper a system which employs a general solution of these problems is presented (SMART3D). It is based on the evaluation of the affinity of a pair of surveyed points with the configuration of the markers in the three-dimensional space. The figure of merit is composed by two terms. The first term is derived from the difference between the surveyed positions of the points and their position as predicted by a FIR filter, the second term is related to the change of the distances between the corresponding marker and the others. The assignment is subject to a stereopsis constraint: two points belong to the same marker if the distance between their projections and their co-ordinates is less than the quantization error. To avoid combinatorial explosion, a preliminary test is carried out and all the surveyed points that are distant from the projections of the other markers are extracted and assigned. When a marker, temporarily hidden, reappears, it is successfully classified by a congruence test on the distances. The system, connected with the Elite system in a configuration of 4 independent TV cameras, is undergoing extensive testing in our laboratory during routine use. Preliminary results will be shown.