3d Movement Detecticn: A Hierarchical Approach

Abstract

To study quantitatively human body movement the modelling approach is used in biomechanics; body's segments are schematically represented by links, and joint articulations by hinges. For an exhaustive description of the movement of such a structure, it is enough to detect and follow hinges movement from which the trajectories of all the structure can be recovered. A common approach is to mark these points by active or passive markers and then to use a suitable machine to detect them. For this purpose, a 3D movement detection system (Elite) has been developed. It is a vision system with a two level architecture; the first level, hardware implemented, receives the image from a set of TVcameras and recognizes, in real-time. (100Hz), any number of markers on the scene. By a cross-correlation filtering algorithm, the markers are recognized on the basis of their shape. This process gives a high signal to noise ratio, which allows the operator to disregard reflexes and other objects present in the environment (which are rejected); moreover the markers (small,light pieces of reflecting paper) do not cause any constraints on the subject to be analyzed. The second level, software implemented, performs a more intelligent task, that of matching the surveyed markers' coordinates to the marker arrangement predefined in a model. Each model constituted by links and hinges interconnected among them is the visual schematic representation of the subject to be analyzed. Starting from it, a special designed program (KAT - Knowledge based Automatic Tracking) automatically tracks down each marker frame by frame reconstructing the coordinates even when the overlapping of body segments occurs. After this procedure, a 3D reconstruction is carried out by means of a generalized triangulation algorithm starting from the images picked up by a couple of TVcameras. The description of the system is completed with the 3D resection and calibration algorithm. The first allows the recovering of TVcameras geometrical internal and external parameters which are requested in 3D reconstruction; calibration algorithm allows distortions correction and leads to a final high accuracy in the computation of 3D markers position.