PROCESSING OF IMAGES IN PASSIVE MARKER BASED MOTION ANALYSIS

Abstract

The majority of non-contacting motion analysers process images of CCD cameras. Hardware based feature extraction can be utilised to alleviate the necessary computation: markers are attached to the landmark points of the moving objects and the marker images are separated from the environment based on their high luminosity. Data reduction can be achieved by characterising the extracted marker images with the co-ordinates of a single point. When a spherical marker is used then the centre point of its image must be determined. Real-time motion analysers usually can afford simple data processing only. This explains that a simple geometric centroid calculation of binarised images is so popular. The height and width of the marker images are used to characterise image distortion: these measures of ideal circular images are equal. This paper summarises the results of our theoretical research work. For the centre estimation of binarised images a new method with high accuracy, the ring fitting is recommended. Two methods are shown for the processing of distorted images. Both methods can recognise if a distorted image derives from the overlapping of two (or more) marker images. Furthermore, the methods give good estimates of the centres of the individual marker images. The features of multiple level thresholding for the video/digital conversion has also been studied. The results show that the achievable accuracy does not increase proportionally with the increase in resolution.