NEW MARKER CENTRE ESTIMATION ALGORITHM OF HIGH ACCURACY IN MOTION ANALYSIS

Abstract

The non-contact measurement method of motion means almost no discomfort to the moving person or animal. Furthermore, the use of CCD cameras and stroboscopic lighting makes daylight applications possible. As a result a popular and widely used solution in motion analysis is to place markers to landmark points and to characterise motion on the basis of tracking the markers. The actual position of a marker is determined by estimating its centre. The estimation is based on the covered pixels, if several pixels are covered then sub-pixel resolution and accuracy can be achieved. The paper surveys the most commonly used geometric centroid calculation and shows that this method gives a distorted estimation if the set of covered pixels is asymmetrical. In the case of ideal marker images circle fitting does not give a better result, it can be useful if the marker image is partly occluded. A new marker centre estimation algorithm is introduced. It is optimal in the sense that the expected value of the deviation between the actual and the estimated value of a circular marker image is minimal.