A method for detecting and thinning a contour image of an object in a video frame comprises the steps of: (a) calculating edge magnitudes for pixels in the video frame and detecting edge points, (b) generating a contour image constituted by the detected edge points, and (c) thinning the contour image by comparing an edge magnitude for each of the edge points with each of the edge magnitudes included in a plurality of edge magnitude sets, each of the edge magnitude sets having a series of three edge magnitudes selected from the edge magnitudes corresponding to the neighboring pixels surrounding said each of the edge points.