1. A computer-implemented method for recognising a human head in a source image, the source image having depth values for each pixel thereof, the method comprising the steps of: a) segmenting at least part of a human body from a background in said source depth image; b) detecting edges of said segmented part of a human body to determine a contour of said part; and c) determining the position of a human head within the source image; characterised in that step c) further comprises the steps of:— c1) calculating a depth of said part of a human body in said source image; c2) calculating a major radius size and a minor radius size of an ellipse corresponding to the size of a human head at said calculated depth in said source image; c3) generating an accumulator array for at least one segment of an ellipse centred on the positions of a set of contour pixels of said contour, said at least one segment of said ellipse having said major and minor radius sizes; and c4) selecting a position of a local intensity maximum in said accumulator array that corresponds to the position of a human head in the source image.