1. A method of analysing a captured digital image comprising an instance of a target object, the method comprising the steps of: for each of a plurality of different brightness threshold levels, generating contours from the captured digital image that indicate where in the captured digital image the pixel values of the captured digital image cross the respective brightness threshold level; identifying instances of a contour corresponding to a characteristic feature of said target object, the instances being detected at similar image positions in the contours derived using at least two of the respective brightness threshold levels; and estimating a homography which maps the characteristic feature of the target object to its representation in the captured image, based upon the two or more instances of that target object's corresponding contour.