1. A method for constructing a virtual model of an object based on video of the object in motion, the method comprising: identifying in an image from the video a set of reference points on the object; for each reference point identified, observing a displacement of that reference point in response to a motion of the object; grouping together those reference points for which a common translational or rotational motion of the object results in the observed displacement; and fitting the grouped-together reference points to a shape.