1. A method of tracking the position of the upper body of a user from a video image stream, said upper body comprising the torso and the head of the user, said head comprising the face of the user, the method comprising a determination of the position of the torso in a first image, wherein: a virtual reference frame is associated with the torso in said first image, wherein in order to associate a virtual reference frame with the first image, the position of the face of the user is determined in said first image, and from this the position of the torso in said first image is deduced; and, for a second image: a new position of the torso is determined in said second image; a new position of the virtual reference frame is determined in said second image; and a relative position of the head relative to said new position of the virtual reference frame is measured by comparison with the position of the virtual reference frame in said first image, wherein the relative position of the head is characterized by a rotation and a translation of the head between the first and the second images and wherein the translation of the head is defined to be identical to the translation of the torso; wherein the head of the user comprises the face of the user and at least one position of the face is measured relative to the new position of the reference frame in the second image in order to deduce from this the relative position of the head and wherein the relative position of the head is measured from a comparison between the positions of the torso and of the face in the first image, and the new positions of the torso and of the face in said second image.