1. A 3D human machine interface comprising: an image sensor to acquire a set of two-dimensional images of a user body from a single viewing angle, wherein substantially each image is associated with a different point in time; and a processing unit to: (1) derive estimated three dimensional coordinates of elements of the user body, during the acquisition of an image of the set, by correlating and fitting two dimensional projections of a three-dimensional data model of a template human body to appearances of the user within the set of two-dimensional images acquired by the image sensor, wherein the data model includes definitions of spatial relationships between elements of the model, which definitions are used to facilitate said correlating and fitting of the model to appearances of the user; (2) determine a movement of one or more body parts of the user during acquisition of the set of images, based on the estimated three dimensional coordinates of elements of the user body; and (3) transmit a signal representing a user input in response to the determined user body movement.