1. An apparatus for image based human machine interfacing by estimating a 3D representation of at least a portion of a user, based on a set of 2D image data of the at least a portion of the user, comprising: image processing circuits for: (1) identifying non-rigid structures of the user within said image data; and (2) associating three-dimensional skeleton model elements with identified structures, such that model defined spatial constraints between skeleton model elements and spatial relations between identified non-rigid structures in the 2D image data set are used to fit the model to the 2D image data set and to approximate 3D coordinates of at least one of said non-rigid structures of the user.