An apparatus for 3D representation of image data, comprises: a structure identifier for identifying structures in motion within image data, and a skeleton insertion unit, which associates three-dimensional skeleton elements with the identified structures. The skeleton elements are able to move with the structures to provide a three-dimensional motion and structure understanding of said image data which can be projected back onto the input data. As well as individual elements, complex bodies can be modeled by complex skeletons having multiple elements. The skeleton elements themselves can be used to identify the complex objects.