Surface computer user interaction is described. In an embodiment, an image of a user's hand interacting with a user interface displayed on a surface layer of a surface computing device is captured. The image is used to render a corresponding representation of the hand. The representation is displayed in the user interface such that the representation is geometrically aligned with the user's hand. In embodiments, the representation is a representation of a shadow or a reflection. The process is performed in real-time, such that movement of the hand causes the representation to correspondingly move. In some embodiments, a separation distance between the hand and the surface is determined and used to control the display of an object rendered in a 3D environment on the surface layer. In some embodiments, at least one parameter relating to the appearance of the object is modified in dependence on the separation distance.