An apparatus to generate data relating to a specular surface of an object, the apparatus including a screen that is movable relative to the object, a graphic rendered on the screen' wherein the graphic is notionally divided into contiguous segments such that the graphic content in each segment allows that segment to be distinguished from a plurality of other segments, at least one camera for capturing successive frames of the object' illuminated by said graphic, during said relative movement of the screen, and a computing device that accepts data from pixels of the captured frames.