An interface surface for user interaction with a computer system via a sensing device, the interface comprising: visible graphical information; and, substantially invisible coded data readable by the sensing device, the coded data comprising a plurality of substantially undifferentiated marks positioned relative to a predetermined pattern with a target structure extending radially outwardly from a center of the pattern for allowing relative rotation of the predetermined pattern to be determined; wherein, the visible graphical information indicates an operation that the user can select and transmit to the computer system by reading the coded data with the sensing device.