A data processing unit generates graphics data that are sent to a display screen of a head-mountable structure worn by a user. Thereby, the user can observe the image data, which reflect a virtual reality environment implemented by the data processing unit, namely image data representing a field of view as seen by the user from a particular position and in a particular direction in the virtual reality environment. The head-mountable structure includes a first light source projecting a well-defined light pattern on a light-reflecting surface. The data processing unit is associated with an image registering unit recording image data representing the first well-defined light pattern. The data processing unit calculates the graphics data based on the image data.