1. A simulator for training in simulation to a user, said simulator comprising: a display screen arrangement supported so as to be viewed by the user; a simulation computer system having computer accessible data storage storing scene data defining a virtual environment of the simulation and virtual objects therein, said simulation computer system generating out-the-window (OTW) scenery video from the scene data, and causing the OTW scenery video to be displayed on the display screen arrangement in real time as OTW scenery visible to the user; a tracking apparatus continually sensing a position of the head of the user or a portion thereof in real time and generating from said sensing a tracking signal containing tracking data from which current time-varying locations of the eyes of the user can be determined, and transmitting said tracking signal to the simulation computer system; a head-mounted display (HMD) configured to be supported on the head of the user, said HMD having an eye display over one of the eyes of the user configured to display to said eye of the user HMD imagery video so that the eye of the user sees the OTW scenery video on the screen arrangement therethrough with the HMD imagery video superimposed thereon; said simulation computer system supporting a three-dimensional model of the simulator including data defining a virtual screen arrangement corresponding to the display screen arrangement in the simulator; the simulator computer system generating said HMD imagery video as a sequence of HMD images each corresponding to a respective point in time by rendering each frame by first generating a starting HMD image corresponding to HMD imagery for a predetermined projection point that is fixed relative to the position of the head of the user based on the data of the tracking signal, and then rendering the HMD image as a view from the location of said eye in the three-dimensional model of the virtual screen arrangement with said starting HMD image projected thereagainst from the projection point.