Figure 2.1.1 depicts the most important parts of human-computer-human interaction loop fundamental to every immersive system. The user is equipped with a head mounted display, tracker and optionally a manipulation device (e.g., three-dimensional mouse, data glove etc.).As the human performs actions like walking, head rotating (i.e. changing the point of view), data describing his/her behavior is fed to the computer from the input devices. The computer processes the information in real-time and generates appropriate feedback that is passed back to the user by means of output displays.In general: input devices are responsible for interaction, output devices for the feeling of immersion and software for a proper control and synchronization of the whole environment.