Navigation is the challenge in VR applications: it needs to be as intuitive as possible so that sufficient capacity is left for the primary task, perception of and interaction with the virtual environment. But an intuitive solution is not easy to find, because of the conflict between the restrictions of a conventional mouse, which allows movements in two dimensions, and the six axes of navigating in a virtual environment (the three translation and three rotation axes). Thus, the conventional mouse is not the right device to navigate in 3D space, in principle. To solve this conflict, one could refer to alternative 3D interaction devices, like 3Dconnexion's SpaceMouse. But such a solution would seriously limit the spectrum of potential users, because of the still low dissemination of such equipment.

Some developers draw from this problem the conclusion to allow no continuous navigation at all, even in space environments, where users expect to be able to steer their own ship. Examples are the Planet 10 Interactive Solar System and the 3D Tour of the Planets. That way, one of the key features of virtual environments 'volatizes'.

A similar solution is realized in many VRML models, by providing a number of viewpoints: predefined camera locations describing a position and orientation for viewing the scene. They are usually placed at particularly interesting places from which user may want to see the virtual environment. But this concept is not very immersive, as under these conditions the scene is perceived rather as a set of renderings than a continuous space. Consequently, predefined viewpoints should be used as providing particularly convenient starting points from which the virtual environment can be explored.

Given the restrictions of a conventional mouse, one straightforward solution is to reduce the user's degree of freedom to just two dimensions. Commonly, this approach results in something like a walk mode, in which the user can go forward and backward as well as rotate around the vertical axis. Meanwhile, this is the standard in most computer games, but on the other hand, four of the six degrees of freedom are eliminated.

To allow the user to access the full potential of navigating in virtual environments, one can define additional navigation modes, like flying or examining, as has been realized in many VRML viewers, e.g., Blaxxun Contact, Cortona, or the Cosmo Player. In principle, mapping the six degrees of freedom would require the definition of two additional navigation modes. Despite this, in Blaxxun Contact, for example, five additional modes were realized, and this redundancy makes it quite difficult for the user to develop a mental model of all these options, as a precondition to select a mode in a targeted way.

On this background, we came to the conclusion to provide the standard walk mode and just two additional modes, covering the remaining four degrees of freedom (see table below). The slide mode allows moving the viewpoint up and down and to the left and the right, similar to viewing a two-dimensional picture. The rotate mode resembles the steering of an aircraft, moving around the longitudinal (roll) and lateral axis (pitch).

Walk	Forward / backward     or Turn left / right     or
Slide	Up / down and or Left / right and or
Rotate	Pitch up / down and or Roll left / right and or