When a user sees himself in the VR he wants to navigate through the scene in the most natural way that is possible. Even in the largest CAVEs the freedom to walk around is limited by the borders (screens) of the CAVE. And secondly not all sides of the CAVE may have projection screens, normally there is no screen behind the user, but he may want to look behind him (or wants to see what is behind him)
We must find a way to navigate through the VR scene using the input device (3d mouse) that is available.
The first method that comes to mind, is to go in the direction that is pointed to by the mouse (point plus button press). This solution would be quite appropriate for VR environments like a head mounted display, but in the CAVE one will get some problems, if the user wants to go in a direction where there is no screen (backwards). There has to be a way to turn around without physically turning around, that means we need a way to rotate the whole scene.
That can be acomplished by using the trackball/joystick on the pointing device or by rotating the scene whenever the user is turning around far enough (e.g. whenever the user turns around more than 80 degrees, the scene will rotate into the opposite direction)
Our navigation through the real world is restricted by several means, one is for example gravitation, as much as someone may wish, we cannot simply fly away just by pointing our hands into that direction and say some magical spell. That is not so in the VR, we can move in every direction through the sky and even through walls or the ground. Sometimes that will be desireable (e.g. to look into some hidden parts of a machine) but usually we want to navigate in the VR as natural as possible that is walking just on the ground.
Another restriction we have in reality but we may not have in the virtual environment is the restriction of speed. In VR we can travel with any speed we wish, we can even jump from one point in space to another in no time at all.
Collision detection in the CAVE is a more serious and more complex problem than many people may see at once.
This problem results from the reason that the user can walk around inside the CAVE physically. And although that motion can be detected, there are no means to prohibit the user from walking through an object that is just virtually somewhere inside the CAVE.
One way to hinder the user from walking through objects could be to black out the whole scene whenever there is an unwanted collision.
Another way could be to make a warning noise that tells the user he has made a forbidden movement.
As the user can walk around inside the CAVE as he wishes and there are no means to prohibit this, we may restrict movement of the user by giving him another way to physically move around. This could be a bicycle he can use to navigate through the VR without moving around in the CAVE.
Another more complicated possibility would be a spherical CAVE. A CAVE that is not a cube but a sphere which rotates as the user walks through the virtual world.