I can't seem to find the paper atm, but during my VR coursework, we reviewed this technology that basically put someone inside a large sphere (think hampster in a rolling ball), and it sat atop rollers that allowed the person to move, walk forward, in any direction, and on the surface of the sphere would be numerous projectors displaying an image, so you were completely immersed...no need for a VR helmet. It was really cool, the only prob was your feet would always have a slight uphill climb that would push the ball down, i.e. to move you forward.