Mismatched obstacles in VR

Although collision detection is a standard, well-solved problem, VR once again poses unusual challenges. One of the main difficulties is the matched zone, in which the real and virtual worlds share the same space. This leads to three interesting cases:

1. Real obstacle only: In this case, an obstacle exists in the real world, but not in the virtual world. This is potentially dangerous! For example, you could move your arm and knock over a real, hot cup of coffee that is not represented in the virtual world. If you were walking with a VR headset, then imagine what would happen if a set of real downward stairs were not represented. At the very least, the boundary of the matched zone should be rendered if the user gets close to it. This mismatch motivated the introduction of the Chaperone system in the HTC Vive headset, in which an outward-facing camera is used to detect and render real objects that may obstruct user motion.
2. Virtual obstacle only: This case is not dangerous, but can be extremely frustrating. The user could poke her head through a wall in VR without feeling any response in the real world. This should not be allowed in most cases. The VWG could simply stop moving the viewpoint in the virtual world as the virtual wall is contacted; however, this generates a mismatch between the real and virtual motions, which could be uncomfortable for the user. It remains a difficult challenge to keep users comfortable while trying to guide them away from interference with virtual obstacles.
3. Real and virtual obstacle: If obstacles are matched in both real and virtual worlds, then the effect is perceptually powerful. For example, you might stand on a slightly raised platform in the real world while the virtual world shows you standing on a building rooftop. If the roof and platform edges align perfectly, then you could feel the edge with your feet. Would you be afraid to step over the edge? A simpler case is to render a virtual chair that matches the real chair that a user might be sitting in.