You can 'billboard items' toward the user if they end up at too much of an angle. This helps users know where they can best place their holograms, and shows if the spot they're trying to place the hologram isn't mapped. Visualize surfaces when placing or moving holograms (use a projected grid). Holograms floating in space or moving with the user won't feel as real. Occluding your holograms based on other holograms and real world objects helps convince the user that these holograms are actually in their space.
This helps anchor objects in the user's world and takes advantage of real world depth cues. Spatial mapping makes it possible to place objects on real surfaces.
