If the projection surface in front of the eye is also an LCD, it's possible to block out part of the background at the same time you project something onto it.

If the projection surface in front of the eye is also an LCD, it's possible to block out part of the background at the same time you project something onto it. I don't know what happens in this particular product, but from the Mars demo, it sounds like they can do some display of dark objects: "The sun shines brightly over the rover, creating short black shadows on the ground beneath its legs." It's possible that they are just getting that effect by making the Mars surface bright, but they could also be actively blocking light from the shadow regions. We'll have to wait for more details.