They are based on stereopsis... two seperate images that when viewed with both eyes things look 3D. But the image is still really 2D... if you cover one eye, you wont see the effect.
Thats exactly how your real eyes work duh!
The image each eye gets is only 2d. Each eye is basically just a pinhole camera.
IF you cover one eye, you drastically lose depth perception. Although you wont notice it immediately as you remember your surroundings. Placed in a completely new room with 1 eye covered you would notice it tho.
I'm waiting for cerebral implant technology, to completely bypass our eyes.. yeah!!