It's simple. Ever see those 3D post cards or 3D Jesus on the Cross paintings.
Now, I wondered this when I was a kid. I dissected one of those Jesus 3D post cards.
On the front of the card is a square of plastic, with ridges in it.
These ridges act as lens to focus your eye on a particular perspective of the picture.
The picture behind the plastic is actually TWO pictures.
Anyway, with one of these square of plastic on a computer monitor, it would be possible to write computer games that don't require glasses. They just interlace 2 images on the screen.
Simple, sweet, and no one's made it yet. Why?