[dspillett]

Actually depth perception is possible with only one view-port. Both my eyes work (for varying definitions of "work") but they don't work together very well (tests show I only tend to use one at a time, which one gets control depending on the location of the object(s) I'm trying to focus on and how tired I am) and I can perceive depth just not quite as well.

The main clue the brain uses for depth perception on top of the binocular input (of instead of the binocular input for those who don't have it) is parallax effects gleaned from moving objects within the view passing behind each other. On top of that comes knowledge of the world (how big things should be relative to each other) though that is very easily fooled.

On top of that the perception of parallax exploited further by moving your head. Ever seen a cat size up a jump is isn't entirely confident with? You seem them bob their heap up and down a bit giving them extra parallax clues effectively extended the effect of multi-eye vision into a second degree of freedom. This effect can be used by single ey eby moving the head more.

Binocular vision can help 3D perception a lot, which is why most creatures use it, but it is far from the be-all and end-all of it.