When we analyse a scene we need to make hundreds of discriminations between lines, shadows, colours, saturation levels, fineness of grain. We need to figure out what is out there based on a ton of visual information. As usual, we do an amazing analysis on all these data and take it for granted. It seems easy.
As humans we rely very heavily on visual information to identify objects and people and to navigate through space. Our colour vision is very good enabling us to make more distinctions in our visual scene than if we had to rely on brightness alone. Even if two objects are the same brightness we may be able to tell them apart based on colour. So, colour vision gives us the ability to pick out many shapes and objects in a visual scene.
Stereo vision or binocular vision allows our visual system to make other very useful and fine distinctions between objects in terms of their distance from our viewpoint. This is kind of important because we usually make our navigation decisions based on visual information. The fact that our eyes are separated by a stable distance allows us to capture TWO slightly different images of the same scene. Our visual system and brain do some heavy computation on the slight differences in the two images and we perceive the endpoint of this analysis as depth.
In particular, objects that are close to us will be in very different parts of the two "pictures", whereas objects in the distance will occupy the same part of the two "pictures". You can demonstrate this to yourself by holding up a finger (an object close to you) and alternately close your left and right eye. Your finger will look like it is jumping back and forth compared to the computer screen (an object further away).
So, what if you took two photographs of a scene, one a few inches from the other (say about the distance your eyes are apart) and then presented one photo to your left eye and the other to your right eye. Your brain wouldn't know that you were looking at two flat photographs and would automatically analyse the difference between the two images and you would perceive "depth". Of course, this would be a just an illusion because both pictures would be flat. And even if you KNOW they are flat, it is a very convincing illusion. Once you see some real stereo pictures you may understand why I am hooked.
Before you look at the pictures, here is an introduction to the methods I use on my image pages.