Our binocular vision is what allows most of us to see the world in 3D and also makes it possible to see the 3D effect in stereoscopic photographs and 3D TV and movies. This stereovision
, or stereopsis
, uses our two eyes, only a few inches apart, to view a scene from slightly different angles. As a result each eye picks up some visual information the other doesn’t—a phenomenon called parallax
. To demonstrate this effect hold your thumb up with your arm outstretched in front of you. Alternately open one eye and close the other and you will see your thumb jump against the background. What you are seeing is parallax in action.
As each eye captures a slightly different 2D view of a scene it's up to the brain to process the views into a single 3D view that adds depth and volume. It's this depth and volume that makes objects look solid—hence the origin of the term "stereo" that's derived from the Greek "stereos" meaning firm or solid. Stereopsis and convergence (sensing the eyes' muscles converging the eyes for close subjects and diverging them for more distant ones) let us directly perceive depth.
However, in addition to stereopsis and convergence, there are other ways we can judge depth even when we can't directly perceive it. All of these depth cues
are at work in parts of the scene distant from the camera in 3D photos and are the only depth cues available in 2D photography, and to people without stereo vision. The more cues at work, the easier it is to judge depth. The cues include:
- The occlusion of more distant objects by closer ones.
- Perspective from parallel lines such as railroad tracks converging into the distance.
- Scale, or angular size, lets us perceive the distance of subjects with known heights. The apparent size as objects changes as they get closer or farther away.
- Shading and shadows that delineate the shape of objects.
- Aerial perspective that blurs and softens more distant subjects. A variant of this is selective focus.
- Motion parallax that shows the relationship of subjects as you move through a scene or move your head from side to side.
- Change in size of texture and patterns.
- Vertical position that leads us to believe taller objects are further away.
STEREO ADDS JUST TWO THINGS
The only depth cues that stereo vision adds are parallax and accommodation—the refocusing of the eyes as their vision shifts from one distance plane to another. As you explore depth in a fused 3D stereo pair your eyes converge to focus on parts of the scene at different distances just as if you were viewing the original scene. Your brain senses this convergence and uses it to determine depths. This doesn't happen when you view a 2D photo.
Following are some very simple graphics that you should be able to fuse into a single image—one of them in 3D. To begin, focus your eyes through the images to an imaginary point behind them. When three images appear concentrate on the center one and the left and right images should merge into one to form a cross or a circle pierced by a line. After the two images have merged into a sharply focused central image you may still be able to see the out of focus original pair on either side.
IT TAKES TIME TO GO DEEP
Oliver Sacks in his book "The Mind''s Eye states "If a stereo photograph is flashed on the screen for as little as twenty milliseconds, a person with normal stereoscopy can perceive some stereo depth straightway. But what one sees in a flash is not full depth; the perception of this requires several seconds, even minutes, in which the picture seems to deepen as one gazes at it—it is as if the stereo system takes a certain time to warm up, to come to its full capacity."
Try merging these two dots into a single dot (top), two bars into a cross (middle) a 3D image of the line piercing the circle.
In an article he wrote for the Atlantic Monthly in 1861, Oliver Wendall Holmes wrote about viewing stereo images that "the shutting out of surrounding objects, and
the concentration of the whole attention, which is a consequence of this, produce a dreamlike exaltation…in which we seem to leave the body behind us and sail away
into one strange scene after another, like disembodied spirits".