In this pair of images the right image camera viewpoint is slightly to the right of the left image camera viewpoint (look closely at the relationship of the blue arrow to one of the red balls).
If you could 'stare at infinity' in a way that causes your right eye to gaze directly at the right image, and your left eye at the left, then your doubled vision may see a third merged image in the center that has the illusion of depth. Many people find this to be quite difficult. We do not have much practice in intentionally diverging our eyes while maintaining a close focal plan. (note 3)
More people are able to use the 'cross-eyed' viewing technique. This pair of images has been transposed left to right.
If you can converge (cross) the lines of sight of your eyes while maintaining your focal plane on the images, then your doubled vision may see a third merged image in the center that has the illusion of depth.
All systems that allow you to see 3D in virtual worlds use this technique to create the illusion of depth. They differ in the methods by which they allow each eye to see one of the images. They may
- alternately flicker the two images while you wear glasses with alternating shutters (note 4)
- show both images but use goggle optics to diverge and focus your eyes to each image (note 2)
- merge images while tinting each with distinct colors while you wear color screening glasses (note 5)
Note 1: Beyond several meters distance our binocular vision mechanism becomes ineffective and our depth perception relies on other cues such as parallax, scale consistency, atmospheric degradation, light shading, and memory. When you watch a conventional movie, all of your sense of depth in the scenes comes from these alternate cues.
Note 3: For diverging ("infinity") viewing it may help to hold up a piece of paper that prevents each eye from seeing the image intended for the other eye. It may also help to download the images on this post and reduce their size.
Note 4: http://en.wikipedia.org/wiki/Active_shutter_3D_system
Note 5: http://en.wikipedia.org/wiki/Anaglyph_3D