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Binocular Vision - Retinal Disparity

Figure 1-2u. Illustration of the slightly different images the two eyes get of the same thing.

The visual system can extract information about depth very successfully and accurately, at least up to about 100 feet. Yet the eyes get two-dimensional images of the visual world. To understand how it is so successful, you need to figure out how the visual system can extract this information. Many sources of information about depth have been identified. There are two general classes: binocular, which involve two eyes, and monocular, which use only one eye.

Binocular (~2-eyed) disparity is the main binocular cue for depth. The two eyes look at an object from slightly different angles, so they get slightly different views of it. This difference is the disparity (imperfect match) between the two views. The visual system normally fuses these two images into a single perception and converts the disparity between the two images into perception of depth. The closer an object is the larger the disparity (error in matching) between the images it produces on the two retinas.

You can see the disparity of (~difference in) the images to your two eyes. Close your right eye, and hold up a pencil (or pen) with a clip on it about 18 inches in front of your nose. Turn it so that the clip is just visible, as in the left side of Figure 1-2u. Remember to look only with your left eye. Now close your left eye and open your right eye. You will notice that the right eye dies not see the clip, which the left eye does see. Bring the pencil closer; the difference between the images gets bigger. The farther away an object is, the smaller the difference; beyond about 30 feet, the difference is vanishingly small and provides no depth information.

A stereoscope is an optical device uses artificially induced binocular disparity to create a very strong impression of depth. In the mid 19th century, it was a popular entertainment. Today stereoscopes are mainly children's toys. The stereoscope works by using pictures of the same view made from slightly different positions. The optical system of the stereoscope feeds one of these pictures to each eye. The brain compares the two images and creates depth from the disparity created between them from the small difference in the position of the original pictures.

Figure 2-2u. Demonstration of binocular stereoscopic depth.

You don't need a stereoscope to get the 3-D effect of binocular disparity. A feature that has appeared in the Sunday comics called the Magic Eye shows a large picture in which is hidden two images. You can fuse the two hidden images by getting your eyes into proper position to create the right amount of binocular disparity. When your eyes positioned properly, the hidden image appears in front of the page. You can get this effect in Figure 1-2u. With a bit of practice you can learn to relax your eyes, the way they relax as you start to fall asleep, and fuse the two images into one. As your eyes drift apart, you will see two images of the two pencils, one from each eye. try to move your eyes so that the inner images from each eye superimpose (~are on top of each other) to make a single image. The single image stands out in striking 3-D.

Chemists use binocular disparity to make the 3-D structure of complex molecules visible, like the one in Figure 2-2u. Journals print two images of the molecular structure viewed from slightly different angles. Use the technique described above to fuse the two images and see the middle one in 3-D. This one is much easier to fuse than are the pencils in Figure 1-2u.

Figure 2-2u. Depth from retinal disparity of random dots. Source: http://zeus.rutgers.edu/julesz.html

Figure 3 shows another example of binocular depth effects. It shows two squares made up of columns of randomly placed dots. The one on the right has set of dots that make up the shape of a flower displaced by a few columns. If you use the same technique that lets you see the 3-D in the preceding figures, this form will appear floating above the other random dots.

To go to a website about 3-D vision using binocular retinal disparity, click HERE.
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