Stereopsis

Note:

To do this activity you will need a pair of red-cyan glasses. Red-cyan glasses have a piece of red, transparent gel for one lens, and a piece of cyan (bluish-green), transparent gel for the other lens. Many perception textbooks come with such a pair of glasses. You can also purchase them (along with an anaglyph / 3D book) at most bookstores or online.

Background:

Because your eyes are in two slightly different positions (the right eye is about 6cm to the right of the left eye) they get slightly different views of the world. You can confirm this by placing two similar objects (I typically use white board markers) on a table. One marker should be a little to the right and about 30 cm (1 foot) behind the other. At a viewing distance of about 30 cm from the closer object and directly in front of the objects, look at the objects with just the right eye open and notice how close, left to right, the objects seem to be. Now look at the objects with just the left eye open and notice how close, left to right, the objects seem to be. With one of your eyes, the objects should appear much closer than with the other eye.

The difference between the image seen by the right eye and by the left eye is called binocular disparity. When the visual system uses binocular disparity information to help it recreate the perception of depth it is called stereopsis. The larger the binocular disparity (the bigger the difference in what the two eyes see), the larger the difference in depth of the two objects. Take your two objects and move the farther one closer -- about 15 cm (half a foot) from the closer object. Repeat the left eye / right eye viewing. You should notice that the difference in the left to right distances of the objects, the binocular disparity, should be smaller than when the two objects were far apart from each other in depth. Move the two objects so that they have the same distance from you and view them with just the left eye and with just the right eye. The binocular disparity should be 0 -- the distance between the two objects should be the same in the left eye's view as in the right eye's view.

We can take advantage of binocular disparity to create the perception of depth in a flat object. But to do so, we need to present one image to the left eye and a different image to the right eye. The image presented to the left eye must be taken from a vantage point about 6 cm to the left of the vantage point used to take the right eye's image. Such a pair of images is called a stereogram. When you view a stereogram in a device called a stereoscope, which presents only the left eye's image to the left eye and only the right eye's image to the right eye, most people perceive depth. If you have a ViewMaster, you have a stereoscope, and the disks that you put into the ViewMaster contain multiple stereograms.

In this activity, the two images, the left and right eye's images, are combined into a single image called an anaglyph. In a red-cyan anaglph, such as the one on this page, the right eye's image is filtered so that it contains only shades of red and the left eye's image is filtered so that it contains only shades of cyan. The two images are then superimposed on each other.

If you view the red-cyan anaglphy through red-cyan glasses, each eye will see a separate image. The cyan gel in the left lens is cyan because it blocks the longer, reddish, wavelengths while allowing the shorter, bluish, and medium, greenish, wavelengths to pass through. Thus the cyan lens blocks all of the red image. The opposite is true for the red lens. Each eye sees a separate image. If those images contain binocular disparity information, depth should be perceived.

How is disparity information created in the two images? Certain parts, those that you want to appear closer or farther away, of one image are slid either left or right. The more you slide the image left or right, the greater the disparity will be and the farther away or closer the three dimensional image will appear. The fact that creating binocular disparity in the stereogram leads to a strong perception of depth is evidence that the visual system uses binocular disparity information in recreating the third dimension in vision.

The image on this page is a random dot anaglyph which is functionally equivalent to a random dot stereogram. The image is initally created randomly -- each pixel is randomly colored either black or white. Then, a set of pixels is slid either left or right to create binocular disparity.

Look at the random dot anaglyph with just the cyan lens (e.g. close your right eye). You should not see an object -- a rectangle and you should not perceive depth. Look at the random dot anaglyph with just the red lens (e.g. close your left eye). Again, you should not see and object and you should not perceive depth. With your red-cyan glasses on and both eyes open, you should see objects and depth.

The fact that you can perceive depth where there are no objects tells us that object perception is not strictly necessary for depth perception to occur. Mayhew and Frisby (1976) showed that the spatial frequencies in the two images in the stereogram must be similar in order for people to perceive depth in a random dot stereogram. Recall that spatial frequency is also important for some theories of object perception.

The Activity:

Don your 3D, red/cyan glasses and look the figure below. After a few moments you should see two rectangles. One should appear closer to you than the computer monitor while the other should appear farther from you than the computer monitor. If you are having problems getting the depth to appear, try moving a little closer or farther from the monitor and/or decreasing or increasing the magnification of the web page (Ctrl - / Ctrl +). Converging (crossing) your eyes a little can help. If you still are having problems, make sure that your 3D glasses are indeed red / cyan and not red / blue -- one lens should have a decidedly greenish blue appearance. About 5% of the population is stereoblind -- their visual system never developed stereopsis, and if you are stereoblind, then you will never see the depth in the figure.

Once you see the depth, slide the disparity control left or right (depending on the speed of your computer, it may take a few seconds for the image to refresh with the new disparity). For the left rectangle, sliding the disparity control to the left moves the left eye's image to the right. For the right rectangle, sliding the disparity control to the left moves the left eye's image to the left. This creates a disparity between what the left and right eyes see. This disparity in interpreted as depth by the visual system.

Binocular disparity (in pixels): -10 +10

Reference

Mayhew, J. E. W. & Frisby, J. P. (1976). Rivalrous texture stereograms. Nature, 264, 53-56. http://dx.doi.org/10.1038/264053a0