How to view the stereo images presented here.
Although several techniques (colored glasses, polarizing glasses, stereopticons, crossed-eye viewing, etc.) are commonly used to view stereo images , the stereo images presented on this Web site are set up for viewing with the aid of a simple hand mirror. To view them, first get yourself an ordinary hand mirror, about 4 by 5 or 4 by 6 inches in size, with a handle. The front view of this mirror will be roughly as follows.
Most U.S. drugstores will carry this item, which should cost no more than $5 U.S.
Each stereo image presented to you for viewing will consist of a slightly differing left half image and right half image, which will be presented side-by-side in two panels of your main browser window. The left half image will be approximately a mirror image of the right half image - but not quite. Stereo results from the slight differences (or 'disparity') between these two images, and from their fusion, when one is viewed by your right eye and the other by your left.
What is 'Fusion'?.
'Fusion' is what happens automatically when your brain combines the images seen by your left and your right eye, suppressing these two partial images and presenting your with a single combined image, seen 3-dimensionally. Normally this happens so automatically that you are unaware that it is going on, but you can make yourself aware of the process by holding your finger near your nose and gazing past it into the distance. Do this, and you will notice that you see a 'double image', two fingers, each a bit out of focus and transparent. Now look directly at your finger; concentrate on it. This will turn each of your eyes inward, as they both come to point at your finger. The two blurry finger images will move together, and as soon as they come close enough together your brain will combine them into one, and repress the two separate images that have entered into the combination, so you will see just one 3-D finger - which is exactly what you are most used to seeing!
This mental event, combination of two nearly overlapping, similar but not identical images, into one percept seen in depth is exactly what is meant by the term 'fusion'. You know it has happened whey you stop 'seeing double'. As long as you are 'seeing double', it has not happened.
Fusion of the image pairs that this Web site will present to you for stereo viewing must obviously go a bit differently, since neither image of a pair (presented on the computer screen) is really 3-dimensional; but together they will look that way when properly viewed. To view these images, proceed as follows.
- Bring up a stereo pair for viewing. The 'Bullseye Tile' image that you get by clicking on the main window anchor labeled 'The Bullseye Tile in Stereo and Color' and then on the anchor labeled 'Click here to see these Images in stereo' is a good one to try first.
- Once you have done this, sit in front of your computer monitor, in the position that you normally use, but take your mirror, and hold it in front of your nose, with one eye on each side of the mirror. The front of the mirror should point to the left, as shown in the following diagram.
OK, I'm holding the mirror. What next?
Look with your right eye at the right-hand copy of the image, and look in the same place with your left eye. This means that your left eye will actually be looking in the mirror, so what you will see through your left eye, if the mirror is lined up properly (we will explain how to line it up in a minute), is the reflection of the left-hand computer-screen image. You will need to line up the mirror, by turning it slightly back and forth while holding it vertically just in front of our nose, to make this reflection fall on top of the image you see through your right eye. When this has been done, the geometry will be as seen in the following figure, and stereo fusion will take place automatically. (Once you have done this a few times, it will be as easy as tying your shoelaces. But to help you get started, we explain the whole situation in gruesome detail.)
So how do I line up the mirror?
The easiest way to line up the mirror is this.
- Still holding the mirror, shut your left eye for a minute. Look at the right-hand image with your right eye. Remember where it is.
- Now open your left eye and shut your right eye. Look in the mirror with your left eye. Ignore everything except what you see in the mirror.If the mirror is properly positioned, you should see the reflected left hand image in the mirror. If you can't see this, turn the mirror slightly back and forth, keeping its edge right in front of your nose, until you do. As soon as you can see this in the mirror, you are getting warmer.
- Turn the mirror the remaining small bit necessary to bring the reflection of the left-hand screen image exactly to where you remember seeing the right hand image. The easiest way to do this may be to open your right eye again, so you can see exactly where this right-hand image is. (Note: always turn the mirror, holding its edge vertical and just in front of your nose. Don't wave it back and forth.)
- Now open both eyes and (we said it before) look with both of them toward the right hand image (as shown by the black 'lines of sight' in the figure above.) To see in binocular stereo, you must obviously keep both eyes open. Ignore everting else. If the mirror is properly positioned, you should no longer see this double, but in stereo depth (as you see every real 3-D object in the world.) If the mirror is still not properly positioned, you will see the right-hand image doubled (like your finger too close to you nose.) In this case, turn the mirror the last small bit needed to bring the two parts of the doubled image you see together.
What will I see while all this is going on?
If your eyes are properly balanced (see note on eye problems below) this is what you might see the while following the above instructions:
This is seen if you are 'Way Off'. If you see this (nothing in the mirror), you have turned the mirror much too far to the right or left (it should first be pointed straight between the right and left-hand parts of the image pair you are viewing, and then turned only slightly from this position.) You need to turn the mirror until you see the reflection of the left-hand image in it.
This is seen if you are 'getting warmer'. If you see this (the reflection of the left-hand figure is seen in the mirror, but doesn't overlap the right-hand image), you have the mirror turned into almost, but not quite, the correct position. Move it cautiously left or right to bring the image seen in the mirror on top of the image seen by your right eye. Ignore the image seen at the left of your field of vision, which is your left eye's direct view of the left-hand half image.
This is seen when you are holding the mirror 'just right'. When the reflected copy seen in the mirror is brought close enough to the image copy seen thru your right eye, the two images should fuse, and you will no longer be seeing double. You should then see the combined image in 3-dimensional depth. You will still be able to see another (not 3-D) copy of the left image in the left part of your field of view, but just ignore it.
If you don't get a sensation of 3-D depth even when the reflected copy has been brought very close to the image copy seen thru your right eye, you may have an eye problem of some sort (for example, glasses that adjust one eye too strongly or weakly relative to the other.) In this case, see the comments on eye problems made below. Good stereo vision depends on a fairly close balance between your two eyes.
Complaints and Kudos Department. If these instructions don't work for you, you can E-mail the author to explain your problem. Or, if they work just fine, you can send him a congratulatory note. Or, as a happy new mirror-equipped stereo viewer, you can just go on to view the collection of stereo images provided.
What if I have eye problems, or wear glasses or contact lenses?
Successful stereo (or 'binocular') viewing depends on the balanced cooperation of both eyes. Obviously, if you shut one eye, you will no longer see things binocularly. The same is true if one of your eyes is partially impaired, because of cataracts, retinal problems, poor eyesight uncorrected by suitable glasses, or what have you. If you wear glasses for reading or for working at the computer screen, be sure to put them on before trying to view the stereo images. Success in stereo viewing depends rather sensitively on the 'match' between your eyes, and so makes a fairly good 'eye test'. If you follow the preceding instructions carefully, but never get the 3-D sensation that they describe, your eyes may not be properly balanced, perhaps because you need glasses and don't know that you do, or because you need new glasses, since one of your eyes has changed a bit since the last time they were examined.
