There are 2 types of stereo viewing:
1. Convergent or Cross-eye stereo viewing. Mostly for computer monitors or projected screen viewing
2. Divergent or relaxed stereo viewing. Mostly used in Texts and Periodicals.
Note that viewing a pair of relaxed stereo image with the cross-eye method of viewing (or vice versa) will transpose left and right and back and front.
After looking into stereo viewing I found that the best is to spend some time to train your eyes to see stereo.
The stereo viewing aids are usually for relaxed stereo and still the eyes need to fuse the two pictures into one. The only viewer that would work for both (relaxed or cross-eye) is a dual mirror viewer where the mirrors can be adjusted to bring both pictures into one, thus relieving eye strain. The only source that was recommended (Nu 3D Vu) sent me a pair that did not have enough mirror adjustment to see the cross-eye format.
Convergent or Cross-eye viewing
Viewing Stereo Pairs On Computers by Gale Rhodes
Stereo Viewing by:
Gale Rhodes
Department of Chemistry
University of Southern Maine
rhodes@usm.maine.edu
Too often, people try only briefly and halfheartedly to view in stereo, and never try again. Almost anyone can view in stereo with a little effort and practice. The only ones who simply cannot are those who have acute amblyopia (one very weak eye). And those who say they can see just as much without stereo simply cannot imagine what they are missing. You can be a much more effective explorer of molecular structure if you learn to see in 3D. Most structural biologists learn to view stereo views by two methods:
convergent and divergent viewing.
Viewing In 3D On A Computer Monitor Or Classroom Screen
This is a 3D stereo image, in the cross-eye format, showing the polyhedral concentric hierarchy
How To Do It
To view most computer and projected stereo images, you need to look at the left-hand image with your right eye, and at the right-hand image with your left eye (called convergent or cross-eye viewing). Here's how to develop the skill at your computer.
Gaze at the stereo pair, keeping your head level (don't tilt it left or right), and cross your eyes slightly. As you know, crossing your eyes makes you see double, so you will see four images. Try to cross your eyes slowly, so that the two images in the center come together. When they converge or fuse, you will see them as a single 3D image. The fused image will appear to lie between two flat images, which you should ignore. When you are viewing correctly, you see three images instead of four. The center image is three-dimensional. At first, the 3D image may be blurred. Keep trying to hold the stereo pair together while you focus. The longer you can hold it, the more time your eyes have to adjust their focus. Usually, even before you begin to get the hang of focusing, the two central images lock together, because your mind begins to interpret them as a single 3D object.
Having trouble? Here's another approach. With your head level and about 2.5 feet from the screen, hold up a finger, with its tip about 6 inches in front of your face, and centered between the stereo pair on the screen. Focus on your finger tip. Without focusing on the screen, notice how many images you see there (they will be blurred). If you see four images, move your finger slowly toward or away from you eyes, keeping focused on your finger tip, until the middle pair of images converge. With your finger still in place, partly covering the converged pair, change your focus to the screen. The image partly hidden by your finger should appear three-dimensional. Your finger should still appear single, but blurred. With some practice, you can remove your finger and still keep the screen images converged into a stereo image.
Practice, Practice, Practice
Viewing either convergently or divergently becomes easier with practice. Once your mind sees a pair as a single 3D image, it recalls the experience, and resists your efforts less with each try. I can automatically snap a stereo pair -- convergent or divergent -- into superimposition, and I no longer know think about exactly what I am doing. Suddenly, there is a vivid, three-dimensional object floating above the screen or page. I've been doing it since 1969 (that's a hint about the source of the first biochemical stereos!), and I've suffered no damage to my vision. It's very handy to be able to use the stereo pairs in Science and other journals without digging up a viewer. Such important tasks as interpreting electron-density maps from x-ray crystallography are practically impossible without this skill.
Cross-eye stereo viewing aid
There is a simple aid that will get rid of the distraction from the two side images, and maybe make it easier to see 3D stereo:
Cut a 1-1/2" square hole in the middle of a 5" x 7" index card (white or dark). For a quick test use scissors and 2 index cards and cut a 1-1/2" slot on both cards and staple the two cards together to get a square hole. It does not have to be precise!
To view the cross-eye stereo image above:
Hold card slightly in front of your eyes as you look at the computer screen normally. Move it closer or further away from you to the spot where the right eye sees the left image only, and the left eye sees the right image only. Close one eye at a time to check.
Now, with both eyes open, focus on the computer screen itself where it reads "Viewing In 3D On A Computer Monitor Or Classroom Screen" there will be two blurred images below.
Move your eyes to focus on the square hole on the card that you are holding. About midway between the card and the computer screen is the focus for the stereo image. At that point the two images automatically become one and in 3D. Relax your eyes, it is the same as looking at objects at different distances from you. Try it.
Looking through the hole try focusing at different distances between the screen and the card. You can use your finger to help focusing, by moving back and forth between the screen and the card.
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