Some 3-D Facts!
by Duncan Woods, Cygnus Graphic
ONE OF THE MOST fascinating aspects of our vision is that we can see in three dimensions, which permits us to judge distances of objects from us and from one another. This ability is known as stereopsis, and although there are a number of factors that contribute to it, the most important - at least for distances up to about 200 feet (60 m) - is binocular vision ("two-eyed seeing"). Our eyes are separated by an average of 2.5 inches (64 mm) and each one sees a slightly different view than the other. These two images are relayed by the optic nerves to the brain, where they are merged into a single view and the perception of depth is created in a process that isn't yet fully under-stood. You can easily see this difference in views by looking at a scene with a lot of objects at different distances in it and alternately closing one eye and opening the other; each object in your view will appear to shift its location relative to the others as you close one eye and open the other, and the amount of shift will vary with the distances of the various objects from you. Most of us see in 3-D all the time, but we're not usually aware of this ability and just take it for granted. It's estimated that only about 6 to 8 percent of people can't see three-dimensionally, usually because of uncorrectable physiological eye and vision conditions. It's been found that some adults have apparently developed psychological barriers to seeing depth in 3-D pictures, movies, etc., al-though they seem to be able to see stereoscopically in all other situations. Children can almost always see 3-D images quickly and clearly - their eyes are very flexible and adapt rapidly, and they haven't had enough time or experience to develop any mental barriers against this visual process.
Many people believe that viewing 3-D will somehow damage their eyes or vision. Nothing could be farther from the truth! In viewing 3-D pictures and movies, you're just extending your normal stereoscopic vision ability (usually with the aid of simple viewing devices) to be able to see depth where you don't normally see it - on the flat page or the movie screen. Some people do feel a slight discomfort at first, particularly when viewing anaglyphs through red/blue or red/green glasses, but this soon passes as they adapt both visually and mentally to being able to see depth in an unusual presentation. It's very important, then, - especially if you're new to 3-D - to take your time and study each 3-D view carefully for a few minutes. This will give your eyes time to adapt properly so you can see the full 3-D effect. All too often, people glance at an anaglyph momentarily or flip through a View-Master reel in just a few seconds and are very disappointed. Little wonder - they haven't even come close to allowing enough time for the 3-D image to develop! To view 3-D images, time, patience, and practice are essential; as you gain experience, you'll find you're able to see 3-D images much more easily and quickly. You'll also find you're much more aware of your spatial vision in everyday life; thinking "in-depth" and observing relative distances will become second nature to you. Two primary forms of 3-D presentation are used in just about all publications. Each form has its own pluses and minuses, but each will provide an excellent spatial image when viewed properly.
One of these forms is the stereo anaglyph. You've no doubt seen anaglyphs in posters, comic books, etc., where you've had to use glasses with red and blue (or red and green) lenses to see the 3-D image. The two images are printed in superimposition with colors matching the viewer lenses. Without the viewer, an anaglyph appears to be a blurred picture, but when the viewer is placed before the eyes, each eye can see only its own appropriate image and the 3-D depth can be seen. Since color is used to separate the two images, anaglyphs are usually seen only as black and white (monochromatic) images, but a lot of recent experimentation with computer color scanning and enhancement has resulted in "full-color" anaglyphs. When properly created with limited color ranges, they can provide excellent 3-D images. A similar system is used for 3-D slide shows and movies, but rather than using color, this system uses light polarizing filters in both the projection system and the viewers to achieve image separation. Since the polarizers are colorless neutral gray, the projection and viewing of full, bright colors is possible, and the combination of depth and color is often quite spectacular. Random dot stereograms (RDS images) are similar to anaglyphs in that the two images are super-imposed, but in this case, the images are hidden within an overall pattern of dots or even figures. They can be reproduced as standard anaglyphs that must be viewed with the colored glasses, but more often, they're created in the form of prints that must be "free-viewed" without any type of device at all. Interestingly, an early use of these images was a foolproof test of stereoscopic ability: in most 3-D images, you can figure out which images lie at which distances, even if you can't see in 3-D, but with-out this ability, you can't possibly identify the various images and their relative distances from one another in an RDS image. Because of the way RDS images are produced, their resolution (the ability to show fine detail) is much lower than the resolution of 3-D photographic or drawn images. Anaglyphs and random dot stereograms have a unique property that other 3-D forms don't have: they can be viewed from practically any distance, and as you move toward or away from them, the apparent depth decreases or increases correspondingly. It's a bit complicated to explain why this is so, but in a nutshell, it's caused by geometric properties peculiar to these 3-D forms.
The other popular 3-D form is side-by-side stereo pairs; the parlor stereoscopes that were popular from the 1860s through the 1920s used this method by providing two disparate views on a card measuring about 7 inches long by 4 inches high. Nowadays, side-by-side pairs are often smaller, but the principle is the same: when the pairs are viewed with a twin-lens viewer, each eye is limited to its own view and the 3-D image is created. Bright, full color is also possible in this system, since the viewer lenses are colorless. Both the View-Master and 3Discover systems are actually highly evolved side-by-side systems; the pairs exist as transparencies inserted in reels or cassettes rather than being printed on paper. The reel or cassette is inserted into a special viewer and then advanced to each of its 3-D scenes by mechanisms built into the viewers. These systems provide excellent 3-D images and are very popular among all age groups. There are other forms of 3-D, such as holograms, lenticular prints, over-and-under pairs, vectographs, etc., but it's unlikely you'll run across any of them except for holograms and lenticular prints. You'll also encounter chroma-stereoscopic images, which are viewed with special diffraction grating viewers; the degree of depth is determined by the actual placement of the colors of the spectrum. This system has been widely used to view laser light shows and is now appearing in more and more publications. Its development still has quite a way to go, though, so anaglyphs and side-by-side pairs (in all of their various forms) will remain the primary forms of 3-D presentation, at least for a while yet. We hope you'll find stereoscopy as fascinating as we do and that your interest in it will continue and expand - it's truly a subject of great depth and dimension for people of all ages!
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