Digital 3D Stereo
3D stereo photography is both easy and tricky. It is very easy to make 3D images, but it is not so easy to make great 3D images.
Here I provide tips and insights here that will guide the beginner and resolve problems that might even trip up the most advanced stereographer.
Examples and explanations are provided relative to the 3 major stereo presentations; parallel view, cross-view, and anaglyph.
I plan to post more tips periodically, but I may take a vacation now and then, so no promises.
Tip # 1 Distance to Subject Disparity
The second photo of a stereo pair must NOT be taken from a point closer to or farther from the subject than is the first. That is, keep your distance to the subject exactly the same for both photos. When you move sideways to take the second photo, take care to move exactly perpendicular to the line of sight to the subject. Failure to observe this rule will result in an aberration which cannot be corrected.
Why? To illustrate; Suppose you are taking a photo of a bikini clad beauty holding a large beach ball toward you. In the first picture you just can see her eyes over the top of the ball and her . . . navel below the ball. If the second shot is taken slightly closer, you will obscure either her eyes or her navel. The resulting stereo pair will have rivalries which cannot be corrected due to the missing image data.
Even small errors of this type will degrade your stereos and make them uncomfortable to view. This error is often the cause of images that just won't align properly.
Tip # 2 Vertical Parallax (Disparity)
A very frustrating obstacle to stereo alignment occurs when the second photo of a stereo pair is made from a point either higher or lower than the first. This disparity introduces vertical parallax into the stereo. This error cannot be repaired with computer software and the two images can never be properly aligned for horizontal presentation. This is also known as a "tilted shift line."
Note: A stereo pair with a tilted shift line actually can be aligned perfectly, but only if you are willing to present the pair tilted to match the angle of the tilted shift.
3D Stereo Tips 1 - 6
Tip # 3 Avoiding Headaches While Watching 3D Movies
Some persons suffer headaches and/or nausea while watching 3D movies. The most likely cause is that 3D movies are not true binocular seeing. In a real 3 dimensional setting, people look at a near object and their eyes converge on it. At the same time, each eye changes shape slightly to bring the object into sharp focus. These are two separate actions, but in most people they are coupled. That is, when the eyes converge, the focus is near, and when the eyes are less converged (sight lines almost parallel), the focus is far. This is not what happens in artificial 3D (movies).
In artificial 3D, such as in a 3D movie, the eyes still converge to see objects that seem near and converge less to see objects that seem farther away. But here's the problem; all the images in a movie are at a fixed distance . . . on the screen. The eyes must learn to focus at a constant distance while vergence changes rapidly to look at objects that seem to be at varying distances. Some people, particularly older people, cannot easily disassociate focus from vergence and consequently suffer considerable discomfort, including headaches and nausea.
There are several things you can do to minimize this miasma. First, sit as far as possible from the theater screen. Second, try to keep your attention (eyes) on those parts of the movie action which seem to you to be occurring roughly at the distance where you think the actual physical movie screen is located. Third, and certainly obvious, take a time-out now and then.
You have 3 options. If you want to keep viewing the movie, this is the best
option; close or cover 1 eye (with the glasses still on) and view the movie
in 2D. Removing the glasses is not the best option as the image will be
blurred. The last option is, of course, to just close your eyes. My experience is that near sighted persons (who use glasses to drive) have less trouble than far sighted persons (who use glasses to read).
If you can't view 3D movies without discomfort, don't worry that you are missing something. Most people find that 3D actually distracts
their attention from the subject matter of a movie. Why? Because regular 2D focuses
your attention by reducing sensory overload. People watching 3D movies often admit to losing track of the plot and missing dialog due to the increased sensory input . . . they become overly absorbed in the 3D effect itself. On the other hand, movies with no other substance are only good to watch in 3D.
All photographs, images and text on this website page copyright Michael Beech
Tip # 4 Fixing Mismatched Image Rotation
Often, one image of a stereo 3d pair is rotated (tilted) relative to the other. Even in twin-cam systems, keeping the cameras in perfect alignment is almost impossible. The best result is usually obtained with a single camera on a rigid slide-bar. Regardless, it is wise to always assume that the one image is slightly rotated relative to the other. Stereo pairs with a rotation error are very uncomfortable to view.
Detecting a rotation error is easy. Every object or point in one image must be the identical distance from the bottom of the stereo window as the homologous (same) object or point in the other. For example: The tip of a flagpole near the left side of one photo is 55 mm from the bottom of the photo. It must be the same in the other photo. Moving to the right edge of the photo, the top of a signpost is 37 mm from the bottom and it must be the same in the other. If points at the left edge of the photos and points at the right edge of the photo are confirmed to match in this manner, then the stereo pair is free from rotation error.
Rotation errors are easy to fix with image editing software by simply rotating one image or the other until homologous points are on the same horizontal line. The borders of the images must then be trimmed to match.
Detailed information, including techniques for avoiding alignment errors, proper 3D photography techniques, methods for correcting alignment errors, and easy ways to align 3D pairs without window violations can be found in the Digital 3D Stereo Guide and other publications.
Tip # 5 How 3D Movies Work
A considerable misunderstanding exists about 3D movies and how they work. Lets clarify exactly what happens on the 3D movie screen by discussing the 3 primary types of 3D movies.
Anaglyph 3D movies. These require the red and blue (or red/green or red/cyan) glasses with which we are all familiar. Just exactly as is done in comic books, each frame of the movie contains both the left and the right views superimposed. Consequently, the movie appears to be a bit of a garish mess if viewed without the glasses. With the glasses on, one of the images cannot be seen through the red filter and the other cannot be seen through the blue filter. Each eye thus has its own perspective view and this then provides an artificial 3D similar to what we experience in the real world.
Anaglyph 3D movies must be projected onto the screens extra brightly to compensate for the light reduction caused by the red/blue filters. Worse, the colors in an anaglyph movie are unrealistic. In fact some colors cannot be used at all because they closely match the filters in the glasses, resulting in visual discomfort and headaches.
Shutter glasses 3D movies. This is the Imax type of 3D that requires the electronic headgear. In this version the view for the left eye is projected on the screen and then the view for the right eye. At no time are both images on the screen at the same time. Meanwhile, in the headgear, there is a clear screen over the eyes. The left and right halves of this screen can be made opaque or transparent by electrical stimulation. So, in perfect synchronization with the alternating images on the screen, the head gear blocks first one eye then the other. In this way the viewer sees with the left eye only the image intended for that eye and likewise the right eye sees only its appropriate image.
The colors in a shutter glass movie are normal and the result is spectacular. When viewed without glasses, the images on the screen are blurred.
Closing one eye allows you to view the movie in 2D.
Polarized glasses 3D movies. These movies use the glasses with dark gray lenses. Polarized 3D movies have been around forever. In the 1950's such classics as The House of Wax were released in a linear polarized version. You can tell if 3D glasses are linear polarized by placing two pairs face-to-face and trying to look through them. Doing this should totally block the passage of light. Again, like the anaglyph movie, both images are projected simultaneously on the screen. Each frame of the movie contains two images . . . the image for the left eye and the image for the right eye but they are projected through a linear polarized filter. Thus, the left eye sees only the image whose filter direction matches the filter over the left eye and likewise for the right eye.
A new version of polarized 3D, called Real-D, is now becoming the norm. It is almost the same, but uses a new type of polarized filtering called circular polarization. These are still dark gray glasses, but reversing two pairs face-to-face does not totally block all light.
With both types of polarized 3D, the colors are perfectly natural and the result is spectacular. When viewed without glasses, the images on the screen are blurred.
Closing one eye allows you to view the movie in 2D.
Tip # 6 Seeing 3D movies if you have strabismus and/or amblyopia
Strabismus and amblyopia. Those who have strabismus often suffer also from amblyopia. For those who do not have these difficulties, a short explanation follows.
Strabismus means that the eyes do not track or converge properly. This presents as crossed-eyes (over converged), wall-eyes (diverged beyond parallel), as one eye tracking above the other (hypertropia), or even as one or both eyes rotated. Those with strabismus experience double vision and have little and usually not any 3D perception.
Amblyopia means that one eye is ignored either partially or totally by the brain. This is not a focus issue per se, but more a neurological issue called suppression. Amblyopia often develops in the strabismic person because the brain, to avoid confusion, learns to ignore one of the eyes, resulting in a 2D vision of the world.
The bottom line is that persons with strabismus are unlikely to have a good experience watching either polarized or anaglyph movies. However, persons with amblyopia can view both shutter glass and polarized 3D movies, but not anaglyph movies. They must wear the glasses and the movie will appear normal, but It will be only 2D to them. This is because their good eye will only see the image intended for that eye (see Tip # 5, How 3D Movies Work for an explanation of how a shutter glasses movie works). The brain will ignore, as always, the input from the suppressed eye and see nothing with that eye. If there is conflict for any reason, the strabismic person can simply cover one eye and view the movie in 2D, like any regular 2D movie. Persons with hypertropia with prism corrections might be able to view 3D movies if they see 3D in the real world. Even so, 3D on a movie screen is not like the real world 3D and problems might occur. For an explanation why, see Tip # 3, Avoiding Headaches While Watching 3D Movies.
A person with only amblyopia may be able to view 3D movies, particularly the polarized type and the shutter glasses type. The movie may or may not appear to them to be 3D, but it should be no worse than their normal experience in the real world. A good clue would be this; if the amblyopic person can see 3D to any extent in the real world, then they should see 3D in a polarized movie or a shutter glass movie. It might possibly be even better.
Now, for the good news. 3D movies can actually be therapeutic for some strabismus and amblyopia sufferers. In mild cases, I have heard that persons with these disorders have suddenly experienced 3D; some for the first time in their lives. Strange things occur in 3D images (too complex to explain here), but a strabismic person can often see 3D in parts of the image that seem to be very far behind the screen or projecting very near ahead of the screen. It would be a worthwhile experiment for them to test this.
I am sure that every strabismus sufferer in the world has heard of the book Fixing My Gaze by Susan R. Barry, a longtime
victim of strabismus. If not, it is a must read as it shows how visual
3D therapy has the potential to correct this problem, even for someone
who, as she, has had it for decades.
A final caution. It may be hazardous to take young children to see 3D movies. Until their eyes have developed enough to track strongly in the real world, 3D movies (being artificial 3D) may cause abnormal linkages to form in the brain leading to severe strabismus problems. See Tip # 3, Avoiding Headaches While Watching 3D Movies, to understand how 3D movies differ from the real world. In Tip # 3 you will see how artificial 3D (movies and pictures) rely on disassociating focus from convergence, and this can be troublesome, particularly for very young children.
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Tips & How-To # 1 - 6
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