Stereoscopy, stereoscopic imaging or 3-D (three-dimensional) imaging is a technique to create the illusion of depth in a photograph, movie, or other two-dimensional image, by presenting a slightly different image to each eye. It was first invented by Sir Charles Wheatstone in 1838. Stereoscopy is used in photogrammetry and also for entertainment through the production of stereograms. Stereoscopy is useful in viewing images rendered from large multi-dimensional data sets such as are produced by experimental data. Complex three-dimensional objects such as molecular models that exist only in computer data sets may also be rendered using stereoscopy as an aid to creating medications.
The basic technique consists of creating a 3-D illusion starting from a pair of 2-D images. The easiest way to create depth perception in the brain is to provide to the eyes of the viewer two different images, representing two perspectives of the same object, with a minor deviation similar to the perspectives that both eyes naturally receive in binocular vision.
Little or no additional image processing is required. Under some circumstances, such as when exchanged images are presented for crossed eye viewing, no additional equipment is needed.
The principal advantage of side-by-side viewers is that there is no diminution of brightness and images may be presented at very high resolution and in full colour. The principal disadvantage is that only a single observer is accommodated.
Stereographic cards and the stereoscope
Two separate images are printed side-by-side. When viewed without a stereoscopic viewer the user is required to force his eyes either to cross, or to diverge, so that the two images appear to be three. Then as each eye sees a different image, the effect of depth is achieved in the central image of the three. This is the oldest method of stereoscopy, having been discovered in the mid-19th century by Charles Wheatstone. In the late 19th and early 20th century stereo cards or stereographs were popularly sold. The cards had a pair of photographs, usually taken with a special camera that took the pair of images from slightly separated views simultaneously. Cards were printed with these views (often with explanatory text); when the cards were looked at through the double-lensed viewer, called a stereoscope or a stereopticon, a three-dimensional image could be seen.
A simple stereoscope is limited in the size of the image that may be used. A more complex stereoscope uses a pair of horizontal periscope-like devices, allowing the use of larger images that can present more detailed information in a wider field of view.
A moving image extension of the stereoscope has a large vertically mounted drum containing a wheel upon which are mounted a series of stereographic cards which form a moving picture. The cards are restrained by a gate and when sufficient force is available to bend the card it slips past the gate and into view, obscuring the preceding picture. These coin-enabled devices were found in arcades in the late 19th and early 20th century and were operated by the viewer using a hand crank. These devices can still be seen and operated in some museums specializing in arcade equipment.
The stereoscope offers several advantages:
- Using positive curvature (magnifying) lenses, the focus point of the image is changed from its short distance (about 30 to 40 cm) to a virtual distance at infinity. This allows the focus of the eyes to be consistent with the parallel lines of sight, greatly reducing eye strain.
- The card image is magnified, offering a wider field of view and the ability to examine the detail of the photograph.
- The viewer provides a partition between the images, avoiding a potential distraction to the user.
Similar advantages are offered by the transparency viewers described below.
By exchanging the right and left views it is possible to obtain (with some strain) a 3-D effect without any equipment. To view the crossed-eye view shown here, move slightly back from your normal viewing distance and place your viewpoint on a line perpendicular to the center of the image. Place your finger halfway between your eyes and the image and view your finger. The three bright spots between the pictures should become four spots, and the two images become three. If the focus of the eyes is now allowed to drift to the surface of the screen without uncrossing the eyes a three dimensional depth illusion will appear in the central image. The finger may now be removed from the view. A viewer may find that the extra side images disappear once in-depth view of the central image is stable. This is a popular way of presenting images on computers but it is difficult to learn and for many viewers the method is not comfortable enough for extended viewing. It also offers none of the advantages enumerated above that are provided by the stereoscope.
In the 1940s, a modified and miniaturized variation of this technology was introduced as "The View-Master ®". Pairs of stereo views are printed on translucent film which is then mounted around the edge of a cardboard disk, images of each pair being diametrically opposite. A lever is used to move the disk so as to present the next image pair. A series of seven views can thus be seen on each card when it was inserted into the View-Master® viewer. These viewers were available in many forms both non-lighted and self-lighted and may still be found today. One type of material presented is children's fairy tale story scenes or brief stories using popular cartoon characters. These use photographs of three dimensional model sets and characters. Another type of material is a series of scenic views associated with some tourist destination, typically sold at gift shops located at the attraction.
Low cost folding cardboard viewers with plastic lenses have been used to view images from a sliding card and have been used by computer technical groups as part of their annual convention proceedings. These have been supplanted by the DVD recording and display on a television set. By exhibiting moving images of rotating objects a three dimensional effect is obtained through other than stereoscopic means.
An advantage offered by transparency viewing is that a wider field of view is may be presented since the images, being illuminated from the rear, may be placed much closer to the lenses. Note that with simple viewers the images are limited in size as they must be adjacent and so the field of view is determined by the distance between each lens and its corresponding image.
Good quality wide angle lenses are not inexpensive and so are not found in most stereo viewers.
The user typically wears a helmet or glasses with two small LCD displays with magnifiers, one for each eye. The technology can be used to show stereo films, images or games, but it can also be used to create a virtual display. Head-mounted displays may also be coupled with head-tracking devices to allow the user "look around" the virtual world naturally by moving the head without the need for separate controller. Performing this update quickly enough to avoid inducing nausea in the user requires a great amount of computer image processing. If six axis position sensing (direction and position) is used then wearer may move about within the limitations of the equiment used. Owing to rapid advancements in computer graphics and the continuing miniaturization of video and other equipment this will likely become available at reasonable cost within a decade.
Head-mounted or wearable glasses may be used to view a see-through image imposed upon the real world view, creating what is called augmented reality. This is done by reflecting the video images through partially reflective mirrors. The real world view is seen through the mirrors' reflective surface. Experimental systems have been used for gaming, where virtual opponents may peek from real windows as a player moves about. This type of system is expected to have wide application in the maintenance of complex systems, as it can give a technician what is effectively "x-ray vision" by combining computer graphics rendering of hidden elements with the technician's natural vision. Additionally, technical data and schematic diagrams may be delivered to this same equiment, eliminating the need to obtain and carry bulky paper documents.
LCD shutter glasses
Glass containing liquid crystal and a polarizing filter has the property that it becomes dark when voltage is applied, but otherwise is translucent. A pair of eyeglasses can be made using this material and connected to a computer video card. The video card alternately darkens over one eye, and then the other, in synchronization with the refresh rate of the monitor, while the monitor alternately displays different perspectives for each eye. At sufficiently high refresh rates, the viewer's visual system does not notice the flickering, each eye receives a different image, and the effect is achieved.
The problems associated are the cost for the additional equipment, and that the flickering can be noticeable if the refresh rate is not sufficiently high, as each eye is effectively receiving only half of the monitor's actual refresh rate. As with other single image methods the brightness is considerably diminished.
To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through orthogonal polarizing filters. The viewer wears low-cost eyeglasses which also contain a pair of orthogonal polarizing filters. As each filter only passes light which is similarly polarized and blocks the orthogonally polarized light, each eye only sees one of the images, and the effect is achieved.
The difficulty arises because light reflected from a motion picture screen tends to lose a bit of its polarization. Aluminized screens are said to be better than glass bead screens in this respect.
When stereo images are to be presented to a single user, it is practical to construct an image combiner, using partially silvered mirrors and two image screens at right angles to one another. One image is seen directly through the angled mirror whilst the other is seen as a reflection. Polarised filters are attached to the image screens and appropriately angled filters are worn as glasses. A similar technique uses a single screen with an inverted upper image, viewed in a horizontal partial reflector, with an upright image presented below the reflector, again with appropriate polarizers. Polarizing techniques are most simply used with cathode ray technology, as polarizers are used within ordinary LCD screens for control of pixel presentation - this can interfere with these techniques.
In 2003 Keigo Iizuka discovered an inexpensive implementation of this principle on laptop computer displays using cellophane sheets. See the reference below for more information.
Anaglyph images have seen a recent resurgence due to the presentation of images on the internet, coupled with the availabilty of low cost paper frames that hold accurate color filters. Practical images, where depth perception is useful, include the presentation of complex multi-dimensional data sets and stereographic images from (for example) the surface of mars, but for the most part, the materials are presented for entertainment. Anaglyph images are much easier to view than either parallel sighting or crossed eye stereograms, although the later types offer bright and accurate color rendering, not possible with anaglyphs. Anaglyphs could be useful as introductory materials for the merchandising of stereo image cards. With the techniques outlined below it is possible to convert stereo pairs from any source into anaglyph images.
Producing anaglyph images
In historical methods using camera filters, two images from the perspective of the left and right eyes are projected or printed together as a single image, one side through a red filter and the other side through a contrasting color such as blue or green or cyan. As outlined below, one may also use an image processing computer program to simulate the effect of using color filters, using as a source image a pair either color or monochrome images.
As the eye is less sensitive to red than to the other primary colors the viewing of an image is somewhat improved if the non-red color is made less intense in brightness to compensate.
Using a digital image processing computer program
Anaglyphs from monochrome images
For the example above the entire stereo card image was first converted to grayscale (shades varying between black and white). The right image was selected and pasted into a new document. This new document was then converted to color (it still looks black and white, with all channels identical). Returning to the original stereo pair document the selection was dragged to the left card image. The red channel was selected and the selection with that channel copied to the clipboard. The new color anaglyph document was selected as the destination window. Using the channels window the red channel was selected and the left image was pasted in. The images were flattened and saved as a jpeg without further adjustment.
Anaglyphs containing color information
Using color images, select the entire right eye image (for crossed eye stereograms this will be on the left) and make a new document. Paste the right eye image in. Move the selection to the left eye image (with consideration as above for crossed eye stereograms) and using the channels dialog select the red channel. Copy the red channel from this source image. Select the new document and select the red channel. Paste the left eye image into the red channel. Depending upon the colors this might look rather good as a color image (but not exactly true to the original color). Eye sensitivity balance can be improved by selecting the green channel and reducing it using a linear curve selection (e.g. reduce to 12.5%). Select the blue channel and reduce somewhat less (e.g. reduce by 5%). This action compensates for the eye's lower sensitivity to red and its high sensitivity to green, but may induce bleed through (you may see ghost images on one side or to each side of an object). Without the anaglyph glasses the picture will appear reddish and somewhat dimmer, but the overall effect is improved when the glasses are used. The dimness can be corrected by increasing the overall brightness 15 to 20% and the contrast 10 to 15%. All of these adjustments will depend upon color balance, brightness, and contrast of the original image and the nature of the subject material.
Using color information, it is possible to obtain reasonable (but not accurate) blue sky, green vegetation, and appropriate skin tones. Color information appears disruptive when used for brightly colored and high contrast objects such as signage, toys, and patterned clothing when these contain colors that are close to red or cyan.
(The adjustment suggested in this is section is applicable to any type of stereogram but is particularly appropriate when anaglyphed images are to be viewed on a computer screen or on printed matter.)
Those portions of the left and right images that are coincident will appear to be at the surface of the screen. Depending upon the subject matter and the composition of the image it may be appropriate to make this align to something slightly behind the nearest point of the principal subject (as when imaging a portrait). This will cause the near points of the subject to "pop out" from the screen. For best effect, any portions of a figure to be imaged forward of the screen surface should not intercept the image boundary, as this can lead to a discomforting "amputated" appearance. It is of course possible to create a three-dimensional "pop out" frame surounding the subect in order to avoid this condition.
If the subject matter is a landscape, you may consider putting the frontmost object at or slightly behind the surface of the screen. This will cause the subject to be framed by the window boundary and recede into the distance. Once the adjustment is made, trim the picture to contain only the portions containing both left and right images. In the example shown above, the upper image appears (in a visually disruptive manner) to spill out from the screen, with the distant mountains appearing at the surface of the screen. In the lower modification of this image the red channel has been translated horizontally to bring the images of the nearest rocks into coincidence (and thus appearing at the surface of the screen) and the distant mountains now appear to recede into the image. This latter adjusted image appears more natural, appearing as a view through a window onto the landscape.
In the toy images to the right, the shelf edge was selected as the point where images are to coincide and the toys were arranged so that ony the central toy was projecting beyond the shelf. When the image is viewed the shelf edge appears to be at the screen, and the toy's feet project toward the viewer, creating a "pop out" effect.
A pair of eyeglasses with two filters of the same colors used on the camera (or simulated by the image processing software manipulations) is worn by the viewer. In the case above the red lens over the left eye allows only the red part of the anaglyph image through to that eye, while the cyan (blue/green) lens over the right eye allows only the cyan part of the image through to that eye. Portions of the image that are red will appear dark thrugh the cyan filter, while cyan portions will appear dark through the red filter. Each eye therefore sees only the perspective it is supposed to see.
These techniques have been used to produce 3-dimensional comic books, mostly during the early 1950s, using carefully constructed line drawings printed in colors appropriate to the filter glasses provided. The material presented were typically short graphic novels of a war story, horror, or crime/detective nature - similar in content to some modern Japanese manga. These genres were largely eliminated in the US by the rise of the comic book code authorities. Anaglyphed images were of little interest for use in the remaining comics, which emphasized bright and colorful images, unsuited for use with the viewing and production methods available at the time, which were usually red-green rather than red-cyan.
In fine arts
Some maintainers of internet web sites have added depth information to images of famous paintings, further processing these to produce color anaglyph images.
Other artists are producing original artwork to be viewed using stereoscopic devices.
Several fine arts photographers are producing and marketing photographic stereoscope cards, but these seem to fall within a rather narrow genre, appearing to be mostly high quality Censored page.
Other display methods
This method, possibly the most simple sterogram viewing technique, is to simply alternate between the left and right images of a stereogram. On a computer, this can easily be accomplished with an animated .gif image  or a flash applet  . Most people can get a crude sense of dimensionality from such images, due to persistence of vision and parallax. To understand why this works, try closing one eye and move your head from side-to-side. Objects that are closer appear to move more than those further away.
Advantages of the wiggle viewing method include:
- No glasses or special hardware required
- Most people can "get" the effect much quicker than cross-eyed and parallel viewing techniques
- It is the only method of stereoscopic visualisation for people with limited or no vision in one eye
This effect may also be observed by a passenger in a vehicle or low-flying aircraft, where distant hills or tall buildings appear in three-dimensional relief, a view not seen by a static observer as the distance is beyond the range of effective binocular vision.
More recently, random-dot autostereograms have been created using computers to hide the different images in a field of apparently random noise, so that until viewed using this technique, the subject of the image remains a mystery. A popular example of this is the Magic Eye series, a collection of stereograms based on distorted colorful and interesting patterns instead of random noise.
The Pulfrich effect is a consequence of the fact that at low light levels the eye-brain visual response is slower. By placing a neutral (transparent gray) filter over one eye, a moving image perceived by that eye will lag behind the image perceived by the unimpeded eye. This lag will induce a difference in the images perceived by each eye, inducing a binocular vision illusion of depth. The observation made by Pulfrich was that if a pendulum is swung across the visual field (i.e., perpendicular to the line of sight) and one eye is viewing through a light-reducing filter, that the pendulum will be perceived to be swinging in an eliptical orbit, rather then the linear arc in which it actually swings. This effect was exploited in a "3D" motion television commercial in the 1990s, where objects moving in one direction appeared to be nearer to the viewer (actually in front of the television screen) due to the binocular vision of the user. To view this, the advertiser provided a large number of viewers with a pair of filters in a paper frame. One eye's filter was a rather dark neutral gray while the other was transparent. The commercial was in this case restricted to objects (such as refrigerators and skateboarders) moving down a steep hill from left to right across the screen, a directional dependency determined by which eye was covered by the darker filter.
Displays with filter arrays
The LCD display is covered with an array of prisms that divert the light from odd and even pixel columns to left and right eyes respectively. As of 2004, several manufacturers, including Sharp Corporation, offer this technology in their notebook and desktop computers. These displays usually cost upwards of 1000 dollars and are mainly targeted at science or medical professionals.
Another technique, for example used by the X3D company  , is simply to cover the LCD display with two layers, the first being closer to the LCD than the second, by some millimeters. The two layers are transparent with black strips, each strip about one millimeter wide. One layer has its strips about ten degrees to the left, the other to the right. That allow seeing different pixels depending on the viewer's position.
Taking the pictures
In the 1950s, stereoscopic photography regained popularity when a number of manufacturers began introducing stereoscopic cameras to the public. These cameras were marketed with special viewers that allowed for the use of transparency film, or slides, which were similar to View-Master® reels but offered a much larger image. With these cameras the public could easily create their own stereoscopic memories. Although their popularity has waned somewhat, these cameras are still in use today.
In the 1980s stereoscopic photography was again revived but to a lesser extent when point-and-shoot stereo cameras were introduced. Because these cameras suffered from poor optics and plastic construction they never gained the popularity of the 1950s stereo cameras. This type of stereo camera typically is used with print film. Over the last few years they have been improved upon and now produce good images.
The beginning of the 21st century marked the coming of age of digital photography. Stereo lenses were introduced which could turn a digital or print film single lens reflex camera into a stereo camera.
The side-by-side method is extremely simple to create, but it can be difficult or uncomfortable to view without optical aids. Devices such as the stereoscope, View-Master, and stereoscopic glasses make viewing easy.
If anything is in motion within the field of view it is necessary to take both images at once, either through use of a specialized two-lens camera, or by using two identical cameras, operated as close as possible to the same moment.
Longer base line
For making stereo images of a distant object (e.g., a mountain with foothills), one can separate the camera positions by a larger distance than usual. This will enhance the depth perception of these distant objects, but is not suitable for use when foreground objects are present. In the red-cyan anaglyphed example at right, a ten-meter baseline atop the roof ridge of a house was used to image the mountain. The two foothill ridges are about 6.5km (4mi.) distant and are separated in depth from each other and the background. The baseline is still too short to resolve the depth of the two more distant major peaks from each other. Owing to various trees that appeared in only one of the images the final image had to be severely cropped at each side and the bottom.
In the wider image, taken from a different location, a single camera was walked about 100 ft (30m) between pictures. The images were converted to monochrome before combination.
Base line selection
There is a specific optimal distance for viewing of natural scenes (not stereograms), which has been estimated by some to have the closest object at a distance of about 30 times the distance between the eyes. This interocular distance will vary between individuals. If one assumes that it is 2.5 inches (6.35 cm), then the closest object in a natural scene by this criterion would be 30 x 2.5 = 75 inches (1.9 m). It is this ratio (30:1) that determines the inter-camera spacing appropriate to imaging scenes. Thus if the nearest object is 30 feet away, this ratio suggests an inter-camera distance of one foot. It may be that a more dramatic effect can be obtained with a lower ratio, say 20:1 (in other words, the cameras will be spaced further apart), but with some risk of having the overall scene appear less "natural". This unnaturalness can often be seen in old stereoscope cards, where a landscape will have the appearance of a stack of cardboard cutouts.
- 3-D film for 3D movie history
- List of optical topics
- Stereoscopy.com - The World of 3D-Imaging!
- National Stereoscopic Association
- World 3-D Film Expo 2003
- Using cellophane to convert a laptop computer screen into a three-dimensional display
- Loreo Stereo Cameras, Lenses and Viewers
- Transcontinental Railroad Stereoview Exhibits (at the Central Pacific Railroad Photographic History Museum)
- Stereoviews of 19th Century U.S. Cities
- A demonstration of the Pulfrich effect (requires Java support within your browser)
- Create your own Stereogram (German)
- "Wiggle" Stereogram creator/viewer (requires Macromedia Flash 6+)
- Mars, Historical, Experimental Anaglyphs
- "Stereoscopy, principles and applications" article at VResources