Stereoscopy

Stereo card image modified for crossed eye viewing View of Manhattan, c. 1909 Stereoscopy, stereoscopic imaging or 3-D (three-dimensional) imaging is any technique capable of recording three-dimensional visual information or creating the illusion of depth in an image. The illusion of depth in a photograph, movie, or other two-dimensional image is created by presenting a slightly different image to each eye. Many 3D displays use this method to convey images. 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. Modern industrial three dimensional photography may use laser or other advanced techniques to detect and record 3 dimensional information.

Traditional stereoscopic photography 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. Modern industrial three dimensional photography may use laser or other advanced techniques to detect and record 3 dimensional information.

Characteristics
Little or no additional image processing is required. Under some circumstances, such as when a pair of images is presented for crossed or diverged eye viewing, no device or additional optical equipment is needed.

The principal advantages of side-by-side viewers is that there is no diminution of brightness so images may be presented at very high resolution and in full spectrum color. The ghosting associated with polarized projection or when color filtering is used is totally eliminated. The images are discretely presented to the eyes and visual center of the brain, with no co-mingling of the views. The recent advent of wider HD and computer flat screens has made wider 3D digital images practical in this side by side mode, which hitherto has been used mainly with paired photos or in print form.

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.

Stereograms cards are frequently used by orthoptists and vision therapists in the treatment of many binocular vision and accommodative disorders.

Crossed-eye viewing


By exchanging the right and left views, so that the image to be viewed with the left eye is on the right side, it is possible to obtain a 3-D effect with some effort but without any equipment. To view the crossed-eye view shown here, the viewer should move slightly back from his or her normal viewing distance and place his viewpoint on a line perpendicular to the center of the image. A finger should be placed halfway between the eyes and the image, then the finger should be viewed. 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 produces substantial eye-strain, and is not comfortable enough for extended viewing. It also offers none of the advantages enumerated above that are provided by the stereoscope. When images are presented as for the stereoscope, with the image to be viewed by the left eye on the left, they can be viewed by diverging the eyes. This gives much less eye-strain than crossing the eyes, and requires a smaller adjustment of focus, but can be even harder to learn. Without the use of viewing equipment, the size of a stereoscopic image viewable is significantly limited by one's eye-spacing and the inability of one's eyes to diverge to a large extent. The major advantage of cross-eye viewing is that the images can be substantially larger, and no glasses are needed by those who have the viewing knack. Prismatic glasses, with built-in masking, make the trick easy for most people, but they tend to be a bit expensive.

Transparency viewers


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 very inexpensive and so are not found in most stereo viewers.

Head-mounted displays
The user typically wears a helmet or glasses with two small LCD or OLED displays with magnifying lenses, 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 these devices are beginning to become available at more reasonable cost.

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.

Augmented stereoscopic vision is also expected to have applications in surgery, as it allows the combination of radiographic data (CAT scans and MRI imaging) with the surgeon's vision.

LC shutter glasses
Glasses containing liquid crystal that will let light through in synchronization with the images on the computer display, using the concept of Alternate-frame sequencing. See also Time-division multiplexing.

Linearly polarized glasses
To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through orthogonal polarizing filters. It is best to use a silver screen so that polarization is preserved. The projectors can receive their outputs from a computer with a dual-head graphics card. 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. Linearly polarized glasses require the viewer to keep her head level, as tilting of the viewing filters will cause the images of the left and right channels to bleed over to the opposite channel - on the other hand, viewers learn very quickly not to tilt their heads. In addition, since no head tracking is involved, several people can view the stereocopic images at the same time.

There are several commercial systems offering products like the above, and one can also put one together by oneself using instructions on the GeoWall Consortium site.

Circularly polarized glasses
To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through circular polarizing filters of opposite handedness. The viewer wears low-cost eyeglasses which contain a pair of analyzing filters (circular polarizers mounted in reverse) of opposite handedness. Light that is left-circularly polarized is extinguished by the right-handed analyzer; while right-circularly polarized light is extinguished by the left-handed analyzer. The result is similar to that of steroscopic viewing using linearly polarized glasses; except the viewer can tilt his head and still maintain left/right separation.

Real D Cinema System (used recently with the sterescopic Disney movie, "Chicken Little 3D") uses electronically driven circular polarizers that alternate between left- and right- handedness, and does so in sync with the left or right image being displayed by the (digital) movie projector.

Two-color anaglyph


Anaglyph images have seen a recent resurgence due to the presentation of images on the internet. Where traditionally, this has been a largely black & white format, recent digital camera and processing advances have brought very acceptable color images to the internet and DVD field. With the online availabilty of low cost paper glasses with improved red-cyan filters, and even better plastic framed glasses, the field is growing fast. Scientific 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 due to recent release of 3D DVDs, they are increasingly used 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, which is not quite obtainable with even good color anaglyphs.

Anachrome "compatible" method
A recent variation on the anaglyph technique is called "Anachrome method". This approach is an attempt to provide images that look fairly normal without glasses as 2D images to be "compatible" for posting in conventional websites or magazines. The 3D effect is generally more subtle, as the images are shot with a narrower stereo base, (the distance between the camera lenses). Pains are taken to adjust for a better overlay fit of the two images, which are layered one on top of another. Only a few pixels of non-registration give the depth cues. The range of color is perhaps three times wider in Anachrome due to the deliberate passage of a small amount of the red information through the cyan filter. Warmer tones can be boosted,and this provides warmer skin tones and vividness.

Plastic glasses frequently have better contrast and much better focus through the red filter, due to the 250 nanometer difference in the wave lengths of the red-cyan filters. With paper glasses, the red filter is blurry when viewing a close computer screen or printed image. Plastic glasses can offer a compensating diopter power to equalize the red filter focus shift relative to the cyan. Only molded plastic glasses can provide this focus fix. As of January 2006, more than 3,000 educational, or scientific images were offered on-line in this and similar "compatible" formats.

Anachrome on HDTV
In late 2005, NBC, a major US television network, offered up the first high definition (HDTV) 3D program using anaglyph full color technology. The advent of HD disks (HD DVD & Blu-ray), available in early 2006, will likely accelerate the wider use of 3D in home entertinment. The superior color presentation and detail of HDTV will make anaglyph a better 3D viewing experience than previous attempts.

Chromadepth glasses
The Chromadepth procedure of American Paper Optics is based on the fact that with a prism colors are separated by varying degrees. The ChromaDepth eyeglasses contain special view foils, which consist of microscopically small prisms. This causes the image to be translated a certain amount that depends on its color. If one uses a prism foil now with one eye but not on the other eye, then the two seen pictures - depending upon color - are more or less widly separated. The brain produces the spatial impression from this difference. The advantage of this technology consists above all of the fact that one can regard ChromaDepth pictures also without eyeglasses (thus two-dimensional) problem-free (unlike with two-color anaglyph). However the colors are only limitedly selectable, since they contain the depth information of the picture. If one changes the color of an object, then its observed distance will also be changed.

Autostereograms
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 by diverging the eyes, 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.

Pulfrich effect
The Pulfrich effect is a consequence of the fact that at low light levels the eye-brain visual response is slower. The ultimate effect of this is the illusion of depth. A single screen direction must be maintained, or the effect is seen in pseudo-stereo, only very limited "real-world" practical use with this method.

Prismatic crossview glasses
Cross viewing is a skill that must be learned to be used. New prismatic glasses now make cross-viewing easier, and also mask off the non-3d images, that otherwise show up on either side of the 3D image. The most recent glasses optically widens the image by about 20% so that 2 conventional cross view frames can be displayed on a new widescreen HD or computer monitor. The best of these glasses flip up the filters when not viewing a 3D image. Cross viewing provides true "Ghost-free 3D" with maximum clarity and color range.

Lenticular Prints
Lenticular printing is a technique by which one places an array of lenses over a specially made and carefully aligned print such that different viewing angles will produce different angles, producing the illusion of three dimensions, over a certain limited viewing angle. This can be done cheaply enough that it is sometimes used on stickers, album covers, etc.

Displays with filter arrays
The LCD 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 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. This allows seeing different pixels depending on the viewer's position.

Wiggle stereoscopy
This method, possibly the most simple sterogram viewing technique, is to simply alternate between the left and right images of a stereogram. In a web browser, 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. Closing one eye and moving the head from side-to-side helps to understand why this works. Objects that are closer appear to move more than those further away.

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.

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

Disadvantages of the "wiggle" method:
 * Does not provide true binocular stereoscopic depth perception
 * Not suitable for print, limited to displays that can "wiggle" between the two images
 * Difficult to appreciate details in images that are constantly "wiggling"

Although the "wiggle" method is an excellent way of previewing stereoscopic images, it cannot actually be considered a true three-dimensional stereoscopic format. An individual looking at a wiggling image is not at all experiencing stereoscopic viewing, they are still only seeing a flat two-dimensional image that is "wiggling". To experience binocular depth perception as made possible with true stereoscopic formats, each eyeball must be presented with a different image at the same time - this is not the case with "wiggling" stereo. The "wiggle" effect is similar to walking around one's environment while blinking one eye and then the other.

To illustrate the difference between true stereoscopic formats and the two-dimensional "wiggle" method, consider what happens when a stereophonic music CD is played through only one loudspeaker: It is no longer possible to hear the stereophonic audio signal since it is now only coming out of one loudspeaker. Flipping between the Left and Right audio channels of the stereophonic signal through the one loudspeaker, the listener is still only hearing a monaural signal. By listening to the stereophonic music CD through stereophonic headphones that deliver the proper audio signal to each ear, the listener can experience true stereophonic audio. Similarly, the only way to experience binocular stereoscopic depth perception when viewing stereoscopic images is to use a device (stereoscope, anaglyph glasses, polarized glasses, shutter glasses) that presents each of the two eyes with the corresponding Left or Right image.

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&reg; 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. Although there are not any out-of-the-box digital stereocameras available, it is possible to create a twin camera rig, together with a "shepherd" device to synchronise shutter and flash of the two cameras.

The side-by-side method is extremely simple to create, but it can be difficult or uncomfortable to view without optical aids. One such aid for non-crossed images is the modern Pokescope tm. Traditional stereoscopes such as the Holmes can be used as well. Cross view technique now has the Prisma HD viewing glasses to facilitate viewing.

Imaging methods
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.

A single digital camera can also be used if the subject remains perfectly still,( such as an object in a museum display). Two exposures are required. The camera can be moved on a sliding bar for offset, or with practice, the photographer can simply shift the camera while holding it straight and level. A good rule of thumb is to shift sideways 30 to one for side by side or just 60 to one ratio if the image is to be also used for color anaglyph or anachrome image display.

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. An object at this distance will appear on the picture plane, the apparent surface of the image. Objects closer than this will appear in front of the picture plane, or popping out of the image. All objects at greater distances appear behind the picture plane. This interpupilar or 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. Where images may also be used for anaglyph display a narrower base, say 50 to 60 to 1 will allow for less ghosting in the display.