Stereoscopy

Pocket stereoscope with original test image. Used by military to examine stereoscopic pairs of aerial photographs.
Stereo card image modified for crossed eye viewing.
View of Manhattan, c. 1909
View of Boston, c. 1860
An 1893-era World's Columbian Exposition viewer
Company of ladies watching stereoscopic photographs (Jacob Spoel, before 1868) Probably the earliest depiction of people using a stereoscope

Stereoscopy (also called stereoscopic or 3-D imaging) is any technique capable of recording three-dimensional visual information or creating the illusion of depth in an image.

Human vision uses several cues to determine relative depths in a perceived scene[1]. Some of these cues are:

All the above cues, with the exception of the first two, are present in traditional two-dimensional images such as paintings, photographs, and television. Stereoscopy is the enhancement of the illusion of depth in a photograph, movie, or other two-dimensional image by presenting a slightly different image to each eye, and thereby adding the first of these cues (stereopsis) as well. It is important to note that the second cue is still not satisfied and therefore the illusion of depth is incomplete.

Many 3D displays use this method to convey images. It was first invented by Sir Charles Wheatstone in 1838.[2] 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 3D scanners to detect and record 3 dimensional information.[3] The three-dimensional depth information can be reconstructed from two images using a computer by corresponding the pixels in the left and right images. Solving the Correspondence problem in the field of Computer Vision aims to create meaningful depth information from two images.

Traditional stereoscopic photography consists of creating a 3-D illusion starting from a pair of 2-D images. The easiest way to enhance depth perception in the brain is to provide the eyes of the viewer with two different images, representing two perspectives of the same object, with a minor deviation exactly equal to the perspectives that both eyes naturally receive in binocular vision. If eyestrain and distortion are to be avoided, each of the two 2-D images preferably should be presented to each eye of the viewer so that any object at infinite distance seen by the viewer should be perceived by that eye while it is oriented straight ahead, the viewer's eyes being neither crossed nor diverging. When the picture contains no object at infinite distance, such as a horizon or a cloud, the pictures should be spaced correspondingly closer together.

Contents

Etymology

The word stereoscopy derives from the Greek "στερεός" (stereos), "firm, solid"[4] + "σκοπέω" (skopeō), "to look", "to see".[5]

Visual requirements

Anatomically, there are 3 levels of binocular vision required to view stereo images:

These functions develop in early childhood. Some people who have strabismus disrupt the development of stereopsis, however orthoptics treatment can be used to improve binocular vision. A person's stereoacuity determines the minimum image disparity they can perceive as depth.

Side-by-side (non-shared viewing scenarios)

Characteristics

Stereograph published in 1900 by North-Western View Co. of Baraboo, Wisconsin, digitally restored.

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 flat screens and "software stereoscopes" has made larger 3D digital images practical in this side by side mode, which hitherto had been used mainly with paired photos in print form.

Freeviewing

Freeviewing is viewing a side-by-side image without using a viewer.[6]

Several methods are available to freeview.[7][8]

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.

The stereoscope offers several advantages:

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

Vintage Stereoscopic Picture (for parallel viewing)

Vintage Stereoscopic Picture (for cross viewing)

Narrow paired images (for cross viewing)

Stereoscopic Picture of Piano Keys (for cross viewing)

Transparency viewers

Stereoscope and case – during WWII this tool was used by Allied photo interpreters to analyze images shot from aerial photo reconnaissance platforms.

The practice of viewing transparencies in stereo via a viewer dates to at least as early as 1931, when Tru-Vue began to market filmstrips that were fed through a handheld device made from Bakelite. 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.

Another important development in the late 1940s was the introduction of the Stereo Realist camera and viewer system. Using color slide film, this equipment made stereo photography available to the masses and caused a surge in its popularity. The Stereo Realist and competing products can still be found (in estate sales and elsewhere) and utilized today.

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 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 may be presented since 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 quite expensive and they are not found in most stereo viewers.

Head-mounted displays

An HMD with a separate video source displayed in front of each eye to achieve a stereoscopic effect

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, allowing the user to "look around" the virtual world by moving their head, eliminating the need for a 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 equipment 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 equipment, 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.

3D viewers

There are two categories of 3D viewer technology, active and passive. Active viewers have electronics which interact with a display.

Active

Liquid crystal shutter glasses

Glasses containing liquid crystal that block or pass light through in synchronization with the images on the computer display, using the concept of alternate-frame sequencing. See also Time-division multiplexing.

"Red eye" shutterglasses method

The Red Eye Method reduces the ghosting caused by the slow decay of the green and blue P22-type phosphors typically used in conventional CRT monitors. This method relies solely on the red component of the RGB image being displayed, with the green and blue component of the image being suppressed.

Passive

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 his 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 – therefore, viewers learn very quickly not to tilt their heads. In addition, since no head tracking is involved, several people can view the stereoscopic images at the same time.

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 or her head and still maintain left/right separation.

The RealD Cinema system uses an electronically driven circular polarizer, mounted in front of the projector and alternating between left- and right- handedness, in sync with the left or right image being displayed by the (digital) movie projector. The audience wears passive circularly polarized glasses.

Infitec glasses

Infitec stands for interference filter technology. Special interference filters (dichromatic filters) in the glasses and in the projector form the main item of technology and have given it this name. The filters divide the visible color spectrum into six narrow bands - two in the red region, two in the green region, and two in the blue region (called R1, R2, G1, G2, B1 and B2 for the purposes of this description). The R1, G1 and B1 bands are used for one eye image, and R2, G2, B2 for the other eye. The human eye is largely insensitive to such fine spectral differences so this technique is able to generate full-color 3D images with only slight colour differences between the two eyes.[9] Sometimes this technique is described as a "super-anaglyph" because it is an advanced form of spectral-multiplexing which is at the heart of the conventional anaglyph technique.

Dolby uses a form of this technology in its Dolby 3D theatres.

Complementary color anaglyphs

Full color Anachrome red (left eye)
and cyan (right eye) filters
3d glasses red cyan.svg 3D red cyan glasses are recommended to view this image correctly.

Complementary color anaglyphs employ one of a pair of complementary color filters for each eye. The most common color filters used are red and cyan. Employing tristimulus theory, the eye is sensitive to three primary colors, red, green, and blue. The red filter admits only red, while the cyan filter blocks red, passing blue and green (the combination of blue and green is perceived as cyan). If a paper viewer containing red and cyan filters is folded so that light passes through both, the image will appear black. Another recently introduced form employs blue and yellow filters. (Yellow is the color perceived when both red and green light passes through the filter.)

Anaglyph images have seen a recent resurgence because of 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 availability of low cost paper glasses with improved red-cyan filters, and plastic framed glasses of increasing quality, the field of 3D imaging is growing quickly. Scientific images where depth perception is useful include, for instance, the presentation of complex multi-dimensional data sets and stereographic images of the surface of Mars. With the recent release of 3D DVDs, they are more commonly being used for entertainment. Anaglyph images are much easier to view than either parallel sighting or crossed eye stereograms, although these types do offer more bright and accurate color rendering, most particularly in the red component, which is commonly muted or desaturated with even the best color anaglyphs. A compensating technique, commonly known as Anachrome, uses a slightly more transparent cyan filter in the patented glasses associated with the technique. Processing reconfigures the typical anaglyph image to have less parallax to obtain a more useful image when viewed without filters.

Compensating diopter glasses for red-cyan method

Simple sheet or uncorrected molded glasses do not compensate for the 250 nanometer difference in the wave lengths of the red-cyan filters. With simple glasses, the red filter image can be blurry when viewing a close computer screen or printed image since the retinal focus differs from the cyan filtered image, which dominates the eyes' focusing. Better quality molded plastic glasses employ a compensating differential diopter power to equalize the red filter focus shift relative to the cyan. The direct view focus on computer monitors has been recently improved by manufacturers providing secondary paired lenses fitted and attached inside the red-cyan primary filters of some high end anaglyph glasses. They are used where very high resolution is required, including science, stereo macros, and animation studio applications. They use carefully balanced cyan (blue-green) acrylic lenses, which pass a minute percentage of red to improve skin tone perception. Simple red/blue glasses work well with black and white, but blue filter unsuitable for human skin in color.

ColorCode 3D
Michelle Obama and Barack Obama and their party watch the commercials using ColorCode 3D during Super Bowl XLIII on February 1, 2009 in the White House theatre.

ColorCode 3D is a newer, patented[10] stereo viewing system deployed in the 2000s that uses amber and blue filters. Notably, unlike other anaglyph systems, ColorCode 3D is intended to provide perceived nearly full colour viewing (particularly within the RG color space) with existing television and paint mediums. One eye (left, amber filter) receives the cross-spectrum colour information and one eye (right, blue filter) sees a monochrome image designed to give the depth effect. The human brain ties both images together.

Images viewed without filters will tend to exhibit light-blue and yellow horizontal fringing. The backwards compatible 2D viewing experience for viewers not wearing glasses is improved, generally being better than previous red and green anaglyph imaging systems, and further improved by the use of digital post-processing to minimise fringing. The displayed hues and intensity can be subtly adjusted to further improve the perceived 2D image, with problems only generally found in the case of extreme blue.

The blue filter is centred around 450 nm and the amber filter lets in light at wavelengths at above 500 nm. Wide spectrum colour is possible because the amber filter lets through light across most wavelengths in spectrum. When presented via RGB color model televisions, the original red and green channels from the left image are combined with a monochrome blue channel formed by averaging the right image with the weights \{r:0.15, g:0.15, b:0.7\}.

In the United Kingdom, television station Channel 4 commenced broadcasting a series of programmes encoded using the system during the week of 16 November 2009.[11] Previously the system had been used in the United States for an "all 3-D advertisement" during the 2009 Super Bowl for SoBe, Monsters vs. Aliens animated movie and an advertisement for the Chuck television series in which the full episode the following night used the format.

Chromadepth method and 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 widely 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.

Anachrome "compatible" color anaglyph method

Anachrome optical diopter glasses.

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 is claimed to provide warmer skin tones and vividness.

Printable cross eye viewer

3dviewer.gif.

Other display methods

Autostereograms

A random dot autostereogram encodes a 3D scene which can be "seen" with proper viewing technique
Main article: autostereogram

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 or converging the eyes in a manner similar to naked eye viewing of stereo pairs, 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 effects

In the classic Pulfrich effect paradigm a subject views, binocularly, a pendulum swinging perpendicular to his line of sight. When a neutral density filter (e.g., a darkened lens -like from a pair of sunglasses) is placed in front of, say, the right eye the pendulum appears to take on an elliptical orbit, being closer as it swings toward the right and farther as it swings toward the left.

The widely accepted explanation of the apparent motion with depth is that a reduction in retinal illumination (relative to the fellow eye) yields a corresponding delay in signal transmission, imparting instantaneous spatial disparity to moving objects. This occurs because the eye, and hence the brain, respond more quickly to brighter objects than to dimmer ones.[12][13][14][15]

So if the brightness of the pendulum is greater in the left eye than in the right, the retinal signals from the left eye will reach the brain slightly ahead of those from the right eye. This makes it seem as if the pendulum seen by the right eye is lagging behind its counterpart in the left eye. This difference in position over time is interpreted by the brain as motion with depth: no motion, no depth.

The ultimate effect of this, with appropriate scene composition, is the illusion of motion with depth. Object motion must be maintained for most conditions and is effective only for very limited "real-world" scenes.

Prismatic & self-masking crossview glasses

"Naked-eye" cross viewing is a skill that must be learned to be used. New prismatic glasses now make cross-viewing as well as over/under-viewing easier, and also mask off the secondary non-3D images, that otherwise show up on either side of the 3D image. The most recent low-cost glasses mask the images down to one per eye using integrated baffles. Images or video frames can be displayed on a new widescreen HD or computer monitor with all available area used for display. HDTV wide format permits excellent color and sharpness. Cross viewing provides true "ghost-free 3D" with maximum clarity, brightness and color range, as does the stereopticon and stereoscope viewer with the parallel approach and the KMQ viewer with the over/under approach. The potential depth and brightness is maximized. A recent cross converged development is a new variant wide format that uses a conjoining of visual information outside of the regular binocular stereo window. This allows an efficient seamless visual presentation in true wide-screen, more closely matching the focal range of the human eyes.

Lenticular prints

Lenticular printing is a technique by which one places an array of lenses, with a texture much like corduroy, over a specially made and carefully aligned print such that different viewing angles will reveal different image slices to each eye, 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. It is the classic technique for 3D postcards.

A variant of this for portable electronic devices, the parallax barrier, has begun deployment.

Displays with filter arrays

The LCD is covered with an array of prisms that divert the light from 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

Wiggle stereoscopy

This method, possibly the simplest stereogram 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, flash applet or a specialized java applet. Most people can get a crude sense of dimensionality from such images, due to parallax.

Closing one eye and moving the head from side-to-side when viewing a selection of objects helps one understand how 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:

Disadvantages of the "wiggle" method:

Most wiggle images use only two images, leading to an annoyingly jerky image. A smoother image, more akin to a motion picture image where the camera is moved back and forth, can be composed by using several intermediate images (perhaps with synthetic motion blur) and longer image residency at the end images to allow inspection of details. Another option is a shorter time between the frames of a wiggle image through the use of an animated .png.

Although the "wiggle" method is an excellent way of previewing stereoscopic images, it cannot actually be considered a true three-dimensional stereoscopic format. 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 apparent "stereo like effect" comes from syncing the timing of the wiggle and the amount of parallax to the processing done by the visual cortex. Three or five images with good parallax produce a much better effect than simple left and right images.

Wiggling works for the same reason that a translational pan (or tracking shot) in a movie provides good depth information: the visual cortex is able to infer distance information from motion parallax, the relative speed of the perceived motion of different objects on the screen. Many small animals bob their heads to create motion parallax (wiggling) so they can better estimate distance prior to jumping[16][17][18]. You can see this for yourself in a 3D movie by removing the glasses during a scene where the camera is moving: the glasses have very little additional effect at such a time.

Piku-Piku

A Piku-Piku is a new technique for viewing 3D photos on a computer screen pioneered by 3D photo sharing site "Start 3D". Similar to a "wiggle", a Piku-Piku first converts a stereo photo into a multiview 3D photo and then uses gentle animation to display the 3D effect. Viewers can also stop the animation and interact with the Piku-Piku using a slider giving the viewer control of the viewpoint. As with a "wiggle" the advantage of this technique is that anyone can view a 3D photo on a normal screen without the need for any special 3D display equipment.

Taking the pictures

It is necessary to take two photographs for a stereoscopic image. This can be done with two cameras, with one camera moved quickly to two positions, or with a stereo camera such as the Fujifilm FinePix Real 3D W1.

When using two cameras there are two prime considerations to take into account when taking stereo pictures; How far the resulting image is to be viewed from and how far the subject in the scene is from the two cameras.

How far you are intending to view the pictures from requires a certain separation between the cameras. This separation is called stereo base or stereo base line and results from the ratio of the distance to the image to the distance between your eyes. The mean interpupillary distance (IPD) is 63 mm (about 2.5 inches), but varies with age, race and gender. The vast majority of adults have IPDs in the range 50–75 mm. Almost all adults are in the range 45–80 mm. The minimum IPD for children as young as five is around 40 mm. [19] In any case the farther you are from the screen the more the image will pop out. The closer you are to the screen the flatter it will appear. Personal anatomical differences can be compensated for by moving closer or farther from the screen .

For example if you are going to view a stereo image on your computer monitor from a distance of 1000 mm you will have an eye to view ratio of 1000/63 or about 16. To set your cameras the correct distance apart you take the distance to the subject (say a person at a distance from the cameras of 3 metres) and divide by 16 which gives you a stereo base of 188 mm between the cameras.

If you intend to view the stereo image from the same distance as it is captured (e.g. a subject photographed three meters away, projected on a movie screen at a distance from the viewer of three meters) then the stereo base separation will be the same as the distance between the viewer's eyes (about 63 mm).

In the 1950s, stereoscopic photography regained popularity when a number of manufacturers began introducing stereoscopic cameras to the public. The new cameras were developed to use 135 film, which had gained popularity after the close of World War II. Many of the conventional cameras used the film for 35 mm transparency slides, and the new stereoscopic cameras utilized the film to make stereoscopic slides. The Stereo Realist camera was the most popular, and the 35 mm picture format became the standard by which other stereo cameras were designed. The stereoscopic cameras were marketed with special viewers that allowed for the use of such 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.

The 1980s saw a minor revival of stereoscopic photography extent when point-and-shoot stereo cameras were introduced. These cameras suffered from poor optics and plastic construction, so they never gained the popularity of the 1950s stereo cameras. 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 the age of digital photography. Stereo lenses were introduced which could turn an ordinary film camera into a stereo camera by using a special double lens to take two images and direct them through a single lens to capture them side-by-side on the film. Although current digital stereo cameras cost thousands of dollars,[20] cheaper models also exist, for example those produced by the company Loreo. It is also possible to create a twin camera rig, together with a "shepherd" device to synchronize the shutter and flash of the two cameras. By mounting two cameras on a bracket, spaced a bit, with a mechanism to make both take pictures at the same time. Newer cameras are even being used to shoot "step video" 3D slide shows with many pictures almost like a 3D motion picture if viewed properly. A modern camera can take five pictures per second, with images that greatly exceed HDTV resolution.

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. Traditional stereoscopes such as the Holmes can be used as well. Cross view technique now has the simple Perfect-Chroma cross 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. In practice the hand-held method works very well. This method of taking stereo photos is sometimes referred to as the "Cha-Cha" method.

A good rule of thumb is to shift sideways 1/30th of the distance to the closest subject for 'side by side' display, or just 1/60th if the image is to be also used for color anaglyph or anachrome image display. For example, if you are taking a photo of a person in front of a house, and the person is thirty feet away, then you should move the camera 1 foot between shots.

The stereo effect is not significantly diminished by slight pan or rotation between images. In fact slight rotation inwards (also called 'toe in') can be beneficial. Bear in mind that both images should show the same objects in the scene (just from different angles) - if a tree is on the edge of one image but out of view in the other image, then it will appear in a ghostly, semi-transparent way to the viewer, which is distracting and uncomfortable. Therefore, you can either crop the images so they completely overlap, or you can 'toe-in' the cameras so that the images completely overlap without having to discard any of the images. However, be a little cautious - too much 'toe-in' can cause eye strain for reasons best described here.[21]

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 (commonly called the "interocular") than the adult human norm of 62-65mm. This will effectively render the captured image as though it was seen by a giant, and thus will enhance the depth perception of these distant objects, and reduce the apparent scale of the scene proportionately. However, in this case care must be taken not to bring objects in the close foreground too close to the viewer, as they will require the viewer to become cross-eyed to resolve them.

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 four miles (6.5 km) 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.

This technique can be applied to 3D imaging of the Moon: one picture is taken at moonrise, the other at moonset, as the face of the Moon is centered towards the center of the Earth and the diurnal rotation carries the photographer around the perimeter.

In the wider image, taken from a different location, a single camera was walked about one hundred feet (30 m) between pictures. The images were converted to monochrome before combination.(below)

Long base line image showing prominent foothill ridges; click the image for more information on the technique3d glasses red cyan.svg 3D red cyan glasses are recommended to view this image correctly.
Small anaglyphed image3d glasses red cyan.svg 3D red cyan glasses are recommended to view this image correctly.

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 thirty times the distance between the eyes (when the scene extends to infinity). 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 interpupillar or interocular distance will vary between individuals. If one assumes that it is 2.5 inches (about 6.5 cm), then the closest object in a natural scene by this criterion would be 30 × 2.5 = 75 inches (about 2 m). It is this ratio (1:30) that determines the inter-camera spacing appropriate to imaging scenes. Thus if the nearest object is thirty 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 1:20 (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 cut-outs. Where images may also be used for anaglyph display a narrower base, say 1:50 or 1:60 will allow for less ghosting in the display..

Precise stereoscopic baseline calculation methods

Recent research has led to precise methods for calculating the stereoscopic camera baseline [22]. These techniques consider the geometry of the display/viewer and scene/camera spaces independently and can be used to reliably calculate a mapping of the scene depth being captured to a comfortable display depth budget. This frees up the photographer to place their camera wherever they wish to achieve the desired composition and then use the baseline calculator to work out the camera inter-axial separation required to produce a high quality 3D image.

This approach means there is no guess work in the stereoscopic setup once a small set of parameters have been measured, it can be implemented for photography and computer graphics and the methods can be easily implemented in a software tool. One such tool is available freely from the Durham Visualization Laboratory.

Multi-rig stereoscopic cameras

The precise methods for camera control have also allowed the development of multi-rig stereoscopic cameras where different slices of scene depth are captured using different inter-axial settings [23], the images of the slices are then composed together to form the final stereoscopic image pair. This allows important regions of a scene to be given better stereoscopic representation while less important regions are assigned less of the depth budget. It provides stereographers with a way to manage composition within the limited depth budget of each individual display technology.

See also

References

  1. ↑ Flight Simulation, J.M. Rolfe and K.J. Staples, Cambridge University Press, 1986, page 134
  2. ↑ Welling, William. Photography in America, page 23
  3. ↑ Fay Huang, Reinhard Klette, and Karsten Scheibe: Panoramic Imaging (Sensor-Line Cameras and Laser Range-Finders). Wiley & Sons, Chichester, 2008
  4. ↑ στερεός, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library
  5. ↑ σκοπέω, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library
  6. ↑ The Logical Approach to Seeing 3D Pictures. www.vision3d.com by Optometrists Network. Retrieved 2009-08-21
  7. ↑ How To Freeview Stereo (3D) Images. Greg Erker. Retrieved 2009-08-21
  8. ↑ How to View Photos on This Site. Stereo Photography - The World in 3D. Retrieved 2009-08-21
  9. ↑ Jorke, Helmut; Fritz M. (2006). "Stereo projection using interference filters". Stereoscopic Displays and Applications Proc. SPIE 6055. http://spie.org/x648.xml?product_id=650348. Retrieved 2008-11-19. 
  10. ↑ Sorensen, Svend Erik Borre; Hansen, Per Skafte; Sorensen, Nils Lykke (2001-05-31). "Method for recording and viewing stereoscopic images in color using multichrome filters". United States Patent 6687003. Free Patents Online. http://www.freepatentsonline.com/6687003.html. 
  11. ↑ "Announcements". 3D Week. 2009-10-11. http://www.channel4.com/programmes/3d-week/articles/3d-week. Retrieved 2009-11-18. "glasses that will work for Channel 4’s 3D week are the Amber and Blue ColourCode 3D glasses" 
  12. ↑ Lit A. (1949) The magnitude of the Pulfrich stereo-phenomenon as a function of binocular differences of intensity at various levels of illumination. Am. J. Psychol. 62:159-181.
  13. ↑ Rogers B.J. Anstis S.M. (1972) Intensity versus Adaptation and the Pulfrich Stereophenomenon Vision Res. 12:909-928.
  14. ↑ Williams JM, Lit A. (1983) Luminance-dependent visual latency for the Hess effect, the Pulfrich effect, and simple reaction time. Vision Res. 23(2):171-9.
  15. ↑ Deihl Rolf R. (1991) Measurement of Interocular delays with Dynamic Random-Dot stereograms. Eur. Arch. Psychiatry Clin. Neurosci. 241:115-118.
  16. ↑ Steinman & Garzia 2000, p. 180
  17. ↑ Legg and Lambert 1990
  18. ↑ Ellard, Goodale and Timney & Behavioral Brain Research 14(1) October 1984, pp.29-39
  19. ↑ Dodgson, Neil A. (2003). "Variation and extrema of human interpupillary distance" (PDF). Stereoscopic Displays and Applications Proc. SPIE 5291: 36–46. http://www.cl.cam.ac.uk/~nad10/pubs/EI5291A-05.pdf. Retrieved 2008-11-20. 
  20. ↑ Stereo Video - V2_Lab Projects - Trac
  21. ↑ How a 3-D Stereoscopic Movie is made - 3-D Revolution Productions
  22. ↑ Jones, G.R.; Lee, D., Holliman, N.S., Ezra, D. (2001). "Controlling perceived depth in stereoscopic images" (PDF). Stereoscopic Displays and Applications Proc. SPIE 4297A. http://www.dur.ac.uk/n.s.holliman/Presentations/EI4297A-07Protocols.pdf. 
  23. ↑ Holliman, N.S. (2004). "Mapping perceived depth to regions of interest in stereoscopic images" (PDF). Stereoscopic Displays and Applications Proc. SPIE 5291. http://www.dur.ac.uk/n.s.holliman/Presentations/EI5291A-12.pdf. 

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