Stereoscopy

From Academic Kids

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. 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. 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.

Contents

Techniques

Side-by-side

Characteristics

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. See stereoscope for more details.

Crossed-eye viewing

Missing image
XEyeStCdNYCSmall.jpg
Image:XEyeStCdNYCSmall.jpg


Stereo card image modified for crossed eye viewing
View of Manhattan, c. 1909

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. 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. With cross-eye viewing, the images can be substantially larger. This is the major advantage of this method.

Transparency viewers

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

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 LCDs 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.

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 glasses

Missing image
3d_glasses.jpg
A pair of 3D glasses (color anaglyph)

LCD shutter glasses

Glasses containing liquid crystal that will let light through in synchronization with the images on the computer display. See LCD shutter glasses for more information.

Polarized glasses

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. See polarized glasses for further information.

Two-color anaglyph

Missing image
AnaglyphNYC.jpg
Stereo card image anaglyphed for red (left eye)
and cyan (right eye] filters
Main article: Anaglyph image

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. See Anaglyph image for more information.

Other display methods

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 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.

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.

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 [1] (http://www.nypost.com/postopinion/opedcolumnists/23542.htm), 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. That allow seeing different pixels depending on the viewer's position.

Wiggle stereoscopy

Missing image
Wiggle.gif
Wiggle stereograph in animated .gif format.

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 [2] (http://www.burningmanopera.org/2002/2002_stereo_wiggle1.html) or a flash applet [3] (http://wiggle.sourceforge.net/). 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.

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 electronic 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 you play a stereophonic music CD 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. Even if you flip 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® 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.

Missing image
TDC.jpg
A TDC Vivid stereo camera, one of the many models produced during the 1950's.

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.

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.

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.

Missing image
LBLFoothillsBWAna.jpg
Long base line image showing prominent foothill ridges; click the image for more information on the technique


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.

See also

External links

fr:Stéréoscopie pt:Estereocopia

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