Information AboutStereoscopy |
| CATEGORIES ABOUT STEREOSCOPY | |
| photographic techniques | |
| 3d imaging | |
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, 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 Display s 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 Stereogram s. Stereoscopy is useful in viewing images rendered from large multi- Dimension al data sets such as are produced by experimental data. Modern industrial three dimensional photography may use 3D Scanner s to detect and record 3 dimensional information. 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 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 . 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. SIDE-BY-SIDE 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. The stereoscope offers several advantages:
Stereograms cards are frequently used by Orthoptist s and Vision Therapist s in the treatment of many binocular vision and Accommodative disorders. Cross converged viewing, with new "masking" glasses By exchanging the right and left views, and the opposite eye converged to the shifted images, it is possible to obtain a true color 3-D effect with some effort, without glasses or a viewer. Very recently, low cost glasses became available to aid the viewer in cross convergence viewing. An unusual effect of these optical glasses is to substantially widen the field of view to include a part of the peripheral area not visible to both eyes. There is a wrap-around effect produced, without the ghosting and the depth perspective can be greatly deepened. These new glasses provide acrylic lenses, plastic frames, that integrate a set of adjustable masking elements, that channel the view to only one image per eye. This allows full use of the screen width in the way that live vision treats stereo. The cross view image can easily fill any screen including 16 by 9 HDTV screens, working like a mirror to allow a dual perspecitive & wider view, as the images are mentally processed. Several non-commercial demonstration sites, such as bymal.com Malcom Patterson are showing extended 3D, sound, perfect color slide-shows, of subjects such as the Renaisance Pleasure Faire. They can be viewed with the glasses or without by masking with two hands turned upward, one per side. The technique is easily applied to full motion video as well. There are examples of the ultra wide aspect cross glasses images, as well as narrower "portrait" format, which fills a 4×3 computer screen efficiently. Cross viewing without glasses 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 become unimportant 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. Another disadvantage is that after prolonged viewing, the eyes may become accustomed to "close-convergence", as it requires the ability to direct the eyes as if viewing an object about eight inches away. This very close angle may lead to momentary double-vision. It also offers few of the advantages enumerated above that are provided by the stereoscope or Pokescope. 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 a different kind of "naked eye stress" than crossing the eyes (known as "wall-eyed divergence") but may require 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 painlessly. The major advantage of cross-eye viewing is that the images can be more than twice the area, and no glasses are needed by those who have the viewing knack. Prismatic cross glasses, with built-in masking, make the convergence very easy for most people, but they tend to be expensive, something like 5 times the cost of the simpler arcylic masking glasses. 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. 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 See Also: Head-mounted display 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 Scan s and MRI imaging) with the surgeon's vision. 3D GLASSES Liquid Crystal shutter glasses See Also: LCD 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 See Also: 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 – 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 – they refer to such a system as a GeoWall . Circularly polarized glasses See Also: 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 See Also: Anaglyph image 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 availability 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 latter types offer bright and accurate color rendering, which is not quite obtainable with even good color anaglyphs. Compensating anaglyph glasses 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 also 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 are very unsuitable for human skin in color. 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 A recent variation on the anaglyph technique is called "Anachrome method". Anachrome – Advanced Plastic Anaglyph 3D Glasses and Anachrome 3D Technology 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. As of April 2007, more than 4,500 educational, or scientific images were offered on-line in this and similar "compatible" formats. More than 40 public photo groups on www.flickr.com, the free photo archive, accept or feature "compatible" or more conventional anaglyph photos. |
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