Stereoscopy

Background[edit]

Stereoscopy creates the illusion of three-dimensional depth from given two-dimensional images. Human vision, including the perception of depth, is a complex process which only begins with the acquisition of visual information taken in through the eyes; much processing ensues within the brain, as it strives to make intelligent and meaningful sense of the raw information provided. One of the very important visual functions that occur within the brain as it interprets what the eyes see is that of assessing the relative distances of various objects from the viewer, and the depth dimension of those same perceived objects. The brain makes use of a number of cues to determine relative distances and depth in a perceived scene, including:[4]

Stereopsis

Accommodation of the eye

Overlapping of one object by another

Subtended visual angle of an object of known size

Linear perspective (convergence of parallel edges)

Vertical position (objects higher in the scene generally tend to be perceived as further away)

Haze, desaturation, and a shift to bluishness

Change in size of textured pattern detail

(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 production 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. Both of the 2D offset images are then combined in the brain to give the perception of 3D depth. It is important to note that since all points in the image focus at the same plane regardless of their depth in the original scene, the second cue, focus, is still not duplicated and therefore the illusion of depth is incomplete. There are also primarily two effects of stereoscopy that are unnatural for the human vision: first, the mismatch between convergence and accommodation, caused by the difference between an object's perceived position in front of or behind the display or screen and the real origin of that light and second, possible crosstalk between the eyes, caused by imperfect image separation by some methods.

Although the term "3D" is ubiquitously used, it is also important to note that the presentation of dual 2D images is distinctly different from displaying an image in three full dimensions. The most notable difference is that, in the case of "3D" displays, the observer's head and eye movement will not increase information about the 3-dimensional objects being displayed. Holographic displays or volumetric display are examples of displays that do not have this limitation. Similar to the technology of sound reproduction, in which it is not possible to recreate a full 3-dimensional sound field merely with two stereophonic speakers, it is likewise an overstatement of capability to refer to dual 2D images as being "3D". The accurate term "stereoscopic" is more cumbersome than the common misnomer "3D", which has been entrenched after many decades of unquestioned misuse. Although most stereoscopic displays do not qualify as real 3D display, all real 3D displays are also stereoscopic displays because they meet the lower criteria as well.

Most 3D displays use this stereoscopic method to convey images. It was first invented by Sir Charles Wheatstone in 1838,[5][6] and improved by Sir David Brewster who made the first portable 3D viewing device.[7]

Wheatstone mirror stereoscope

Wheatstone originally used his stereoscope (a rather bulky device)[8] with drawings because photography was not yet available, yet his original paper seems to foresee the development of a realistic imaging method:[9]

For the purposes of illustration I have employed only outline figures, for had either shading or colouring been introduced it might be supposed that the effect was wholly or in part due to these circumstances, whereas by leaving them out of consideration no room is left to doubt that the entire effect of relief is owing to the simultaneous perception of the two monocular projections, one on each retina. But if it be required to obtain the most faithful resemblances of real objects, shadowing and colouring may properly be employed to heighten the effects. Careful attention would enable an artist to draw and paint the two component pictures, so as to present to the mind of the observer, in the resultant perception, perfect identity with the object represented. Flowers, crystals, busts, vases, instruments of various kinds, &c., might thus be represented so as not to be distinguished by sight from the real objects themselves.[5]

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. An early patent for 3D imaging in cinema and television was granted to physicist Theodor V. Ionescu in 1936. Modern industrial three-dimensional photography may use 3D scanners to detect and record three-dimensional information.[10] The three-dimensional depth information can be reconstructed from two images using a computer by corresponding the pixels in the left and right images (e.g.,[11]). Solving the Correspondence problem in the field of Computer Vision aims to create meaningful depth information from two images.

Visual requirements[edit]

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

Simultaneous perception

Fusion (binocular 'single' vision)

Stereopsis

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. It is believed that approximately 12% of people are unable to properly see 3D images, due to a variety of medical conditions.[12][13] According to another experiment up to 30% of people have very weak stereoscopic vision preventing them from depth perception based on stereo disparity. This nullifies or greatly decreases immersion effects of stereo to them.[14]

Side-by-side[edit]

"The early bird catches the worm" Stereograph published in 1900 by North-Western View Co. of Baraboo, Wisconsin, digitally restored.

Traditional stereoscopic photography consists of creating a 3D illusion starting from a pair of 2D images, a stereogram. The easiest way to enhance depth perception

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