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Stereoscopy

Stereoscopy (also called stereoscopics, or stereo imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision.[2] The word stereoscopy derives from Greek στερεός (stereos) 'firm, solid', and σκοπέω (skopeō) 'to look, to see'.[3][4] Any stereoscopic image is called a stereogram. Originally, stereogram referred to a pair of stereo images which could be viewed using a stereoscope.

Several terms redirect here. For other uses, see autostereogram, phonogram, and stereographic projection.

Most stereoscopic methods present a pair of two-dimensional images to the viewer. The left image is presented to the left eye and the right image is presented to the right eye. When viewed, the human brain perceives the images as a single 3D view, giving the viewer the perception of 3D depth. However, the 3D effect lacks proper focal depth, which gives rise to the Vergence-Accommodation Conflict.


Stereoscopy is distinguished from other types of 3D displays that display an image in three full dimensions, allowing the observer to increase information about the 3-dimensional objects being displayed by head and eye movements.

Vergence

Accommodation

Stereopsis

Occlusion - The overlapping of one object by another

Subtended visual angle of an object of known size

Linear perspective (convergence of parallel edges)

Vertical position (objects closer to the horizon in the scene tend to be perceived as farther away)

Haze or contrast, saturation, and color, greater distance generally being associated with greater haze, desaturation, and a shift toward blue

Change in size of textured pattern detail

The parallel viewing method uses an image pair with the left-eye image on the left and the right-eye image on the right. The fused three-dimensional image appears larger and more distant than the two actual images, making it possible to convincingly simulate a life-size scene. The viewer attempts to look through the images with the eyes substantially parallel, as if looking at the actual scene. This can be difficult with normal vision because eye focus and binocular convergence are habitually coordinated. One approach to decoupling the two functions is to view the image pair extremely close up with completely relaxed eyes, making no attempt to focus clearly but simply achieving comfortable stereoscopic fusion of the two blurry images by the "look-through" approach, and only then exerting the effort to focus them more clearly, increasing the viewing distance as necessary. Regardless of the approach used or the image medium, for comfortable viewing and stereoscopic accuracy the size and spacing of the images should be such that the corresponding points of very distant objects in the scene are separated by the same distance as the viewer's eyes, but not more; the average interocular distance is about 63 mm. Viewing much more widely separated images is possible, but because the eyes never diverge in normal use it usually requires some previous training and tends to cause eye strain.

The cross-eyed viewing method swaps the left and right eye images so that they will be correctly seen cross-eyed, the left eye viewing the image on the right and vice versa. The fused three-dimensional image appears to be smaller and closer than the actual images, so that large objects and scenes appear miniaturized. This method is usually easier for freeviewing novices. As an aid to fusion, a fingertip can be placed just below the division between the two images, then slowly brought straight toward the viewer's eyes, keeping the eyes directed at the fingertip; at a certain distance, a fused three-dimensional image should seem to be hovering just above the finger. Alternatively, a piece of paper with a small opening cut into it can be used in a similar manner; when correctly positioned between the image pair and the viewer's eyes, it will seem to frame a small three-dimensional image.

Stereo window[edit]

The concept of the stereo window is always important, since the window is the stereoscopic image of the external boundaries of left and right views constituting the stereoscopic image. If any object, which is cut off by lateral sides of the window, is placed in front of it, an effect results that is unnatural and is undesirable, this is called a "window violation". This can best be understood by returning to the analogy of an actual physical window. Therefore, there is a contradiction between two different depth cues: some elements of the image are hidden by the window, so that the window appears as closer than these elements, and the same elements of the image appear as closer than the window. So that the stereo window must always be adjusted to avoid window violations.


Some objects can be seen in front of the window, as far as they do not reach the lateral sides of the window. But these objects can not be seen as too close, since there is always a limit of the parallax range for comfortable viewing.


If a scene is viewed through a window the entire scene would normally be behind the window, if the scene is distant, it would be some distance behind the window, if it is nearby, it would appear to be just beyond the window. An object smaller than the window itself could even go through the window and appear partially or completely in front of it. The same applies to a part of a larger object that is smaller than the window. The goal of setting the stereo window is to duplicate this effect.


Therefore, the location of the window versus the whole of the image must be adjusted so that most of the image is seen beyond the window. In the case of viewing on a 3D TV set, it is easier to place the window in front of the image, and to let the window in the plane of the screen.


On the contrary, in the case of projection on a much larger screen, it is much better to set the window in front of the screen (it is called "floating window"), for instance so that it is viewed about two meters away by the viewers sit in the first row. Therefore, these people will normally see the background of the image at the infinite. Of course the viewers seated beyond will see the window more remote, but if the image is made in normal conditions, so that the first row viewers see this background at the infinite, the other viewers, seated behind, will also see this background at the infinite, since the parallax of this background is equal to the average human interocular.


The entire scene, including the window, can be moved backwards or forwards in depth, by horizontally sliding the left and right eye views relative to each other. Moving either or both images away from the center will bring the whole scene away from the viewer, whereas moving either or both images toward the center will move the whole scene toward the viewer. This is possible, for instance, if two projectors are used for this projection.


In stereo photography window adjustments is accomplished by shifting/cropping the images, in other forms of stereoscopy such as drawings and computer generated images the window is built into the design of the images as they are generated.


The images can be cropped creatively to create a stereo window that is not necessarily rectangular or lying on a flat plane perpendicular to the viewer's line of sight. The edges of the stereo frame can be straight or curved and, when viewed in 3D, can flow toward or away from the viewer and through the scene. These designed stereo frames can help emphasize certain elements in the stereo image or can be an artistic component of the stereo image.

Cloud stereoscopy

Simmons, Gordon (March–April 1996). "Clarence G. Henning: The Man Behind the Macro". Stereo World. 23 (1): 37–43.

Willke, Mark A.; Zakowski, Ron (March–April 1996). "A Close Look into the Realist Macro Stereo System". Stereo World. 23 (1): 14–35.

Morgan, Willard D.; Lester, Henry M. (October 1954). Stereo Realist Manual. and 14 contributors. New York: Morgan & Lester. :1954srm..book.....M. OCLC 789470.

Bibcode

Special Collections and Archives, The UC Irvine Libraries, Irvine, California.

Guide to the Edward R. Frank Stereograph Collection.

Brock University Library Digital Repository

Niagara Falls Stereo Cards RG 541

from the University of Louisville Archives & Special Collections

Stereographic Views of Louisville and Beyond, 1850s-1930