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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Binocular vision | 5/6 | https://en.wikipedia.org/wiki/Binocular_vision | reference | science, encyclopedia | 2026-05-05T15:53:38.467964+00:00 | kb-cron |
=== Stereoscopy === When viewing the recorded or generated images, care must be taken to ensure that each eye sees only the image that was recorded for each eye. This can be done with many different techniques. These techniques ensure that the eyes can focus on the physical distance of the stereo image, and at the same time converge on the corresponding point in space. In doing so, the observer must sometimes learn to overcome conflicts between both reflexes (vergence-accommodation conflict). Nowadays, stereograms are often offered via VR glasses.
==== Vergence-accommodation conflict ==== When looking around in a natural 3D environment, the eyes fixate on different spatial points in succession. The eyes automatically focus and converge on the fixated point. When looking at a stereogram, the eyes must focus on the distance of the images and not on the distance of the fixation point. The vergence point must also move with the fixation point to ensure that the area for stereopsis is around the fixation point. This means that the vergence and accommodation reflexes must be decoupled. This can be trained, but can cause eye strain or headaches in the beginning.
==== Stereoscope ==== A stereoscope is a tool to be able to offer the two images of a stereogram separately and sharply to both eyes and at a different distance than where the eyes converge. Different stereoscopes are suitable for different types of stereograms. There are different types of stereoscopes, based on lenses, mirrors, prisms, color filters and polaroid filters. The first stereoscope was invented by Wheatstone in 1838. Greater depth experience. To increase the sense of depth, the left and right images of stereo photographs are sometimes placed slightly further apart than they should be for a realistic image. The explanation for the greater sense of depth at a greater convergence distance is that the eyes have to look slightly farther away (converge) than is appropriate for the original situation and the monocular perspective in the images. The brain "corrects" this by perceiving objects as larger and with more depth. A similar mechanism underlies the explanation of pseudocopy.
==== Pseudoscopy ==== Pseudoscopy is viewing a stereogram of a natural scene in which the images for both eyes have been swapped. This reverses the binocular depth (disparity), convex becomes concave, and vice versa. The monocular perspective is unchanged, and therefore conflicts with the binocular depth information. This results in nearby objects appearing larger than normal and more distant objects appearing smaller. This gives a surreal feeling.
==== Vergence-Lock Stereoscope ====
The two stereo images of a stereogram can, with some practice, be viewed without a stereoscope. A common way of doing this is with a stereogram in which the image for the left eye is on the right and the image for the right eye is on the left (R-L stereogram).
The practice now is to cross the eye axes at a point in front of the stereogram in such a way that the left eye looks at the center of the right image and the right eye looks at the center of the left image. It helps to hold the point of a pencil at the intersection and focus attention on this point, and then wait until the image becomes sharp and depth is perceived. Krol (1982, p. 16-17) uses a piece of cardboard with a round or square recess instead of a pencil. This allows each eye to only see its own image. Moreover, the hole helps to automatically converge correctly.
== History == Alhazen (Ibn al-Haytham), an Arab scholar from the 11th century, was the first to propose that vision is possible because light reflects off objects and then enters the eye after which perceptions arise in the brain that are partly the result of activities of the observer, such as directing the eyes. In the 19th century, this idea was elaborated by Ewald Hering. He assumed that each eye saw direction (visual direction) and introduced the idea of a "cyclopean eye" for egocentric direction, as if we saw the world from a single central point between both eyes. Combined with the results of research on the horopter this can explain single and double images. The explanation of sensory fusion, in which two double images merge into one new image with a new direction, was only possible when nerve cells were found in the brain that become active when a specific direction is stimulated simultaneously in the left and right eyes. The perception of depth based on differences in the directions between both eyes (disparity) is discussed in a separate article, Stereopis.
=== Egocentric direction ===
Hering described in 1861 that we seem to perceive the world from a point, midway between both eyes, instead of from each eye separately. He called this point a cyclopean eye, after the cyplopes in Greek mythology. Hering also described a method to show how the two retinal images in this cyclopean eye are apparently merged into a combined image. He called this method cyclopean projection. He illustrated this method with a pencil held so that it points away from the observer, see the figure. The method basically involves rotating the images of the two eyes around the fixation point towards each other until they coincide in the cyclopean eye. Geometrically this does not change the direction, but it does give a concise description of how we seem to see directions. This is also called Hering's law of common binocular direction.
=== Horopter ===