Stereopsis (from the Greek στερεο- stereo- meaning "solid", and
ὄψις opsis, "appearance, sight") is a term that is most often
used to refer to the perception of depth and 3-dimensional structure
obtained on the basis of visual information deriving from two eyes by
individuals with normally developed binocular vision. Because the
eyes of humans, and many animals, are located at different lateral
positions on the head, binocular vision results in two slightly
different images projected to the retinas of the eyes. The differences
are mainly in the relative horizontal position of objects in the two
images. These positional differences are referred to as horizontal
disparities or, more generally, binocular disparities. Disparities are
processed in the visual cortex of the brain to yield depth perception.
While binocular disparities are naturally present when viewing a real
3-dimensional scene with two eyes, they can also be simulated by
artificially presenting two different images separately to each eye
using a method called stereoscopy. The perception of depth in such
cases is also referred to as "stereoscopic depth".
The perception of depth and 3-dimensional structure is, however,
possible with information visible from one eye alone, such as
differences in object size and motion parallax (differences in the
image of an object over time with observer movement), though the
impression of depth in these cases is often not as vivid as that
obtained from binocular disparities. Therefore, the term stereopsis
(or stereoscopic depth) can also refer specifically to the unique
impression of depth associated with binocular vision; what is
colloquially referred to as seeing "in 3D".
It has been suggested that the impression of "real" separation in
depth is linked to the precision with which depth is derived, and that
a conscious awareness of this precision – perceived as an impression
of interactability and realness – may help guide the planning of
1.1 Coarse and fine stereopsis
1.2 Static and dynamic stimuli
1.3 Research on perception mechanisms
2 Prevalence and impact of stereopsis in humans
3 History of investigations into stereopsis
4 Human stereopsis in popular culture
5 Geometrical basis
6 Computer stereo vision
7 Computer stereo display
8.1 Random dot stereotests
8.2 Contour stereotests
9 Deficiency and treatment
10 In non-human animals
11 See also
14 External links
Coarse and fine stereopsis
There are two distinct aspects to stereopsis: coarse stereopsis and
fine stereopsis, and provide depth information of different degree of
spatial and temporal precision.
Coarse stereopsis (also called gross stereopsis) appears to be used to
judge stereoscopic motion in the periphery. It provides the sense of
being immersed in one's surroundings and is therefore sometimes also
referred to as qualitative stereopsis. Coarse stereopsis is
important for orientation in space while moving, for example when
descending a flight of stairs.
Fine stereopsis is mainly based on static differences. It allows the
individual to determine the depth of objects in the central visual
area (Panum's fusional area) and is therefore also called quantitative
stereopsis. It is typically measured in random-dot tests; persons
having coarse but no fine stereopsis are often unable to perform on
random-dot tests, also due to visual crowding which is based on
interaction effects from adjacent visual contours. Fine stereopsis is
important for fine-motor tasks such as threading a needle.
The stereopsis which an individual can achieve is limited by the level
of visual acuity of the poorer eye. In particular, patients who have
comparatively lower visual acuity tend to need relatively larger
spatial frequencies to be present in the input images, else they
cannot achieve stereopsis. Fine stereopsis requires both eyes to
have a good visual acuity in order to detect small spatial
differences, and is easily disrupted by early visual deprivation.
There are indications that in the course of the development of the
visual system in infants, coarse stereopsis may develop before fine
stereopsis and that coarse stereopsis guides the vergence movements
which are needed in order for fine stereopsis to develop in a
subsequent stage. Furthermore, there are indications that coarse
stereopsis is the mechanism that keeps the two eyes aligned after
Static and dynamic stimuli
It has also been suggested to distinguish between two different types
of stereoscopic depth perception: static depth perception (or static
stereo perception) and motion-in-depth perception (or stereo motion
perception). Some individuals who have strabismus and show no depth
perception using static stereotests (in particular, using Titmus
tests, see this article's section on contour stereotests) do perceive
motion in depth when tested using dynamic random dot
stereograms. One study found the combination of motion
stereopsis and no static stereopsis to be present only in exotropes,
not in esotropes.
Research on perception mechanisms
There are strong indications that the stereoscopic mechanism consists
of at least two perceptual mechanisms, possibly three. Coarse
and fine stereopsis are processed by two different physiological
subsystems, with a coarse stereopsis being derived from diplopic
stimuli (that is, stimuli with disparities well beyond the range of
binocular fusion) and yielding only a vague impression of depth
magnitude. Coarse stereopsis appears to be associated with the
magno pathway which processes low spatial frequency disparities and
motion, and fine stereopsis with the parvo pathway which processes
high spatial frequency disparities. The coarse stereoscopic system
seems to be able to provide residual binocular depth information in
some individuals who lack fine stereopsis. Individuals have been
found to integrate the various stimuli, for example stereoscopic cues
and motion occlusion, in different ways.
How the brain combines the different cues – including stereo,
motion, vergence angle and monocular cues – for sensing motion in
depth and 3D object position is an area of active research in vision
science and neighboring disciplines.
Prevalence and impact of stereopsis in humans
Not everyone has the same ability to see using stereopsis. One study
shows that 97.3% are able to distinguish depth at horizontal
disparities of 2.3 minutes of arc or smaller, and at least 80% could
distinguish depth at horizontal differences of 30 seconds of arc.
Stereopsis has a positive impact on exercising practical tasks such as
needle-threading, ball-catching (especially in fast ball games),
pouring liquids, and others. Professional activity may involve
operating stereoscopic instruments such as a binocular microscope.
While some of these tasks may profit from compensation of the visual
system by means of other depth cues, there are some roles for which
stereopsis is imperative. Occupations requiring the precise judgment
of distance sometimes include a requirement to demonstrate some level
of stereopsis; in particular, there is such a requirement for airplane
pilots (even if the first pilot to fly around the world alone, Wiley
Post, accomplished his feat with monocular vision only). Also
surgeons normally demonstrate high stereo acuity. As to car
driving, a study found a positive impact of stereopsis in specific
situations at intermediate distances only; furthermore, a study on
elderly persons found that glare, visual field loss, and useful field
of view were significant predictors of crash involvement, whereas the
elderly persons' values of visual acuity, contrast sensitivity, and
stereoacuity were not associated with crashes.
Binocular vision has further advantages aside from stereopsis, in
particular the enhancement of vision quality through binocular
summation; persons with strabismus (even those who have no double
vision) have lower scores of binocular summation, and this appears to
incite persons with strabismus to close one eye in visually demanding
It has long been recognized that full binocular vision, including
stereopsis, is an important factor in the stabilization of
post-surgical outcome of strabismus corrections. Many persons lacking
stereopsis have (or have had) visible strabismus, which is known to
have a potential socioeconomic impact on children and adults. In
particular, both large-angle and small-angle strabismus can negatively
affect self-esteem, as it interferes with normal eye contact, often
causing embarrassment, anger, and feelings of awkwardness. For
further details on this, see psychosocial effects of strabismus.
It has been noted that with the growing introduction of 3D display
technology in entertainment and in medical and scientific imaging,
high quality binocular vision including stereopsis may become a key
capability for success in modern society.
Nonetheless, there are indications that the lack of stereo vision may
lead persons to compensate by other means: in particular, stereo
blindness may give people an advantage when depicting a scene using
monocular depth cues of all kinds, and among artists there appear to
be a disproportionately high number of persons lacking stereopsis.
In particular, a case has been made that Rembrandt may have been
History of investigations into stereopsis
Stereopsis was first explained by
Charles Wheatstone in 1838: “…
the mind perceives an object of three dimensions by means of the two
dissimilar pictures projected by it on the two retinæ …”. He
recognized that because each eye views the visual world from slightly
different horizontal positions, each eye's image differs from the
other. Objects at different distances from the eyes project images in
the two eyes that differ in their horizontal positions, giving the
depth cue of horizontal disparity, also known as retinal disparity and
as binocular disparity. Wheatstone showed that this was an effective
depth cue by creating the illusion of depth from flat pictures that
differed only in horizontal disparity. To display his pictures
separately to the two eyes, Wheatstone invented the stereoscope.
Leonardo da Vinci
Leonardo da Vinci had also realized that objects at different
distances from the eyes project images in the two eyes that differ in
their horizontal positions, but had concluded only that this made it
impossible for a painter to portray a realistic depiction of the depth
in a scene from a single canvas. Leonardo chose for his near
object a column with a circular cross section and for his far object a
flat wall. Had he chosen any other near object, he might have
discovered horizontal disparity of its features. His column was
one of the few objects that projects identical images of itself in the
Stereoscopy became popular during
Victorian times with the invention
of the prism stereoscope by David Brewster. This, combined with
photography, meant that tens of thousands of stereograms were
Until about the 1960s, research into stereopsis was dedicated to
exploring its limits and its relationship to singleness of vision.
Researchers included Peter Ludvig Panum, Ewald Hering, Adelbert Ames
Jr., and Kenneth N. Ogle.
In the 1960s,
Bela Julesz invented random-dot stereograms. Unlike
previous stereograms, in which each half image showed recognizable
objects, each half image of the first random-dot stereograms showed a
square matrix of about 10,000 small dots, with each dot having a 50%
probability of being black or white. No recognizable objects could be
seen in either half image. The two half images of a random-dot
stereogram were essentially identical, except that one had a square
area of dots shifted horizontally by one or two dot diameters, giving
horizontal disparity. The gap left by the shifting was filled in with
new random dots, hiding the shifted square. Nevertheless, when the two
half images were viewed one to each eye, the square area was almost
immediately visible by being closer or farther than the background.
Julesz whimsically called the square a
Cyclopean image after the
Cyclops who had only one eye. This was because it was as
though we have a cyclopean eye inside our brains that can see
cyclopean stimuli hidden to each of our actual eyes. Random-dot
stereograms highlighted a problem for stereopsis, the correspondence
problem. This is that any dot in one half image can realistically be
paired with many same-coloured dots in the other half image. Our
visual systems clearly solve the correspondence problem, in that we
see the intended depth instead of a fog of false matches. Research
began to understand how.
Also in the 1960s, Horace Barlow, Colin Blakemore, and Jack Pettigrew
found neurons in the cat visual cortex that had their receptive fields
in different horizontal positions in the two eyes. This
established the neural basis for stereopsis. Their findings were
David Hubel and Torsten Wiesel, although they eventually
conceded when they found similar neurons in the monkey visual
cortex. In the 1980s, Gian Poggio and others found neurons in V2
of the monkey brain that responded to the depth of random-dot
In the 1970s,
Christopher Tyler invented autostereograms, random-dot
stereograms that can be viewed without a stereoscope. This led to
Magic Eye pictures.
In 1989 Antonio Medina Puerta demonstrated with photographs that
retinal images with no parallax disparity but with different shadows
are fused stereoscopically, imparting depth perception to the imaged
scene. He named the phenomenon "shadow stereopsis". Shadows are
therefore an important, stereoscopic cue for depth perception. He
showed how effective the phenomenon is by taking two photographs of
the Moon at different times, and therefore with different shadows,
making the Moon to appear in 3D stereoscopically, despite the absence
of any other stereoscopic cue.
Human stereopsis in popular culture
A stereoscope is a device by which each eye can be presented with
different images, allowing stereopsis to be stimulated with two
pictures, one for each eye. This has led to various crazes for
stereopsis, usually prompted by new sorts of stereoscopes. In
Victorian times it was the prism stereoscope (allowing stereo
photographs to be viewed), while in the 1920s it was red-green glasses
(allowing stereo movies to be viewed). In 1939 the concept of the
prism stereoscope was reworked into the technologically more complex
View-Master, which remains in production today. In the 1950s
polarizing glasses allowed stereopsis of coloured movies. In the 1990s
Magic Eye pictures (autostereograms) - which did not require a
stereoscope, but relied on viewers using a form of free fusion so that
each eye views different images - were introduced.
Stereopsis appears to be processed in the visual cortex of mammals in
binocular cells having receptive fields in different horizontal
positions in the two eyes. Such a cell is active only when its
preferred stimulus is in the correct position in the left eye and in
the correct position in the right eye, making it a disparity detector.
When a person stares at an object, the two eyes converge so that the
object appears at the center of the retina in both eyes. Other objects
around the main object appear shifted in relation to the main object.
In the following example, whereas the main object (dolphin) remains in
the center of the two images in the two eyes, the cube is shifted to
the right in the left eye's image and is shifted to the left when in
the right eye's image.
The two eyes converge on the object of attention.
The cube is shifted to the right in left eye's image.
The cube is shifted to the left in the right eye's image.
We see a single, Cyclopean, image from the two eyes' images.
The brain gives each point in the
Cyclopean image a depth value,
represented here by a grayscale depth map.
Because each eye is in a different horizontal position, each has a
slightly different perspective on a scene yielding different retinal
images. Normally two images are not observed, but rather a single view
of the scene, a phenomenon known as singleness of vision.
Nevertheless, stereopsis is possible with double vision. This form of
stereopsis was called qualitative stereopsis by Kenneth Ogle.
If the images are very different (such as by going cross-eyed, or by
presenting different images in a stereoscope) then one image at a time
may be seen, a phenomenon known as binocular rivalry.
There is a hysteresis effect associated with stereopsis. Once
fusion and stereopsis have stabilized, fusion and stereopsis can be
maintained even if the two images are pulled apart slowly and
symmetrically to a certain extent in the horizontal direction. In the
vertical direction, there is a similar but smaller effect. This
effect, first demonstrated on a random dot stereogram, was initially
interpreted as an extension of Panum's fusional area. Later it was
shown that the hysteresis effect reaches far beyond Panum's fusional
area, and that stereoscopic depth can be perceived in random-line
stereograms despite the presence of cyclodisparities of about 15 deg,
and this has been interpreted as stereopsis with diplopia.
Computer stereo vision
Main article: Computer stereo vision
Computer stereo vision
Computer stereo vision is a part of the field of computer vision. It
is sometimes used in mobile robotics to detect obstacles. Example
applications include the
ExoMars Rover and surgical robotics.
Two cameras take pictures of the same scene, but they are separated by
a distance – exactly like our eyes. A computer compares the images
while shifting the two images together over top of each other to find
the parts that match. The shifted amount is called the disparity. The
disparity at which objects in the image best match is used by the
computer to calculate their distance.
For a human, the eyes change their angle according to the distance to
the observed object. To a computer this represents significant extra
complexity in the geometrical calculations (epipolar geometry). In
fact the simplest geometrical case is when the camera image planes are
on the same plane. The images may alternatively be converted by
reprojection through a linear transformation to be on the same image
plane. This is called image rectification.
Computer stereo vision
Computer stereo vision with many cameras under fixed lighting is
called structure from motion. Techniques using a fixed camera and
known lighting are called photometric stereo techniques, or "shape
Computer stereo display
Many attempts have been made to reproduce human stereo vision on
rapidly changing computer displays, and toward this end numerous
patents relating to
3D television and cinema have been filed in the
USPTO. At least in the US, commercial activity involving those patents
has been confined exclusively to the grantees and licensees of the
patent holders, whose interests tend to last for twenty years from the
time of filing.
3D television and cinema (which generally require more
than one digital projector whose moving images are mechanically
coupled, in the case of IMAX 3D cinema), several stereoscopic LCDs are
going to be offered by Sharp, which has already started shipping a
notebook with a built in stereoscopic LCD. Although older technology
required the user to don goggles or visors for viewing
computer-generated images, or CGI, newer technology tends to employ
Fresnel lenses or plates over the liquid crystal displays, freeing the
user from the need to put on special glasses or goggles.
In stereopsis tests (short: stereotests), slightly different images
are shown to each eye, such that a 3D image is perceived in case
stereovision is present. This can be achieved by means of vectographs
(visible with polarized glasses), anaglyphs (visible with red-green
glasses), lenticular lenses (visible with the naked eye), or
head-mounted display technology. The type of changes from one eye to
the other may differ depending on which level of stereoacuity is to be
detected. A series of stereotests for selected levels thus constitutes
a test of stereoacuity.
There are two types of common clinical tests for stereopsis and
stereoacuity: random dot stereotests and contour stereotests.
Random-dot stereopsis tests use pictures of stereo figures that are
embedded in a background of random dots. Contour stereotests use
pictures in which the targets presented to each eye are separated
Random dot stereotests
Random dot stereogram
Random dot stereogram § Random dot stereotests
The ability of stereopsis can be tested by, for example, the Lang
stereotest, which consists of a random-dot stereogram upon which a
series of parallel strips of cylindrical lenses are imprinted in
certain shapes, which separate the views seen by each eye in these
areas, similarly to a hologram. Without stereopsis, the image
looks only like a field of random dots, but the shapes become
discernible with increasing stereopsis, and generally consists of a
cat (indicating that there is ability of stereopsis of 1200 seconds of
arc of retinal disparity), a star (600 seconds of arc) and a car (550
seconds of arc). To standardize the results, the image should be
viewed at a distance from the eye of 40 cm and exactly in the
frontoparallel plane. There is no need to use special spectacles
for such tests, thereby facilitating use in young children.
Examples of contour stereotests are the Titmus stereotests, the most
well-known example being the Titmus Fly Stereotest, where a picture of
a fly is displayed with disparities on the edges. The patient uses a
3-D glasses to look at the picture and determine whether a 3-D figure
can be seen. The amount of disparity in images vary, such as 400-100
sec of arc, and 800-40 sec arc.
Deficiency and treatment
Deficiency in stereopsis can be complete (then called stereoblindness)
or more or less impaired. Causes include blindness in one eye,
amblyopia and strabismus.
Vision therapy is one of the treatments for people lacking in
Vision therapy will allow individuals to enhance their
vision through several exercises such as by strengthening and
improving eye movement. There is recent evidence that stereoacuity
may be improved in persons with amblyopia by means of perceptual
learning (see also: treatment of amblyopia).
In non-human animals
There is good evidence for stereopsis throughout the animals kingdom.
It occurs in many mammals, birds, reptiles, amphibia, fish,
crustaceans, spiders, and insects.
Computer stereo vision
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Middlebury Stereo Vision Page
VIP Laparoscopic / Endoscopic Video Dataset (stereo medical images)
What is Stereo Vision?
Learn about Stereograms then make your own Magic Eye
International Orthoptic Association
Kinetic depth effect
Active shutter 3D system
Polarized 3D system
Virtual retinal display
2D to 3D conversion
2D plus Delta
Computer stereo vision
Multiview Video Coding
Stereo photography techniques
Stereoscopic depth rendition
Stereoscopic Video Coding
3D-enabled mobile phones
Stereoscopic video game
Virtual reality headset
Fujifilm FinePix Real 3D
Nvidia 3D Vision
Sharp Actius RD3D
International Stereoscopic Union