Binocular Disparity

Binocular disparity refers to the difference in image location of similar features seen by the left and right eyes resulting from the eyes' horizontal separation (parallax).

In visual perception, binocular disparity refers to edges and small blobs with equal contrast sign in the retinal images. The mind extracts binocular disparity for these edges and blobs and then fills in the depth and forms of surfaces, resulting in stereopsis.

Related terms

There exists also vertical disparities which result from height level differences and which can also invoke a depth sensation.Matthews N;Meng X.; Xu P; Qian Q.(2003) “A physiological theory of depth perception from vertical disparity”, Vision Research. Volume 43, Issue 1, January 2003, Pages 85-99.

In stereoscopy and computer vision, binocular disparity refers to the difference in coordinates of similar features within two stereo images.

A similar disparity can be used in rangefinding by a coincidence rangefinder to determine distance and/or altitude to a target. In astronomy, the disparity between different locations on the Earth can be used to determine various celestial parallax, and Earth's orbit can be used for stellar parallax.

Geometric terms

In the following situation, the left eye (LE) and the right eye (RE) see the same two objects X and Y. In that case the following definitions apply:Koenderink J.J.;van Doorn A.J. (1976) “Geometry of binocular vision and a model of stereopsis.”, Biol. Cybern. 21, 29-35.

* the egocentric distance of object X: the metric distance from the observer to X. In the figure: Dx.

* the depth between two objects X and Y: the difference of the egocentric distances to X and Y. In the figure: dXY.
* angle αXY: the direction of X relative to Y in the left eye.

* angle βXY: the direction of X relative to Y in the right eye.

* binocular disparity: the difference between angle αXY and angle βXY. In the figure: δXY. If the eyes fixate X or Y, dispariy is defined releative to the horopter.
* horopter: points with a disparity = 0 relative to the fixation point. These points lie on a circle through the fixation point and both eyes (Vieth Műller circle).

Perceived depth as function of disparity

The quality and quantity of depth experienced from disparity depends on a number of factors, see stereopsis. When moving around in space, disparity δXY varies with egocentric distance but perceived depth dXY appears invariant. This is caused by eye vergence; the visual system uses egocentric distance, as measered by eye vergence, to scale the perceived depth from disparity. This phenomenon is used in stereophotograpy to increase the experienced depth effect: the pictures for the two eys are presented such that the viewer is forced to converge further away then the distance at which the initial scene was photographed.

Illusory disparities

Two identical objects behind each other have the same retinal images as two similar objects next to each other. At a small distance between A and B the brain chooses to see option C,D. This results in an illusion if the real objects are present at positions A,B and not at C,D ( double-nail illusion).

Disparity calculation in computer vision

Human eyes are horizontally separated by about 50–75 mm (interpupillary distance) depending on each individual. Thus, each eye has a slightly different view of the world around. This can be easily seen when alternately closing one eye while looking at a vertical edge. The binocular disparity can be observed from apparent horizontal shift of the vertical edge between both views.

At any given moment, the line of sight of the two eyes meet at a point in space. This point in space projects to the same location (i.e. the center) on the retinae of the two eyes. Because of the different viewpoints observed by the left and right eye however, many other points in space do not fall on corresponding retinal locations. Visual binocular disparity is defined as the difference between the point of projection in the two eyes and is usually expressed in degrees as the visual angle.Qian, N.
Binocular Disparity and the Perception of Depth
Neuron, 18, 359–368, 1997.

The term "binocular disparity" refers to geometric measurements made external to the eye. The disparity of the images on the actual retina depends on factors internal to the eye, especially the location of the nodal points, even if the cross section of the retina is a perfect circle. Disparity on retina conforms to binocular disparity when measured as degrees, while much different if measured as distance due to the complicated structure inside eye.

Figure 1: The full black circle is the point of fixation. The blue object lies closer to the observer. Therefore, it has a "near" disparity ''d_n''. Objects lying more far away (green) correspondingly have a "far" disparity ''d_f''. Binocular disparity is the angle between two lines of projection. One of which is the real projection from the object to the actual point of projection. The other one is the imaginary projection running through the nodal point of the fixation point.

In computer vision, binocular disparity is calculated from stereo images taken from a set of stereo cameras. The variable distance between these cameras, called the baseline, can affect the disparity of a specific point on their respective image plane. As the baseline increases, the disparity increases due to the greater angle needed to align the sight on the point. However, in computer vision, binocular disparity is referenced as coordinate differences of the point between the right and left images instead of a visual angle. The units are usually measured in pixels.

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* Sum of absolute differences: $\sum$

The disparity with the lowest computed value using one of the above methods is considered the disparity for the image feature. This lowest score indicates that the algorithm has found the best match of corresponding features in both images.

The method described above is a brute-force search algorithm. With large patch and/or image sizes, this technique can be very time consuming as pixels are constantly being re-examined to find the lowest correlation score. However, this technique also involves unnecessary repetition as many pixels overlap. A more efficient algorithm involves remembering all values from the previous pixel. An even more efficient algorithm involves remembering column sums from the previous row (in addition to remembering all values from the previous pixel). Techniques that save previous information can greatly increase the algorithmic efficiency of this image analyzing process.

Uses of disparity from images

Knowledge of disparity can be used in further extraction of information from stereo images. One case that disparity is most useful is for depth/distance calculation. Disparity and distance from the cameras are inversely related. As the distance from the cameras increases, the disparity decreases. This allows for depth perception in stereo images. Using geometry and algebra, the points that appear in the 2D stereo images can be mapped as coordinates in 3D space.

This concept is particularly useful for navigation. For example, the Mars Exploration Rover uses a similar method for scanning the terrain for obstacles. The rover captures a pair of images with its stereoscopic navigation cameras and disparity calculations are performed in order to detect elevated objects (such as boulders)."Spacecraft: Surface Operations: Rover ." JPL.NASA.GOV. JPL/NASA, n.d. Web. 5 Jun 2011

Additionally, location and speed data can be extracted from subsequent stereo images by measuring the displacement of objects relative to the rover. In some cases, this is the best source of this type of information as the encoder sensors in the wheels may be inaccurate due to tire slippage.

See also

* Binocular vision
* Cyclodisparity
* Epipolar geometry

References

{{DEFAULTSORT:Binocular Disparity
Psychology
Perception
Vision
Computer vision
Parallax

lb:Parallax
ja:両眼視差