Stereoscopy, also called stereoscopics or stereo imaging, refers to making images appear three-dimensional. The most popular kind of stereoscopy is two-view stereoscopy, which causes partial stereopsis in an image from a set of two two-dimensional images by using binocular disparity. The word stereoscopy derives .στερεός Tufts.edu, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Libraryσκοπέω, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library A stereoscopic image is called a stereogram. Originally, stereogram referred to a pair of two-dimensional images that could be viewed using a stereoscope.
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.
Background Stereoscopy creates the impression of three-dimensional depth. The most popular type of stereoscopy, two-view stereoscopy, creates the impression from a pair of two-dimensional images.Exercises in Three Dimensions: About 3D, Tom Lincoln, 2011 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 happens in the brain, as it strives to make sense of the raw information. One of the functions that occur within the brain as it interprets what the eyes see is assessing the relative distances of objects from the viewer, and the depth dimension of those objects. The cues that the brain uses to gauge relative distances and depth in a perceived scene include:Flight Simulation, J. M. Rolfe and K. J. Staples, Cambridge University Press, 1986, page 134 Binocular disparity - The differences in images in each eye caused by the horizontal distance between the two eyes Vergence Accommodation 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
(All but the first three of the above cues exist in traditional two-dimensional images, such as paintings, photographs, and television.)Exercises in Three Dimensions, Tom Lincoln, 2011
Two-view stereoscopy is the production of the impression of depth in a photograph, movie, or other image by the presentation of a slightly different image to each eye, also known as stereopsis from binocular disparity. The two images are combined in the brain to give the perception of stereopsis. However, because all points in the image produced by stereoscopy from binocular disparity focus at the same plane regardless of their depth in the original scene, the third cue, focus, also known as accommodation, is not replicated and therefore the appearance of depth is incomplete.
There are two main elements of stereopsis from binocular disparity that are unnatural for human vision: (1) the mismatch between convergence and accommodation, caused by the difference between objects' perceived position in front of or behind the display or screen and the real origin of that light; and (2) possible crosstalk between the eyes, caused by imperfect image separation in some methods of binocular fixed-focus stereoscopy. The presentation of dual 2D images is distinctly different from displaying an image in three full dimensions by using a volumetric display, or recreating a light field.
Most 3D displays use the two-view stereoscopic method to convey images. The method was first invented by Sir Charles Wheatstone in 1838,Contributions to the Physiology of Vision.—Part the First. On some remarkable, and hitherto unobserved, Phenomena of Binocular Vision. By CHARLES WHEATSTONE, F.R.S., Professor of Experimental Philosophy in King's College, London. Stereoscopy.comWelling, William. Photography in America, page 23 and improved by Sir David Brewster who made the first portable 3D viewing device.International Stereoscopic Union, 2006, "Stereoscopy", Numbers 65–72, p.18
Wheatstone originally used his stereoscope (a rather bulky device)Stereo Realist Manual, p. 375. with drawings because photography was not yet available, yet his original paper seems to foresee the development of a realistic imaging method:Stereo Realist Manual, pp. 377–379.
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.
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. Modern industrial three-dimensional photography may use 3D scanners to detect and record three-dimensional information.Fay Huang, Reinhard Klette, and Karsten Scheibe: Panoramic Imaging (Sensor-Line Cameras and Laser Range-Finders). Wiley & Sons, Chichester, 2008 The three-dimensional depth information can be reconstructed from two images using a computer by correlating the pixels in the left and right images. Solving the Correspondence problem in the field of Computer Vision aims to create meaningful depth information from two images.
Visual requirements There are 3 components of binocular vision involved in viewing two-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. 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.
Stereoscopic viewing may be artificially created by the viewer's brain, as demonstrated with the Van Hare Effect, where the brain perceives stereo images even when the paired photographs are identical. This "false dimensionality" results from the developed stereoacuity in the brain, allowing the viewer to fill in depth information even when few if any 3D cues are actually available in the paired images.
Side-by-side
Traditional stereoscopic photography consists of creating a binocular stereogram, which can show a 3D effect starting from a pair of 2D images. The easiest way to trigger stereopsis is to provide the eyes of the viewer with two different images, representing two perspectives of the same object, with a minor deviation equal or nearly equal to the perspectives that both eyes naturally receive in binocular vision. This is known as binocular disparity.
To avoid eyestrain and distortion, each of the two 2D images should be presented to the viewer so that any object at infinite distance is perceived by the eye as being straight ahead, the viewer's eyes being neither crossed nor diverging. When the picture contains no object at infinite distance, such as a horizon or a cloud, the pictures should be spaced correspondingly closer together.
The advantages of side-by-side viewers is the lack of diminution of brightness, allowing the presentation of images at very high resolution and in full spectrum color, simplicity in creation, and little or no additional image processing is required. Under some circumstances, such as when a pair of images is presented for freeviewing, no device or additional optical equipment is needed.
The principal disadvantage of side-by-side viewers is that large image displays are not practical and resolution is limited by the lesser of the display medium or human eye. This is because as the dimensions of an image are increased, either the viewing apparatus or viewer themselves must move proportionately further away from it in order to view it comfortably. Moving closer to an image in order to see more detail would only be possible with viewing equipment that adjusted to the difference.
Freeviewing Freeviewing is viewing a side-by-side image pair without using a viewing device.The Logical Approach to Seeing 3D Pictures. www.vision3d.com by Optometrists Network. Retrieved 2009-08-21
Two methods are available to freeview: parallel viewing and cross-eyed viewing.How To Freeview Stereo (3D) Images. Greg Erker. Retrieved 2009-08-21How to View Photos on This Site. Stereo Photography – The World in 3D. Retrieved 2009-08-21 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.