Stereoscopic depth rendition
Stereoscopic depth rendition specifies how the depth of a three-dimensional object is encoded in a stereoscopic reconstruction. It needs attention to ensure a realistic depiction of the three-dimensionality of viewed scenes and is a specific instance of the more general task of 3D rendering of objects in two-dimensional displays. A stereogram consists of a pair of one for each eye. Common to both are the heights of objects; the geometric relationship between an object's third dimension and these position differences is presented below and depends on the location of the stereo-camera lenses and the observer's eyes. Other factors, contribute to the depth seen in a stereoscopic view and whether it corresponds to that in the actual object; the right and left eyes' panels in a stereoscopic reconstruction are created by projection from the principal points of the twin recording camera. The geometrical situation is most understood by analyzing how the screens are generated when a small cubical element of side length dx = dy = dz is photographed from a distance z with a twin camera whose lenses are a distance a apart.
In the left eye panel of the stereogram the distance AB is the representation of the front face of the cube, in the right eye panel, there is in addition BC, the representation of the cube's depth, i.e. the intercept on the screen of the rays from the cameras’ principal points to the back of the cube. This interval computes to the first order to dz×a/z. Hence the depth/width ratio of the cube’s view, as embodied in its representation on the viewing screen, is r = a×dz/z×dx = a/z since dx=dz and depends on the distance of the target from the twin lenses and their separation and remains constant with scale or magnification changes; the depth/width ratio of the actual object, of course, is 1.00. This stereogram with the cube, whose depth/width ratio had been captured with recording parameters ac and zc and embodied in the ratio BC/AB = rc=ac/zc, is now viewed by an observer with interocular separation ao at a distance zo. An overall scale change in BC/AB does not matter, but unless ro = rc, i.e. ao/zo = ac/zc. this no longer represents a cube but rather becomes, for this observer at this distance, a configuration for which R = rc/ro...... i.e. whose depth is R times that of a cube.
The stereoscopic depth rendition r is a measure of the flattening or expansion in depth for a display situation and is equal to the ratio of the angles of depth and width subtended at the eye in the stereogram reconstruction of a small cubical element. A value r > 1 says. A numerical example will illustrate: a structure is photographed by a stereocamera with interlens separation ac = 25 cm from a distance of 1 m, zc = 100. Hence rc = ac/zc = 0.25 and on the screens the right and left representation of the cube's far edge will be separated by ¼ the distance of the width. This stereogram is now viewed from a distance of 39 cm by an observer with interocular distance 6.5 cm, i.e. ro = 6.5/39 = 0.167. According to equation for this view the structure has a stereoscopic depth rendition given by R = rc/ro = 0.25/0.167 = 1.5, meaning that the observer is presented with the geometrical situation not of a cube but of a structure 1.5× as deep as it is wide. For this to become a cube ro needs to be 0.25 which occurs for an observation distance zo = 6.5/0.25 = 26 cm.
This example illustrates that a given stereoscopic presentation for a given observer gains in depth/width ratio with increasing observation distance. Observers, who can fuse the twin images of the rings by voluntarily changing their convergence, can verify this by moving away and towards the viewing screen. Only when the recording and viewing situations have the same r value, i.e. only when ac/zc = ao/zo will the depth/width ratios of the actual structure and its view be identical. This particular condition has been termed homeomorphic by Moritz von Rohr and was contrasted by him with the heteromorphic one in which the r values of the stereoscopic and actual views differ, but homeomorphic rendition with geometrical parameters identical to the original does not assure that an observer's perception of depth in a stereoscopic image is the same as that in the actual three-dimensional structure. An observer judgment of the apparent disposition of objects in space depends on many factors other than the geometrical ones that pertain to the angles subtended by the components at the two eyes.
This was well described in the classical study by Wallach and Zuckerman who pointed out that the depth in the view through binoculars seems foreshortened. Scenes appear flattened through field glasses non-prismatic ones without artificial extension of the base, which provide overall magnification and leave the r value unchanged. In contrast to the rules, laid out above, for calculating the geometrically defined stereoscopic depth rendition, the perceived depth involves factors — context, previous experience — that are individual and not specifiable with the same degree of generality. Chief among them is the distance; this is by no means fixed: the subjective z is only vaguely related to the actual object distance, as is obvious in watching 3D film. Because apparent distance is the main source of judging object size (size or subjective constanc
A head-mounted display, both abbreviated HMD, is a display device, worn on the head or as part of a helmet, that has a small display optic in front of one or each eye. A HMD has many uses, including in gaming, aviation and medicine lift. A head-mounted display is the primary component of virtual reality headsets. There is an optical head-mounted display, a wearable display that can reflect projected images and allows a user to see through it. A typical HMD has one or two small displays, with lenses and semi-transparent mirrors embedded in eyeglasses, a visor, or a helmet; the display units are miniaturised and may include cathode ray tubes, liquid crystal displays, liquid crystal on silicon, or organic light-emitting diodes. Some vendors employ multiple micro-displays to increase total field of view. HMDs differ in whether they can display only computer-generated imagery, or only live imagery from the physical world, or combination. Most HMDs can display only a computer-generated image, sometimes referred to as virtual image.
Some HMDs can allow a CGI to be superimposed on real-world view. This is sometimes referred to mixed reality. Combining real-world view with CGI can be done by projecting the CGI through a reflective mirror and viewing the real world directly; this method is called optical see-through. Combining real-world view with CGI can be done electronically by accepting video from a camera and mixing it electronically with CGI; this method is called video see-through. An optical head-mounted display uses an optical mixer, made of silvered mirrors, it can reflect artificial images, let real images cross the lens, let a user look through it. Various methods have existed for see-through HMD's, most of which can be summarized into two main families based on curved mirrors or waveguides. Curved mirrors have been used by Laster Technologies, by Vuzix in their Star 1200 product. Various waveguide methods have existed for years; these include diffraction optics, holographic optics, polarized optics, reflective optics.
Augmented reality systems guru Karl Guttag compared the optics of diffractive waveguides against the competing technology, reflective waveguides. Major HMD applications include military and civilian-commercial. In 1962, Hughes Aircraft Company revealed the Electrocular, a compact CRT, head-mounted monocular display that reflected a TV signal in to transparent eyepiece. Ruggedized HMDs are being integrated into the cockpits of modern helicopters and fighter aircraft; these are fully integrated with the pilot's flying helmet and may include protective visors, night vision devices, displays of other symbology. Military and firefighters use HMDs to display tactical information such as maps or thermal imaging data while viewing a real scene. Recent applications have included the use of HMD for paratroopers. In 2005, the Liteye HMD was introduced for ground combat troops as a rugged, waterproof lightweight display that clips into a standard US PVS-14 military helmet mount; the self-contained color monocular organic light-emitting diode display replaces the NVG tube and connects to a mobile computing device.
The LE has see-through ability and can be used as a standard HMD or for augmented reality applications. The design is optimized to provide high definition data under all lighting conditions, in covered or see-through modes of operation; the LE has a low power consumption, operating on four AA batteries for 35 hours or receiving power via standard Universal Serial Bus connection. The Defense Advanced Research Projects Agency continues to fund research in augmented reality HMDs as part of the Persistent Close Air Support Program. Vuzix is working on a system for PCAS that will use holographic waveguides to produce see-through augmented reality glasses that are only a few millimeters thick. Engineers and scientists use HMDs to provide stereoscopic views of computer-aided design schematics. Virtual reality, when applied to engineering and design, is a key factor in integration of the human in the design. By enabling engineers to interact with their designs in full life-size scale, products can be validated for issues that may not have been visible until physical prototyping.
The use of HMDs for VR is seen as supplemental to the conventional use of CAVE for VR simulation. HMDs are predominantly used for single-person interaction with the design, while CAVEs allow for more collaborative virtual reality sessions. Head Mounted Display systems are used in the maintenance of complex systems, as they can give a technician a simulated x-ray vision by combining computer graphics such as system diagrams and imagery with the technician's natural vision. There are applications in surgery, wherein a combination of radiographic data is combined with the surgeon's natural view of the operation, anesthesia, where the patient vital signs are within the anesthesiologist's field of view at all times. Research universities use HMDs to conduct studies related to vision, balance and neuroscience; as of 2010, the use of predictive visual tracking measurement to identify mild traumatic brain injury was being studied. In visual tracking tests, a HMD unit with eye tracking ability shows an object moving in a regular pattern.
People without brain injury are able to track the moving object with smooth pursuit eye movements and corr
A stereo display is a display device capable of conveying depth perception to the viewer by means of stereopsis for binocular vision. The basic technique of stereo displays is to present offset images that are displayed separately to the left and right eye. Both of these 2D offset images are combined in the brain to give the perception of 3D depth. Although the term "3D" is ubiquitously used, it is important to note that the presentation of dual 2D images is distinctly different from displaying an image in three full dimensions; the most notable difference to real 3D displays is that the observer's head and eyes movements will not increase information about the 3-dimensional objects being displayed. For example, holographic displays do not have such limitations. Similar to how in sound reproduction it is not possible to recreate a full 3-dimensional sound field with two stereophonic speakers, it is an overstatement of capability to refer to dual 2D images as being "3D"; the accurate term "stereoscopic" is more cumbersome than the common misnomer "3D", entrenched after many decades of unquestioned misuse.
It is to note that although most stereoscopic displays do not qualify as real 3D display, all real 3D display are stereoscopic displays because they meet the lower criteria as well. Based on the principles of stereopsis, described by Sir Charles Wheatstone in the 1830s, stereoscopic technology provides a different image to the viewer's left and right eyes; the following are some of the technical details and methodologies employed in some of the more notable stereoscopic systems that have been developed. Traditional stereoscopic photography consists of creating a 3D illusion starting from a pair of 2D images, a stereogram; the easiest way to enhance depth perception in the brain is to provide the eyes of the viewer with two different images, representing two perspectives of the same object, with a minor deviation equal to the perspectives that both eyes receive in binocular vision. If eyestrain and distortion are to be avoided, each of the two 2D images preferably should be presented to each eye of the viewer so that any object at infinite distance seen by the viewer should be perceived by that eye while it is oriented 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 side-by-side method is simple to create, but it can be difficult or uncomfortable to view without optical aids. A stereoscope is a device for viewing stereographic cards, which are cards that contain two separate images that are printed side by side to create the illusion of a three-dimensional image. Pairs of stereo views printed on a transparent base are viewed by transmitted light. One advantage of transparency viewing is the opportunity for a wider, more realistic dynamic range than is practical with prints on an opaque base; the practice of viewing film-based stereoscopic transparencies dates to at least as early as 1931, when Tru-Vue began to market sets of stereo views on strips of 35 mm film that were fed through a hand-held Bakelite viewer. In 1939, a modified and miniaturized variation of this technology, employing cardboard disks containing seven pairs of small Kodachrome color film transparencies, was introduced as the View-Master.
The user wears a helmet or glasses with two small LCD or OLED displays with magnifying lenses, one for each eye. The technology can be used to show stereo images or games. Head-mounted displays may be coupled with head-tracking devices, allowing the user to "look around" the virtual world by moving their head, eliminating the need for a separate controller. Owing to rapid advancements in computer graphics and the continuing miniaturization of video and other equipment these devices are beginning to become available at more reasonable cost. Head-mounted or wearable glasses may be used to view a see-through image imposed upon the real world view, creating what is called augmented reality; this is done by reflecting the video images through reflective mirrors. The real world can be seen through the partial mirror. A recent development in holographic-waveguide or "waveguide-based optics" allows a stereoscopic images to be superimposed on real world without the uses of bulky reflective mirror. Head-mounted projection displays is similar to head-mounted displays but with images projected to and displayed on a retroreflective screen, The advantage of this technology over head-mounted display is that the focusing and vergence issues didn't require fixing with corrective eye lenses.
For image generation, Pico-projectors is used instead of OLED screen. In an anaglyph, the two images are superimposed in an additive light setting through two filters, one red and one cyan. In a subtractive light setting, the two images are printed in the same complementary colors on white paper. Glasses with colored filters in each eye separate the appropriate image by canceling the filter color out and rendering the complementary color black. A compensating technique known as Anachrome, uses a more transparent cyan filter in the patented glasses associated with the technique. Process reconfigures the typical anaglyph image to have less parallax. An alternative to the usual red and cyan filter system of anaglyph is ColorCode 3-D, a patented anaglyph system, invented in order to present an anaglyph image in conjunction with the NTSC television standard, in which the red channel is compromised. ColorCode uses the complementary color
Stereoscopic acuity stereoacuity, is the smallest detectable depth difference that can be seen in binocular vision. Stereoacuity is most explained by considering one of its earliest test, a two-peg device, named Howard-Dolman test after its inventors: The observer is shown a black peg at a distance of 6m. A second peg, below it, can be moved back and forth until it is just detectably nearer than the fixed one. Stereoacuity is this difference in the two positions, converted into an angle of binocular disparity, i.e. the difference in their binocular parallax. Conversion to the angle of disparity dγ is performed by inserting the position difference dz in the formula d γ = c a d z / z 2 where a is the interocular separation of the observer and z the distance of the fixed peg from the eye. To transfer dγ into the usual unit of minutes of arc, a multiplicative constant c is inserted whose value is 3437.75. In the calculation a, dz and z must be in the same units, feet, inches, cm or meters. For the average interocular distance of 6.5 cm, a target distance of 6m and a typical stereoacuity of 0.5 minute of arc, the just detectable depth interval is 8 cm.
As targets come closer, this interval gets smaller by the inverse square of the distance, so that an equivalent detectable depth interval at ¼ meter is 0.01 cm or the depth of impression of the head on a coin. These small values of normal stereoacuity, expressed in differences of either object distances, or angle of disparity, makes it a hyperacuity. Since the Howard-Dolman test described above is cumbersome, stereoacuity is measured using a stereogram in which separate panels are shown to each eye by superimposing them in a stereoscope using prisms or goggles with color or polarizing filters or alternating occlusion. A good procedure is a chart, analogous to the familiar Snellen visual acuity chart, in which one letter in each row differs in depth sequentially increasing in difficulty. For children the fly test is ideal: the image of a fly is transilluminated by polarized light. There is no equivalent in stereoacuity of the normal 20/20 visual acuity standard. In every case, the numerical score if expressed in disparity angle, depends to some extent on the test being used.
Superior observers under ideal conditions can achieve 0.1 arc min or better. The distinction between screening for the presence of stereopsis and a measurement of stereoacuity is valuable. To ascertain that depth can be seen in a binocular views, a test must be administered and not subject to deception; the random-dot stereogram is used for this purpose and has the advantage that for the uninitiated the object shape is unknown. It is made of random small pattern elements. A population study revealed a high incidence of good stereoacuity. Out of 188 biology students, 97.3 % could perform at 2.3 minutes of better. Optimum stereoacuity requires that the following mitigating factors be avoided: Low contrast Short duration exposures Fuzzy or spaced pattern elements. Uncorrected or unequally corrected refractive errors More than other such visual capabilities, the limits of stereopsis depend on the observer's familiarity with the situation. Stereo thresholds always improve several-fold, with training and involve perceptual factors, differing in their particulars for each test.
This is most vividly evident in the time it takes to "solve" a random-dot stereogram decreases between the first exposure and subsequent views Computer vision Visual acuity Review of 3D displays and stereo vision
A hologram is an image that appears to be three dimensional and which can be seen with the naked eye. Holography is the practice of making holograms. A hologram is a photographic recording of a light field, rather than an image formed by a lens; the holographic medium, i.e. the object produced by a holographic process is unintelligible when viewed under diffuse ambient light. It is an encoding of the light field as an interference pattern of variations in the opacity, density, or surface profile of the photographic medium; when suitably lit, the interference pattern diffracts the light into an accurate reproduction of the original light field, the objects that were in it exhibit visual depth cues such as parallax and perspective that change realistically with the relative position of the observer. That is, the view of the image from different angles represents the subject viewed from similar angles. In its pure form, holography requires the use of laser light for illuminating the subject and for viewing the finished hologram.
A microscopic level of detail throughout the recorded scene can be reproduced. In common practice, major image quality compromises are made to eliminate the need for laser illumination to view the hologram, in some cases, to make it. Holographic portraiture resorts to a non-holographic intermediate imaging procedure, to avoid the hazardous high-powered pulsed lasers otherwise needed to optically "freeze" moving subjects as as the motion-intolerant holographic recording process requires. Holograms can now be computer-generated to show objects or scenes that never existed. Holography is distinct from lenticular and other earlier autostereoscopic 3D display technologies, which can produce superficially similar results but are based on conventional lens imaging. Images requiring the aid of special glasses or other intermediate optics, stage illusions such as Pepper's Ghost and other unusual, baffling, or magical images are incorrectly called holograms; the Hungarian-British physicist Dennis Gabor was awarded the Nobel Prize in Physics in 1971 "for his invention and development of the holographic method".
His work, done in the late 1940s, was built on pioneering work in the field of X-ray microscopy by other scientists including Mieczysław Wolfke in 1920 and William Lawrence Bragg in 1939. The discovery was an unexpected result of research into improving electron microscopes at the British Thomson-Houston Company in Rugby and the company filed a patent in December 1947; the technique as invented is still used in electron microscopy, where it is known as electron holography, but optical holography did not advance until the development of the laser in 1960. The word holography comes from the Greek words ὅλος and γραφή; the development of the laser enabled the first practical optical holograms that recorded 3D objects to be made in 1962 by Yuri Denisyuk in the Soviet Union and by Emmett Leith and Juris Upatnieks at the University of Michigan, USA. Early holograms used silver halide photographic emulsions as the recording medium, they were not efficient as the produced grating absorbed much of the incident light.
Various methods of converting the variation in transmission to a variation in refractive index were developed which enabled much more efficient holograms to be produced. Several types of holograms can be made. Transmission holograms, such as those produced by Leith and Upatnieks, are viewed by shining laser light through them and looking at the reconstructed image from the side of the hologram opposite the source. A refinement, the "rainbow transmission" hologram, allows more convenient illumination by white light rather than by lasers. Rainbow holograms are used for security and authentication, for example, on credit cards and product packaging. Another kind of common hologram, the reflection or Denisyuk hologram, can be viewed using a white-light illumination source on the same side of the hologram as the viewer and is the type of hologram seen in holographic displays, they are capable of multicolour-image reproduction. Specular holography is a related technique for making three-dimensional images by controlling the motion of specularities on a two-dimensional surface.
It works by reflectively or refractively manipulating bundles of light rays, whereas Gabor-style holography works by diffractively reconstructing wavefronts. Most holograms produced are of static objects but systems for displaying changing scenes on a holographic volumetric display are now being developed. Holograms can be used to store and process information optically. In its early days, holography required high-power expensive lasers, but nowadays, mass-produced low-cost semi-conductor or diode lasers, such as those found in millions of DVD recorders and used in other common applications, can be used to make holograms and have made holography much more accessible to low-budget researchers and dedicated hobbyists, it was thought that it would be possible to use X-rays to make holograms of small objects and view them using visible light. Today, holograms with x-rays are generated by using synchrotrons or x-ray free-electron lasers as radiation sources and pixelated detectors such as CCDs as recording medium.
The reconstruction is retrieved via computation. Due to the shorter wavelength of x-rays compared to visible light, this approach allows imaging objects with higher spatial resolution; as free-electron lasers can provide ultrashort and x-ray pulses in the range of femtoseconds which are intense and coherent, x-ray holography
Depth perception is the visual ability to perceive the world in three dimensions and the distance of an object. Depth sensation is the corresponding term for animals, since although it is known that animals can sense the distance of an object, it is not known whether they "perceive" it in the same subjective way that humans do. Depth perception arises from a variety of depth cues; these are classified into binocular cues that are based on the receipt of sensory information in three dimensions from both eyes and monocular cues that can be represented in just two dimensions and observed with just one eye. Binocular cues include stereopsis, eye convergence and yielding depth from binocular vision through exploitation of parallax. Monocular cues include size: distant objects subtend smaller visual angles than near objects, grain and motion parallax. Monocular cues provide depth information. Motion parallax When an observer moves, the apparent relative motion of several stationary objects against a background gives hints about their relative distance.
If information about the direction and velocity of movement is known, motion parallax can provide absolute depth information. This effect can be seen when driving in a car. Nearby things pass while far off objects appear stationary; some animals that lack binocular vision due to their eyes having little common field-of-view employ motion parallax more explicitly than humans for depth cueing. Depth from motion When an object moves toward the observer, the retinal projection of an object expands over a period of time, which leads to the perception of movement in a line toward the observer. Another name for this phenomenon is depth from optical expansion; the dynamic stimulus change enables the observer not only to see the object as moving, but to perceive the distance of the moving object. Thus, in this context, the changing size serves as a distance cue. A related phenomenon is the visual system’s capacity to calculate time-to-contact of an approaching object from the rate of optical expansion – a useful ability in contexts ranging from driving a car to playing a ball game.
However, calculation of TTC is speaking, perception of velocity rather than depth. Kinetic depth effect If a stationary rigid figure is placed in front of a point source of light so that its shadow falls on a translucent screen, an observer on the other side of the screen will see a two-dimensional pattern of lines, but if the cube rotates, the visual system will extract the necessary information for perception of the third dimension from the movements of the lines, a cube is seen. This is an example of the kinetic depth effect; the effect occurs when the rotating object is solid, provided that the projected shadow consists of lines which have definite corners or end points, that these lines change in both length and orientation during the rotation. Perspective The property of parallel lines converging in the distance, at infinity, allows us to reconstruct the relative distance of two parts of an object, or of landscape features. An example would be standing on a straight road, looking down the road, noticing the road narrows as it goes off in the distance.
Relative size If two objects are known to be the same size but their absolute size is unknown, relative size cues can provide information about the relative depth of the two objects. If one subtends a larger visual angle on the retina than the other, the object which subtends the larger visual angle appears closer. Familiar size Since the visual angle of an object projected onto the retina decreases with distance, this information can be combined with previous knowledge of the object's size to determine the absolute depth of the object. For example, people are familiar with the size of an average automobile; this prior knowledge can be combined with information about the angle it subtends on the retina to determine the absolute depth of an automobile in a scene. Absolute size Even if the actual size of the object is unknown and there is only one object visible, a smaller object seems further away than a large object, presented at the same location Aerial perspective Due to light scattering by the atmosphere, objects that are a great distance away have lower luminance contrast and lower color saturation.
Due to this, images seem hazy the farther they are away from a person's point of view. In computer graphics, this is called "distance fog"; the foreground has high contrast. Objects differing only in their contrast with a background appear to be at different depths; the color of distant objects are shifted toward the blue end of the spectrum. Some painters, notably Cézanne, employ "warm" pigments to bring features forward towards the viewer, "cool" ones to indicate the part of a form that curves away from the picture plane. Accommodation This is an oculomotor cue for depth perception; when we try to focus on far away objects, the ciliary muscles stretch the eye lens, making it thinner, hence changing the focal length. The kinesthetic sensations of the contracting and relaxing ciliary muscles is sent to the visual cortex where it is used for interpreting distance/depth. Accommodation is only effective for distances less than 2 meters. Occultation Occultation (also referred to as interposit