Stereo display
Stereo display (also: 3D display , obsolete: spatial image projection ) refers to the display of stereoscopic or three-dimensional images for an impression of depth through stereoscopic viewing using a screen or through a projection . A widespread use is the display of 3D movies . There are also many areas of application, for example in research, development, medicine and the military.
Differentiation between stereoscopy and 3D
The technique of stereoscopic displays consists in showing two images that are slightly different for the left and right eye. The two images are then combined in the brain for depth perception. Although the term “3D” is often used for this, the term “stereoscopic” is usually more correct. Because there is a difference whether two 2D images are displayed or whether several images or even a spatial projection are displayed. The biggest difference is that the head and eye movements of a viewer in a stereoscopic image do not lead to more information about a displayed object, i.e. the movement parallax is neither simulated nor can it be accommodated at a variable viewing distance. Autostereoscopy, holography and volume displays are examples of methods that do not have such restrictions or only have some of them.
Stereo displays
General
In stereoscopic viewing methods, the channel separation describes the separation of the left and right image necessary for the eyes. Each eye is only allowed to see the image belonging to it. The ideal case is one hundred percent channel separation in which the right image is completely invisible to the left eye and vice versa. If the channel separation is insufficient, ghosting, known as crosstalk or crosstalk , occurs, which cloud the 3D perception and, in the worst case, can lead to headaches for the viewer. Crosstalk is therefore a faulty display in which the image is visible to one eye also to the other eye. This effect occurs especially in high-contrast areas of the image. Various methods deal with reducing the undesired crosstalk.
Glasses-based procedures
The channel separation is achieved with the help of stereo or 3D glasses that the viewer must wear. There are several different technical processes that can bring about the separation into left and right image.
Anaglyph projection
"Anaglyph projection" (from the Greek ἀνά aná "on", "on top of" and γλύφω glýphō "chisel", "engrave", also "represent") in the original sense is basically any stereo projection in which both partial images are simultaneously on the same projection surface (the polarization projection is strictly speaking an "anaglyph projection"), but mostly with "anaglyphic" a color anaglyphic representation is meant: To separate the two individual images, different color filters are used in 3D glasses, originally red in front of the right eye and Green in front of the left. When watching the film, the red filter erases the red film image and the green image becomes black - the green filter erases the green color image and the red becomes black. Since both eyes now see different images, a three-dimensional image is created again in the brain.
In the late 1970s, Stephen Gibson improved the color anaglyph technique with his patented "Deep Vision" system that uses different filter colors: red in front of the right eye and cyan in front of the left. The Danish company "Color Code" now also offers its own color anaglyph system. The filter colors of the “ColorCode” glasses are blue in front of the right eye and yellow in front of the left. Another color anaglyph process (“Trio Scopics”) was introduced in England in 2008 for the feature film “Journey to the Center of the Earth”, with green in front of the left eye and magenta in front of the right.
Polarization filter technology
With this projection technique, the channel separation is achieved with linear (obsolete) or circularly polarized light . There are oppositely offset polarizing filter foils in front of the projection lenses and in the polarizing filter glasses of the viewer. Two projectors are used for this in cinemas.
A metal-coated canvas is required to maintain the polarization status of the light. A normal (matt) white canvas reflects the light with loss of polarization, which means that the channel separation is lost. The advantage of the polarization projection technology lies in the high color fidelity of the images shown. The disadvantage is the loss of brightness due to the filters used and the higher costs of the metallic screen.
With linearly polarized light, the head must be kept straight while viewing the image. If you hold your head at an angle, the 90 ° angle required to separate the channels changes between the foils in front of the projection lenses and the filters in the glasses. As a result, there is no longer any channel separation, "ghost images" appear. This no longer applies to modern methods such as Real-D , as they use circularly polarized light - you can move your head freely without any disadvantages.
Another problem is the inconsistent use of filters by different manufacturers of glasses and projectors. The filters in the glasses must match the filters in the projector, otherwise the channels will be mixed up. In addition to the private sector , this technology is used in many IMAX 3D presentations and, since 2009, in many 3D cinemas.
Interference filter technology
The interference filter technology is a system for stereoscopic reproduction developed at DaimlerChrysler at the time . The system is being further developed and sold by INFITEC GmbH. The company DOLBY Inc. uses the process under the name Dolby 3D . It works according to a light wavelength filter system (wavelength division multiplex). For each eye, a part of the wavelengths perceived by the eye as red-green-blue is transmitted and that of the other eye is blocked very effectively. Here, the basic colors of the images are reduced to different, overlap-free wavelength ranges for the left and right eyes. The process appears to be color-neutral for the viewer, i.e. H. there are no visible color changes.
With this viewing technique, the head can be tilted at will and no silver screen is required. This is why the process is also used in planetariums. The lenses and filters consist of glass coated in a vacuum process and are therefore comparatively expensive. In the early days, this method required a video processor that changed the color components of the left and right view in order to compensate for the color distortions that occurred due to this method. In the meantime, a new generation of filters is used that allows four wavelength ranges to pass on one side. This means that color distortions are largely or completely avoided even without processing the video signal, depending on the actual light spectrum of the respective projector. The color processor built into most projectors is sufficient to correct any remaining color deviations. The basis for this is metamerism , which makes it possible to create the same color impression in the eye from different spectra. The wavelength division multiplex method is only suitable for projections, but not for printing 3D images.
Shutter technology
With this method both images are projected one after the other onto the white screen. For a film with 24 frames per second, 48 images have to be brought onto the screen in the same time, which is no problem for modern projectors. In order to avoid flicker, higher frequencies are usually chosen, with each individual image then being shown several times. During the performance, the projector sends control impulses to the shutter glasses worn by the audience via infrared signal generators located above the screen . These glasses alternately darken the built-in LCD glass and ensure that each eye only sees the image that is specific to itself. The advantages are the high color fidelity and the usability of a normal screen as well as the independence from the head tilt of the viewer. In addition, despite the higher costs for the shutter glasses, such a system is more cost-effective up to a certain audience size, since, in contrast to the polarization process, neither a second projector nor a polarizing filter for the projectors nor a metalized screen are required and the synchronization effort is eliminated.
With the "3D-ready" home theater projector (mostly DLP projector), the HDMI 1.3 connection i. d. Usually a 120 Hz 3D video signal is supplied from a 3D compatible PC graphics card and projected sequentially as 2x60 Hz 3D video. In addition to suitable 3D shutter glasses (e.g. Nvidia 3D-Vision with its own USB infrared transmitter), inexpensive so-called "DLP-Link" shutter glasses can be used, which are synchronized between the video images by a projector white pulse and therefore do not need an infrared transmitter. Only 3D projectors with an HDMI 1.4a connection can be fed directly with 3D HD signals from 3D Blu-ray players or HDTV receivers.
Head-mounted display
Head-mounted displays or video glasses are visual output devices worn in front of the eyes, with which there are usually separate screens for the left and right eyes and the two stereo images can be displayed directly in front of the respective eye. See e.g. B. Oculus Rift .
Prismatic glasses
A number of methods also use the effect that prisms deflect the beam path. For example, B. the stereo viewing device SSG1b , also known under the name KMQ since the 1980s, this effect. Primarily for books and posters where color accuracy and simplicity are important. But it could be used earlier on the screen or for projection with a few viewers. However, the user must maintain the appropriate distance to the picture and keep his head permanently horizontal. Otherwise the lines of sight of both eyes will not coincide with the two partial images which are arranged one below the other. Hence the name of the process: Over-Under . These restrictions are to be lifted in the future by an OpenHardware or OpenSource project called openKMQ for working on computers.
Autostereoscopy
With autostereoscopy, no glasses are required for spatial viewing. The left and right image are separated by means of an optical beam splitter, which is located directly in front of the screen. Lenses, prisms or strips ensure that each eye perceives a different part of the screen. By cleverly distributing the image information, the interlacing, the viewer always sees two matching stereo images, even if he moves his head to the side. Movement parallax can also be simulated within certain limits, so that these are real 3D displays as defined above.
An early application was the projection on "wire mesh canvases", which was carried out for the first time in Moscow in 1930. Such a mechanical image separation system was postulated as early as 1906 by Estanawe , who proposed a fine grid of metal lamellas as a canvas. During the projection, the audience must be placed very precisely in front of the screen, otherwise the eyes cannot see the image that is intended for them. The system was improved by Noaillon , who inclined the grid to the viewer and moved the grid strips , which were now radially arranged, back and forth. The system was further developed by Ivanov , who instead of a mechanical parallel grid used 30,000 very fine copper wires as a canvas. The elaborate process was not ready for series production. Only one cinema was converted for the system, the Moskva in Moscow, and only a few films were shown there using this procedure, for example in 1940 Zemlja Molodosti, Koncert (The Land of Youth / Concert) and in 1947 the Russian feature film Robinzon Kruzo which was able to boast over 100,000 spectators.
In the meantime, autostereoscopy is finding new uses again, for example on Nintendo 3DS , cell phones, computer screens and televisions.
3D displays
Volume display
In a volume display, physical mechanisms are used to display points of light floating in space, e.g. B. via luminous voxels in gas, fog or on a rapidly rotating frosted glass disc or helix. Other solutions use multiple LCDs .
One of the principles of a volume display is to move the projection surface so that it covers an entire volume . If this happens fast enough and, depending on the position of the surface, other content is projected, the human eye, due to its inertia, combines everything into a closed 3D image.
One approach is to use a helically wound surface, shaped like a turn of an Archimedean screw . This surface is mounted vertically and rotates around a vertical axis. A 2-D projector projects the image onto a sector of this area, i.e. onto an inclined area that is higher or lower depending on the current angle of rotation. The projector must project the information on this section, synchronized with the rotation, that matches the respective height points on the inclined surface. Scottish TV pioneer JL Baird applied for a patent for his volumetric 3D and color TV system in 1941. Current laboratory systems (Japan, UK) may use a. Laser light sources.
Spheres display
A spherical display is a playback device in the form of a sphere that enables users to view and interact with three-dimensional objects.
This interactive visualization is made possible by eight pocket-size projectors (pico projectors), which are housed in the foot part of the sphere, as well as powerful software that coordinates the interaction of the individual projectors and ensures automatic calibration of the projectors. This makes it possible to seamlessly combine the resolution and brightness of the individual projected images in such a way that a decrease in quality is prevented and a uniform representation of a 3D animation is possible almost everywhere on the spherical surface. This is achieved primarily through the use of a webcam, which determines the position of the individual projections and specifically calculates their contribution to the overall image in the sphere.
The display uses six infrared cameras to enable interaction with people in the area. They track the movement of the audience, who are marked with the help of a sensor. Using the data from the cameras, a computer can constantly correct the perspective of the virtual scene based on the position of an observer. In addition, a Leap Motion interface enables control and interaction of 3-D scenes and animations with gestures. This makes it possible, for example, to start an animation or move it back and forth.
Holographic display
Pseudostereoscopic procedures
Pulfrich method
So-called "Pulfrich glasses" with light / dark filters (e.g. "Nuoptix"), use the "Pulfrich effect" for a 3D impression when tracking shots from the side. For example, the RTL television program Tutti Frutti made it very popular in the early 1990s. The Pulfrich method is not a real stereoscopic display, since the image is only recorded with a single camera. The two perspectives for the left and right eye come about through the darkened spectacle lens, which is based on the Pulfrich principle. The darkened view is passed on to the brain with a time delay, so that two views from different perspectives (but offset in time) form the spatial impression. This method can only be used to a very limited extent because important requirements must be met for this method to work as a 3D method at all. The camera or the objects must always (fundamentally and continuously) carry out a constant, slow, exclusively horizontal movement. If only one of these requirements is not met, no more 3D effect occurs.
ChromaDepth method
American Paper Optics' ChromaDepth process is based on the fact that colors are refracted to different degrees in a prism . The ChromaDepth glasses contain special viewing foils, which consist of microscopic prisms. As a result, light rays are deflected differently depending on the color. The light rays hit the eye in different places. However, since the brain assumes straight rays of light, the impression arises that the different colors come from different points of view. The brain thus creates the spatial impression (3D effect) from this difference. The main advantage of this process is that you can view ChromaDepth images without glasses (i.e. two-dimensionally) without any problems - there are no annoying double images. In addition, ChromaDepth images can be rotated as desired without losing the 3D effect. However, the colors can only be selected to a limited extent because they contain the depth information of the image. If you change the color of an object, then its perceived distance also changes. This means that a red object always comes before z. B. green or blue objects.
Pseudoholography
Occasionally the tricky implementation of the magic trick known as Pepper's ghost is also referred to as a "holographic display". But it is only a 2D or stereo projection.
Individual evidence
- ↑ Visual comfort of binocular and 3D displays, Displays, Volume 25, Issues 2-3, August 2004, Pages 99-108
- ↑ Woods, AJ (2010): Understanding Crosstalk in Stereoscopic Displays (Keynote Presentation) at 3DSA (Three-Dimensional Systems and Applications) conference, Tokyo, Japan, 19–21 May 2010 PDF .
- ↑ Helmut Jorke, Markus Fritz: Infitec - A new Stereoscopic Visualization Tool by Wavelenth Multiplex Imaging. ( Memento of the original from February 25, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 966 kB)
- ↑ Example Spheree ( Memento of the original from September 12, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Snoop Dogg resurrects Tupac. In: Spiegel Online. April 16, 2012, accessed December 2, 2014 .
- ↑ Play me the song of the dead. In: Spiegel Online. August 21, 2012, accessed December 2, 2014 .
literature
- Leo.H. Groom: stereo photography with a 35mm camera. A practice-oriented introduction to analog and digital 3D photography. 2nd edition 2004, with addendum "Digital Stereo-3D-Photography 2014". Wittig reference book, ISBN 978-3-930359-31-8 .
- Gerhard Kuhn: stereo photography and spatial image projection. (Theory and practice, equipment, materials). 2nd, expanded and completely redesigned edition. vfv, Gilching 1999, ISBN 3-88955-119-X .
- Peter A. Hagemann: The 3-D film. (= Retrospective 1980, ZDB -ID 997681-4 ). Published by the Deutsche Kinemathek Foundation for the 1980 Berlinale . Nüchtern, Munich 1980.
- David Hutchison (Ed.): Fantastic 3-D. A Starlog Photo Guidebook. Starlog Press, New York NY 1982, ISBN 0-931064-53-8 (English).
- Thomas Abé: Basic course in 3D images. Analog and digital techniques. vfv, Gilching 1998, ISBN 3-88955-099-1 .