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to the invention of the stereoscope by Sir David Brewster14 [9], where two displaced images could convey 3D information to the human observer.
1.2.3 Stereoscopic Displays
Since Brewster’s stereoscope, a wealth of technical devices for presenting stereoscopic images to the human observer have been designed. Virtually all of modern stereoscopic display technologies are based on the same principle, namely that of presenting two different views to the two eyes on a single display. To do this, techniques have been employed that:
•separate views by color-coding (the anaglyph technique with red-green glasses or spectral comb filters),
•use polarization properties of light (circularly or linearly polarized eye glasses),
•perform time-coding with left-right time-interleaving and actively synchronized shutter glasses, or
•exploit lens systems to project the different images directly into the eyes of the observer.
While the first three techniques all use separating glasses to be worn by the user, the latter lens projection systems allow glasses-free, auto-stereoscopic perception, even for more than two different views. Figure 1.2 sketches the stereoscopic perception with either two-view stereoscopic or glasses-free auto-stereoscopic multiview displays. In the binocular display, polarization serves to decouple left and right eye information. The auto-stereoscopic display exploits optical lenticular sheet lenses or light barrier systems to selectively project the displaced images into different angular sections in front of the display. If the observer moves in front of the display, each eye receives a differently displaced image, resulting in a look-around capability.
Binocular stereoscopic movies and selected stereoscopic television programs have now entered the market quite successfully. These display techniques are commonly given the branding 3D, but actually they do not contain or need true 3D information. In a stereo movie recording, two displaced movie cameras are synchronously used to capture left and right eye views and stereoscopic digital movie projectors utilize polarization filters to separate both views. The spectator needs to wear a similar set of polarized glasses for binocular perception. The perceived depth impression is fixed by the inter-camera eye distance of the recording stereo camera and can only be adjusted during recording. This is a drawback of binocular stereo camera recordings because it is difficult to scale depth perception later on. Hence, different recordings must be undertaken for large screen movie theaters and for home TV settings. Even more severe is the stereoscopic image capture for autostereoscopic displays. In this case, not two but many slightly displaced views need
14Sir David Brewster, 1781–1868, Scottish physicist and inventor.
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Fig. 1.2 Left: Stereoscopic displays use glasses-based polarization light separators to produce the two images required for stereoscopic reception. Right: Lens-based auto-stereoscopic displays project multiple, slightly displaced images by use of lenses or parallax barrier systems, allowing glasses-free stereoscopic reception. Such systems allow for slight head motion
to be recorded simultaneously. Typical displays require 8 to 28 simultaneous views and it is not feasible to record all views directly, because the amount of data would grow enormously. Also, the design of such multi-ocular cameras is difficult and expensive. Instead, a true 3D movie format is needed that allows us to synthesize the required views from a generic 3D image format. Currently, 3D data formats like Multi-View Depth (MVD) or Layered Depth Video (LDV) are under discussion [3]. MVD and LDV record both depth and color from few camera positions that capture the desired angular sections in front of the display. The many views needed to drive the display are then rendered by depth-compensated interpolation from the recorded data. Thus, a true 3D format will greatly facilitate data capture for future 3D-TV systems.
There is another obstacle to binocular perception that was not discussed in early binocular display systems. The observed disparity is produced on the image plane and both eyes of the human observer are accommodating their focus on the display plane. However, the binocular depth cue causes the eyes to physically converge towards the virtual 3D position of the object, which may be before or behind the display plane. Both, eye accommodation and eye convergence angle, are strong depth cues to our visual system and depth is inferred from both. In the real world, both cues coincide since the eyes focus and converge towards the same real object position. On a binocular display, the eyes always accommodate towards the display, while the convergence angle varies with depth. This conflict causes visual discomfort and is a major source of headaches when watching strong depth effects, especially in front of the screen. Stereographers nowadays take great care to balance these effects during recording. The only remedy to this disturbing effect is to build volumetric displays where the image is truly formed in 3D space rather than on the 2D display. In this case, the convergence and accommodation cues coincide and yield stress-free stereoscopic viewing. There is an active research community underway developing volumetric or holographic displays, that rely either on spatial pattern interference, on volume-sweeping surfaces, or on 3D lightfields. Blundell and Schwarz give a good classification of volumetric displays and sketch current trends [7]. All these 3D displays need some kind of 3D scene representation and binocular imaging is not sufficient. Hence, these displays also are in need of true 3D data formats.