Diálogo con el autor Según Daniel Cassany:

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(i) (ii)

Figure 3.24: Schematic diagrams of: (i) time-multiplexed and (ii) polarisation-multiplexed display, for producing stereo images with video.

Other LCD systems have also been used for displaying flicker-free, stereoscopic images. At present, the most successful commercial product is CrystalEyes®,

manufactured by Stereographies Corporation [41]. In this system, stereoscopic pairs are displayed by mapping a vertically compressed left eye sub-field to the top half o f the display, and a vertically compressed right eye sub-field to the bottom half (see Figure 3.24(i)). With this video formatting method the refresh rate o f image per eye doubles, thus making the display system flicker-free. The above can be applied only to computer monitors that can display interlaced images at high frequencies. However, for PAL or NTSC transmission, such a technique would fail to resolve good images since the number o f raster lines would be halved. Instead, for real-time video viewing a side-by- side video format is preferred (see Figure 3.25(ii)) but unfortunately this requires

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doubling the video bandwidth'^. As a result, the system is not compatible for real-time viewing and can only act as a playback video interface.

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(i) (ii)

Figure 3.25:CrystalEyes® stereo vision formats: (i) above-and-below images: when displayed at 120 Hz/sec a stereo image results, (ii) side-by-side images: left and right images fit into a

standard video field.

CrystalEyes® and other off-head display (OHD) systems that use similar display technology may have not been proved very useful in real-time video applications but have had a tremendous impact in computer graphics applications, particularly in virtual reality. Current VR systems that use this technology include the CAVE™ [42] and ImmersaDesk™ [43], shown in Figure 3.26(i) and Figure 3.26(ii) respectively.

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(i) (ii)

Figure 3.26: Commercial VR system s using stereoscopic displays: (i) CAVE: a computer­ generated environment using a multi-wall display to access high-resolution 3D models and

computer graphics, (ii) ImmersaDesk: a fixed-angle display allowing a physical ‘heads-on’ interaction with the 3 0 computer environment. Both display system s are commercially available

(Fakespace Systems).

T he two-fold video bandw idth increase explained here results from capturing a video image using tw o cam eras. Since the PAL or N T S C system s have to be used, the tw o video signals have to be ‘squeezed’ into the norm al bandw idth.

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3.4.2 Autostereoscopic displays

The use o f liquid crystal glasses for stereo viewing has been fundamental, especially in VR applications. Nowadays, Head Mounted Displays (HMD) can immerse the viewer into a computer-generated 3D scene while continuously tracking any changes in his/her position, orientation or viewpoint [44]. However, there are some serious drawbacks in their use, the most important ones being uncomfortable eyewear, reduced image brightness, image flicker and cross-talk levels o f up to 10% [45]. Even in highly sophisticated systems, there are physiological [46] and psychological [47] effects that prohibit long periods o f viewing, such as nausea, headache and tired eyes.

Autostereoscopic displays do not require specialised headsets and are generally considered as a more realistic approach to 3D viewing. They offer greater viewing freedom than immersive systems, such as stereo microscope eyepieces and HMDs, and could be useful for multi-view image presentations. There are three main scientific types o f 3D autostereoscopic displays: electro-holographic, volumetric and direction- multiplexed displays. Holographic techniques reproduce the properties o f light waves (luminance, chrominance and phase difference) almost to perfection, making them a very close approximation o f an ideal free viewing world. Volumetric displays project image points to definite locations in a physical volume o f space where they appear either on a real surface, or in translucent images forming a stack o f distinct depth planes. Direction-multiplexed displays apply optical effects like diffraction, refraction, reflection and occlusion in order to direct light emitted by pixels o f different perspective views exclusively to the correct eye. In his paragraph, only refraction-based direction- multiplexed techniques are explained as the remaining ones exceed the purposes o f this thesis.

Image quality o f displays based on optical refraction can be affected by several factors. Cross-talk of the two image channels may occur due to aberrations o f the optical system. A limited display bandwidth causes degradation o f image quality which results in lower depth perception. Bandwidth constraints are also responsible for the number o f views that can be simultaneously displayed.

Head movement problems can be overcome by using observer-tracking displays [48]. The observer’s position is normally measured by infrared or video image

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processing tracking devices. The current trend in autostereoscopic 3D display developm ent is found in the use o f TFT monitors. These offer significant advantages such as flatness and thinness, high image resolution, high image contrast and fidelity, good colour quality and acceptable cost.

The design concept o f an autostereoscopic 3D display is based on lenticular imaging

and field-lens imaging, with recent systems separated into two main categories: flat

panel 3D-LCD and Twin-LCD screens.

A flat panel autostereoscopic 3D display consists o f a lenticular array, placed directly above a TFT monitor. With this arrangement, specific display pixels are visible only under specific horizontal viewing angles (see Figure 3.27). If these pixels are charged accordingly with stereo information to match human stereopsis, a 3D viewing window is created in front o f the view er’s eyes. The former is provided through spatial m ultiplexing o f the LCD screen'^ while the latter is achieved by aligning each lenticular refractive lens with at least two columns o f the LCD pixels [49].

Figure 3.27: Picture projection on conventional 3D-LCD screens. Pixels and pixel boundaries are projected to form discrete viewing zones.

The main side effect o f the viewing conditions illustrated in the figure above is that, along w ith the autostereoscopic effect, the pixel boundaries are also imaged. This leads to the irritating effect o f vertical bars through which the view er has to look at the display. The visual effect o f this is that, with sideways movement. M oire-like fringes appear to run over the screen. A way to overcome this problem is to put the lenticular

The spatial m ultiplexing process described here is different from the tim e m ultiplexing process in stereoscopic displays. In HM D system s the sam e pixels provide inform ation for different eyes at different tim es w hile in autostereoscopic displays left and right eye pixels are spatially separated.

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sheet at a slant [50]. Some attempts have been made to create multi-view stereoscopic displays using lenticular flat panels [51], but existing products suffer from poor resolution.

An alternative method o f viewing 3D autostereoscopic images is to place a field-lens at the locus o f a 2D image in order to collimate the rays o f light passing through that image, without affecting its geometrical properties. Such display systems, project fully resolved left-right eye images on two separate LCD screens respectively. The screens, which have a viewing arrangem ent identical to the one illustrated in Figure 3.23(ii), are optically combined with a beam splitter to create the 3D effect. Twin-LCD micro-optic systems are considered o f great prospect as they can offer full resolution o f the LCD to each o f the observer’s eyes [52].

The development o f autostereoscopic display systems (see Figure 3.28) runs in parallel with that o f VR applications. The ability to display high-resolution, com puter­ generated images on LCD screens stereoscopically has initiated large-scale objects in various industrial and scientific fields, from military research to industrial assembly units. In medicine, stereoscopic displays look to improve, or even replace, conventional techniques o f medical imaging [53], surgical navigation and intervention [54], and education. Their im plementation with other VR com ponents and relative applications are discussed in the following chapter.

(i) (ii)

Figure 3.28; Flat-panel 3D autostereoscopic displays from (i) SHARP and (ii) NEC-GWT.

Vir t u a l Re a l i t yf o r He a l t h c a r es y s t e m s

CHAPTER 4