In this section, we discuss some common display technologies and artefacts may be perceived in each of them.
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2.6.1 Display Technologies
There are several approaches and techniques available in display technologies for stereoscopic video. The basic principle is to project the distinct image into each eye of a viewer.
• Coloured anaglyph (Figure 2.7) is the simplest method used for displaying stereoscopic images. Just an ordinary RGB image is displayed on the screen. The image is composed of two colour elements. One is red and the other green. An observer should wear glasses with red filter over one eye and green filter over the other one. Advantages of this system are simple equipment like ordinary monitor and cheap glasses. Disadvantages include colour reproduction inaccuracy and minor depth perception.
Figure 2.7 Red/Green glasses.
• Active shutter glasses (Figure 2.8) are another method. The technology was developed mainly for use with computer games by NVIDIA, because it is quite cheap and gives much better quality comparing to coloured anaglyphs. This technology blocks image for each eye while showing image for the other eye. As an example, if image for right eye is displayed, the shutter glasses block the image for the left eye and vice versa. The switch is fast (nearly 100 Hz refresh rate), so that human eyes are unable to notice it. The switching causes that the proper image, and nothing else, is displayed in the respective eye with half frequency. There are two approaches to stereoscopic image displaying with LCD shutter glasses known as field-sequential and frame-sequential stereo. The field-sequential method uses interlaced display mode. It means that the image for one eye is displayed in even rows and the image for the second one in odd rows of the image. The frame-sequential
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method displays full im ages at very high refresh rates. There are su ccessiv ely changing im ages for left and right eye.
A ctive shutter glasses m ostly elim inate 3-D crosstalk, w hich is a problem w ith other 3-D display technologies such as linearly polarised glasses. A lso, unlike anaglyph glasses, active shutter glasses are colour neutral, enabling 3-D view in g in the full colour spectrum.
Figure 2.8 A ctive shutter glasses manufactured by N V ID IA [38].
• Polarised glasses (Figure 2.9) are sim ple glasses with two polarising filters. The polarisation axis o f one filter m ust be vertical w ith respect to the second one. Polarisation glasses can be used with CRT m onitor or w ith projection display. To present stereoscopic contents, tw o im ages are projected superim posed onto the same screen or display through different polarising filters. The view er wears low -cost eyeglasses, w hich contain a pair o f different polarising filters. A s each filter passes only that light, w h ich is sim ilarly polarised, and blocks the light polarised in the opposite direction, each eye sees a different image. This is used to produce a three-dim ensional effect by projecting the sam e scene into both eyes, but depicted from slightly different perspectives. Several people can v iew the stereoscopic im ages at the same time.
This technology is inexpensive and does not consum e power. It does not require a transmitter to synchronise them with the display. Flickering and v isib le crosstalk are less. Finally, it produces brighter im ages. Disadvantages include that im ages for polarised glasses have to share the screen sim ultaneously, and therefore cannot have full resolution delivered to each eye sim ultaneously. It has a narrow vertical view in g angle.
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Figure 2,9 Polarised glasses
• A utostereoscopy (Figure 2.10) rem oves the need o f glasses to see the stereoscopic im age. Lenticular lens and parallax barrier technologies involve im posing tw o im ages on the sam e sheet, in narrow, alternating strips, and using a screen that either blocks one o f the two im ages strips (in the case o f parallax barriers) or uses equally narrow len ses to bend the strips o f im age and m ake it appear to fill the entire im age (in the case o f lenticular prints). To produce the stereoscopic effect, the person must be positioned so that one eye sees one o f the two im ages and the other sees the other.
L enticular L ens lets
Right Left Right Left Pixels P arallax Barrier Right Left R ight Left Pixels
Figure 2.10 Autostereoscopy used in different display technologies. Image is taken from [39].
M any autostereoscopic displays are sin gle-view displays and are thus not capable o f reproducing the sense o f m ovem ent parallax, except for a single view er in system s capable o f eye tracking. Som e autostereoscopic displays are m ulti-view displays and capable o f
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providing the perception o f left-right m ovem ent parallax. Eight and sixteen v iew s are typical for such displays. W hile it is theoretically possib le to sim ulate the perception o f up-dow n m ovem ent parallax, no current display system s are know n to do so, and the up- dow n effect is w idely seen as less important than left-right m ovem ent parallax. One consequence o f not including parallax about both axes becom es m ore evident as objects increasingly distant from the plane o f the display are presented, for as the view er m oves closer to or farther away from the display such objects w ill more obviously exhibit the effects o f perspective shift about one axis but not the other, appearing variously stretched or squashed to a view er not positioned at the optimum distance from the display.
• H ead-m ounted display (Figure 2.11) that utilises eye-tracking technology is a recent technology. This category o f technology is used w hile very im m ersive experience o f stereoscopic content is required. Latest advancem ents in software and eye tracking equipment have made these devices to becom e available at more reasonable cost. Head- m ounted or wearable glasses m ay be used to v iew a see-through im age im posed upon the real-world view , creating what is called augm ented reality. This is done by reflecting the video im ages through partially reflective mirrors. The real-world v iew is seen through the mirrors reflective surface.
Figure 2.11 Head mounted display with eye tracking technology. Image is taken from [40].
2.6.2 Visualisation Artefacts
Similar to previous stages in stereoscopic video provisioning system , visualisation can cause som e artefacts to stereoscopic contents. A m ong the artefact, important ones are m entioned here.
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• Blurring or colour bleeding is a result of colour sub-sampling techniques in different display technologies. In this case, a visible smearing of luminance or chrominance components occurs.
• Colour inaccuracy occurs while using technologies like coloured anaglyph. This is due to hardware limitations that cannot adapt actual range of luminance and changes darkest and brightest colours.
• Flickering is a visible fading between cycles displayed on stereoscopic video displays, especially the switch interval when a stereoscopic display uses the field-sequential method.
• Display specific distortions include limitations incurred by a specific display. As an example, autostereoscopic displays impose improper depth perception if viewer distance and position to the screen is not adjusted.