Output Displays that Create Virtual Reality (part-III)
Output Displays that Create Virtual Reality (part-III)
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  • 승인 2005.06.01 12:01
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In the last article, I explained the concept of virtual reality and its applications. In order to create a virtual and whole body experience, special displays are often employed to provide "multimodal" feedback that stimulates user's sensory organs and body parts (output). The term "display" is usually associated with the "visual" output. However, in the context of virtual reality, it refers to any modality output, that is, visual, aural, haptic (a term referring to the sense of touch and force feedback), olfactory, thermal, or concerns taste. The most important VR display is the visual display. Visual displays for VR are designed to be large and immersive and provide wide field of view (FOV) to mimic the visual span of humans as much as possible. VR visual displays are also usually stereoscopic, that is, 3D. In order to provide 3D imagery, the visual system must provide two separate images to the right and the left eye. This is because humans perceive depth due to the image difference as felt from the two eyes. For instance, the "active" stereoscopic display system displays the right and the left image in an alternate fashion (one at a time) and the user must wear special glasses that block the right and left views (by making it literally "black") in synchrony with the display. This way, the user's left or right eye sees only the appropriate image at a given time. Figure 1(a) and 1(b) shows users wearing the special glasses for 3D viewing. Figure 1(c) shows an immersive display system called the CAVE, originally conceived by the researchers at the University of Illinois, Chicago in the mid 1990s. Ideally, a small cubical room is constructed with each of the sides serving as a large projection screen. The user is thus surrounded by images. Rather than using multiple tiles of rectangular and upright screens, specially made large spherical or cylindrical screens can be used also. Figure 1(b) shows the workbench type of a display where one projection surface is used in a table like manner using a mirror underneath. Such table displays are very suitable for tasks such as painting or surgery. Figure 1(d) shows the head mounted display (HMD). An HMD usually employs two separate display devices and is designed to provide an isolated display for each eye. Unlike large displays, HMDs are worn on the head for that purpose. Two separate (synchronized) images (for the right and left eye respectively) are fed into the respective display channel to create the stereo effect. The display isolation also creates the feeling of immersion aside from the stereoscopic effect. To create the display device itself, miniature LCDs or CRTs are used. The small images that appear on these LCDs or CRTs are magnified through the use of optics. While 3D viewing requires expensive set ups or special devices to be worn, research and development is now focused on "autostereoscopic" displays that do not require either. Sharp Corporation, for instance, is already offering auto-stereoscopic displays for their laptop computers at an added cost of only $500. Convenient and cheap 3D displays will further foster the use of VR in various areas. Perhaps the second most important display for VR is the aural display. Adding sound effects can dramatically improve the virtual experience. While faithful sound reproduction technology has come a long way (e.g., 5.1 Dolby Surround), VR requires producing sound with dynamic directionality (e.g. "Oh, there is a car coming from the left side!") for virtual environments with moving objects and changing sound sources. It has been discovered that the 3D sound perception of humans is based on the difference in energy levels within the frequency distribution of sound waves. This sparked the invention of a concept called "Head Related Transfer Function (HRTF)" and HRTF-based technology can produce spatial sound with new energy distributions according to the (possibly varying) locations of the sound sources (for the right and left ears). Today's computer sound cards include this functionality and can be easily programmed to produce reasonable 3D sound effects. The third of the "big three" modalities is the haptic (the other two being visual and aural). The word "haptics" in the context of virtual reality refers to the sense of "force feedback". Humans rely on their haptics to explore environments in which there is poor or no visibility. For instance, pegs can be inserted into a hole by feeling for the surrounding surface and chamfers that lead into the hole. Force feedback or haptic displays interact with the muscles and tendons to give humans a sensation of a force being applied. In most cases, haptics works the best when used with other modalities like the visual and aural display. Most force feedback devices are in the form of robotic devices, like high degrees of freedom (DOF) manipulators and exoskeleton mechanisms, force feedback joysticks, and motion platforms that can generate and stimulate a user with the various types of forces, at the point of interaction. Figure 2 shows a few examples. No practical sensory display systems or devices exist for simulating the effects of smell, wind, taste and other more "exotic" human sensory modalities. For instance, very little work, commercial or research, has been done on olfactory display systems. Unlike color, it is not possible to compose a new scent from the basic component scents and once a scent is diffused into the air, the problem of quickly removing the residuals is hard to solve. However, ingenuity can go a long way to overcome these problems. At a recent VR conference, Yanagida, et al., presented a novel olfactory display system called the "Air Cannon." The Air Cannon, detached from the user by some distance, tracks the location of the nose by a camera, and "shoots" a potion of small scent packet using an aerodynamic pump system so that the potion arrives near the nose of the user. Likewise, active researches are on-going for display technologies for such modalities. In the next article, I will introduce some of the sensors and input devices for virtual reality systems.

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