1. INTRODUCTION
Augmented reality (AR) refers to computer displays that add virtual information to a user’s sensory perception. Most AR research focuses on see-through devices, usually worn on the head that overlay graphics and text on the user’s view of his or her surroundings. In general it superimposes graphics over a real world environment in real time.
Getting the right information at the right time and the right place is key in all these applications. Personal digital assistants such as the Palm and the Pocket PC can provide timely information using wireless networking and Global Positioning System (GPS) receivers that constantly track the handheld devices. But what make Augmented Reality different is how the information is presented: not on a separate display but integrated with the user’s perceptions. This kind of interface minimizes the extra mental effort that a user has to expend when switching his or her attention back and forth between real-world tasks and a computer screen. In augmented reality, the user’s view of the world and the computer interface literally become one.
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Between the extremes of real life and Virtual Reality lies the spectrum of Mixed Reality, in which views of the real world are combined in some proportion with views of a virtual environment. Combining direct view, stereoscopic videos, and stereoscopic graphics, Augmented Reality describes that class of displays that consists primarily of a real world environment, with graphic enhancement or augmentations.
In Augmented Virtuality, real objects are added to a virtual environment. In Augmented Reality, virtual objects are added to real world. An AR system supplements the real world with virtual (computer generated) objects that appear to co-exist in the same space as the real world. Virtual Reality is a synthetic environment.
1.1 Comparison between AR and virtual environments
The overall requirements of AR can be summarized by comparing them against the requirements for Virtual Environments, for the three basic subsystems that they require.
1. Scene generator : Rendering is not currently one of the major problems in AR. VE systems have much higher requirements for realistic images because they completely replace the real world with the virtual environment . In AR, the virtual images only supplement the real world. Therefore, fewer virtual objects need to be drawn, and they do not necessarily have to be realistically rendered in order to serve the purposes of the application.
2. Display devices: The display devices used in AR may have less stringent requirements than VE systems demand, again because AR does not replace the real world. For example, monochrome displays may be adequate for some AR applications, while virtually all VE systems today use full color. Optical see-through HMD’s with a small field-of-view may be satisfactory because the user can still see the real world with his peripheral vision; the see-through HMD does not shut off the user’s normal field-of-view. Furthermore, the resolution of the monitor in an optical see-through HMD might be lower than what a user would tolerate in a VE application, since the optical see-through HMD does not reduce the resolution of the real environment.
3. Tracking and sending: While in the previous two cases AR had lower requirements than VE that is not the case for tracking and sensing. In this area, the requirements for AR are much stricter than those for VE systems. A major reason for this is the registration problem.
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BASIC SUBSYTEMS |
VR | AR |
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SCENE GENERATOR |
MORE ADVANCED |
LESS ADVANCED |
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DISPLAY DEVICE |
HIGH QUALITY |
LOW QUALITY |
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TRACKING AND SENSING |
LESS ADVANCED |
MORE ADVANCED |
Table 1: Comparison of requirements of Augmented Reality and Virtual Reality
2. EVOLUTION
- Although augmented reality may seem like the stuff of science fiction, researchers have been building prototype system for more than three decades. The first was developed in the 1960s by computer graphics pioneer Ivan Surtherland and his students at Harvard University.
- In the 1970s and 1980s a small number of researchers studied augmented reality at institution such as the U.S. Air Force’s Armstrong Laboratory, the NASA Ames Research Center and the university of North Carolina at Chapel Hill.
- It wasn’t until the early 1990s that the term “Augmented Reality “was coined by scientists at Boeing who were developing an experimental AR system to help workers assemble wiring harnesses.
- In 1996 developers at Columbia University develop ‘The Touring Machine’
- In 2001 MIT came up with a very compact AR system known as “MIThrill”.
- Presently research is being done in developing BARS (Battlefield Augmented Reality Systems) by engineers at Naval Research Laboratory, Washington D.C.
3. WORKING
AR system tracks the position and orientation of the user’s head so that the overlaid material can be aligned with the user’s view of the world. Through this process, known as registration, graphics software can place a three dimensional image of a tea cup, for example on top of a real saucer and keep the virtual cup fixed in that position as the user moves about the room. AR systems employ some of the same hardware technologies used in virtual reality research, but there’s a crucial differences: whereas virtual reality brashly aims to replace the real world, augmented reality respectfully supplement it.
Augmented Reality is still in an early stage of research and development at various universities and high-tech companies. Eventually, possible by the end of this decade, we will see first mass-marketed augmented reality system, which one researcher calls “The Walkman of the 21st century”. What augmented reality attempts to do is not only super impose graphics over a real environment in real-time, but also change those graphics to accommodate a user’s head- and eye- movements, so that the graphics always fit and perspective.
Here are the three components needed to make an augmented-reality system work:
- Head-mounted display
- Tracking system
- Mobile computing power
3.1 Head-Mounted Display
Just as monitor allow us to see text and graphics generated by computers, head-mounted displays (HMD’s) will enable us to view graphics and text created by augmented-reality systems.
There are two basic types of HMD’s
- Optical see-through
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Video see-through
Optical Display Video Display
Fig 1: Optical and Video Display
3.1.1 Optical see-through display
Fig 2: Optical see-through HMD conceptual diagram.
A simple approach to optical see-through display employs a mirror beam splitter- a half silvered mirror that both reflects and transmits light. If properly oriented in front of the user’s eye, the beam splitter can reflect the image of a computer display into the user’s line of sight yet still allow light
from the surrounding world to pass through. Such beam splitters, which are called combiners, have long been used in head up displays for fighter-jet- pilots (and, more recently, for drivers of luxury cars). Lenses can be placed between the beam splitter and the computer display to focus the image so that it appears at a comfortable viewing distance. If a display and optics are provided for each eye, the view can be in stereo. Sony makes a see-through display that some researchers use, called the “Glasstron”.
3.1.2 Video see-through displays
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Fig 3: Video see-through HMD conceptual diagram
In contrast, a video see through display uses video mixing technology, originally developed for television special effects, to combine the image from a head worn camera with synthesized graphics. The merged image is typically presented on an opaque head worn display. With careful design the camera can be positioned so that its optical path is closed to that of the user’s eye; the video image thus approximates what the user
would normally see. As with optical see through displays, a separate system can be provided for each eye to support stereo vision. Video composition can be done in more than one way. A simple way is to use chroma-keying: a technique used in many video special effects. The background of the computer graphics images is set to a specific color, say green, which none of the virtual objects use. Then the combining step replaces all green areas with the corresponding parts from the video of the real world. This has the effect of superimposing the virtual objects over the real world. A more sophisticated composition would use depth information at each pixel for the real world images; it could combine the real and virtual images by a pixel-by-pixel depth comparison. This would allow real objects to cover virtual objects and vice-versa.
A different approach is the virtual retinal display, which forms images directly on the retina. These displays, which Micro Vision is developing commercially, literally draw on the retina with low power lasers modulated beams are scanned by microelectro-mechanical mirror assemblies that sweep the beam horizontally and vertically. Potential advantages include high brightness and contrast, low power consumption, and large depth of field.
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Fig 4: Two views of a combined augmented and virtual environment
Fig 5: Two optical see-through HMD’s, made by Hughes Electronics
3.1.3 Comparison of optical see through and video see through displays
Each of approaches to see through display design has its pluses and minuses. Optical see through systems allows allow the user to see the real world with resolution and field of view. But the overlaid graphics in current optical see through systems are not opaque and therefore cannot completely obscure the physical objects behind them. As result, the superimposed text may be hard to read against some backgrounds, and three-dimensional graphics may not produce a convincing illusion. Furthermore, although a focuses physical objects depending on their distance, virtual objects are all focused in the plane of the display. This means that a virtual object that is intended to be at the same position as a physical object may have a geometrically correct projection, yet the user may not be able to view both objects in focus at the same time.
In video see-through systems, virtual objects can fully obscure physical ones and can be combined with them using a rich variety of graphical effects. There is also discrepancy between how the eye focuses virtual and physical objects, because both are viewed on same plane. The limitations of current video technology, however, mean that the quality of the visual experience of the real world is significantly decreased, essentially to the level of the synthesized graphics, with everything focusing at the same apparent distance. At present, a video camera and display is no match for the human eye.
An optical approach has the following advantages over a video approach
- Simplicity: Optical blending is simpler and cheaper than video blending. Optical approaches have only one “stream” of video to worry about: the graphic images. The real world is seen directly through the combiners, and that time delay is generally a few nanoseconds. Video blending, on the other hand, must deal with separate video streams for the real and virtual images. The two streams of real and virtual images must be properly synchronized or temporal distortion results. Also, optical see through HMD’s with narrow field of view combiners offer views of the real world that have little distortion. Video cameras almost always have some amount of distortion that must be compensated for, along with any distortion from the optics in front of the display devices. Since video requires cameras and combiners that optical approaches do not need, video will probably be more expensive and complicated to build than optical based systems.
- Resolution: Video blending limits the resolution of what the user sees, both real and virtual, to the resolution of the display devices. With current displays, this resolution is far less than the resolving power of the fovea. Optical see-through also shows the graphic images at the resolution of the display devices, but the user’s view of the real world is not degraded. Thus, video reduces the resolution of the real world, while optical see-through does not.
- Safety: Video see-through HMD’s are essentially modified closed-view HMD’s. If the power is cut off, the user is effectively blind. This is a safety concern in some applications. In contrast, when power is removed from an optical see-through HMD, the user still has a direct view of the real world. The HMD then becomes a pair of heavy sunglasses, but the user can still see.
- No eye offset: With video see-through, the user’s view of the real world is provided by the video cameras. In essence, this puts his “eyes” where the video cameras are not located exactly where the user’s eyes are, creating an
offset between the cameras and the real eyes. The distance separating the cameras may also not be exactly the same as the user’s interpupillary distance (IPD). This difference between camera locations and eye locations introduces displacements from what the user sees compared to what he expects to see. For example, if the cameras are above the user’s eyes, he will see the world from a vantage point slightly taller than he is used to.
Video blending offers the following advantages over optical blending
1. Flexibility in composition strategies: A basic problem with optical see-through is that the virtual objects do not completely obscure the real world objects, because the optical combiners allow light from both virtual and real sources. Building an optical see-through HMD that can selectively shut out the light from the real world is difficult. Any filter that would selectively block out light must be placed in the optical path at a point where the image is in focus, which obviously cannot be the user’s eye. Therefore, the optical system must have two places where the image is in focus: at the user’s eye and the point of the hypothetical filter. This makes the optical design much more difficult and complex. No existing optical see-through HMD blocks incoming light in this fashion. Thus, the virtual objects appear Ghost-like and semi-transparent. This damages the illusion of reality because occlusion is one of the strongest depth cues. In contrast, video see-through is far more flexible about how it merges the real and virtual images. Since both the real and virtual are available in digital form, video see-through compositors can, on a pixel-by-pixel basis, take the real, or the virtual, or some blend between the two to simulate transparency.
2. Wide field-of-view: Distortions in optical systems are a function of the radial distance away from the optical axis. The further one looks away from the center of the view, the larger the distortions get. A digitized image taken through a distorted optical system can be undistorted by applying image processing techniques to unwrap the image, provided that the optical distortion is well characterized. This requires significant amount of computation, but this constraint will be less important in the future as computers become faster. It is harder to build wide field-of-view displays with optical see-through techniques. Any distortions of the user’s view of the real world must be corrected optically, rather than digitally, because the system has no digitized image of the real world to manipulate. Complex optics is expensive and add weight to the HMD. Wide field-of-view systems are an exception to the general trend of optical approaches being simpler and cheaper than video approaches.
3. Real and virtual view delays can be matched: Video offers an approach for reducing or avoiding problems caused by temporal mismatches between the real and virtual images. Optical see-through HMD’s offer an almost instantaneous view of the real world but a delayed view of the virtual. This temporal mismatch can cause problems. With video approaches, it is possible to delay the video of the real world to match the delay from the virtual image stream.
4. Additional registration strategies: In optical see-through, the only information the system has about the user’s head location comes from the head tracker. Video blending provides another source of information: the digitized image of the real scene. This digitized image means that video approaches can employ additional registration strategies unavailable to optical approaches.
5. Easier to match the brightness of the real and virtual objects: Both optical and video technologies have their roles, and the choice of technology depends upon the application requirements. Many of the mismatch assembly and repair prototypes use optical approaches, possibly because of the cost and safety issues. If successful, the equipment would have to be replicated in large numbers to equip workers on a factory floor. In contrast, most of the prototypes for medical applications use video approaches, probably for the flexibility in blending real and virtual and for the additional registration strategies offered.
3.2 Tracking and Orientation
The biggest challenge facing developers of augmented reality the need to know where the user is located in reference to his or her surroundings. There’s also the additional problem of tracking the movement of users eyes and heads. A tracking system has to recognize these movements and project the graphics related to the real-world environment the user is seeing at any given movement. Currently both video see-through and optical see-through displays optically have lag in the overlaid material due to the tracking technologies currently available.
3.2.1 Indoor Tracking
Tracking is easier in small spaces than in large spaces. Trackers typically have two parts: one worn by the tracked person or object and other built into the surrounding environment, usually within the same room. In optical trackers, the targets - LED’s or reflectors, for instance - can be attached to the tracked person or to the object, and an array of optical sensors can be embedded in the room’s ceiling. Alternatively the tracked users can wear the sensors, and targets can be fixed to the ceiling. By calculating the distance to reach visible target, the sensors can determine the user’s position and orientation.
Researchers at the University of North Carolina-Chapel Hill have developed a very precise system that works within 500 sq feet. The HiBall Tracking System is an optoelectronic tracking system made of two parts:
§ Six user-mounted, optical sensors.
§ Infrared-light-emitting diodes (LED’s) embedded in special ceiling panels.
The system uses the known location of LED’s the known geometry of the user-mounted optical sensors and a special algorithm to compute and report the user’s position and orientation. The system resolves linear motion of less than 0.2 millimeters, and angular motions less than 0.03 degrees. It has an update rate of more than 1500Hz, and latency is kept at about one millisecond. In everyday life, people rely on several senses-including what they see, cues from their inner ears and gravity’s pull on their bodies- to maintain their bearings. In a similar fashion, “Hybrid Trackers” draw on several sources of sensory information. For example, the wearer of an AR display can be equipped with inertial sensors (gyroscope and accelerometers) to record changes in head orientation. Combining this information with data from optical, video or ultrasonic devices greatly improve the accuracy of tracking.
3.2.2Out door Tracking
Head orientation is determined with a commercially available hybrid tracker that combines gyroscopes and accelerometers with magnetometers that measure the earth’s magnetic field. For position tracking we take advantage OF a high-precision version of the increasingly popular Global Positioning system receiver.
A GPS receiver can determine its position by monitoring radio signals from navigation satellites. GPS receivers have an accuracy of about 10 to 30 meters. An augmented reality system would be worthless if the graphics projected were of something 10 to 30 meters away from what you were actually looking at.
User can get better result with a technique known as differential GPS. In this method, the mobile GPS receiver also monitors signals from another GPS receiver and a radio transmitter at a fixed location on the earth. This transmitter broadcasts the correction based on the difference between the stationary GPS antenna’s known and computed positions. By using these signals to correct the satellite signals, the differential GPS can reduce the margin of error to less than one meter.
The system is able to achieve the centimeter-level accuracy by employing the real-time kinematics GPS, a more sophisticated form of differential GPS that also compares the phases of the signals at the fixed and mobile receivers. Trimble Navigation reports that they have increased the precision of their global positioning system (GPS) by replacing local reference stations with what they term a Virtual Reference Station (VRS). This new VRS will enable users to obtain a centimeter-level positioning without local reference stations; it can achieve long-range, real-time kinematics (RTK) precision over greater distances via wireless communications wherever they are located. Real-time kinematics technique is a way to use GPS measurements to generate positioning within one to two centimeters (0.39 to 0.79 inches). RTK is often used as the key component in navigational system or automatic machine guidance.
Unfortunately, GPS is not the ultimate answer to position tracking. The satellite signals are relatively weak and easily blocked by buildings or even foliage. This rule out useful tracking indoors or in places likes midtown Manhattan, where rows of tall building block most of the sky. GPS tracking works well in wide open spaces and relatively low buildings.
GPS provide far too few updates per second and is too inaccurate to support the precise overlaying of graphics on nearby objects. Augmented Reality system places extra ordinary high demands on the accuracy, resolution, repeatability and speed of tracking technologies. Hardware and software delays introduce a lag between the user’s movement and the update of the display. As a result, virtual objects will not remain in their proper position as the user moves about or turns his or her head. One technique for combating such errors is to equip AR system with software that makes short-term predictions about the user’s future motion by extrapolating from previous movements. And in the long run, hybrid trackers that include computer vision technologies may be able trigger appropriate graphics overlays when the devices recognize certain objects in the user’s view.
4. MOBILE COMPUTING POWER
For a wearable augmented realty system, there is still not enough computing power to create stereo 3-D graphics. So researchers are using whatever they can get out of laptops and personal computers, for now. Laptops are just now starting to be equipped with graphics processing unit (GPU’s). Toshiba just now added a NVIDIA to their notebooks that is able to process more than 17-million triangles per second and 286-million pixels per second, which can enable CPU-intensive programs, such as 3D games. But still notebooks lag far behind- NVIDIA has developed a custom 300-MHz 3-D graphics processor for Microsoft’s Xbox game console that can produce 150 million polygon per second-and polygons are more complicated than triangles. So you can see how far mobiles graphics chips have to go before they can create smooth graphics like the ones you see on your home video-game system.
5. APPLICATIONS
Only recently have the capabilities of real-time video image processing, computer graphics systems and new display technologies converged to make possible the display of a virtual graphical image correctly registered with a view of the 3D environment surrounding the user. Researchers working with the AR system have proposed them as solutions in many domains. The areas have been discussed range from entertainment to military training. Many of the domains, such as medical are also proposed for traditional virtual reality systems. This section will highlight some of the proposed application for augmented reality.
5.1 Medical
Because imaging technology is so pervasive throughout the medical field, it is not surprising that this domain is viewed as one of the more important for augmented reality systems. Most of the medical application deal with image guided surgery. Pre-operative imaging studies such as CT or MRI scans, of the patient provide the surgeon with the necessary view of the internal anatomy. From these images the surgery is planned. Visualization of the path through the anatomy to the affected area where, for example, a tumor must be removed is done by first creating the 3D model from the multiple views and slices in the preoperative study. This is most often done mentally though some systems will create 3D volume visualization from the image study. AR can be applied so that the surgical team can see the CT or MRI data correctly registered on the patient in the operating theater while the procedure is progressing. Being able to accurately register the images at this point will enhance the performance of the surgical team.
Another application for AR in the medical domain is in ultra sound imaging. Using an optical see-through display the ultrasound technician
can view a volumetric rendered image of the fetus overlaid on the abdomen of the pregnant woman. The image appears as if it were inside of the abdomen and is correctly rendered as the user moves.
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Fig 6: Virtual fetus inside womb of pregnant patient.
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Fig 7: Mockup of breast tumor biopsy. 3-D graphics guide needle insertion.
5.2 Entertainment
A simple form of the augmented reality has been in use in the entertainment and news business for quite some time. Whenever you are watching the evening weather report the weather reporter is shown standing in the front of changing weather maps. In the studio the reporter is standing in front of a blue or a green screen. This real image is augmented with the computer generated maps using a technique called chroma-keying. It is also possible to create a virtual studio environment so that the actors can appear to be positioned in a studio with computer generated decorating.
Movie special effects make use of digital computing to create illusions. Strictly speaking with current technology this may not be considered augmented reality because it is not generated in the real-time. Most special effects are created off-line, frame by frame with a substantial amount of user interaction and computer graphics system rendering. But some work is progressing in
computer analysis of the live action images to determine the camera parameters and use this to drive the generation of the virtual graphics objects to be merged.
Princeton Electronics Billboard has developed an augmented reality system that allows broadcasters to insert advertisement into specific areas of the broadcast image. For example, while broadcasting a baseball game this system would be able to place an advertisement in the image so that it appears on the outfield wall of the stadium. By using pre-specified reference points in the stadium, the system automatically determines the camera angles being used and referring to the pre-defined stadium map inserts the advertisement into the current place. AR QUAKE, 76 designed using the same platform, blends users in the real world with those in a purely virtual environment. A mobile AR user plays as a combatant in the computer game Quake, where the game runs with a virtual model of the real environment.
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Fig 8: AR in sports broadcasting. The annotations on the race cars and the yellow first down line are inserted into the broad cast in real time.
5.3 Military Training
The military has been using display in cockpits that present information to the pilot on the windshield of the cockpit or the visor of their flight helmet. This is a form of Augmented Reality display. SIMNET, a distributed war games simulating system, is also embracing augmented reality technology. By equipping military personnel with helmet mounted visor displays or a special purpose rangefinder the activities of other units participating in the exercise
can be imaged. While looking at the horizon, for example, the display equipped soldier could see a helicopter rising above the tree line. This helicopter could be being flown in simulation by another participant. In war time, the display of the real battlefield scene could be augmented with annotation information or highlighting to emphasize hidden enemy units.
5.4 Engineering Design
Imagine that a group of designers are working on the model of a complex device for their clients. The designers and clients want to do a joint design reviews even though they are physically separated. If each of them had a conference room that was equipped with an augmented re4ality display this could be accomplished. The physical prototype that the designers have mocked up is imaged and displayed in the client’s conference room in 3D. The clients can walk around display looking at different aspects of it. To hold the discussion the client can point at the prototype to highlight sections and this will be reflected on the real model in the augmented display that the designers are using. Or perhaps in an earlier stage of the design, before a prototype is built, the view in each conference room is augmented with a computer generated image of the current design built from the CAD file describing it. This would allow real time interactions with elements of the design so that either side can make adjustments and change that are reflected in the view seen by both groups.
5.5 Robotics and Telerobotics
In the domain of robotics and Telerobotics an augmented display can assist the user of the system. A Telerobotics operator uses a visual image of the remote workspace to guide the robot. Annotation of the view would still be useful just as it is when the scene is in front of the operator. There is an added potential benefit. Since often the view of the remote scene is monoscopic, augmentation with wire frame drawings of structures in the view can facilitate visualization of the remote 3D geometry. If the operator is attempting a motion it could be practiced on a virtual robot that is visualized as an augmentation to the real scene. The operator can decide to proceed
with the motion after seeing the results. The robot motion could then be executed directly which in a telerobotics application would eliminate any oscillations caused by long delays to the remote site.
Fig 9: Virtual lines show a planned motion of a robot arm
5.6 Manufacturing, maintenance and repair
When the maintenance technician approaches a new or unfamiliar piece of equipment instead of opening several repair manuals they could put on an augmented reality display. In this display the image of the equipment would be augmented with annotations and information pertinent to the repair. For example, the location of fasteners and attachment hardware that must be removed would be highlighted. Then the inside view of the machine would highlight the boards that need to be replaced. The military has developed a wireless vest worn by personnel that is attached to an optical see-through display. The wireless connection allows the soldier to access repair manuals and images of the equipment. Future versions might register those images on the live scene and provide animation to show the procedures that must be performed.Boeing researchers are developing an augmented reality display to replace the large work frames used for making wiring harnesses for their aircraft. Using this experimental system, the technicians are guided by the augmented display that shows the routing of the cables on a generic frame used for all harnesses. The augmented display allows a single fixture to be used for making the multiple harnesses.
5.7 Consumer design
Virtual reality systems are already used for consumer design. Using perhaps more of a graphics system than virtual reality, when you go to the typical home store wanting to add a new deck to your house, they will show you a graphical picture of what the deck will look like. It is conceivable that a future system would allow you to bring a video tape of your house shot from various viewpoints in your backyard and in real time it would augment that view to show the new deck in its finished form attached to your house. Or bring in a tape of your current kitchen and the augmented reality processor would replace your current kitchen cabinetry with virtual images of the new kitchen that you are designing.
Applications in the fashion and beauty industry that would benefit from an augmented reality system can also be imaged. If the dress store does not have a particular style dress in your size an appropriate sized dress could be used to augment the image of you. As you looked in the three sided mirror you would see the image of the new dress on your body. Changes in hem length, shoulder styles or other particulars of the design could be viewed on you before you place the order. When you head into some high-tech beauty shops today you can see what a new hair style would look like on a digitized image of yourself. But with an advanced augmented reality system you would be able to see the view as you moved. If the dynamics of hair are included in the description of the virtual object you would also see the motion of hair as your head moved.
5.8 Instant information
Tourists and students could use these systems to learn more about a certain historical event. Imagine walking onto a Civil War battlefield and seeing a re-creation of historical events on a head-mounted, augmented reality display. It would immerse you in the event, and the view would be panoramic. The recently started Archeoguide project is developing a wearable AR system for providing tourists with information about a historical site in Olympia, Greece.
6. future directions
This section identifiers areas and approaches that require further researches to produce improved AR systems.
Hybrid approach
Further tracking systems may be hybrids, because combining approaches can cover weaknesses. The same may be true for other problems in AR. For example, current registration strategies generally focus on a single strategy. Further systems may be more robust if several techniques are combined. An example is combining vision-based techniques with prediction. If the fiducially are not available, the system switches to open-loop prediction to reduce the registration errors, rather than breaking down completely. The predicted viewpoints in turn produce a more accurate initial location estimate for the vision-based techniques.
Real time systems and time-critical computing
Many VE systems are not truly run in real time. Instead, it is common to build the system, often on UNIX, and then see how fast it runs. This may be sufficient for some VE applications. Since everything is virtual, all the objects are automatically synchronized with each other. AR is different story. Now the virtual and real must be synchronized, and the real world “runs” in real time. Therefore, effective AR systems must be built with real time performance in mind. Accurate timestamps must be available. Operating systems must not arbitrarily swap out the AR software process at any time, for arbitrary durations. Systems must be built ton guarantee completion within specified time budgets, rather than just “running as quickly as possible”. These are characteristics of flight simulators and a few VE systems. Constructing and debugging real-time systems is often painful and difficult, but the requirements for AR demand real-time performance.
Perceptual and psychophysical studies
Augmented reality is an area ripe for psychophysical studies. How much lag can a user detect? How much registration error is detectable when the head is moving? Besides questions on perception, psychological experiments that explore performance issues are also needed. How much does head-motion prediction improve user performance on a specific task? How much registration error is tolerable for a specific application before performance on that task degrades substantially? Is the allowable error larger while the user moves her head versus when she stands still? Furthermore, no much is known about potential optical illusion caused by errors or conflicts in the simultaneous display of real and virtual objects.
Portability
It is essential that potential AR applications give the user the ability to walk around large environments, even outdoors. This requires making the requirement self-continued and portable. Existing tracking technology is not capable of tracking a user outdoors at the required accuracy.
Multimodal displays
Almost all work in AR has focused on the visual sense: virtual graphic objects and overlays. But augmentation might apply to all other senses as well. In particular, adding and removing 3-D sound is a capability that could be useful in some AR applications.
7. CONCLUSION
Augmented reality is far behind Virtual Environments in maturity. Several commercial vendors sell complete, turnkey Virtual Environment systems. However, no commercial vendor currently sells an HMD-based Augmented Reality system. A few monitor-based “virtual set” systems are available, but today AR systems are primarily found in academic and industrial research laboratories.
The first deployed HMD-based AR systems will probably be in the application of aircraft manufacturing. Both Boeing and McDonnell Douglas are exploring this technology. The former uses optical approaches, while the letter is pursuing video approaches. Boeing has performed trial runs with workers using a prototype system but has not yet made any deployment decisions. Annotation and visualization applications in restricted, limited range environments are deployable today, although much more work needs to be done to make them cost effective and flexible.
Applications in medical visualization will take longer. Prototype visualization aids have been used on an experimental basis, but the stringent registration requirements and ramifications of mistakes will postpone common usage for many years. AR will probably be used for medical training before it is commonly used in surgery.
The next generation of combat aircraft will have Helmet Mounted Sights with graphics registered to targets in the environment. These displays, combined with short-range steer able missiles that can shoot at targets off-bore sight, give a tremendous combat advantage to pilots in dogfights. Instead of having to be directly behind his target in order to shoot at it, a pilot can now shoot at anything within a 60-90 degree cone of his aircraft’s forward centerline. Russia and Israel currently have systems with this capability, and the U.S is expected to field the AIM-9X missile with its associated Helmet-mounted sight in 2002.
Augmented Reality is a relatively new field, where most of the research efforts have occurred in the past four years. Because of the numerous challenges and unexplored avenues in this area, AR will remain a vibrant area of research for at least the next several years.
After the basic problems with AR are solved, the ultimate goal will be to generate virtual objects that are so realistic that they are virtually indistinguishable from the real environment. Photorealism has been demonstrated in feature films, but accomplishing this in an interactive application will be much harder. Lighting conditions, surface reflections, and other properties must be measured automatically, in real time. More sophisticated lighting, texturing, and shading capabilities must run at interactive rates in future scene generators. Registration must be nearly perfect, without manual intervention or adjustments.
While these are difficult problems, they are probably not insurmountable. It took about 25 years to progress from drawing stick figures on a screen to the photorealistic dinosaurs in “Jurassic Park.” Within another 25 years, we should be able to wear a pair of AR glasses outdoors to see and interact with photorealistic dinosaurs eating a tree in our backyard.
8. REFERENCES
Ø A survey of Augmented Reality by Ronald T. Azuma
Ø Recent Advances in Augmented Reality by Ronald T.Azuma, Yohan Baillot, Reinhold Beringer, Simon Julier and Blair MacIntyre
Ø Augmented Reality: A new way of seeing. Steven K Feiner
Ø Augmented Reality and computer Augmented Environment, available at
http://www.csl.sony.co.jp/project/ar/ref.html