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A Survey of Augmented Reality

In Presence: Teleoperators and Virtual Environments 6, 4 (August 1997), 355-385.A Survey of Augmented RealityRonald T. AzumaHughes Research Laboratories3011 Malibu Canyon Road, MS RL96Malibu, CA 90265azuma@/~azumaW: (310) 317-5151Fax: (310) 317-5695AbstractThis paper surveys the field of Augmented Reality, in which 3-D virtual objects are integrated into a 3-D real environment in real time. It describes the medical, manufacturing, visualization, path planning, entertainment and military applications that have been explored. This paper describes the characteristics of Augmented Reality systems, including a detailed discussion of the tradeoffs between optical and video blending approaches. Registration and sensing errors are two of the biggest problems in building effective Augmented Reality systems, so this paper summarizes current efforts to overcome these problems. Future directions and areas requiring further research are discussed. This survey provides a starting point for anyone interested in researching or using Augmented Reality.1.Introduction1.1GoalsThis paper surveys the current state-of-the-art in Augmented Reality. It describes work performed at many different sites and explains the issues and problems encountered when building Augmented Reality systems. It summarizes the tradeoffs and approaches taken so far to overcome these problems and speculates on future directions that deserve exploration.A survey paper does not present new research results. The contribution comes from consolidating existing information from many sources and publishing an extensive bibliography of papers in this field. While several other introductory papers have been written on this subject [Barfield95] [Bowskill95] [Caudell94] [Drascic93b] [Feiner94a] [Feiner94b] [Milgram94b] [Rolland94], this survey is more comprehensive and up-to-date. This survey provides a good beginning point for anyone interested in starting research in this area.Section 1 describes what Augmented Reality is and the motivations for developing this technology. Six classes of potential applications that have been explored are described in Section 2. Then Section 3 discusses the issues involved inbuilding an Augmented Reality system. Currently, two of the biggest problems are in registration and sensing: the subjects of Sections 4 and 5. Finally, Section 6 describes some areas that require further work and research.1.2DefinitionAugmented Reality (AR) is a variation of Virtual Environments (VE), or Virtual Reality as it is more commonly called. VE technologies completely immerse a user inside a synthetic environment. While immersed, the user cannot see the real world around him. In contrast, AR allows the user to see the real world, with virtual objects superimposed upon or composited with the real world. Therefore, AR supplements reality, rather than completely replacing it. Ideally, it would appear to the user that the virtual and real objects coexisted in the same space, similar to the effects achieved in the film "Who Framed Roger Rabbit?" Figure 1 shows an example of what this might look like. It shows a real desk with a real phone. Inside this room are also a virtual lamp and two virtual chairs. Note that the objects are combined in 3-D, so that the virtual lamp covers the real table, and the real table covers parts of the two virtual chairs. AR can be thought of as the "middle ground" between VE (completely synthetic) and telepresence (completely real) [Milgram94a] [Milgram94b].Figure 1: Real desk with virtual lamp and two virtual chairs. (Courtesy ECRC)Some researchers define AR in a way that requires the use of Head-Mounted Displays (HMDs). To avoid limiting AR to specific technologies, this survey defines AR as systems that have the following three characteristics:1) Combines real and virtual2) Interactive in real time3) Registered in 3-DThis definition allows other technologies besides HMDs while retaining the essential components of AR. For example, it does not include film or 2-D overlays. Films like "Jurassic Park" feature photorealistic virtual objects seamlessly blended with a real environment in 3-D, but they are not interactive media. 2-D virtual overlays on top of live video can be done at interactive rates, but the overlays are not combined with the real world in 3-D. However, this definition does allow monitor-based interfaces, monocular systems, see-through HMDs, and various other combining technologies. Potential system configurations are discussed further in Section 3.1.3MotivationWhy is Augmented Reality an interesting topic? Why is combining real and virtual objects in 3-D useful? Augmented Reality enhances a user's perception of and interaction with the real world. The virtual objects display information that the user cannot directly detect with his own senses. The information conveyed by the virtual objects helps a user perform real-world tasks. AR is a specific example of what Fred Brooks calls Intelligence Amplification (IA): using the computer as a tool to make a task easier for a human to perform [Brooks96].At least six classes of potential AR applications have been explored: medical visualization, maintenance and repair, annotation, robot path planning, entertainment, and military aircraft navigation and targeting. The next section describes work that has been done in each area. While these do not cover every potential application area of this technology, they do cover the areas explored so far.2.Applications2.1MedicalDoctors could use Augmented Reality as a visualization and training aid for surgery. It may be possible to collect 3-D datasets of a patient in real time, using non-invasive sensors like Magnetic Resonance Imaging (MRI), Computed Tomography scans (CT), or ultrasound imaging. These datasets could then be rendered and combined in real time with a view of the real patient. In effect, this would give a doctor "X-ray vision" inside a patient. This would be very useful during minimally-invasive surgery, which reduces the trauma of an operation by using small incisions or no incisions at all. A problem with minimally-invasive techniques is that they reduce the doctor's ability to see inside the patient, making surgery more difficult. AR technology could provide an internal view without the need for larger incisions.AR might also be helpful for general medical visualization tasks in the surgical room. Surgeons can detect some features with the naked eye that they cannot see in MRI or CT scans, and vice-versa. AR would give surgeons access to both types of data simultaneously. This might also guide precision tasks, such as displaying where to drill a hole into the skull for brain surgery or where to perform a needle biopsy of a tiny tumor. The information from the non-invasive sensors would be directly displayed on the patient, showing exactly where to perform the operation.AR might also be useful for training purposes [Kancherla95]. Virtual instructions could remind a novice surgeon of the required steps, without the need to look away from a patient to consult a manual. Virtual objects could also identify organs and specify locations to avoid disturbing [Durlach95].Several projects are exploring this application area. At UNC Chapel Hill, a research group has conducted trial runs of scanning the womb of a pregnant woman with an ultrasound sensor, generating a 3-D representation of the fetus inside the womb and displaying that in a see-through HMD (Figure 2). The goal is to endow the doctor with the ability to see the moving, kicking fetus lying inside the womb, with the hope that this one day may become a "3-D stethoscope" [Bajura92][State94]. More recent efforts have focused on a needle biopsy of a breast tumor. Figure 3 shows a mockup of a breast biopsy operation, where the virtual objects identify the location of the tumor and guide the needle to its target [State96b]. Other groups at the MIT AI Lab [Grimson94] [Grimson95] [Mellor95a] [Mellor95b], General Electric [Lorensen93], and elsewhere [Betting95] [Edwards95] [Taubes94] are investigating displaying MRI or CT data, directly registered onto the patient. Figure 2: Virtual fetus inside womb of pregnant patient. (Courtesy UNC ChapelHill Dept. of Computer Science.)Figure 3: Mockup of breast tumor biopsy. 3-D graphics guide needle insertion.(Courtesy UNC Chapel Hill Dept. of Computer Science.)2.2Manufacturing and repairAnother category of Augmented Reality applications is the assembly, maintenance, and repair of complex machinery. Instructions might be easier to understand if they were available, not as manuals with text and pictures, but rather as 3-D drawings superimposed upon the actual equipment, showing step-by-step the tasks that need to be done and how to do them. These superimposed 3-D drawings can be animated, making the directions even more explicit.Several research projects have demonstrated prototypes in this area. Steve Feiner's group at Columbia built a laser printer maintenance application [Feiner93a], shown in Figures 4 and 5. Figure 4 shows an external view, and Figure 5 shows the user's view, where the computer-generated wireframe is telling the user to remove the paper tray. A group at Boeing is developing AR technology to guide a technician in building a wiring harness that forms part of an airplane's electrical system. Storing these instructions in electronic form will save space and reduce costs. Currently, technicians use large physical layout boards to construct such harnesses, and Boeing requires several warehouses to store all these boards. Such space might be emptied for other use if this application proves successful [Caudell92] [Janin93] [Sims94]. Boeing is using a Technology Reinvestment Program (TRP) grant to investigate putting this technology onto the factory floor [BoeingTRP94]. Figure 6 shows an external view of Adam Janin using a prototype AR system to build a wire bundle. Eventually, AR might be used for any complicated machinery, such as automobile engines [Tuceryan95].Figure 4: External view of Columbia printer maintenance application. Note that all objects must be tracked. (Courtesy Steve Feiner, Blair MacIntyre, and DoréeSeligmann, Columbia University.)Figure 5: Prototype laser printer maintenance application, displaying how to remove the paper tray. (Courtesy Steve Feiner, Blair MacIntyre, and DoréeSeligmann, Columbia University.)Figure 6: Adam Janin demonstrates Boeing's prototype wire bundle assembly application. (Courtesy David Mizell, Boeing)2.3Annotation and visualizationAR could be used to annotate objects and environments with public or private information. Applications using public information assume the availability of public databases to draw upon. For example, a hand-held display could provide information about the contents of library shelves as the user walks around the library [Fitzmaurice93] [Rekimoto95a] [Rekimoto95b]. At the European Computer-Industry Research Centre (ECRC), a user can point at parts of an engine model and the AR system displays the name of the part that is being pointed at [Rose95]. Figure 7 shows this, where the user points at the exhaust manifold on an engine model and the label "exhaust manifold" appears.Figure 7: Engine model part labels appear as user points at them. (CourtesyECRC)Alternately, these annotations might be private notes attached to specific objects. Researchers at Columbia demonstrated this with the notion of attaching windows from a standard user interface onto specific locations in the world, or attached to specific objects as reminders [Feiner93b]. Figure 8 shows a window superimposed as a label upon a student. He wears a tracking device, so the computer knows his location. As the student moves around, the label follows his location, providing the AR user with a reminder of what he needs to talk to the student about.Figure 8: Windows displayed on top of specific real-world objects. (Courtesy Steve Feiner, Blair MacIntyre, Marcus Haupt, and Eliot Solomon, ColumbiaUniversity.)AR might aid general visualization tasks as well. An architect with a see-through HMD might be able to look out a window and see how a proposed new skyscraper would change her view. If a database containing information about a building's structure was available, AR might give architects "X-ray vision" inside a building, showing where the pipes, electric lines, and structural supports are inside the walls [Feiner95]. Researchers at the University of Toronto have built a system called Augmented Reality through Graphic Overlays on Stereovideo (ARGOS) [Milgram95], which among other things is used to make images easier to understand during difficult viewing conditions [Drascic93a]. Figure 9 shows wireframe lines drawn on top of a space shuttle bay interior, while in orbit. The lines make it easier to see the geometry of the shuttle bay. Similarly, virtual lines and objects could aid navigation and scene understanding during poor visibility conditions, such as underwater or in fog.Figure 9: Virtual lines help display geometry of shuttle bay, as seen in orbit.(Courtesy David Drascic and Paul Milgram, U. Toronto.)2.4Robot path planningTeleoperation of a robot is often a difficult problem, especially when the robot is far away, with long delays in the communication link. Under this circumstance,instead of controlling the robot directly, it may be preferable to instead control a virtual version of the robot. The user plans and specifies the robot's actions by manipulating the local virtual version, in real time. The results are directly displayed on the real world. Once the plan is tested and determined, then user tells the real robot to execute the specified plan. This avoids pilot-induced oscillations caused by the lengthy delays. The virtual versions can also predict the effects of manipulating the environment, thus serving as a planning and previewing tool to aid the user in performing the desired task. The ARGOS system has demonstrated that stereoscopic AR is an easier and more accurate way of doing robot path planning than traditional monoscopic interfaces [Drascic93b] [Milgram93]. Others have also used registered overlays with telepresence systems [Kim93] [Kim96] [Oyama93] [Tharp94] [Yoo93]. Figure 10 shows how a virtual outline can represent a future location of a robot arm. Figure 10: Virtual lines show a planned motion of a robot arm (Courtesy DavidDrascic and Paul Milgram, U. Toronto.)2.5EntertainmentAt SIGGRAPH '95, several exhibitors showed "Virtual Sets" that merge real actors with virtual backgrounds, in real time and in 3-D. The actors stand in front of a large blue screen, while a computer-controlled motion camera records the scene. Since the camera's location is tracked, and the actor's motions are scripted, it is possible to digitally composite the actor into a 3-D virtual background. For example, the actor might appear to stand inside a large virtual spinning ring, where the front part of the ring covers the actor while the rear part of the ring is covered by the actor. The entertainment industry sees this as a way to reduce production costs: creating and storing sets virtually is potentially cheaper than constantly building new physical sets from scratch. The ALIVE project from the MIT Media Lab goes one step further bypopulating the environment with intelligent virtual creatures that respond to user actions [Maes95].2.6Military aircraftFor many years, military aircraft and helicopters have used Head-Up Displays (HUDs) and Helmet-Mounted Sights (HMS) to superimpose vector graphics upon the pilot's view of the real world. Besides providing basic navigation and flight information, these graphics are sometimes registered with targets in the environment, providing a way to aim the aircraft's weapons. For example, the chin turret in a helicopter gunship can be slaved to the pilot's HMS, so the pilot can aim the chin turret simply by looking at the target. Future generations of combat aircraft will be developed with an HMD built into the pilot's helmet [Wanstall89].3.CharacteristicsThis section discusses the characteristics of AR systems and design issues encountered when building an AR system. Section 3.1 describes the basic characteristics of augmentation. There are two ways to accomplish this augmentation: optical or video technologies. Section 3.2 discusses their characteristics and relative strengths and weaknesses. Blending the real and virtual poses problems with focus and contrast (Section 3.3), and some applications require portable AR systems to be truly effective (Section 3.4). Finally, Section 3.5 summarizes the characteristics by comparing the requirements of AR against those for Virtual Environments.3.1AugmentationBesides adding objects to a real environment, Augmented Reality also has the potential to remove them. Current work has focused on adding virtual objects to a real environment. However, graphic overlays might also be used to remove or hide parts of the real environment from a user. For example, to remove a desk in the real environment, draw a representation of the real walls and floors behind the desk and "paint" that over the real desk, effectively removing it from the user's sight. This has been done in feature films. Doing this interactively in an AR system will be much harder, but this removal may not need to be photorealistic to be effective.Augmented Reality might apply to all senses, not just sight. So far, researchers have focused on blending real and virtual images and graphics. However, AR could be extended to include sound. The user would wear headphones equipped with microphones on the outside. The headphones would add synthetic, directional 3–D sound, while the external microphones would detect incoming sounds from the environment. This would give the system a chance to mask or cover up selected real sounds from the environment by generating a masking signal that exactly canceledthe incoming real sound [Durlach95]. While this would not be easy to do, it might be possible. Another example is haptics. Gloves with devices that provide tactile feedback might augment real forces in the environment. For example, a user might run his hand over the surface of a real desk. Simulating such a hard surface virtually is fairly difficult, but it is easy to do in reality. Then the tactile effectors in the glove can augment the feel of the desk, perhaps making it feel rough in certain spots. This capability might be useful in some applications, such as providing an additional cue that a virtual object is at a particular location on a real desk [Wellner93].3.2Optical vs. videoA basic design decision in building an AR system is how to accomplish the combining of real and virtual. Two basic choices are available: optical and video technologies. Each has particular advantages and disadvantages. This section compares the two and notes the tradeoffs. For additional discussion, see [Rolland94].A see-through HMD is one device used to combine real and virtual. Standard closed-view HMDs do not allow any direct view of the real world. In contrast, a see-through HMD lets the user see the real world, with virtual objects superimposed by optical or video technologies.Optical see-through HMDs work by placing optical combiners in front of the user's eyes. These combiners are partially transmissive, so that the user can look directly through them to see the real world. The combiners are also partially reflective, so that the user sees virtual images bounced off the combiners from head-mounted monitors. This approach is similar in nature to Head-Up Displays (HUDs) commonly used in military aircraft, except that the combiners are attached to the head. Thus, optical see-through HMDs have sometimes been described as a "HUD on a head" [Wanstall89]. Figure 11 shows a conceptual diagram of an optical see-through HMD. Figure 12 shows two optical see-through HMDs made by Hughes Electronics.The optical combiners usually reduce the amount of light that the user sees from the real world. Since the combiners act like half-silvered mirrors, they only let in some of the light from the real world, so that they can reflect some of the light from the monitors into the user's eyes. For example, the HMD described in [Holmgren92] transmits about 30% of the incoming light from the real world. Choosing the level of blending is a design problem. More sophisticated combiners might vary the level of contributions based upon the wavelength of light. For example, such a combiner might be set to reflect all light of a certain wavelength and none at any other wavelengths. This would be ideal with a monochrome monitor. Virtually all the light from the monitor would be reflected into the user's eyes, while almost all the light from the real world (except at the particular wavelength) would reach the user's eyes. However, most existing optical see-through HMDs do reduce the amount of light from the real world, so they act like a pair of sunglasses when the power is cut off.RealworldFigure 12:Two optical see-through HMDs, made by Hughes ElectronicsIn contrast, video see-through HMDs work by combining a closed-view HMD with one or two head-mounted video cameras. The video cameras provide the user's view of the real world. Video from these cameras is combined with the graphic images created by the scene generator, blending the real and virtual. The result is sent to the monitors in front of the user's eyes in the closed-view HMD. Figure 13shows a conceptual diagram of a video see-through HMD. Figure 14 shows an actualvideo see-through HMD, with two video cameras mounted on top of a Flight Helmet.ofReal WorldFigure 13:Video see-through HMD conceptual diagramFigure 14:An actual video see-through HMD. (Courtesy Jannick Rolland, Frank Biocca, and UNC Chapel Hill Dept. of Computer Science. Photo by AlexTreml.)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 graphic 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. If the system had 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.AR systems can also be built using monitor-based configurations, instead of see-through HMDs. Figure 15 shows how a monitor-based system might be built. In this case, one or two video cameras view the environment. The cameras may be static or mobile. In the mobile case, the cameras might move around by being attached to a robot, with their locations tracked. The video of the real world and the graphic images generated by a scene generator are combined, just as in the video see-through HMD case, and displayed in a monitor in front of the user. The user does not wear the display device. Optionally, the images may be displayed in stereo on the monitor, which then requires the user to wear a pair of stereo glasses. Figure 16 shows an external view of the ARGOS system, which uses a monitor-based configuration.MonitorStereo glasses (optional)Figure 15:Monitor-based AR conceptual diagramFigure 16:External view of the ARGOS system, an example of monitor-basedAR. (Courtesy David Drascic and Paul Milgram, U. Toronto.)Finally, a monitor-based optical configuration is also possible. This is similar to Figure 11 except that the user does not wear the monitors or combiners on her head. Instead, the monitors and combiners are fixed in space, and the user positions her head to look through the combiners. This is typical of Head-Up Displays on military aircraft, and at least one such configuration has been proposed for a medical application [Peuchot95].The rest of this section compares the relative advantages and disadvantages of optical and video approaches, starting with optical. An optical approach has the following advantages over a video approach:1) Simplicity: Optical blending is simpler and cheaper than video blending.Optical approaches have only one "stream" of video to worry about: the graphicimages. 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. Both streams have inherentdelays in the tens of milliseconds. Digitizing video images usually adds at least one frame time of delay to the video stream, where a frame time is how long it takes to completely update an image. A monitor that completely refreshes the screen at 60 Hz has a frame time of 16.67 ms. The two streams of real and virtual images must be properly synchronized or temporal distortion results. Also, optical see-through HMDs 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.2) 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 device, 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.3) Safety: Video see-through HMDs are essentially modified closed-view HMDs. 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.4) 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. In most configurations, the 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 see-through can avoid the eye offset problem through the use of mirrors to create another set of optical paths that mimic the paths directly into the user's eyes. Using those paths, the cameras will see what the user's eyes would normally see without the HMD. However, this adds complexity to the HMD design. Offset is generally not a difficult design problem for optical see-through displays. While the user's eye can rotate with respect to the position of the HMD, the resulting errors are tiny. Using the eye's center of rotation as the viewpoint in the computer graphics model should eliminate any need for eye tracking in an optical see-through HMD [Holloway95].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. In a normal optical system, the objects are designed to be in。

虚拟世界和现实世界英语作文

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高中英语作文《虚拟现实技术的应用》

高中英语作文《虚拟现实技术的应用》The Application of Virtual Reality TechnologyVirtual reality (VR) technology, which has gained immense popularity in recent years, has revolutionized various fields with its immersive and interactive experiences.Its applications range from entertainment and education to healthcare and industry, offering numerous benefits and transforming the way we live and work.In the entertainment industry, VR has made significant strides, allowing users to immerse themselves in virtual worlds and experience things that are otherwise unavailable in reality.Gaming, in particular, has been greatly influenced by VR technology.Players can now don VR headsets and find themselves inside the game, making gaming experiences more realistic and engaging.Furthermore, VR has also made its way into the world of cinema, providing viewers with immersive movie experiences that make them feel like they are part of the story.Education is another field where VR technology has made a significant impact.It provides an effective tool for learning, offering students the opportunity to explore virtual environments and gain hands-on experiences.For instance, medical students can practice surgical procedures in a virtual operating room, enabling them to develop their skills in a safe and controlled environment.Similarly, history students can virtually visit ancient ruins or witness historical events,making their learning experiences more memorable and engaging.In the healthcare industry, VR technology is being used to treat various conditions, including phobias, post-traumatic stress disorder (PTSD), and anxiety.By simulating virtual environments that trigger patients' fears, therapists can help them overcome their phobias and PTSD.Additionally, VR is also used for pain management, distracting patients from their pain during treatments or surgeries.The industrial sector has also embraced VR technology, using it for training purposes and enhancing productivity.Workers can undergo simulations in virtual environments, preparing them for real-life situations and improving their skills.Moreover, VR is used in the design and manufacturing process, allowing engineers to visualize and test products before they are brought to market.Despite the numerous benefits of VR technology, there are also some challenges and concerns.One of the main concerns is the potential for addiction, as users can become too immersed in virtual worlds.There are also concerns about the impact of VR on social interactions and the possibility of users experiencing motion sickness.In conclusion, virtual reality technology has diverse applications and has transformed various fields.Its immersive and interactive nature offers unique experiences that were previously unimaginable.As the technology continues to advance, it is likely to have even greater impacts on our lives,makingVR a promising and exciting field of the future.。

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现实与虚拟英文作文

现实与虚拟英文作文英文：Reality and virtuality are two different worlds that coexist in our lives. Reality is the world we live in, where we can touch, see, hear, and feel things around us. Virtuality, on the other hand, is a digital world created by technology that can simulate reality, but it is not tangible.In my opinion, both reality and virtuality have their advantages and disadvantages. In reality, we can experience things firsthand, and it is more authentic. For example, when we go on a vacation, we can see the beautiful scenery, taste the local food, and interact with the locals. These experiences are unique and cannot be replicated in the virtual world.However, virtuality also has its benefits. It allows us to experience things that are impossible in reality, suchas flying to space or exploring the depths of the ocean. It also provides a platform for us to connect with people from all over the world and learn about different cultures.In terms of education, virtuality has great potential. It can provide a more immersive and interactive learning experience, and allow students to explore and experiment in a safe and controlled environment. For example, medical students can practice surgeries on virtual patients before performing them on real patients.Overall, I believe that reality and virtuality have their own unique roles in our lives. It is up to us to find a balance between the two and use them to enhance our experiences and knowledge.中文：现实和虚拟是我们生活中共存的两个不同世界。

人们沉浸于虚拟世界的英语作文

人们沉浸于虚拟世界的英语作文English:With the advancement of technology, more and more people are immersing themselves in the virtual world. The convenience and entertainment provided by the virtual world are irresistible to many people. Social media, online games, and virtual reality have become an integral part of people's daily lives. The instantaneous connection with friends and strangers, the thrill of online competition, and the escape from reality are all factors that draw people into the virtual world. However, the excessive immersion in the virtual world has negative consequences. It can lead to a decrease in face-to-face communication skills, a lack of physical activity, and a distorted perception of reality. People may become disconnected from their surroundings and their relationships with others may suffer. It is important for individuals to find a balance between the virtual world and the real world.Translated content:随着科技的进步，越来越多的人沉浸在虚拟世界中。

八年级科技前沿英语阅读理解25题

八年级科技前沿英语阅读理解25题1<背景文章>Artificial intelligence (AI) has been making remarkable strides in the medical field in recent years. AI - powered systems are being increasingly utilized in various aspects of healthcare, bringing about significant improvements and new possibilities.One of the most prominent applications of AI in medicine is in disease diagnosis. AI algorithms can analyze vast amounts of medical data, such as patient symptoms, medical histories, and test results. For example, deep - learning algorithms can scan X - rays, CT scans, and MRIs to detect early signs of diseases like cancer, pneumonia, or heart diseases. These algorithms can often spot minute details that might be overlooked by human doctors, thus enabling earlier and more accurate diagnoses.In the realm of drug development, AI also plays a crucial role. It can accelerate the process by predicting how different molecules will interact with the human body. AI - based models can sift through thousands of potential drug candidates in a short time, identifying those with the highest probability of success. This not only saves time but also reduces the cost associated with traditional trial - and - error methods in drug research.Medical robots are another area where AI is making an impact.Surgical robots, for instance, can be guided by AI systems to perform complex surgeries with greater precision. These robots can filter out the natural tremors of a surgeon's hand, allowing for more delicate and accurate incisions. Additionally, there are robots designed to assist in patient care, such as those that can help patients with limited mobility to move around or perform simple tasks.However, the application of AI in medicine also faces some challenges. Issues like data privacy, algorithmic bias, and the need for regulatory approval are important considerations. But overall, the potential of AI to transform the medical field is vast and holds great promise for the future of healthcare.1. What is one of the main applications of AI in the medical field according to the article?A. Designing hospital buildings.B. Disease diagnosis.C. Training medical students.D. Managing hospital finances.答案：B。

虚拟与现实世界英语作文

虚拟与现实世界英语作文Title: The Interplay Between the Virtual and the Real Worlds。

In today's digital age, the boundary between thevirtual and the real world is becoming increasingly blurred. With the advancement of technology, we are witnessing a profound integration of virtual experiences into our daily lives. This phenomenon has sparked debates and discussions regarding the implications it has on society, culture, and individual lives.First and foremost, the virtual world offersunparalleled opportunities for connectivity and communication. Platforms such as social media, online forums, and virtual reality environments enable people to interact and engage with others irrespective ofgeographical boundaries. These virtual interactions have redefined the concept of socialization, allowingindividuals to form relationships and communities based onshared interests and values. However, the virtual nature of these interactions raises concerns about the authenticity and depth of human connections in the digital realm.Moreover, the virtual world serves as a vast repository of information and knowledge. The internet, in particular, has democratized access to information, empowering individuals to learn and explore diverse subjects with unprecedented ease. From online courses to digital libraries, the virtual sphere has revolutionized education and self-improvement. Nonetheless, the abundance of information available online raises challenges related to misinformation, filter bubbles, and information overload, highlighting the need for digital literacy and critical thinking skills.In addition to social and educational aspects, the virtual world has also transformed entertainment andleisure activities. Video games, virtual reality experiences, and online streaming platforms offer immersive and engaging entertainment options for people of all ages. These virtual experiences allow individuals to escape fromreality momentarily and explore fantastical worlds or engage in thrilling adventures. However, excessive immersion in virtual entertainment can lead to issues such as addiction, sedentary lifestyle, and detachment fromreal-world responsibilities.Furthermore, the integration of virtual elements into various aspects of society has given rise to new economic opportunities and challenges. The rise of e-commerce, digital marketplaces, and remote work has reshaped traditional business models and employment practices. While the virtual economy offers flexibility and scalability, it also poses risks such as cybercrime, data privacy breaches, and economic inequalities. Navigating this digital landscape requires innovative solutions and regulatory frameworks to ensure equitable access and fair competition.Despite the undeniable benefits and advancements brought forth by the virtual world, it is crucial to recognize its limitations and potential pitfalls. Striking a balance between the virtual and the real world is essential to maintain our connection with tangibleexperiences, human relationships, and physical environments. As we embrace the opportunities presented by technology, we must also remain mindful of its impact on our well-being, society, and the planet.In conclusion, the interplay between the virtual andthe real worlds is reshaping the way we live, communicate, and interact with the world around us. While the virtual sphere offers unprecedented opportunities for connectivity, knowledge dissemination, and entertainment, it alsopresents challenges related to authenticity, misinformation, and socioeconomic disparities. By fostering digitalliteracy, promoting responsible usage, and cultivating a balanced approach, we can harness the transformative potential of the virtual world while preserving therichness and authenticity of real-world experiences.。

七下英语介绍vt的作文

七下英语介绍vt的作文Title: Introduction to Virtual Reality (VR)IntroductionVirtual Reality (VR) is a technology that has taken the world by storm in recent years. With the ability to transport users to completely new and immersive worlds, VR has become widely popular in various industries, from entertainment to education to healthcare. In this article, we will explore what VR is, how it works, and its potential impact on the future.What is Virtual Reality?Virtual Reality is a computer-generated environment that simulates a real-world experience. By wearing a VR headset, users can interact with and explore this virtual world as if they were actually there. The technology uses a combination of computer graphics, sensors, and tracking devices to create this immersive experience.How Does VR Work?VR works by using a VR headset, which is a device that covers the user's eyes and ears to block out the real world and immerse them in the virtual world. Inside the headset are twoscreens that display the virtual environment in stereoscopic 3D, giving the illusion of depth. Sensors in the headset track the user's head movements, allowing them to look around and interact with the virtual world in real-time.Uses of VRVR has a wide range of applications across different industries. In entertainment, VR is used to create immersive gaming experiences, allowing players to feel like they are inside the game world. In education, VR is used to create interactive learning environments, enabling students to explore historical sites or conduct science experiments in virtual reality. In healthcare, VR is used for training medical professionals and treating patients with conditions such as phobias.Impact on the FutureThe potential impact of VR on the future is vast. As the technology continues to advance, we can expect to see even more immersive and realistic virtual worlds. VR has the potential to revolutionize the way we work, play, and learn, opening up new possibilities for creativity and innovation.In conclusion, Virtual Reality is a groundbreaking technology that has the power to transform our world. By creatingimmersive and interactive virtual environments, VR opens up new possibilities for entertainment, education, and healthcare. As the technology continues to evolve, we can expect to see even more exciting applications of VR in the future.。

虚拟和现实英文作文

虚拟和现实英文作文英文：Virtual and reality are two different concepts thathave been debated for a long time. On one hand, virtual reality allows us to experience things that we may not be able to experience in real life. On the other hand, reality provides us with a sense of authenticity and genuinenessthat cannot be replicated in the virtual world.For example, let's consider the experience of traveling. In virtual reality, we can explore different parts of the world without ever leaving our homes. We can visit places like the Great Wall of China or the Eiffel Tower without having to deal with the crowds or the expenses of actually traveling there. However, while virtual reality may provide us with a glimpse of these places, it cannot replicate the feeling of actually being there, smelling the air, feeling the sun on our skin, and interacting with the locals.Similarly, social interactions in the virtual world may be convenient and comfortable, but they lack the depth and intimacy of face-to-face interactions in reality. Invirtual meetings, we may miss out on the nonverbal cues and body language that are crucial to effective communication. We may also miss out on the spontaneous moments of laughter, tears, and connection that can only happen when we are physically present with others.In conclusion, virtual and reality each have their own strengths and weaknesses. While virtual reality may provide us with convenience and accessibility, it cannot replicate the authenticity and genuineness of reality. Ultimately, it is up to us to find a balance between the two and use eachto enhance our lives in different ways.中文：虚拟和现实是两个不同的概念，长期以来一直备受争议。

2024年初中英语中考复习备考任务型阅读专项练

任务型阅读阅读下面短文，按要求完成下面小题。

Recently, a primary school in Changsha, has put Virtual Reality (VR) technology (虚拟现实技术) to good use in an English class. When the students put on the VR glasses, it seemed that they were taken into a new world where they could see for themselves. They could enjoy the views along the way, allowing more knowledge to be kept in mind.VR language learning is now starting to be widely used in many schools. Think of it like this. Now you are learning English in China, you don’t have enough chances to use English if you are not in English classes, but if you learn English in English-speaking countries like America or the UK, it’ll be much easier for you to make progress.Nowadays, a lot of apps have also been designed to serve VR language learning. The app “House of Languages” is one of them. It is already available (可获得的) on the Samsung (三星) VR. It can sense your movement and the acts of your eyeballs, arms and hands.When you are learning English in it, a lovely racoon (浣熊) called “Mr. Woo”, will guide you to communicate in English. You can use English by finishing tasks Mr. Woo gives you. For example, you will be asked to find certain things at your home according to the English words that Mr. Woo gives you. After you’ve found these things, you should “take” them, “come back” to Mr. Woo and “give” them to him. Then he will say “thanks” to you. Sometimes, Mr. Woo may have some activities like family party so that you will have more tasks to finish in that situation. You will learn English so quickly that you will even forget you are leaning.Virtual Reality is showing increasing promise in language education and the day, isn’t far away when it will become one of the main media in education. What about you? Would you learn a new language in Virtual Reality?1．Recently, VR technology has been put to good use in a primary school in Changsha. 2．In China, if you aren’t in English classes, you don’t to use English.3．What can Samsung VR sense?.4．Who will guide you to communicate in English in the app?.5．Do you like to learn a new language in Virtual Reality? Why or why not?.阅读短文，根据短文内容回答问题，每个题目的答案不超过5个单词。

【范文】Lesson3 Virtual Reality教案

Lesson3 Virtual Reality教案Lesson3VirtualReality教案Teachingaims:Topractiseusingfirstandsecondcondition als.Toassesswhetherwebsitesarereal.Teachingdifficul ties:TopractiseexpressingpreferencesTeachingAids:co mputerandcassetteTeachingprocedures:Ⅰ.warmingup问学生下面的问题激发学生的兴趣。

whatisvirtualreality？whatdopeopleusethetechnologytodo?whichoftheseusesfo rvirtualrealityarepossiblenow?vvirtualrealityholidaysvlearningtoflyusingvirtualrealitytechnologyvplayingvirtualrealitycomputergamesvvisitingvirtualmuseumsontheInternet展示一些图片（Showstudentssomeslidestoprovethesefacts）T:wouldyouliketovisittheSciencemuseumwebsite?whyorw hynot?wouldyouliketogotoavirtualuniversity?S:T:Virtualrealitycanbringuslotsofconvenient.youwillfindlot sofgoodinformationathome,andneedn’tspendtimetravelingthere.Studyinsuchaworld-famousun iversitywithoutgoingoutofyourroom.ⅡReading理解课文TrueorFalse1.ThisweekendTomwillnothelpcathyfinishap rojectonthehistoryoftheInternet.2.cathyismoreintere stedinvirtualholidaythanTom.3.Tomwillhaveavirtualre alityholidaysthisweekend.4.Virtualuniversityisanint erestinggameonlineinsteadofarealuniversity.5.Virtua lrealityholidayswon’tspendlesstimethanrealholidays.Answers:1.T2.T3.F4.F5.FⅢFurtherreadingReadthetextagainandpayattentiontother ightexpression.correctmistakes1.whatareyougoingtodoatthisweekend.2.Ihavetoomanythingstobedone.3.wewon’tgotogocampingifitwillrain4.Iftheyinventvirtualrealityholidays,I’dgoonanaround-the–worldtour.5.DoyouhaveanythingplanforSaturdayandSund ay.6.Tomimagineshimselfgotoafamousuniversity.7.Pers onalI’minterestedinvirtualuniversity.Answers:1.去掉at2.去掉be3.去掉will4.invent改为invented5.plan改为planed6.go改为going7.Personal改为PersonallyDotheexercise3Readthedialogueandanswerthe questionsⅣTalkingwhataretheadvantagesanddisadvantagesofavirtu aluniversity?wouldyouliketogotosuchauniversity?why?AdvantagesofavirtualuniversityNolimittotimeandplaceItisconvenienttoattendtakeyourjobasstudysavetranspo rtationtimeneedn’tbuytextmaterialⅤLanguagepoints1.what’s…upto口语中问对方在做什么。

虚拟现实技术英文作文

虚拟现实技术英文作文英文：Virtual reality (VR) technology has been gaining popularity in recent years, and it's not hard to see why. With VR, users are transported to a completely different world, where they can interact with their surroundings in ways that were previously impossible. As someone who has experienced VR firsthand, I can attest to its incredible capabilities.One of the most impressive things about VR is itsability to create a sense of presence. When you put on a VR headset, you feel like you're really there, in the virtual world. This is achieved through a combination of visual and auditory cues, as well as haptic feedback (vibrations or other physical sensations). For example, if you're playing a VR game where you're walking on a tightrope, you might feel a slight vibration in your feet to simulate the sensation of being on a real rope.Another advantage of VR is its potential for education and training. For example, medical students can use VR to practice surgeries without risking the lives of real patients. Similarly, pilots can use VR to simulate flying without the danger of crashing. In both cases, VR can provide a safe and cost-effective way to gain experience.However, there are also some potential downsides to VR. For one, it can be isolating. When you're wearing a VR headset, you're cut off from the real world, which can be a problem if you're using VR for extended periods of time. Additionally, some people may experience motion sickness or other physical discomfort when using VR.Overall, I believe that VR has the potential to revolutionize many industries, from entertainment to healthcare. However, it's important to use it in moderation and be aware of its limitations.中文：虚拟现实技术近年来越来越受欢迎，原因不难理解。

人们沉浸于虚拟世界的英语作文

人们沉浸于虚拟世界的英语作文英文回答：As the digital landscape expands and technology evolves at an unprecedented pace, the allure of virtual worlds has captivated individuals around the globe. These immersive environments offer an escape from the constraints of reality, providing a sense of control, connection, and fulfillment that many find lacking in their daily lives. However, the allure of virtual worlds also raises concerns about the potential for addiction, social isolation, and the erosion of real-world relationships and experiences.One of the primary reasons for the appeal of virtual worlds lies in their ability to provide a sense of control and autonomy. Within these virtual realms, individuals can create and customize their avatars, choosing their appearance, abilities, and even their virtual backgrounds. This level of personalization empowers users, allowing them to express their creativity and explore different aspectsof their identity. Furthermore, virtual worlds oftenfeature game mechanics and challenges that provide userswith a sense of accomplishment and progress, further enhancing their feelings of control and empowerment.Another key factor contributing to the allure ofvirtual worlds is their ability to foster social connection and a sense of community. Within these virtual environments, individuals can interact with others from around the world, forming friendships, collaborating on projects, and engaging in meaningful conversations. This social aspect of virtual worlds can alleviate feelings of loneliness and isolation, providing users with a sense of belonging and support.In addition to providing a sense of control and social connection, virtual worlds also offer a wide range of immersive experiences that can be both entertaining and educational. These experiences can range from exploringvast virtual landscapes to participating in interactive simulations and role-playing scenarios. The immersivenature of virtual worlds enhances the user's sense ofpresence and engagement, making these experiences feel more real and meaningful.However, despite the numerous benefits and attractions of virtual worlds, concerns have been raised about their potential negative effects. One of the primary concerns is the risk of addiction. The immersive nature of virtual worlds can lead to excessive use, as individuals become absorbed in the virtual environment and neglect their real-world responsibilities and relationships. This can have detrimental consequences for the individual's health, well-being, and social life.Another concern associated with virtual worlds is the potential for social isolation. While virtual worlds can provide a sense of community, they can also lead to decreased engagement with real-world relationships. Spending excessive time in virtual environments candistract individuals from cultivating real-world connections and engaging in meaningful face-to-face interactions. This can result in a gradual withdrawal from social activities and a decline in the quality of real-world relationships.Furthermore, the erosion of real-world experiences is another potential concern associated with the allure of virtual worlds. The immersive nature of these virtual environments can lead to a reduced appreciation for real-world experiences, as individuals become accustomed to the convenience and instant gratification offered by virtual worlds. This can have a detrimental impact on theindividual's ability to fully engage with and derive meaning from the real world.In conclusion, the allure of virtual worlds lies in their ability to provide a sense of control, connection, and immersive experiences. However, concerns about addiction, social isolation, and the erosion of real-world experiences warrant careful consideration. It is important for individuals to be aware of these potential risks and to use virtual worlds in a balanced and responsible manner.中文回答：虚拟世界的魅力源于其提供掌控感、连接他和沉浸式体验的能力。

2020年广州市天河区中考一模英语试题(附答案)

2020届天河区部分学校初三总复习前测试题九年级英语(本试卷共四大题，10页，满分110分。

考试时间为120分钟。

)注意事项:1. 答卷前，考生务必在答题卡上用黑色字迹的钢笔或签字笔填写自己的学校、考生号、姓名、座位号，再用2B铅笔把对应的标号涂黑。

2. 选择题每小题选出答案后，用2B铅笔把答题卡上对应题目的答案标号涂黑。

如需要改动，用橡皮擦干净后，再选涂其他答案。

不能答在试卷上。

3. 非选择题必须用黑色字逵的钢笔或签字笔作答，答案必须写在答题卡各题目制定区域的相应位置上；如需要改动，先划掉原来的答案，然后再写上新的答案，改动的答案也不能超出指定的区域；不准使用铅笔、圆珠笔和涂改液。

不按以上要求作答的答案无效。

4. 考生必须保持答题卡的整洁，考试结束，将本试卷和答题卡一并交回。

一、语法选择(共15小题；每小题1分，滴分15分)阅读下面短文，按照句子结构的语法性和上下文连贯的要求，从1~15各题所给的A、 B、C和D项中选出最佳选项并在答题卡上将该项涂黑。

One evening, I went out for a walk on the path near my house with my husband.I wore a ring __1__ a beautiful diamond. It was a special ring __2__ my grandmother gave me for my 23rd birthday. At the end of the path, __3__ I looked down at it again, I found the diamond was missing. I started back to look for the tiny stone. In my heart, I knew __4__ seemed impossible to find it. The path was long and covered with leaves. As I __5__ the ground, I met an old lady. “What’s the matter, love?” She asked. I explained about the missing diamond of my ring. She said, “Tell me __6__, love, I walk along this path every day, and I’ll keep my eyes __7__ for it.” I thanked her and thought it couldn't __8__.A few days later, my husband and I met __9__ old lady on the path.“Guess what?” She said, “IT found your diamond!” To ou r __10__, she’d found the ti ny stone and was willing to hand it back to me.” __11__ lucky it was for me to find it!” When we provided her with some money, she refused __12__. I was so moved. She could have kept the diamond, or sold it for it's worth __13__ dollars, but she didn’t.Every time I look at may ring and remind __14__ there are kind and beautiful hearts in the world. And I'll spend the rest of my life __15__ to do something like that.1. A. on B. in C. at D. with2. A. who B. whom C. at which D. where3. A. before B. when C. since D. if4. A. it B. that C. this D. there5. A. search B. would search C. am searching D. was searching6. A. what is it like B. what it is like C. what was it like D. what it was like7. A. open B. opening C. opened D. opens8. A. find B. was finding C. be found D. was found9. A. a B. an C. the D. /10. A. surprise B. surprised C. surprising D. surprisingly11. A. What B. How C. What a D. How a12. A. polite B. politely C. more polite D. more politely13. A. several thousand B. several thousandsC. several thousands ofD. several thousand of14. A. I B. me C. mine D. myself15. A. try B. to try C. tried D. trying二、完形填空(共10小题；每小题1.5分，满分15分)通读下面短文，掌握其大意，然后在每小题所给的A、B. C和D项中选出最佳选项并在答题卡上将该项涂黑。

英语作文虚拟世界与现实生活

英语作文虚拟世界与现实生活The Interface of Virtual Worlds and Real Life.In today's digital age, the virtual world has become an integral part of our daily lives. It's a realm where dreams and imaginations are realized, where boundaries are pushed, and where connections are made. However, as we increasingly spend time in these virtual spaces, it's crucial to understand the differences and similarities between them and the real world.The virtual world is a creation of our minds, a projection of our desires and aspirations. It's a place where we can be anyone we want to be, do anything we want to do, and go anywhere our imaginations can take us. The possibilities are endless, and the experiences are often more intense and immersive than what we can achieve in the real world. Whether it's through gaming, social media, or virtual reality, the virtual world provides us with a sense of escape and adventure that can be both thrilling andrewarding.However, the virtual world is not a replacement forreal life. While it can provide us with a sense of fulfillment and accomplishment, it lacks the tangibility and authenticity of real-world experiences. The connections we make in the virtual world may be deep and meaningful, but they often lack the depth and complexity of real-world relationships. The achievements we achieve in the virtual world may be impressive, but they don't translate into the same sense of accomplishment and pride as real-world accomplishments.Moreover, the virtual world can also have negative impacts on our real lives. The overuse of virtual spaces can lead to addiction and dependency, affecting our mental health and well-being. The constant pursuit of virtual validation and approval can lead to feelings of inadequacy and dissatisfaction with our real lives. The anonymity and lack of consequences in the virtual world can also encourage bad behavior and unethical practices that can spill over into our real lives.Therefore, it's important to strike a balance between the virtual world and real life. We need to recognize that while the virtual world can provide us with valuable experiences and connections, it should never replace the real world. We need to make sure that we're investing enough time and energy into our real-world relationships and activities, and that we're using the virtual world as a complement to our real lives, not as a substitute for it.We also need to be mindful of the negative impacts of the virtual world and take steps to mitigate them. This includes setting boundaries and limits on our virtual activities, seeking professional help if we find ourselves becoming addicted or dependent on virtual spaces, and practicing good digital citizenship by adhering to ethical guidelines and respecting the rights and boundaries of others.In conclusion, the virtual world and real life are two distinct yet interconnected domains. The virtual world can provide us with valuable experiences and connections, butit should never replace the real world. By understanding the differences and similarities between these two worlds and striking a balance between them, we can ensure that we're living a balanced and fulfilling life in both the virtual and real worlds.。

八年级现代科技与人类生活英语阅读理解25题

八年级现代科技与人类生活英语阅读理解25题1<背景文章>Smartphones have become an essential part of our lives. They offer many benefits. For example, they allow us to stay connected with friends and family no matter where we are. We can send messages, make calls, and even video chat. Smartphones also provide us with access to a wealth of information. We can look up facts, read news, and watch educational videos. Moreover, they can be used for entertainment. We can play games, listen to music, and watch movies.However, smartphones also have some negative impacts. One of the main concerns is that people can become too addicted to them. They may spend hours on their phones, neglecting other important activities such as exercise, social interaction, and studying. Another issue is that smartphones can be a distraction, especially in places like classrooms and workplaces. They can disrupt concentration and reduce productivity. Additionally, excessive use of smartphones can have negative effects on our physical and mental health. For instance, it can cause eye strain, neck pain, and sleep problems.In conclusion, smartphones have both positive and negative effects on our lives. We need to use them wisely and in moderation to maximize theirbenefits and minimize their drawbacks.1. Smartphones can be used for all of the following EXCEPT _____.A. staying connected with othersB. doing exerciseC. looking up informationD. playing games答案：B。

《虚拟现实技术的应用》高中英语作文

《虚拟现实技术的应用》高中英语作文Virtual Reality: Applications and BenefitsVirtual reality (VR) has become increasingly popular in recent years, with a wide range of applications in various fields.From gaming and entertainment to education and healthcare, VR has revolutionized the way we interact with technology and the world around us.In this essay, we will explore some of the key applications and benefits of virtual reality technology.One of the most well-known applications of VR is in the field of gaming and entertainment.VR allows players to immerse themselves in a virtual world, providing a more interactive and engaging gaming experience.With the help of VR headsets and controllers, players can explore virtual environments, interact with characters, and even feel as if they are part of the game.This has opened up new possibilities for game developers and has attracted a large number of players who are looking for unique and immersive gaming experiences.Another important application of VR is in the field of education.VR can create realistic and interactive learning environments that can enhance the learning experience for students.For example, students can use VR to explore historical sites, learn about different cultures, or even practice surgical procedures in a virtual operating room.This can help to improve understanding and retention of information, as well as makinglearning more engaging and fun.VR also has a wide range of applications in the healthcare industry.It can be used to treat phobias and PTSD by exposing patients to virtual scenarios that can help them overcome their fears.VR can also be used in pain management, as it can distract patients from their pain and provide them with a more comfortable and relaxing environment.Additionally, VR can assist surgeons in performing complex procedures by providing them with a 3D visualization of the patient's anatomy, allowing for more precise and accurate surgeries.Despite these benefits, VR technology is not without its challenges.For example, it can cause motion sickness and eye strain in some users, and there are concerns about the potential for addiction to virtual worlds.However, as the technology continues to develop and improve, it is likely that these issues will be addressed and resolved.In conclusion, virtual reality technology has a wide range of applications and benefits in various fields, including gaming, education, and healthcare.As the technology continues to advance, it is exciting to consider the potential applications and benefits that VR may bring in the future.。

2023年12月全国大学英语CET四级真题和答案解析(第二套)

2023年12月全国大学英语CET四级真题和答案解析（第二套）一、阅读理解阅读理解一题目Welcome to the World of Virtual Reality!文章Virtual reality (VR) is a technology that simulates a user’s physical presence in a virtual environment. By wearing a VR headset, users are immersed in a three-dimensional computer-generated world, which can be a realistic representation of an existing location or an imaginary world. Virtual reality has gained popularity in recent years, with various applications in entertainment, education, and healthcare.One of the main uses of virtual reality is in the field of entertainment. VR gaming allows users to experience gameplay in a fully immersive and interactive manner. Withthe help of motion tracking sensors, users can move aroundand interact with virtual objects as if they were in the game themselves. This adds a new level of excitement and realism to gaming, making it a favorite among many enthusiasts.In addition to entertainment, virtual reality has also found applications in the field of education. Virtual realitysimulations can provide students with a more engaging and interactive learning experience. For example, instead of reading about ancient civilizations in a textbook, students can explore virtual ancient ruins and interact with virtual characters from that time period. This hands-on approach to learning helps students retain information better and makes learning more enjoyable.Virtual reality is also being used in healthcare to assist in the treatment of certain medical conditions. For example, VR therapy has been found to be beneficial in the treatment of anxiety disorders and phobias. By exposing patients to virtual environments that trigger their fears or anxieties in a controlled manner, therapists can help them gradually overcome their fears. This technology has also been used in physical rehabilitation, allowing patients to practice movements and exercises in a virtual environment.In conclusion, virtual reality has opened up a new world of possibilities in various fields. Whether it’s for entertainment, education, or healthcare, VR technology offers a unique and immersive experience that can enhance our lives in many ways.答案解析本文主要介绍了虚拟现实（VR）技术的定义和应用。

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A survey on virtual realityZHAO QinPingState Key Laboratory of Virtual Reality T echnology and System,Beihang University,Beijing 100083,China (email:zhaoqp@)Virtual reality (VR)is a scienti ﬁc method and technology created during the exploration of the nature by human beings to understand,simulate,and better adapt and use the nature.Based on the analysis on the whole process of VR,this paper presents different categories of VR problems and a type of theoretical expression,and abstracts three kinds of scienti ﬁc and technical problems in VR ﬁeld.On the basis of foresaid content,this paper also studies current major research objectives,research results and development trend of VR in the aspects of VR modeling method,VR representation technology,human-machine interaction and devices,VR development suites and supporting infrastructure,as well as VR applications.Finally,several theoretical and technical problems that need to be further studied and solved are addressed.virtual reality,modeling,rendering,human-machine interaction,development suiteBased on computer science and relevant scientiﬁc technologies,virtual reality (VR)produces a digi-tal environment in which visual perception,sense of hearing,and sense of touch are highly similar to those of actual environment within a certain range.With the help of necessary equipment to interact and interfere with objects in a digital en-vironment,users may have feelings and experiences corresponding to those in the actual environment.VR is a scientiﬁc technology for understanding or simulating the nature.With the constant development of social pro-ductivity and scientiﬁc technologies,the VR tech-nology has found more and more applications in various industries.Now VR has become a new science ﬁeld.This paper presents an overview ofthe developing process of VR.First we introduce the thoughts and the related research direction of VR,then we discuss the major contents and cur-rent situation of VR research in the aspects of VR modeling,VR representation technology,human-machine interaction and equipment,VR develop-ment suite and supporting environment,as well as VR applications,and indicate several problems of the theory and technology.1Formation of VR research ﬁeld1.1Formation of VR thoughts and research objectivesThe VR concepts,thoughts and research objectives have close relationship to some relevant scientiﬁcReceived June 10,2008;accepted February 13,2009doi:10.1007/s11432-009-0066-0Supported by the National Basic Research Program of China (Grant No.2009CB320805),the National Natural Science Foundation of China (Grant Nos.60533070,60503066),and the National High-Tech Research &Development Program of China (Grant Nos.2006AA01Z333,2006AA01Z311)Sci China Ser F-Inf Sci |Mar.2009|vol.52|no.3|348-400technologies,especially the computer science.In1929,Edwin A.Link invented a type ofﬂight simulator for making the passengers experience the feeling ofﬂight.It was theﬁrst try that human be-ings simulated or emulated physical reality.With the constant development of control technology thereafter,various simulators came into being in succession.In1956,Morton Heileg invented the Sensorama—a motorcycle emulator that showed 3D display and stereophonic eﬀects and produced vibration feeling.He advanced some basic thought of VR technology[1]in the Sensorama Simulator patent in1962.The development of electronic technology and the miniaturization of computers facilitated the development of simulation technol-ogy.Finally computer simulation technology ap-peared.In1965,the signiﬁcant founder of computer graphics,Dr.Sutherland[2]published a piece of essay The Ultimate Display,portraying a type of new display technology through his sharp insight and abundant imaginations.He assumed that, supported by this display technology,observers may be surrounded by a virtual environment con-trolled by a computer,just like daily life in the real world.Meanwhile,observers may also inter-act with the objects in virtual environment by nat-ural means,like touch perception,control of vir-tual objects,etc.From the angles of computer dis-play and human-machine interaction,Sutherland’s essay put forward the thoughts of simulating the real world,facilitated the development of computer graphics and image technologies,and cultivated the research on a new type of human-machine in-teraction equipment such as head mounted display (HMD)and data gloves.In1966,Sutherland began to develop a device of HMD,and later put the feedback device of sim-ulation force and sense of touch into the system. In1973,Krueger nominated the phrase of“Artiﬁ-cial Reality”—the VR words appearing during the early stage.Due to the constraints of computer technologies,generally speaking,the development of these types of technologies did not go very fast. During the1960s and1970s,the related thoughts, concepts and technologies still stayed in the brew-ing and formation stage.During the1980s,with the development of com-puter technologies,especially the update of PC and computer network,VR technology made much headway.Several typical VR systems came up dur-ing this stage.The implementation of SIMNET (SIMulation NETworking)Plan by US Army and DARPA in1983initiated the research and appli-cations of distributed interaction simulation tech-nology.Some successful technologies and experi-ence of SIMNET signiﬁcantly inﬂuenced the de-velopment of distributed VR technology.In1984, M.McGreevy and J.Humphries developed a visual display for virtual environment and built up a3D virtual surface environment of Mars by inputting the data from Mars probe to the computer.The de-velopment of VIDEOPLACE and VIEW systems facilitated the research on VR theories and tech-nologies.In1986,Fisher et al.[3]published The Virtual Environment Display System concerning VR sys-tems.In1987,Foley[4]published a paper on Scientific American.The journal also published some articles about data gloves that attracted the attention of the public.In1989,Jaron Lanier, the founder of VPL Co.,put forward the phrase of“Virtual Reality”,which was generally accepted by researchers and became the speciﬁc title of this scientiﬁc technologyﬁeld.During the1990s,with the breakthrough and rapid development of computer technology and high performance computation,human-machine interaction technology and equipment,computer network and communication,as well as huge de-mands in the signiﬁcant applicationﬁelds such as military drill,aeronautics and astronautics,and complicated equipment research,VR technology came into a rapid development stage.In1990,VR technology was discussed in SIG-GRAPH conference held in Dallas,USA,where the main contents of VR technology research were settled—formation technology of real-time3D graphics,interaction technology of multi-sensor, high resolution display technology,etc.In1993, Heim[5]portrayed seven characteristics of VR in Metaphysics of Virtual Reality:simulation,in-ZHAO QinPing Sci China Ser F-Inf Sci|Mar.2009|vol.52|no.3|348-400349teraction eﬀect,artiﬁcial reality,immersion,tele-presence,general immersion and network commu-nication.In1994,Burdea and Coiﬀet[6]published their book,Virtual Reality Technology,in which they used3I(Immersion,Interaction,Imagination) to generalize the basic characteristics of VR.A batch of software platforms and modeling lan-guages for VR system development appeared.In 1989,Quantum3D Co.developed Open GVS. Sense8Co.put forward WTK in1992.On the 1st Annual WWW Conference held in Geneva in March1994,VRML was brought forward for the ﬁrst time,initiating the establishment of relevant international standards.The development of computer modeling and sim-ulation started as early as the beginning of the 1970s in China,mainly concentrating in theﬁelds of aeronautics and astronautics.At the beginning of the1990s,the researchers of some universities and scientiﬁc institutions in China started the VR research[7]from diﬀerent points of view.The Min-istry of Science&Technology of the People’s Re-public of China and the National Natural Science Foundation of China started to support the re-search on VRﬁeld.The National863Program for-mulated the“Distributed Virtual Environment”as a key program and implemented DVENET Plan[8] in1996.In the past decade,Beijing University of Aero-nautics&Astronautics University,Zhejiang Uni-versity,Tsinghua University,Peking University, National University of Defense Technology,Bei-jing Institute of Technology,Wuhan University, Shandong University,Beijing Normal University, University of Electronic Science&Technology of China,Institute of Computing Technology&the Chinese Academy of Sciences,Institute of Automa-tion,the Chinese Academy of Sciences,the Sec-ond Academy of China Aerospace,and scientiﬁc researchers of many other application departments and institutions conducted many research eﬀorts with various backgrounds and characteristics[9−16], and achieved remarkable performances in the as-pects of VR theory research,technology innova-tion,system development,application promotion, etc.This scienceﬁeld came into a brand new de-velopment stage in China.Due to the integrity and specialty of VR science,and increasing application demands of economic,social and militaryﬁelds, the Essentials of National Medium and Long-Term Science and Technology Development Plan granted by the Central People’s Government of the People’s Republic of China in2006listed the VR technol-ogy as one of the prior developing frontier tech-nologies of informationﬁeld.The Ministry of Sci-ence and Technology of P.R.China approved the establishment of State Key Lab of Virtual Reality Technology and Systems in Beijing University of Aeronautics&Astronautics University in2007. 1.2Scientiﬁc and technical problems of virtual realityThe objectives of VR are to use computer tech-nologies and other relevant technologies to dupli-cate and emulate the real world(hypothetic world), and build a virtual world similar to the real world. Users experience the corresponding real world and even inﬂuence the real world through the interac-tion with virtual world.This procedure could be described as the following forms:Set W as the aggregate of all states in the real world(hypothetic world),and divide W into the aggregate T of subaggregate without intersection to let diﬀerent VR modeling methods correspond to the states of real world;C is the aggregate of computer state sequence;E is the aggregate of human-machine interaction equipment;e∈E and S(e)are the aggregate of e state sequence.We de-ﬁne an optional operation“see”ﬁrst,which reﬂects the states in W to the divisions it selects:see:W→T.To indicate the simulation of real world,the sim-ulation function“in”is deﬁned,which reﬂects the states in real world to the computer state sequence set:in:T→ρ(C)(ρ(C)is the power set of C.To indicate the eﬀect of object in virtual world on the outside world,deﬁne virtual object to present function“show”that reﬂects the computer state sequence set to the state sequence set of human-machine equipment as350ZHAO QinPing Sci China Ser F-Inf Sci|Mar.2009|vol.52|no.3|348-400show:ρ(C)→S(E)(S(E)is the state sequence set of all interaction equipment).To indicate the human control of virtual environ-ment,deﬁne the control function“do”asdo:C×S(E)→C.Deﬁne VR system as an octet at last:W,T,C,E,see,in,show,doThe functions of“in”,“show”,and“do”are the three major scientiﬁc and technical problems of vir-tual reality.“in”,the modeling of VR,could be an algorithmic or axiomatic system,or a manual computer input process with structures;the pre-sentation of VR object“show”could be an algo-rithmic or data conversion,or a data outputﬂow with structures;“do”is the control of virtual envi-ronment by humans or the outside world,which is the data inputﬂow with structures and related to interaction equipment closely.The following sections of this paper will discuss these three major problems.1.3Classiﬁcation of virtual reality system The research object,research objective,and appli-cation demands of VR all determine this scientiﬁc and technicalﬁeld with comprehensive and crossed subjects,extensive application areas,and various system categories.Diﬀerent classiﬁcations can be determined by diﬀerent points of view of VR sys-tem.1.3.1Classiﬁcation by system functions.Con-sidering the functions and eﬀects of system,which are actually the applications,VR system could be divided into the following categories:training& drill,planning&design,and presentation&enter-tainment.The Training&Drill System can be extensively used for training and drill of technical operation in various dangerous circumstances(such as nuclear facilities and submarine facilities)where operation objects cannot be obtained easily(such as medical operation and spacecraft maintenance)and cost a large amount of money(such as military drill);the Planning&Design System can be used for demon-stration and veriﬁcation of newly-built facilities and equipment,thereby greatly reducing design cost,shortening design cycle and increasing de-sign rationality,such as the planning and design of cities,communities and buildings,and virtual designs and virtual assemblies of equipment and products;the Presentation&Entertainment Sys-tem will realize the digitalization of the scenes in the real world or the hypothetical world,and pro-vide users with realistic enjoying experiences,such as virtual landscape,digital museum,various game manufactures,movie and television manufacture, etc.1.3.2Classiﬁcation by immersion experience. VR system can be divided into non-interactive ex-perience,human-virtual environmental interactive experience and group-virtual environmental inter-active experience by immersion experience.The above point of view emphasizes the interac-tion between users and virtual environment.With regard to non-interactive experience,users’experi-ence of virtual environment is passive and the con-tent of experience is completely planned.Some systems provide visual points for users to make selections to a certain degree so as to guide data dispatching of virtual scenes.But no such mate-rial interactions will happen as scenic wandering, 4D cinema,etc.In a human-virtual environmen-tal interactive experience system,users may inter-act with virtual environment through interaction equipment(such as data gloves,digital weapons, digital scalpels,etc.).Scenes in virtual environ-ment will respond to the interaction in time,so that users may feel the change in virtual envi-ronment and experience corresponding real world, such asﬂight simulator,individual-soldier train-ing environment,etc.The group-virtual environ-mental interactive experience system is the multi-machine and network of human-virtual environ-mental interactive experience system.Multiple or group users may share one virtual environment, and interact with virtual environment that includes user avatars as well so as to experience the inter-action with virtual environment and other users, such as military emulation drill,on-line interaction games,etc.1.3.3Dataﬂow in virtual reality system.VRZHAO QinPing Sci China Ser F-Inf Sci|Mar.2009|vol.52|no.3|348-400351system is a complicated information system pos-sessing various types of data,which can be de-scribed by the modes and trends of diﬀerent types of data ﬂow during operation period.According to this point of view,VR system is an information system that has m types of data interacting with relevant environment and converting,accumulat-ing and ﬂowing constantly in terms of certain rules during n stages.The data in VR system could be divided into four categories according their origins and eﬀects:Platform Data (PD)includes metadata produced by computer system,network system and public platform that support speciﬁc operation of virtual reality system,and administrative data produced by the all-purpose parts of speciﬁc VR system;Model Data (MD)is the master data of VR sys-tem,and also the reﬂection of scenes in the real world to digital space,which can be produced by scanners,and model algorithm,etc.;Sense Data (D),or the rendering data,is produced by various rendering algorithms applied to all types of out-put equipment to let users have visual,auditory,and force feelings;Control Data (CD)means the data produced through input equipment by users,applied to the control of and eﬀects to virtual en-vironment.Figure 1shows the data ﬂow in a scenic wander-ing VR system.Virtual scenes are established by developers using data acquisition equipment and modeling software such as video,image,and spec-trum instruments and scanners.It can use the mouse and control sticks,thus inputting control data to dispatch model data of the scene during wandering so as to select the visual point of wan-dering.Figure 1Scenic wandering VR system.Figure 2shows a data ﬂow in individual-soldier training ers produce control data andinteract with objects in VR scenes through digi-tal bining with the platform data formed by system rules,they also produce new model data and then generate new perception data through renderingalgorithm.Figure 2Individual-soldier VR training system.Multi-person rivalry-drill VR system can be de-veloped and established based on the individual-soldier training system,and operated under the support of the operative and supportive environ-ment of distributed VR system,such as DIS,RTI,etc.2Modeling methods in virtual realityIn order to simulate the objects and states of real world in digital space,the regulations followed by the objects,object relationships,interactions be-tween objects,and development and change in the real world should be reﬂected as various data in digital space for presentation.This process is called modeling,as in function “in”mentioned in VR Octet in section 1.This section mainly in-troduces the methods and development trends of modeling.2.1Classiﬁcation of model data types and modeling methods2.1.1Origins and types of model data.Model-ing refers to model data ﬁrst.Due to the multi-plicity of objects and states in the real world,there are various model data.But with regard to data origins,there are mainly three categories of model data,namely actual measurement,mathematical generation,and artiﬁcial construction.1)Actual measurement.It refers to the data acquired through 2D or 3D scanners,cameras,mo-tion capture equipment,data clothes,data gloves352ZHAO QinPing Sci China Ser F-Inf Sci |Mar.2009|vol.52|no.3|348-400and various special data acquisition equipment (such as CT,B-ultrasound,nuclear magnetic res-onance,satellite remote sensing,etc.)or artiﬁcial measurement.2)Mathematical generation.It refers to the for-mation of mathematical models such as physical equation,analogue algorithm,and axiomatic sys-tem through experimental analysis,physical em-ulation,induction,abstraction,etc.The compu-tation of mathematical models can generate model data,such as position data of objects acquired with equation of motion.3)Artiﬁcial construction.It refers to model data generated artiﬁcially or through hypothesis and imagination by artiﬁcial modeling software(CRE-ATOR,3D Studio,AutoCAD)such as3D models of objects,cartoon,movie and television manufac-ture,etc.During the process of actual modeling,the com-plexity of simulated objects in the real world and the expectation degree of modelﬁdelity should be taken into consideration to combine multiple meth-ods to generate model data.In VR system,according to diﬀerent aspects presented by the objects in the real world,the model data could also be divided into the following categories—spatial structure data,physical prop-erty,behavioral property,dynamic data and mo-tion data.Spatial structure data is to indicate ge-ometric state of objects,such as point cloud,grid, voxel,etc.The data are presented as solid,liquid and gas.Typical spatial structure data is the3D model of object.Physical properties and param-eters are used for describing the properties shown by objects taking part in physical process,such as weight,size,material,force friction,lamella-tion,deformation parameter;behavioral properties and parameters are to describe the properties rep-resented during behavioral process;dynamic and motion data are to describe the motion,deforma-tion,collision,capture,zooming,and breakage of objects,such as the data of an object’s position, speed and acceleration;ﬁeld data is used for de-scribing the states and properties of variousﬁeld composition,such as electromagneticﬁeld,gravita-tionalﬁeld,ﬂuidﬁeld,soundﬁeld,and windﬁeld.Field data could be divided into scalarﬁeld,vector ﬁeld,tensorﬁeld,etc.2.1.2Classiﬁcation of modeling methods.VR modeling methods are related to object category required to be simulated,as well as the application ﬁelds.Therefore,same as the classiﬁcation of VR system,it could also classify VR modeling meth-ods based on diﬀerent starting points.For exam-ple,based on users’perception,modeling methods include visual modeling,auditory modeling,haptic modeling,etc.According to diﬀerent aspects of simulated ob-jects,the modeling methods can be divided into scene appearance modeling,physics-based mod-eling,behavior modeling,virtual-real combining modeling,etc.Scene appearance modeling focuses on the appearance of the scenery,mainly includ-ing those modeling methods based on geometry, image and material and illumination information; physics-based modeling represents object’s phys-ical properties,making the dynamic and static sceneries in virtual environment more vivid,mainly referring to the simulation of physical process,such as simulation of dynamics,collision,and deforma-tion;at present,behavior modeling in VR mainly indicates the modeling of autonomous entity,refer-ring to artiﬁcial puter generated forces(CGF)is a typical application of behavior modeling or VR;the virtual-real combining model-ing is a type of signiﬁcant modeling methods,which allows the virtual sceneries generated by the com-puter to naturally interfuse with actual environ-ment of the real world so as to eﬀectively improve the eﬃciency,ﬂexibility and verisimilitude of VR modeling.The virtual-real combining modeling is the major research content to enhance reality at present.The following part of this chapter will introduce some normal VR modeling methods used nowadays from the foresaid four aspects.2.2Scene appearance modeling method Scene appearance model is used for displaying the spatial structures and appearances of virtual ob-jects,including point cloud model,grid model, voxel model,etc.The major modeling methods in-ZHAO QinPing Sci China Ser F-Inf Sci|Mar.2009|vol.52|no.3|348-400353clude those based on range image,image and voxel. Object’s surface illumination model is included as well.2.2.1Modeling based on depth range image. Each pixel of range image saves the range infor-mation ofﬁrst intersection point of related light and the scene shot,the modeling process of which involves the acquisition,registration,surface recon-struction and repair of range image.(1)Acquisition and registration of range im-age.The most direct way to acquire range im-age is to use3D scanning equipment.3D scanning equipment includes remote type,medium range type,and short range type.All are applicable to those sweep objects with diﬀerent sizes and diﬀer-ent ranges.The contact type and laser type are available at present.3D laser scanner adopts the round trip longimetry or the triangular longime-try to acquire range information,which has rapid speed of data acquisition,high accuracy,and wide range of applications.In order to acquire all the information of3D ob-ject’s surface,we need to conduct scanning and sampling from diﬀerent visual points,and then put these range images together to form complete point cloud data of object surface.This process is called the registration of range image.Because the data acquired from diﬀerent visual points are located at independent coordinate system of their own,we need to transform them to the same co-ordinate system.The registration of range image is to look for the move of these range images to solve the transformation matrix between these co-ordinate systems.At present,the usual registration procedure of range image involves four steps:every two coarse registration,every two precise registration,adja-cent relation determination,and multiple visual points registration.In the aspects of every two coarse registration,Chen[17]adopted the Global Search Algorithm to realize the registration;how-ever,this method was slow and unstable;Huang[18] put forward the registration method based on the characteristics to abstract the characteristics of range image to be registeredﬁrst,toﬁnd out re-liable coincidence relation between characteristics,and to obtain candidate results of corresponding coarse registration,then to combine the precise registration to conduct consistency inspection on candidate registration results,andﬁnally to ac-quire the optimal registration results.Based on algorithmic theory of ICP(iterative closest point), the every two precise registration is mainly to esti-mate the relative position of two images,and then gradually approach to precise registration results through iteration.Relative relation of image is de-termined through the overlap between images and the results of every two registration[19].The ob-jective of multiple visual points registration is to optimize the overall error and eliminate the accu-mulation of every two errors.(2)Surface reconstruction of3D model.As-sume there is a data set D of sampling point coming from surface S.Then surface reconstruc-tion means to construct a surface S similar to S through D.Surface reconstruction algorithm can be divided into two types in general:prox-imal surface(implicit surface)and interpolation surface(explicit surface).The zero iso-surface of (oriented)distanceﬁeld from proximal surface’s construction point to object surface is the recon-struction surface,such as Marching Cubes Algo-rithm.The interpolation surface applies Voronoi Drawing to conduct Delaunay triangularization to sampled data,andﬁnds out the topological relation between sampled points so as to obtain a surface covering the original sampled points[20].(3)Repair of3D model.Repair methods of3D model mainly include those methods based on grid and volume data.Repair method of model based on grid[21]is toﬁnd out the edge of defective part of a model surfaceﬁrst,then look for appropriate grid faces toﬁll up these defects,and conduct re-ﬁnement and smoothness treatment on these grid faces,so that the additive parts can keep con-sistent with the existing grids in the aspects of grid structure and consistency.This method is ap-plicable to repairing those cavities with relatively small areas,sensitive to data noise,but unable to guarantee the completeness of results.The repair method based on volume data[22,23]ﬁrst conducts voxelization treatment on polygonal grid,convert354ZHAO QinPing Sci China Ser F-Inf Sci|Mar.2009|vol.52|no.3|348-400the polygonal grid data into volume data,then repair volume data,and use the method like iso-surface acquisition to regenerate polygonal grids. In addition,there are repair methods based on radial basis function,ﬁnite element and existing model samples[24].These methods are eﬀective to small cavities,but will cause nonconformity from geometric characteristic of original object when re-pairing large cavities.Therefore,Sharf[25]put for-ward a repair method based on the surrounding grid information,which is to use the geometric characteristic information of known area to rebuild defective surface of object and acquire favorable re-pair results.2.2.2Image-based modeling method.Image-based modeling method uses photos to construct vivid3D models,which can be divided into ac-tive type and passive type.Active type obtains 3D information of object actively through the con-trol of illumination of the scene,which has high re-construction accuracy and an easy algorithm.But it requires artiﬁcial reconstruction of clues,whose operating process is complicated.Passive type re-ceives intensity information of sceneries passively, and then conducts3D reconstruction through ana-lyzing passive clues of image such as tone,shadow, focal length,texture,and parallax.This method barely restricts the scale and position of modeling sceneries,but has poor accuracy and complicated algorithm.(1)Reconstruction of geometric model based on single image.People often use tone,texture and focal distance to conduct geometric reconstruction of single images in the aspect of computer vision. As this type of method strictly requires the shape, reﬂection property and exposure degree of image in the scene,it is only applicable to the3D re-construction in certain special sceneries.There are mainly two types of methods of geometric recon-struction based on single image for VR applications at present.One is based on plenty of interactions, ing conventional image editing method to specify depth value of each point in the image;the other is to introduce model base,ing the basic structural model base of architecture to re-construct the model of architecture based on single image[26].(2)Reconstruction of geometric model by adopt-ing stereo vision and structured light method. Stereo vision method is to rebuild geometric model of the object by using two or more images.The most important part of stereo vision method is to seek and match the corresponding points be-tween images.By introducing polar line constraint, the scope of searching for image corresponding points can be reduced from two dimensions to one dimension[27]to obtain the matching with higher stability and accuracy.Scharstein[28]has made classiﬁcation and comparison of all corresponding matching algorithms.The method used to get the corresponding point between light source and im-age by the known light source coding information is called structured light method.The common light source codes include checker,black and white stripes,sine light,etc.Salvi has summarized the common coding method of structured light. (3)Scenery reconstruction by using prior knowl-edge.As it is diﬃcult to make good eﬀect by di-rectly recovering geometric information of scenery only depending on image information,some meth-ods for rebuilding scenery with the aid of prior knowledge were proposed.Wei[29]ﬁrstly used the “accurate and strict”method to recover geomet-ric information and then combined it with prior knowledge toﬁnally recover hair and plant models. Tan adopted the image-based method in combina-tion with prior knowledge to rebuild a tree model. Compared to not using prior knowledge,the re-building eﬀect is greatly improved[30].(4)Reconstruction of geometric model based on silhouette contour line.Silhouette contour line of object on the image is an important clue to un-derstand geometric shape of an object.Silhouette contour line and the corresponding projection cen-ter determine a conical shell of3D space together. The set of all these conical shells is composed a spatial envelop covering this object.This spatial envelop is called the visual shell of object,which is an approximation of this object.Since generation algorithm of visual shell usually requires reference images of multiple visual points,we need to con-duct image calibration.And computation of get-ZHAO QinPing Sci China Ser F-Inf Sci|Mar.2009|vol.52|no.3|348-400355。

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