Abstract: In recent years, China's virtual reality technology with 5G technology, sensor technology and the development of civil graphics processors and rapid development, education, transportation, business, entertainment, industry and other areas of virtual reality demand is increasing. Virtual reality technology is a new comprehensive information technology, in which the positioning technology is the key technology to determine user immersion and interaction, is an important support of virtual reality technology. Therefore, it is necessary to summarize the positioning technology of virtual reality technology. Firstly, virtual reality and location technology are introduced. Secondly, it analyzes and compares the typical positioning technologies used in the current virtual reality system in detail, and introduces the principles of these technologies, relevant research results and their specific application scenarios in virtual reality. Then it introduces the mainstream virtual reality positioning equipment in the market, and discusses the positioning algorithm used by the virtual reality positioning technology. Finally, the existing problems and future development direction of virtual reality positioning technology are introduced.

Key words: Computer application technology; Virtual reality; Positioning technology; Location algorithm; Virtual reality equipment

One, the introduction

Virtual reality system is a realistic imitation application system with 3I characteristics (Immersion, Interaction, Imagination). With the rapid development of virtual reality technology in recent years, it has been widely used in medical, transportation, education, entertainment, military, archaeology and other industries. As a result, users are no longer satisfied with simply interacting with the original way of mouse and keyboard, and a new revolutionary way of interaction has emerged. Current human-computer interaction mainly relies on wIMP interaction paradigm [1], while interaction in the real world is mainly carried out through people's five senses (taste, vision, hearing, smell and touch). Virtual reality aims to completely simulate the interaction in the real world. However, due to the current technical limitations, interactive information can only be obtained through virtual reality devices in a metaphorical way, with poor accuracy. At present, mainstream virtual reality equipment providers provide products including two locators, two handles and one head display. The locator is used to capture the position of the head display, the handle is used to capture the position of the hand, and the head display is used to transmit 3D scenes and capture the head positioning and direction. The positioner and its positioning technology play an important role in virtual reality positioning system.

The information of position, direction and displacement of objects in 3d real space is provided to virtual reality system with high precision, low delay and low computational complexity, and then displayed in virtual space. For example, if the head rotation Angle and three-dimensional space position of the head are captured more accurately, the faster the head rotation Angle and three-dimensional space position of the head are transmitted, the less dizzy the person will be and the more immersed in the virtual space. At present, the main 3d target location technology is to use cameras, sensors, laser, infrared and other objects ranging and positioning. In the past 30 years of development, generally speaking, foreign virtual reality technology including positioning technology is relatively mature, high social acceptance, while domestic research is still in the middle of development. With the popularization of 5G and the rapid development of commercial Gpus, virtual reality technology is now being applied in more and more fields, and in the near future, virtual reality may reshape the way people live.

Second, positioning technology

two 1. Virtual reality positioning in interior space

At present, the object localization algorithm in indoor space includes adjacent information method, algebraic integration method, geometric feature method and so on. Through the use of these algorithms, Bluetooth positioning technology, ultrasonic positioning technology, infrared positioning technology, laser positioning technology, visible light positioning technology, W-I Fi positioning technology, etc. [2]. The comparison of different positioning techniques is shown in Figure 1.

FIG. 1 Virtual reality positioning technology in interior space

two 2. Infrared positioning technology

Infrared positioning technology can capture reflected images by placing multiple infrared emitters in space and infrared reflective points on the target object, so as to determine the user's position in space [3]. The infrared positioning technology is mainly introduced in the following section, taking the Lighthouse of HTC VIVE as an example. The HTC VIVE has two base stations, each with an array of infrared LEDS and two infrared lasers that rotate at a 10ms turn perpendicular to each other. The base station takes 20ms as a working cycle when working. Infrared LED flashes at the beginning of the cycle. The rotating laser of X axis sweeps the whole space in the first 10ms, and the Y axis does not emit light. In the last 10ms, the rotating laser of Y axis sweeps the whole space, and the X axis does not emit light. After the base station's LED flashes, the base station synchronizes the signal, and a photosensor measures the time it takes for the X - and Y-axis lasers to reach the sensor. This time is exactly the Angle of the X - and Y-axis laser to this particular photosensitive sensor, at which point the X - and Y-axis Angle of the sensor relative to the base station can be calculated. Since the position of photosensitive sensors distributed on the head display and controller is known, the position and motion trajectory of the head display can be calculated by the position difference of each sensor. Polona et al. [4] constructed a virtual reality real-time tracking system by using THE positioner and reverse kinematics technology of HTC VIVE, which effectively reduced the delay and improved the accuracy and reliability. Adrian et al. [5] improved Valve's steamVR tracking technology based on infrared positioning in terms of placement and calibration of photosensitive elements, reducing the cost of head display and tracking equipment. The current problem with the HTC VIVE is that its handles are fixed in shape and size, which doesn't give users a realistic grip in virtual reality. Chang et al. [6] improved HTC VIVE handle. By adding Leap Motion sensors and IMU(Inertial MeasurementUnit) sensors to real objects, the team can capture the position and direction of real objects and then represent them in a virtual space, giving users a real feeling in a virtual space. Xu et al. [7] constructed a relatively cheap information system that can monitor accurate position and orientation. In this system, inexpensive infrared cameras are fixed to the ceiling. Each user in the environment wears an infrared LED module. The distance between any two infrared leds in an LED module is different from that of other modules. Using stereo vision theory and recognition algorithms, the researchers were able to identify each user in real time, getting their precise location and orientation. The system strikes a good balance between price and performance. Liu et al. [8] built the Virtual reality system platform of Han Chang 'an City. The system modeled the main buildings of Han Chang 'an city and recreated the real scene in the HTC VIVE headset. Then, kinect-SDK was used to obtain the position information of bone joints, and quaternion was used to describe the rotation of joints. A double quaternion linear hybrid skin algorithm is used to skin 3d character models automatically. Finally, the interactive control between user and Han Chang 'an city is realized by human pose recognition algorithm. Infrared positioning technology realizes the positioning of objects in space with the help of infrared physical properties. Its advantages lie in high positioning accuracy, low delay, and its disadvantages lie in high cost.

two 3. Visible light positioning technology

Visible light positioning technology is a kind of optical positioning technology. LED array with known position in space serves as position reference point. LED codes and modulates position information and sends optical signal while lighting. Then, the time spent in signal transmission, arrival Angle and intensity are calculated through optical signal information, and the coordinate of moving target is obtained by phase positioning algorithm [9]. The visible light positioning process is shown in Figure 2.

FIG. 2 Visible light positioning process

SONY Play Station VR adopts visible light active positioning technology, installs color LED lights on the head display, calculates the position information of the color LED lights captured by using binocular vision principle, and realizes multi-target tracking by distinguishing colors. However, like Oculus Rift using infrared positioning [10], the positioning range of this technology is limited by the visual size of the two cameras, and there are problems of occlusion, as well as the influence of natural light, which may lead to positioning errors or even failure.

The advantage of Visible Light Communications (VLC) technology is that the working frequency band is the Visible spectrum and there is no electromagnetic radiation. It can be directly applied in some areas where electromagnetic radiation is prohibited, such as operating rooms and gas stations. Compared with WiFi base stations, LightEmittingDiode (LED) has a higher layout density and less multipath interference in real life. To meet lighting needs, indoor LED communication has a high signal-to-noise ratio. In addition to being used for indoor positioning systems, LEDS can also meet the requirements of high speed communication. LED positioning accuracy is high, LED based indoor positioning accuracy can reach cm level.

3. The positioning of objects in the virtual world

In distributed virtual reality system, with the development of virtual reality rendering technology, more and more data need to be transmitted, resulting in the virtual reality image seen by users to produce a lag. In view of this problem, Zheng et al. [11] used the calculation positioning method based on SCP model to predict the displacement of objects in virtual reality in advance. Only when the ontology displacement in reality exceeds the threshold value, it will be transmitted through the network, thus effectively reducing the number of transmitted packets and reducing the network load. Wang et al. [12] simulated the geographical environment in 3d space through the images shot by the camera, and reversely deduced the spatial coordinates of objects in reality by using the location of scenes shot by the camera in the virtual world. The difference between object localization in virtual world and object localization in real world lies in that the position and direction of object in virtual world are obtained through simulation calculation, which is used to assist the needs of reality. In reality, object positioning technology uses visible or invisible light, Bluetooth, WiFi, electromagnetic waves and other signals to locate objects and map them to the virtual space.

Fourth, the use of virtual reality positioning technology in mobile platforms

At present, mobile platforms mainly refer to smart phones. Lee et al. [42] captured the information of hands through marks and the built-in camera of mobile phones and transformed it into a matrix, through which they could draw hands in virtual reality. This exploration will contribute to the better application of virtual reality technology in hand-eye collaborative treatment in the medical field. Zayer et al. [13] use the Stereo Track technology based on the Doppler Shifts method to judge the distance through the arrival time of sound, thereby reducing the computational complexity of CPU, delay and development cost of mobile VR by using mobile phone microphones and microphones. Fang et al. [44] proposed a real-time motion tracking method for mobile AR/VR based on visual inertia, and realized real-time capture of mobile AR/VR posture by using the high frequency and passive output of mobile phone's inertial sensor. In addition, in order to alleviate the dithering phenomenon in the process of visual inertial fusion, the researchers established a framework, an adaptive filtering through the jitter and delay the balance of the 6 dof movement of real-time tracking, to enhance the traditional motion tracking robustness based on vision, make its have the better when meeting motion blur mobile AR/VR performance. Chiu et al. proposed a Mobile Augmented Reality Interactive System based on cloud computing. Including MARIS-I (image) for image target tracking and MARIS-H (Hand) for hand motion tracking. Maris-i uses mean-shift algorithm based on feature to estimate the position of the image target, which has the real-time feature detection of small area. Maris-h offers two tracking modes for fingertip and back of hand tracking. In order to enhance UX for interactive operation), the center position of the back of hand and fingertip is estimated by particle filtering technology, and the weight of each particle is calculated according to the hand or fingertip model. Then, in fingertip tracking mode, contour evolution method based on level set is used to estimate fingertip contour. In addition, the researchers proposed implementing a cloud architecture for MARIS to reduce memory requirements and computational complexity on the device side. Experimental results show that MARIS-I and MARIS-H are superior to other existing methods in image and hand motion tracking respectively. Berrie et al. proposed a low-cost virtual reality game system with a smartphone as the display and processing unit and a rear camera as the main sensor.

5. Development trend and direction

Through the analysis of market research reports, it can be found that the current mainstream VR device is HTC VIVE Oculus Rift[14], so HTC and Oculus are dominant in the development direction of VR positioning technology in the future. The HTC VIVE uses laser positioning technology. In laser positioning technology, the matching problem between the reflector scanned by laser and the one with known position in the environment is a key research direction. In addition, in recent studies, map reconstruction algorithm, closed loop detection algorithm, fusion localization algorithm and so on are the focus of laser positioning research. The positioning technology used by Oculus Rift is infrared positioning. Beacon imaging is the key point in infrared positioning technology. Software and hardware planning for beacon imaging has become a research hotspot. In the next step, infrared positioning technology will expand its application field by optimizing the algorithm and developing the hardware, and further improve the positioning accuracy and user experience on the basis of combining the traditional technology. Due to the inherent limitations of all positioning technologies, it has become a development trend to integrate the advantages of multiple positioning technologies to improve positioning accuracy and reliability. Fusion location can be divided into two forms: bayesian filtering based fusion location and machine learning based fusion location. With the development of VR positioning system, VR films will surpass IMAX and 3D films in the entertainment industry, bringing revolutionary changes to the film industry and making audiences feel like they are there. In navigation, 3D GEOGRAPHIC information system combines traditional GIS and virtual reality to form realistic map scenes, so as to improve users' navigation experience. From the perspective of standardization, the current VR positioning system lacks unified standards in both software and hardware, so there will be unified standards around the world like personal computers and smart phones in the future. In terms of the content required for positioning, future VR hardware design will start from human vision, hearing, smell and touch, collect the most basic sensory information of human beings, and design a new interaction paradigm that can replace WIMP. In terms of positioning accuracy, due to the demand of high immersion of virtual reality, positioning accuracy will develop to the level of centimeters and millimeters. At present, virtual reality is mainly used indoors, and the indoor positioning accuracy is higher than outdoor positioning accuracy. With the continuous improvement of infrastructure, the outdoor positioning accuracy of virtual reality applications will approach the indoor positioning accuracy.

reference

[1] ZHANG F J,DAI G Z,PENG X L.A Review of Human-computer Interaction in virtual Reality [J]. Science in China: Information Science,2016,46(12):1711-1736.

[2] KHALAJMEHRABADI A,GATSIS N,AKOPIAND. Modern WLAN Fingerprinting Indoor Positioning Methods and Deployment Challenges [J]. IEEE Communications

[3] WU J,LIU X J,PANG T,et al. The Status Quo of virtual reality and research on key Technologies [J]. Guangdong Communication Technology, 2016, 4 (8), 40-46.

[4] CASERMAN P, GarCIA-Agundeza,KONRAD R,et al. Real-time body tracking based on Vive Tracker [J]. Virtual Reality, 2019, 23 (2) : 155-168.

[5] NG A,CHAN L,LAU H.A low-cost lighthouse based virtual reality head tracking system[C]∥2017International Conference On 3D Immersion (IC3D). Brussels:IEEE,2017:1-5.

[6] CHANG Y K,HUANG J W,CHEN CH,et al. Lightweight and Efficient System for Tracking handheld Objects in virtual Systems Reality [C]∥Proceedings of the 24TH ACM Symposium on Virtual

Reality Software and Technology Article. Tokyo:ACM,2018:1-2.

[7] XU W,WANG B,JIANG Y.Mt-I Target indoor Tracking and recognition System with infrared markers for virtual devices Reality [C]∥Electronic and Automation Control Conference (IAEAC). Chongqing:IEEE,2017:1549-1553.

[8]LIU X X,FENG X Y,PAN S J,et al. Skeleton Tracking Based on Kinect Camera and its Application in Virtual Reality System[C]∥ The 4th International Conference on Virtual Reality

(ICVR2018). Hong Kong,2018:21-25.

[9]WU N,WANG X D,HUQ Q,et al. High-precision indoor visi-ble light positioning method based on Mult-i LED[J]. Journal of Electronics & Information Technology,2015,37(3):727-732.

[10]TAN C T,LEONG T W,SHEN S,et al. Exploring Gameplay Experiences on the Oculus Rift[C]∥Proceedings of the 2015Symposium  on Computer-Human Interaction in Play(CHI

Play '15). 2015:253-263.

[11] Zhang Y,LI S K,HU C J,et al. An improved positioning system for distributed VR systems [J]. Computer Research and Development,1999(2):59-64.

[12]WANG Y z.research on the combination of target location andvirtual reality in virtual geographic environment[D]. Fuzhou:Fuzhou University,2013.

[13] Li F D I Y. EmergingTechnologies for Mobile AR/VR[C]∥ SIGGRAPH '17 ACM SIGGRAPH 2017 EmergingTechnologies Angeles: ACM, 2017:1-2.

[14]LI X, Wang X, Liu X, et al. A Virtual Reality Analysis model for Virtual Reality [J]. China: Shanxi Securities, 2019.