The Renaissance of Augmented Reality in Construction: History, Present Status, and Future Directions Ke Chen, and Fan Xue This is the peer-reviewed post-print version of the paper: Chen, K., & Xue, F. (2021). The renaissance of Augmented Reality in construction: history, present status, and future directions. Smart and Sustainable Built Environment, in press. Doi: 10.1108/SASBE-08-2020-0124 The final version of this paper is available at: https://doi.org/10.1108/SASBE-08-2020-0124 The use of this file must follow the Creative Commons Attribution Non-Commercial No Derivatives License, as required by Emerald’s policy. Any other use requires prior permissions of Emerald. Abstract: Purpose: Augmented Reality (AR) has become one of the most promising technologies in construction since it can seamlessly connect the physical construction environment and virtual contents. In view of the recent research efforts, this study attempts to summarize the latest research achievements and inform future development of AR in construction. Design/methodology/approach: The review was conducted in three steps. First, a keyword search was adopted, and 546 papers were found from Scopus and Web of Science. Secondly, each paper was screened based on the selection criteria, and a final set of 69 papers was obtained. Thirdly, specific AR applications and the associated technical details were extracted from the 69 papers for further analysis. Findings: The review shows that: (1) design assessment, process monitoring, and maintenance management and operation were the most frequently cited AR applications in the design, construction, and operation stages, respectively; (2) information browser and tangible interaction were more frequently adopted than collaborative interaction and hybrid interaction; and (3) AR has been integrated with BIM, computer vision, and cloud computing for enhanced functions. Originality/value: The contributions of this study to the body of knowledge are twofold. First, this study extends the understanding of AR applications in the construction setting. Second, this study identifies possible improvements in the design and development of AR systems in order to leverage their benefits to construction. Keywords: augmented reality; construction management; physical environment; virtual content; interaction; review 1 1. Introduction 2 Modern construction management in its broader sense is to make informed decisions 3 according to a wealth of information. Perhaps since the emergence of modern construction 4 management, the serious disconnection between the physical environment and virtual 5 information has become an intrinsic part of managerial problems in construction projects (Chen 6 et al., 2015). The problem has further escalated throughout the 2000-2010s and enlarged by the 7 increasing complexity in construction projects. In response, the industry started to embrace 8 advanced digital technologies in order to make accurate information readily available to both 9 managers and workers (Newman et al., 2020). Against this backdrop, Augmented Reality (AR), 10 among all digital technologies, has received attention from both researchers and practitioners 11 in construction. 12 AR creates a composite view of the virtual and reality by superimposing the digital 13 representations (e.g., image or model) of objects onto the physical environment (Azuma, 1997). 14 Such alignment of digital representations with people’s view of the real world enables the 15 interpretation of both the virtual and reality simultaneously. By doing this, AR could improve 16 the information perception process and thereby greatly facilitate decision-making. Researchers 17 have recognized that AR can support operational and managerial activities in various industries 18 (Henderson and Feiner, 2010). In construction, AR has many applications such as the 19 communication of design and planning ideas (Wang et al., 2014b), on-site information retrieval 20 (Yeh et al., 2012), and visualization of facility information for operation and maintenance (Baek 21 et al., 2019). 22 Given the existing research efforts, a literature review becomes necessary to evaluate 23 existing knowledge and uncover supplementary directions for future studies (Webster and 24 Watson, 2002). Some researchers subscribing to this point have reviewed the literature of AR 25 in construction. Chi et al. (2013) reviewed 101 studies before 2012, with a focus on technologies 26 influencing the development of AR applications (e.g., localization and user interface). Rankohi 27 and Waugh (2013) reviewed the relevant literature as of 2012 to discover facts about the stage, 28 sector, scope, and devices of AR applications. Wang et al. (2013) reviewed articles published 29 between 2005 and 2011 and found that no article introduced the industrial adoption of AR in 30 construction. Behzadan et al. (2015) investigated challenges faced by AR visualization in 31 infrastructure projects and provided solutions correspondingly. Recent years also see some 32 review articles regarding AR in construction, but nearly all of them focused on a few specific 33 perspectives. For instance, Li et al. (2018) and Moore and Gheisari (2019) reviewed the AR 34 literature solely on construction safety management. Calderon-Hernandez and Brioso (2018) 35 reviewed 48 papers regarding the simultaneous use of BIM and AR in the design and 36 construction stages. Elghaish et al. (2020) reviewed the use of immersive technologies 37 (including AR) and drones to support digital transformation in construction. 38 In view of the ever-updating technologies today, this paper attempts to uncover the recent 39 development and implementation of AR systems from the construction literature published after 40 2012. Three specific objectives will be addressed: 41 42 43 44 45 1. To revisit the research and development trends of AR systems in the construction setting; 2. To synthesize how AR systems receive and react to users’ input, and how they present information to users; and 3. To identify how AR has been integrated with other technologies in construction. 46 47 2. Overview of Augmented Reality 48 As a concept of providing the digitally interactive experience, AR is not new. Dating back 49 to 1901, Frank Baum, an American author, mentioned an electronic spectacle named as 50 “character marker” that enabled overlaps of data and real life. Decades later in the 1950s and 51 1960s, the concept of AR was firstly made true - such as “Sensorama” and “head-mounted 52 display” - by scientists from different disciplines. Attributed to the fast development of 53 technologies, today, when people talk about AR, they are mainly referring to AR systems. A 54 typical AR system must have three main features (Azuma, 1997). First, it must enable 55 interactions between the physical and virtual contents. Secondly, it enables the real-time 56 overlay of virtual contents onto the real world. Thirdly, it should be registered in three 57 dimensions. The realization of these features requires the use of various techniques, including 58 tracking techniques, display techniques, and interaction techniques. By using these 59 technologies, AR can enhance people’s interpretation of the real world (van Krevelen and 60 Poelman, 2010). 61 Tracking techniques are used to log and verify the position and orientation of users, and 62 thus play important role in the alignment and registration of virtual contents onto the physical 63 environment (Azuma et al., 2001). Depending on the specific application scenarios, different 64 types of tracking techniques can be adopted. For example, sensor-based and vision-based 65 methods generally verify the position and orientation by using various sensors (e.g., magnetic, 66 acoustic, and mechanical) and image processing methods, respectively. In addition, hybrid 67 tracking techniques are becoming increasingly popular for AR applications in many different 68 fields. A hybrid tracking technique takes advantage of both sensing and computer vision 69 techniques, which can generate a more robust estimation of user’s position and orientation than 70 using one single type of technique (Zhou et al., 2008). 71 Display techniques combine virtual contents and real-world environment and show both 72 simultaneously. Three types of display techniques are widely adopted, namely the handheld 73 display, the head-mounted display, and the projection-based display. The handheld display 74 employs mobile devices like a smartphone or a tablet (Wagner and Schmalstieg, 2003). These 75 mobile devices use video-see through methods, providing users with a video view of the 76 physical environment that is augmented by the corresponding virtual contents. The head- 77 mounted display is worn on the user’s head and can be a part of a helmet. Apart from video-see 78 through methods, the head-mounted display can use optical see-through methods that allow 79 users to view the real-world environment with their eyes and let them see the overlaid virtual 80 contents by holographic optical elements or half-silvered mirrors. Compared with the video-see 81 through methods, one major benefit of optical see-through methods is that they can create a 82 superior presentation of the physical environment (Zhou et al., 2008). The projection-based 83 display, also called the spatial display, does not require users to equip any devices, leading to 84 minimal intrusiveness. Projection-based displays directly show the virtual contents on the 85 surfaces of real-life objects and can naturally scale up to enable collaboration between a group 86 of people (Tonn et al., 2008). 87 Interaction techniques concern creating appropriate intuitive interactions between users 88 and AR systems, and can be classified as information browser, tangible interaction, 89 collaborative interaction, and hybrid interaction (Billinghurst et al., 2015). Information browser 90 refers to the most basic and straightforward interaction that views the visualized AR scene and 91 browses the information provided. For tangible interaction, objects in the real-world 92 environment could be deployed as the elements of the AR interface. For example, Kato et al. 93 (2000) proposed an AR application, in which the user can use a real paddle to select and place 94 the virtual objects in a living room environment. Collaborative interaction is designed and 95 developed for either co-located or face-to-face collaboration, both can improve physical 96 collaborative workspaces (Billinghurst and Kato, 2002). Multiple users in different roles can 97 use different display devices to look at the same object but be presented with different AR 98 experiences that are tailored to their needs. Hybrid interaction combines complementary 99 interfaces and allows users to interact with AR systems through various types of input such as 100 gesture and speech (Zhou et al., 2008). 101 The diversity in these three types of techniques produces many different types of AR 102 systems for corresponding application scenarios. Throughout a construction project life-cycle, 103 many operational and managerial tasks require enough information to interpret their complex 104 relations to the physical environment and objects (Shin and Dunston, 2008). Both commercial 105 and tailor-made AR systems have been adopted by different stakeholders in order to meet their 106 requirements of integrating the real world and virtual information, but there still remains much 107 room for further improvements (Wang et al., 2013). The understanding of extant efforts thus is 108 necessary before possible improvements can be made to leverage the benefits of AR to 109 construction. 110 111 3. Research methods 112 The review presented in this paper was carried out in three steps that have also been adopted 113 in many existing review studies (e.g., Chen et al., 2015; Mok et al., 2015). The first step is to 114 search the literature exhaustively. The databases for searching are Web of Science (WoS) and 115 Scopus. WoS core collection covers more than 21,100 high-quality journals (Clarivate 116 Analytics, 2019), and Scopus contains more than 23,452 journals with over 610 journals under 117 the fields of ‘architecture’, ‘construction and building’, and ‘civil and structural engineering’ 118 (Elsevier, 2020). Searching both databases guarantees full coverage for the relevant literature 119 and thereby makes it possible to draw broad conclusions (He et al., 2017). The literature search 120 was conducted on 30th December 2019, using the query combination ‘(augmented reality) 121 AND (building OR construction OR civil OR infrastructure)’, i.e., the retrieved papers should 122 explicitly mention AR and a construction term. In addition, the review only considered peer- 123 reviewed journal papers since they tend to be more rigorous and mature than other types of 124 literature (Jesson et al., 2011), and the language of the paper was limited to ‘English’. Following 125 these rules, a total of 546 papers published from 2013 to 2019 were collected initially. 126 The screening of the title and keywords of the collected papers found that many of them 127 did not fit the review objectives. This is unsurprising because either building, construction, or 128 civil are general terms. Therefore, in the second step, one author filtered suitable literature 129 according to three criteria: (1) papers in areas irrelevant to construction, e.g., manufacturing 130 and surgery, were excluded; (2) papers not providing sufficient technical details of AR systems 131 were excluded; (3) papers only discussing the application potentials of AR without evaluation 132 (either in the laboratory or actual field) were excluded. Then, another author double-checked 133 the selected papers to decrease potential bias. Disparities were addressed by further discussions 134 until a joint agreement about the inclusion or exclusion of involved papers arrived. After 135 filtering, 69 papers were finally obtained for further analysis. 136 In the third step, the authors carefully read each paper and manually extract descriptive 137 information, namely, year of publication, project stage(s), and AR application(s). More 138 importantly, attention has been paid to the interaction between human and AR systems and the 139 integration of AR with other technologies. 140 141 4. Results 142 This section begins with an overall description of the literature (Section 4.1). Next, it 143 presents what the recent AR applications in construction are (Section 4.2), how users control 144 the AR systems and how the AR systems present information to users (Section 4.3), and 145 whether AR has been integrated with other technologies to leverage its full potential to 146 construction (Section 4.4). 147 4.1. General descriptions of the literature 148 As shown in Figure 1, the 69 papers came from 28 journals, and the top two journals were 149 Automation in Construction and Journal of Computing in Civil Engineering. Grouped into 150 “Others” were journals that published only one relevant paper each. 151 152 Figure 1. Sources of the 69 papers 153 Figure 2 shows the distribution of papers published between 2013 and 2019. Overall, the 154 number of papers continuously dropped since 2013, reaching its bottom of three in 2016 and 155 then bounced up in 2017. This U-shape curve could reveal a renaissance of AR research in 156 construction, which might be attributable to the increasingly mature AR technologies in recent 157 years (Gartner, 2018). A further investigation in the authors of the reviewed papers found that 158 Wang X., Kamat V.R., and Ayer S.K. were three notable researchers who published more than 159 five papers about AR in construction between 2013 and 2019. Moreover, 26.67% of the studies 160 published between 2017-2019 have the same author (at least one) of the studies between 2013- 161 2016. Such a result indicates that efforts in exploring the AR applications in construction are 162 coming from different researchers or research groups. 163 Figure 2 also shows the number of papers concerning different project stages. Findings can 164 be drawn by integrating Figure 2 and the results of Wang et al. (2013) and Rankohi and Waugh 165 (2013). In studies published before 2013, most of them used AR in the construction stage; less 166 in the operation stage. In studies published from 2013 to 2019, over half of them focused on the 167 construction stage, and the number of studies focusing on the operation stage has become nearly 168 two times as compared to the number of studies focusing on the design stage. These figures 169 indicate that the construction stage is still taking the leading position to embrace AR, and the 170 applicability of AR to project operation has been increasingly recognized. 171 172 173 Figure 2. Number of publications from 2013 to 2019 (by project stage) 4.2. AR applications in construction 174 Figure 3 shows the specific applications of AR in different project stages. Based on 175 authors’ experience and relevant studies, various AR applications are grouped into three 176 categories for the design stage, and seven and six categories for the construction and operation 177 stages respectively. One may notice that several studies consider more than one application, 178 and the comparison of studies published before and after 2013 reveals that AR applications in 179 construction have become more diverse. In the design stage, AR has been popularly used for 180 assessing the designed drawings or models in the physical environment and communicating 181 design ideas to different stakeholders (Alsafouri and Ayer, 2019). In the construction stage, 182 progress monitoring, assembly instruction, and quality management were dominant AR 183 applications. This is consistent with the findings of Rankohi and Waugh (2013) and Wang et 184 al. (2013). However, research efforts have also been focusing on safety management, 185 positioning, and other applications, which were not widely investigated by studies published 186 before 2013. In the operation stage, maintenance management and operation was the main AR 187 application. This is because maintenance is one of the major tasks in the operation stage, which 188 requires a great amount of information to be available in-situ (Hou et al., 2014). AR can 189 seamlessly connect the physical facilities with their corresponding virtual information, 190 facilitating instant and informed decision making (Dong et al., 2013). By providing additional 191 context information, AR has also been used for disaster management and emergency response, 192 localization and navigation, and energy management (e.g., Golparvar-Fard and Ham, 2014; 193 Tsai and Yau, 2014). 194 Acknowledging the rising diversity of AR applications in construction, it is noticeable that 195 some AR applications are becoming more mature for actual projects. Jiao et al (2013) 196 implemented AR for monitoring the construction progress of the Shanghai Center. Zhou et al. 197 (2017) used AR to support quality management in a metro construction project in China. 198 Nevertheless, most of the AR applications in the operation stage are still under lab experiments. 199 In addition, applications such as personnel management and cost control wait for further 200 exploration. 201 202 203 Figure 3. Number of publications by target application 4.3. Interaction between users and AR systems 204 Based on the extracted data, a Sankey chart was drawn to summarize the AR-user 205 interactions in construction. The Sankey chart provides a graphical presentation, in which the 206 width of the series can clearly indicate the number of studies that mentioned the diverse options 207 of how users control the AR systems and how the AR systems present on-demand information 208 to users (see Figure 4). Over 65% of the reviewed studies adopted the handheld display, while 209 the head-mounted display is also a relatively popular choice. In contrast, the projection-based 210 display has not been widely used in construction. A possible explanation could be the 211 difficulties in setting up the projector and the projected texture might not be sufficiently bright 212 or visible in a complex construction environment. 213 214 Figure 4. Overview of AR-user interactions (The total number of display is larger than 215 69 because one paper used two types of displays) 216 Information browser dominates the interaction between users and AR systems in 217 construction. 14 studies used 2D-based input, and 13 studies used 3D-based input. 2D-based 218 input provides simple interaction, and thus are relatively easy to operate. In the AR system 219 developed by Espíndola et al. (2013), users were allowed to click the buttons or use the 220 keyboard to change their view. Other types of 2D-based interactions mentioned in the reviewed 221 studies include: (1) filtering the information to view; (2) navigating into details of virtual 222 contents; and (3) modifying the visualization style and format. These kinds of 2D-based 223 interactions could also be easily performed using traditional input devices (e.g., keyboard and 224 touchpad). 3D-based input benefits users by interacting with virtual construction components 225 and resources in a natural way. The input devices allow six degrees of freedom (DOF) 226 manipulation (e.g., translation and rotation in 3D) of virtual objects. Like 2D-based input, 3D- 227 based input supports information filtering and other functions. For instance, Soria et al. (2018) 228 developed an AR system for the management of underground facilities. In this system, users 229 can rotate the digital 3D model by swiping left and right and can also delete the model from 230 both the view and the database. 231 Following the information browser, tangible interaction has been adopted in 25 studies. 232 Tangible interaction is increasingly preferred in construction because it allows users to interact 233 with virtual contents by manipulating physical objects. This is attractive to construction since 234 stakeholders often require real-time information tied to dynamically changed objects and 235 locations (Chen and Lu, 2019). Thanks to the advances in AR devices, tangible interaction can 236 be more effectively deployed. For example, the Google Tango tablet used by Ratajczak et al. 237 (2019) was equipped with a time-of-flight (ToF) camera and an IR projector, both can help to 238 accurately measure the position of the Google Tango tablet with respect to its surrounding 239 environment. This type of mobile AR device can generate the 3D point clouds of physical 240 objects in a real-time manner, which can be further processed to obtain details of these objects 241 and link the generated information to them through AR systems. 242 The review study conducted by Wang et al. (2013) showed that multidisciplinary 243 collaboration would be one of the trends for AR applications in construction. In the reviewed 244 69 studies, seven of them deployed collaborative interaction and suggested that this kind of 245 interaction is suitable for information communication and exchange in construction. For 246 instance, the engineers can make annotations about existing cables and pipes overlaid with AR 247 visualizations, and the technicians can know the locations and other information of these cables 248 and pipes before they determine whether extra pipes and cables are required (Olbrich et al., 249 2013). Hybrid interaction is also encouraged by many previous studies since it is more adaptive 250 to the changing environment and can relieve users from screen input or external devices (e.g., 251 mouse, keyboard, and touchpad). However, only three recent studies reported success cases of 252 hybrid interaction in construction. In these studies, users were asked to wear the head-mounted 253 device (i.e., Microsoft HoloLens) and interact with the virtual contents by their gestures (Baek 254 et al., 2019; Mascareñas et al., 2019). Despite the diverse interaction mechanisms, no study has 255 used speech or eye movement as inputs for intuitive interaction, leaving a possibility for further 256 investigation. 257 4.4. Integration of AR and other technologies 258 The technical details of the reviewed studies allowed the authors to analyze whether the 259 AR systems were used alone or in conjunction with other technologies. A notable trend is that 260 many recent research works have integrated AR with building information modeling (33 261 papers), computer vision (22 papers), and cloud computing (12 papers). As shown in Figure 5, 262 three papers even integrated AR with all these three technologies to leverage the benefits of AR 263 to construction. Nevertheless, other technologies such as Auto-ID were only mentioned in very 264 few papers and thus were omitted in the following investigations. 265 266 Figure 5. Number of publications integrating AR with BIM, computer vision and cloud 267 computing 268 4.4.1. Building information modeling 269 Throughout the life-cycle of a construction project, the delivery of many operational and 270 management tasks could be improved if the corresponding workers and managers can access to 271 not only the geometric information (e.g., size, shape, etc.) but also the non-geometric 272 information (e.g., material, function, etc.) of construction elements (Jalal et al., 2020). In these 273 cases, the integration of BIM and AR provided an ideal solution to meet such need, through 274 which advantages of BIM and AR to construction were further leveraged. A BIM model is 275 basically a digital model that contains the physical and functional details of a building. AR can 276 superimpose the BIM model onto the physical project. The merge of the as-designed BIM 277 model and real-world environment provides a vivid presentation of geometric information for 278 operational and managerial tasks. Wang et al. (2014a) presented the BIM model in a handheld 279 AR device to guide on-site assembly tasks. The comparison between the as-designed BIM and 280 the as-built situation can significantly ease the progress monitoring. Zhou et al. (2017) used AR 281 and BIM to measure the segment displacement for quality management in tunneling 282 construction. They treated the BIM model as the baseline model and compared this model with 283 actual video through an image-matching program. Likewise, Kwon et al. (2014) used an AR 284 system to match the shape information of BIM objects and images taken on the construction 285 site, and the matching results can support defect detection. 286 Apart from the geometric information, some of the reviewed studies used AR to make non- 287 geometric information readily available on site. This is extremely helpful for supporting 288 complex construction tasks. For example, Chen et al. (2016) integrated BIM into the AR system 289 so that the material information, rigging orders, and construction schedules can be 290 automatically fetched from BIM and augmented as a layer of information over the real-world 291 view of workers. Ratajczak et al. (2019) also suggested that the integration of BIM and AR 292 could provide context-specific information on tasks and relevant building components, and thus 293 enhance the quality of construction works. Regarding maintenance, some examples can be 294 found in Irizarry et al. (2014) and Soria et al. (2018) that integrated BIM and AR to facilitate 295 maintenance and repair operations for buildings and infrastructures. Their proposed systems 296 can simplify the information retrieval process, and facility managers and maintenance workers 297 can have the condition of a facility, maintenance requirements, and all other information they 298 needed for maintenance. 299 4.4.2. Computer vision 300 In its simplest definition, CV refers to training computers to generate and interpret explicit 301 information from images or videos (Ballard and Brown, 1982). CV has remarkably changed the 302 traditional construction management by enabling automatic activity recognition, object 303 tracking, and performance monitoring (Sherafat et al., 2020). The advances in CV technologies 304 and algorithms have also made them extremely suitable for AR applications since they can 305 make use of the images or videos taken by the built-in camera of an AR device to provide 306 functions such as visual tracking and registration. 307 CV technologies can benefit AR by facilitating both marker-based and marker-less tracking 308 and registration. Chi et al. (2013) found that marker-based tracking and registration methods 309 were generally adopted in studies published before 2013 but suggested that marker-less 310 methods should be more suitable for construction fields. However, among the reviewed 69 311 studies, marker-based methods were still used more frequently than marker-less methods. 312 Regular markers can be placed at different locations that are suitable for various construction 313 and operation activities (Portalés et al., 2018). When a marker is recognized, the AR device 314 would display the virtual content relevant to the location of the marker in space. Researchers 315 have also attempted to improve the marker-based tracking and registration methods so that they 316 can be applied to different scenarios. Ahn et al. (2019) proposed a two-step method, including 317 image segmentation for filtering out the markers and object detection for estimating the 318 coordinates, to perform geometric transformation for projection-based display. Mascareñas et 319 al. (2019) adopted various types of markers (e.g., barcode, QR code) in order to cope with the 320 common occurrence of facilities in nuclear infrastructure. Although marker-based methods are 321 relatively straightforward to use, they require markers being placed at suitable locations without 322 any occlusions. 323 The use of CV for marker-less tracking and registration relies on algorithms for extracting 324 features of the scenes and identifying the location and position of AR devices in the real-world 325 coordination system. Depending on the target scenes and their associated features, different 326 feature detectors and descriptors have been adopted in the reviewed studies. Examples include 327 the Speed Up Robust Features (SURF) used in Kim et al. (2018), and the Canny edge detector 328 used in Fazel and Izadi (2018). In addition, deep learning is expected to significantly improve 329 the marker-less tracking and registration for AR applications. For instance, the convolutional 330 neural network – a typical class of deep learning algorithms – has been demonstrated useful for 331 location tracking in AR-based facility management (Baek et al., 2019). Marker-less methods 332 do not need pre-installed markers and can provide accurate tracking and registration if robust 333 features are available. However, for a complex, dynamic environment like construction, the use 334 of CV for marker-less tracking and registration is vulnerable to drift and fact motion. In such 335 situations, a hybrid method that combines sensors and CV should be adopted to overcome 336 problems such as drift and occlusions (Hou et al., 2014). 337 With the tracking and registration enabled by CV, several value-adding AR services have 338 been developed. Bae et al. (2013) used CV to recognize buildings from images taken by the 339 camera of the AR device. Once a target building is recognized, the AR device can query relevant 340 information of that building and show the information to the field personnel for decision 341 making. Kim et al. (2017) proposed an AR-based safety management system in which multiple 342 objects in the images were tracked and their distances were continuously measured. This system 343 allowed managers to timely view the hazard information in an AR device, and make 344 corresponding safety instructions for the sake of risk prevention. Koch et al. (2014) and Baek 345 et al. (2019) used CV to perform indoor localization, based on which location-specific 346 information was represented in the AR device for indoor navigation and maintenance 347 operations. All these examples illustrated the integration of CV and AR can significantly ease 348 the information accessibility for users so that informed decisions can be made in a real-time 349 manner. 350 4.4.3. Cloud computing 351 Mobile AR devices are flexible and convenient for on-site personnel to use, but they 352 generally have limited memory and computing capacities for computation- and data-intensive 353 AR applications. This is extremely serious when AR is integrated with CV and BIM. Mobile 354 AR devices may not be able to run CV algorithms and render the BIM models. Therefore, cloud 355 computing has been used to facilitate the delivery of AR applications. 356 Cloud computing refers to “a model for enabling ubiquitous, convenient, on-demand 357 network access to a shared pool of configurable computing resources that can be rapidly 358 provisioned and released with minimal management effort or service provider interaction” 359 (Mell and Grance, 2011, p.2). In the reviewed studies, the integration of AR and cloud 360 computing has eased information exchange by storing the required information in a centralized 361 cloud platform. Because no information must be stored locally in the memory of an AR device, 362 wasteful duplications of information can be significantly reduced (Chu et al., 2018). This 363 benefit has been recognized in actual practices, Jiao et al. (2013) adopted a cloud-based system 364 to integrate AR and BIM in the 3D web environment and suggested that their proposed system 365 can be dynamically updated and adapted to different usage requirements. Additionally, Hou et 366 al. (2014) linked users to the object-oriented facility information that is stored on a cloud server, 367 and then augmented the information in the AR interface for real-time facility management. 368 By transferring the data processing workload to the cloud server, cloud computing can 369 relief the AR devices from heavy computational tasks and improve the efficiency of data 370 processing. In the cloud computing-based AR system developed by Olbrich et al. (2013), the 371 AR device was only responsible for simple computational tasks including the image acquisition 372 and visualization of the augmented images, and the cloud server was used to address all other 373 computational tasks such as processing the image and tracking the camera. Kim et al. (2017) 374 used a cloud server to conduct most of the processes, and the AR interface could be 375 synchronized with this cloud server by using HTML. Baek et al. (2019) allocated a computer 376 that had powerful graphics processing units as a cloud computing server in order to ensure a 377 fast deep learning-based localization. Acknowledging the benefits of cloud computing to AR 378 applications, it should be noted that the communication between cloud servers and AR devices 379 requires a stable network, which however might be extremely difficult to guarantee in a 380 dynamic construction site. 381 382 5. Future directions 383 In previous review studies, Chi et al. (2013), Rankohi and Waugh (2013), Wang et al. 384 (2013), and Behzadan et al. (2015) described several research directions of AR in construction, 385 such as expanding the applications of AR from the construction stage to other project stages, 386 the use of mobile AR devices, the integration of AR and cloud computing, human-AR 387 interaction, and context-aware AR. The findings of this study reveal that much research efforts 388 have been devoted to these directions. However, the reviewed papers still highlighted several 389 technical challenges associated with interaction and information processing processes (e.g., 390 Zaher et al., 2018; Chalhoub and Ayer, 2019). Therefore, efforts are needed to further upgrade 391 users’ experience of AR in construction. Specifically, four possible future directions of AR in 392 construction are described as follows. 393 First, natural interactions between users and AR systems are important for streamlining the 394 deployment of AR in construction. The design of natural interaction should ensure that users 395 can easily interact with virtual content without problems such as information overload and 396 attention disruption. In a dynamic construction workspace, if workers hold mobile AR devices 397 or wear head-held AR devices while walking within the construction site, they should not be 398 required to pay much attention to the control of AR devices and cannot be disturbed by the 399 information presented to them in order to avoid safety accidents. Therefore, user-centric 400 interaction design has been advocated since it fully considers how to help users interpret the 401 virtual contents overlaid onto the physical environment (Eitoku et al., 2006). Future studies can 402 explore the user-centric design of human-AR interaction according to the nature of cognition 403 on the user’s performance when merging multimodal cues in the real-world environment. 404 Moreover, future studies can propose systematic methods to evaluate the human-AR interaction 405 in construction. Both subjective metrics (e.g., cognitive load and fatigue) and objective metrics 406 (e.g., the time and accuracy of task delivery) should be used to comprehensively measure the 407 effectiveness and efficiency of the control mechanisms and information presentation of AR in 408 various application scenarios. 409 Secondly, two major issues associated with the integration of BIM and AR have been 410 highlighted. One is related to the reduction of the model complexity and the other one is 411 associated with the communication of BIM information to an AR system, both concerning how 412 the model can be correctly visualized without the loss of essential information. Irizarry et al. 413 (2013) introduced some manual complexity reduction techniques, which include the welding 414 of overlapping vertices, the elimination of unnecessary geometry, and the simplification of 415 mesh. Tools including Vizup and Autodesk 3ds Max have also been used for polygon 416 decimation (Singh and Delhi, 2018). Depending on the platforms for developing the BIM 417 models and AR applications, various data formats and the corresponding format transfer 418 methods have been used for BIM-AR integration. Some of these methods need one time of 419 conversion (e.g., “HOOPS-X3D” in Jiao et al. [2013] and “RVT-IFC+DAE” in [Williams et 420 al., 2015]), and others require a series of conversions (e.g., “IFC-OBJ/MTL-L3D” in [Meža et 421 al., 2014] and “RVT-MAX-FBX-FBX-WT3” in [Chu et al., 2018]). Acknowledging the 422 availability of various methods for different scenarios, it should be noted that a full conversion 423 has hardly been achieved, and manual adjustments of the converted BIM models are often 424 needed. Therefore, more robust conversion methods are expected to be developed and validated 425 in actual projects. 426 Thirdly, the augmentation of the real and virtual contents should be more compelling for 427 construction stakeholders. When stakeholders conduct construction activities, they often need 428 information about the surrounding environment in addition to the design drawings or other 429 readily-available documents (Tsai, 2014). However, most of the reviewed AR systems treated 430 the physical environment as background, without deriving any semantic information from it for 431 decision making. It is thus expected that, in future AR applications, the information presented 432 to user-ends should not be derived solely from the virtual content, but the semantic 433 understanding of the surrounding environment. A few studies have used computer vision 434 technologies to help AR systems to understand the semantic information of objects in the 435 construction and operation stages, but an extensive database of reality should be developed to 436 make AR responsive to providing context-aware information. 437 Finally, AR can be integrated with a new computing paradigm – edge computing – to 438 improve the quality of service for computation-intensive applications. Although previous 439 studies have adopted cloud computing to provide such function in construction, the use of cloud 440 computing has been facing challenges such as high latency, resource consumption, and unstable 441 network in construction sites. In contrast, edge computing is a distributed computing paradigm 442 that transfer data processing and analysis close to the edge of data sources or networks 443 (Satyanarayanan, 2017). Advantages of edge computing include shifting the storage and 444 computation load from center to edge, reducing ingress bandwidth into the cloud, enabling a 445 real-time response, reducing latency, and enhancing scalability (Shi et al., 2016). Such 446 advantages make edge computing suitable for the rapid delivery of AR services. In the 447 manufacturing industry, Fernández-Caramés et al. (2018) has integrated AR with edge 448 computing to provide dynamic on-demand information and analyzed the benefits of edge 449 computing over cloud computing. Nevertheless, one cannot simply apply Fernández-Caramés 450 et al. (2018)’s solution to construction since the working environment and application scenarios 451 in the manufacturing and construction are significantly different. Therefore, future studies can 452 focus on how to deploy the edge computing for different scenarios in construction and how to 453 integrate AR and edge computing to enable fast AR device communications for various users 454 in construction projects. 455 456 6. Limitations of the review method 457 Some limitations of the review method should be recognized. In the literature search stage, 458 the construction term used in the search query, e.g., building or construction, is relatively 459 general, which led to the fact that the initial set of collected papers contained many irrelevant 460 studies. Additional search criteria can help to narrow the results but will increase the risk of 461 omitting important references. The combination of database searches and snowballing might be 462 a more suitable method for future review studies. Moreover, the filtering process was done 463 manually in this study by reading the abstracts of all 546 papers. The efficiency of this process, 464 however, can be significantly improved by using advanced text-mining tools such as 465 RobotAnalyst (Przybyła et al., 2018). 466 467 7. Conclusions 468 By taking research work published from 2013 to 2019 as recent advances, this study 469 reviewed 69 journal articles on AR in construction. Several key findings have been drawn from 470 this review. First, a renaissance of AR research in constriction is observed, but most of the 471 reviewed studies still focused on the construction stage. Secondly, design assessment, process 472 monitoring, and maintenance management and operation were the most widely implemented 473 AR applications in the design, construction, and operation stages, respectively. Thirdly, 474 information browser and tangible interaction were the major interaction mechanisms of AR in 475 construction, and hybrid interaction was only enabled by advanced AR systems for a few 476 application scenarios. Fourthly, AR has been integrated with BIM, CV, and cloud computing 477 in order to leverage its benefits to construction. BIM and CV can significantly improve the 478 virtual and physical contents presented to users; cloud computing shares the majority of 479 computation-intensive tasks with AR devices for rapid response in more complex applications. 480 The review also identifies some unaddressed issues, including natural interactions between 481 users and AR systems, seamless conversion of BIM to AR, merging virtual contents with 482 information obtained from the physical environment, and effective strategies for computation- 483 and data-intensive AR applications. Therefore, four possible directions for future search can be 484 proposed: (1) AR systems should follow a user-centric interaction design so that users can enjoy 485 more intuitive interactions for improved user experience; (2) Development of robust conversion 486 methods for BIM and AR integration; (3) Technical progress in semantic understanding of the 487 physical objects and surrounding environment is expected to unleash the full power of AR to 488 construction; and (4) Future deployment of AR in construction could make use of edge 489 computing which provides the ubiquitous capability of heterogeneous computing for AR 490 applications. 491 492 Acknowledgments 493 This research was funded by the Fundamental Research Funds for the Central Universities, grant 494 number 2019kfyXJJS185. 495 496 References 497 Ahn, S., Han, S. and Al-Hussein, M. 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