Copyright ⓒ2018 Korean Society of Civil Engineers
DOI 10.1007/s12205-017-0734-3 pISSN 1226-7988, eISSN 1976-3808
www.springer.com/12205
Improvement of Realism of 4D Objects Using Augmented Reality Objects and Actual Images of a Construction Site
Hyeon Seung Kim
*, Sung-Keun Kim
**, Andre Borrmann
***, and Leen Seok Kang
****Received June 11, 2017/Accepted September 14, 2017/Published Online November 6, 2017
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Abstract
Many construction firms have begun carrying out schedule management by using digital methods, such as 3D CAD and building information model (BIM)s. However, such methods are hampered by discrepancies between virtual models and real world models. This study develops an augmented reality (AR)-based 4D CAD system, which connects 4D and 5D objects with a real field image and an AR object. A method to link 4D objects and AR objects is proposed in order to implement several types of schedule information, as well as to enable the use of constantly changing schedule information through AR objects. Furthermore, a method for implementing AR objects based on connecting real-time field images is presented to help long distance managers become more accustomed to virtual objects and to enable better reflecting field environmental conditions in schedule management. In this study, the AR-based 4D CAD system using these methods is suggested and applied in a case study to verify the applicability of the system. The proposed system can be used for field monitoring for schedule management at job sites and to support cooperation and decision making between long distance managers and field engineers.
Keywords: augmented reality, telepresence, 4D CAD, 5D CAD, BIM
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1. Introduction
Building Information Models (BIM) are being increasingly used in many industries, including the construction industry.
They are being further integrated with advanced IT technologies such as Augmented Reality (AR), Internet of Things (IoT), and 3D scanning (Business advantage group, 2015). As the construction stage is of major importance in civil works, 4D and 5D CAD systems can be highly beneficial if applied properly. However, 4D and 5D objects provided to the field technicians and managers are computer-generated images based on Virtual Reality (VR), not the actual images of sites.
The VR images constructed by human are different from the actual field environment. This problem can lead to difficulties in the communication between the stakeholders, in particular between workers on the site and remotely connected engineers and decision makers. As information on the construction progress is mostly based on the work reports from field workers and digital photos, managers cannot accurately identify errors and work progress at construction sites (Zollmann et al., 2014).
To promote efficient decision making, the existing operation environments of 4D and 5D simulations should be improved and the practical information should be reflected.
Thus, in this study, an AR-based 4D CAD system is developed to reflect real-time construction information derived from the construction site and provide practical 4D and 5D simulation objects. The proposed system is a management system that not only displays 3D objects through AR, but also obtains real-time field images by using web cameras installed at the construction sites and provides 4D and 5D objects through AR based on the obtained images. Accordingly, a method for integrating 4D and 5D objects in real-time field images through AR and a method for reflecting schedule information in the form of AR objects are proposed. Based on these proposed methods, the system of reflecting various types of nD information in the real-time field images is developled to compare 4D and 5D objects with real- time field images and apply practical field information. In addition, the suggested system is applied to a real project using CCTV(Closed-Circuit TeleVision)s and 4D CAD system in order to verify its performance.
TECHNICAL NOTE
*Researcher, Dept. of Civil Engineering, Univesity of Michigan, Ann Arbor, Michigan, USA (E-mail: [email protected])
**Member, Professor, Dept. of Civil Engineering, Seoul National University of Science & Technology, Seoul 01811, Korea (E-mail:
***Professor, Leonhard Obermeyer Center - Center of Digital Methods for the Built Environment, Technical University of Munich, 80639 Munich, Ger- many (E-mail: [email protected])
****Member, Professor, Dept. of Civil Engineering, Engineering Research Institute, Gyeongsang National University, Jinju 52828, Korea (Corresponding Author, E-mail: [email protected])
2. Research Trend and Significance Compared to Existing Researches
2.1 Research Trend on AR Technology in the Construc- tion industry
In construction, initial studies on AR mainly focus on using VR-based construction information through AR to identify problems and propose solutions (Kirchbach and Runde, 2011).
For example, Kim et al. (2013) merged a 3D modeled rebar object with a practical work site to analyze the operation order of rebar assembly and identify construction errors. Kim et al.
(2012) has established a marker-based AR system that can identify the potential interference of construction equipment and structures in advance to establish an efficient construction plan in the field. Carozza et al. (2014) has developed a markerless AR system to reflect practical environments in urban planning by visualizing the practical effects of a change in the urban environment.
Furthermore, studies on connecting mobile devices and web cameras using AR have recently been performed to determine their ability to facilitate the processing of construction tasks through AR at long range or over the Internet. For example, Raajana et al. (2012) has compared structures under construction with virtual 3D objects based on an image tracking technique using web cameras installed at construction sites. Yang et al.
(2013) and Riera et al. (2013) have developed a mobile AR application to analyze the field applicability of mobile AR devices to construction projects of various sizes. Some studies have used drones to obtain images and implement AR (Zollmann et al., 2014).
As such, the application of AR is constantly increasing in that
relevant information can be obtained in AR regardless of time and location via the Internet. In particular, several studies on connecting this advantage with BIMs are being conducted, including gathering various types of schedule information instead of generating 3D objects for AR. For example, Jiao et al.
(2013) has connected an AR technique that used images with the BIM and BSNS and operated it as a cloud-based device in the 3D web environment. Wang et al. (2014) has proposed a method for implementing AR in the BIM environment and established a conceptual framework for visualizing the status of field processes in real time. Kwon et al. (2014) has developed a marker-based AR system using the BIM to perform fault management at construction sites
2.2 Significance Compared to Existing Researches By analyzing existing studies, it has been determined that the application of AR techniques in construction is gradually increasing. However, most existing studies have implemented 3D object-based AR by using shape information and then compared these objects with actual structures for analysis. Moreover, although AR is implemented at long range or in real time by using mobile devices and web cameras, it is regarded as a stage of reviewing information derived from previously stored information in the form of AR models. For this reason, a new AR model should be formed to enable additional activities through AR. As this problem leads to limitation of reflecting various types of construction information in AR objects, existing studies are considered insufficient to guide the implementation of AR for schedule management.
To solve this problem, this study suggests an AR-based 4D CAD system, which the 4D object is linked with the AR and
Fig. 1. AR-based 4D CAD System with Real Field Images
telepresence techniques. By constructing a linkage system between 4D objects and AR objects, this system can easily convert the schedule analysis, progress analysis and cost analysis information into the AR objects by the embedded process analysis module. In addition, the realization of AR based on the real-time images makes it possible to compare the progress of the actual images on the scene compared to the schedule information of the 4D CAD system.
3. Methods for Forming the AR-based 4D CAD System
3.1 Overview of the AR-based 4D CAD System
The AR-based 4D CAD system proposed in this study provides schedule information to many construction participants regardless of time and location, and enables them to review and discuss construction tasks, as shown in Fig. 1. This system can synchronize the nD simulation information such as the schedule and cost of an original plan with the real-time construction site images. In particular, this system applies AR object to reduce the difference between the existing VR-based schedule simulation and the actual site.
The main functions of the proposed system include the schedule simulation extended in 4D and 5D CAD systems, the telepresence- based field monitoring, and the markerless AR objects. In terms of the nD-based schedule simulation, various types of information on the progress of tasks compared to the plan, such as schedule, cost, and resources, is provided through the VR-based simulation.
Moreover, telepresence-based field monitoring is used to provide the status of tasks at construction sites, which is linked with the
nD-based schedule information in the form of real-time field images, enabling users to intuitively identify the state of construction progress. In particular, these nD CAD and telepresence functions are based on the method of connecting a 4D CAD engine and telepresence, which was developed in previous studies (Kang et al., 2013; Kang et al., 2016). Furthermore, this study develops a method for connecting these functions with markerless AR. This method overlays actual field images with nD-based 3D objects to increase the sense of realism and understanding of schedule information and to support effective communication between field workers and managers.
3.2 Method for Connecting the Telepresence Technique and 4D Objects
The system of connecting the 4D CAD and telepresence techniques not only performs field monitoring based on the obtained images but also identifies the status of construction progress by comparing the information from 4D simulation and real-time field images of the same location. In this regard, the method for providing consistent schedule information by synchronizing the coordinates of 3D models used in 4D simulation and the coordinates of field images obtained by web cameras had to be developed. This method (Kang et al., 2016) can be implemented by adjusting the initial coordinates of 4D simulation and web cameras to be the same viewpoint and then converting the moving distance, direction, and angle into 3D model coordinates when the left and right, and upper and lower rotation angles of web cameras change, as shown in Fig. 2. This process enables users to obtain field images when they select a 3D model because web cameras automatically are directed
Fig. 2. Method for Connecting Field Images and 4D Objects (Kang, 2016)
toward the actual site where the corresponding model is located.
As such, this method allows users to compare the initial plan of a desired structure and its target activity with the practical progress of schedule at the construction site, allowing users to visually determine the status of task being performed.
3.3 Method for Connecting the Markerless 4D Objects with Field Images
Because the connection of the telepresence technique with 4D objects simply compares the real-time field images with the 4D objects on divided screens, thus leading to the difference between the virtual images and the real-time field images. To overcome this problem, a method for directly contrasting 4D objects in real-time field images in the AR environment is proposed. In order to link the 4D object with the actual image in the field, the actual image of the field must be marked and stored in the database. Once the image is stored, the simulation is performed assuming that the construction progresses for a certain period of time from the current point of view of the actual image. In this process, marker and markerless methods are used to recognize actual images. The Fig. 3 shows how to construct the marker and markerless objects. The method of using the markers is as shown in the figure. Markers are attached to the actual finished parts. In the markerless method, the entire image of the on-site image is recognized as a marker.
In particular, real-time image information should be obtained by using several web cameras installed at the construction sites which provide their information to the telepresence-based AR environment, and users at a distance should be allowed to use such information. In this sense, a markerless method, which recognizes and tracks objects based on the natural points, lines,
edges, and texture, is judged to be more effective than a marker- based method that tracks artificial markers in the real environment.
Thus, the markerless tracking method is applied in this study. As shown in Fig. 3, the markerless method using telepresence image is based on the web cameras installed at construction sites. In this system, AR is implemented by recognizing real object images such as the photos and pictures of structures at construction sites instead of the marker of basic patterns. The left part of Fig. 3 shows that the field images captured by the web camera or photos directly taken in the field can be used as markerless images. When markerless images are registered, the images are examined by the built-in AR process and compared to the images being taken by the web cameras, and corresponding virtual objects are augmented in the examined location. The process of tracking objects in real-time images indicates that the new location of objects is constantly recognized when objects or cameras move, thus significantly affecting the response rate of interaction between virtual objects and actual objects according to the object tracking methods. The main tracking methods that have been used include the Scale Invariant Feature Transform (SIFT) by Lowe (2004) and Speed Up Robust Features (SURF) by Bay et al. (2008). Because the SURF algorithm derives a feature point by using a fast Hessian detector based on the integral images, it shows faster speed than other algorithms to derive feature points. For this reason, the SURF algorithm is used to examine and track augmented objects in this study.
3.4 Method for Building AR Objects Based on Schedule Information
In the typical AR implementation process, an AR object is formed by separately selecting a marker and a 3D object and
Fig. 3. Method for Connecting Real Site Image and 4D Object Image
then connecting them. When different types of information must be examined according to the schedule and the work section, as in the case of schedule management for construction projects, AR objects should be constantly adjusted and updated due to the applicable range of AR and the changes in information. To efficiently operate AR, the constructability should be considered when an AR object is generated.
Thus, a method for building AR objects connected to process information is proposed for effective project management, as shown in Fig. 4. This method minimizes the process of generating AR objects by integrally registering the entire 3D objects as AR objects and allowing users to freely adjust and operate them after AR is implemented. Accordingly, a 1:1 correspondence method for connecting a Work Breakdown Structure (WBS) code with a 3D object, as well as a 1:n connection method for linking a WBS code with a group of several 3D objects is used. As the WBS code is already defined as a core information connection code for forming 4D and nD objects, the information items of markerless images for AR can be easily connected to nD object information by adding a few fields. 3D objects forming 4D are divided into n object(s), and the divided 3D objects are connected to information on the initial schedule. For this reason, when the users separately select 3D objects, schedule information is synchronized with the corresponding objects, and 4D objects can be implemented through AR within a range of the selected objects. Moreover, 4D objects can be subsequently implemented through AR according to the synchronized schedule information, thereby facilitating AR-based 4D simulation.
Furthermore, this information connection system enables the
users to easily convert the AR-based 4D simulation into nD objects and nD simulation. In this regard, when the AR-based 5D simulation is implemented by referring to cost information, schedule information according to the cost input from the schedule can be implemented through AR. In particular, as cost information is represented in a S-curve and histogram in this study, cost can be easily examined according to schedule. In addition, when only a schedule with a prerequisite or a certain 3D object is implemented through AR as shown in the upper part of Fig. 4, the 3D object should be manually reviewed and registered as an AR object. To solve this problem, a search function for building an AR object is added in this study to effectively generate the AR object. The users can easily use desired activities through AR by examining these activities based on the search conditions, such as the period and process title, and connecting them with markerless image information.
Implementing one or a few processes through AR is to activate the processes as AR objects selected from among all registered AR objects. This method can be effectively used to review and identify subsequent processes when only subsequent processes are selected by applying the search function. These processes are performed through AR in that AR objects are not implemented in the completed structures in the field and that only processes where tasks are not carried out are indicated through AR.
4. Development of the AR-based 4D CAD System
4.1 System Architecture
In this study, the AR-based 4D CAD system for schedule Fig. 4. Method for Examining and Forming AR Objects Based on Schedule Information
management is developed, as shown in Fig. 5. This system consists of the 4D simulation-based site management module, the telepresence-based site management module, and the AR- based site management module.
The 4D simulation-based site management module executes
4D simulations by using the 3D modeler, schedule modeler, and WBS generator to form 4D models. The telepresence-based site management module compares real-time field images obtained by web cameras to 4D and nD simulations to conduct field management. The AR-based site management module uses the Fig. 5. System Architecture
Fig. 6. Main Functions of AR-based 4D System
German Metaio SDK to provide 4D and nD information through AR. Virtual objects for AR are provided by the 4D simulation- based site management module, and the images used include typical camera images as well as real-time field images provided by the telepresence-based site management module. As these modules are interconnected, 4D simulations, where real-time field images are connected with schedule information, and 5D simulations connected with cost information can be effectively implemented through AR. However, only 4D objects that satisfy the conditions defined by the users can be implemented through AR. Thus, a subsequent schedule simulation function, which simulates only the schedule after the schedule is already completed, can be used.
4.2 Main Functions of the AR-based 4D CAD System The main functions of the AR-based 4D CAD system for schedule management are described in Fig. 6.
The first function of the 4D-based AR simulation not only augments 3D objects, but also implements 4D objects by including schedule information in the real-time field images. The process of implementing 4D objects in AR can be effectively applied to identify the current construction process of structures by considering actual field environments and detailed process information. The second function of the 5D-based AR simulation implements 5D objects where 4D objects are connected with cost information, and thus can be used to analyze the status of construction progress according to the cost input in the field. As this function can also switch between existing 4D operations and 5D operations, information on 4D and 5D objects can be effectively analyzed through comparison. The final function of the successor activity-based AR simulation implements schedule simulation
through AR by preventing the completed structures from being overlaid with virtual 3D objects to easily distinguish them. As in this function, the processes irrelevant to the completely established structures as well as the process to be performed after the period selected by the user are implemented through AR, the status of processes being completed can be easily identified.
5. System Application to the Case Study Project
5.1 Overview of Case Project
The AR-based 4D CAD system is applied to an actual project (00 bridge construction) in order to verify its practical applicability.
The selected project is considered appropriate to verify the performance of the proposed system, because it is managed by a 4D-based process management tool and web cameras, which is highly related to the AR technique detailed in this study.
5.2 Procedure of System Operation
First, a preparation stage for implementing AR is carried out through connecting with 4D simulation information, as indicated in Fig. 7.
As for the markerless images, field photos taken by cameras, smartphones, or directly captured by web cameras in real-time can be used. Particularly, the images directly captured by smartphones and web cameras include location information and are preferred to take close photos for implementing AR through the web camera connection. When a markerless image is generated, a 3D object to be connected to this image is examined and registered as an AR object. Subsequently, the real-time field images are obtained by selecting a web camera installed in the field, as shown in Fig. 7. Moreover, a markerless image is
Fig. 7. Procedure of System Operation for Applying the Main Functions to the Case Project
searched in the real-time image through the built-in AR process and a 3D model synthesized through AR. An augmented object obtained in this process, as shown in Fig. 8, can be used to implement AR based on various types of schedule information to verify real-time 4D simulation and subsequent schedule simulation
function.
On the screen of the system to implement AR, as shown in Fig. 8, a 4D object used with the real-time field image through AR is confirmed. In addition, various information windows and controls for examining and adjusting augmented 4D and 5D Fig. 8. Screen of AR-based 4D CAD System
Fig. 9. AR and 4D Simulation Based on Real-time Images
objects are included on the display. The main controls include the nD object type controller, which establishes object types that are to be implemented through AR, such as 4D and 5D, the markerless controller, which generates and registers a markerless image based on real-time images, and an object on-off controller, which adjusts separately augmented objects. The schedule controller can simulate augmented 4D objects according to the schedule and execute AR by focusing on the subsequent schedule.
In addition, the information viewer window includes the schedule bar, which is shown at the lower part of Fig. 8, as well as schedule on-going during AR operation.
5.3 4D Simulation Augmentation Based on Real-time Images
Figure 9 shows the implementation of markerless AR and the connecting of the object generated with the schedule information in order to enable 4D simulation. As the real-time field process management can be performed at a distance, the target activity can be implemented in 4D simulation based on VR and AR. In other words, 4D objects until the current period can be implemented through the VR-based 4D simulation, and the field management tasks can be performed in real time by comparing the implemented object with the real-time field image of the same location from the telepresence-based web camera. When this object is implemented through AR, the degree to which structures in the field correlate with their intended status can be examined based on the image where the real-time field image is synthesized with the VR-based 4D object.
As shown in Fig. 9, it is possible to know how the processes examined in the virtual environment can be applied to the real construction site. As the activity objects directly reflected in the real-time images of construction are confirmed, the adequacy of certain activities and construction methods for actual construction can be effectively analyzed. In addition, the telepresence-based schedule management can be conducted more efficiently through such comparison techniques because the problems of existing VR and AR environments, such as the lack of sense of realism and the blind spot of web cameras, can be solved through creating simulations sufficiently realistic to be mutually compatible to managers and field workers alike.
5.4 5D Simulation Augmentation Based on Real-time Images
Figure 10 shows the implementation of markerless AR and the execution of 5D simulation by connecting the object generated with cost information.
Various tasks should be conducted for long-distance schedule management, and a 5D model connected with cost information can be displayed through AR, as indicated in Fig. 10. Cost information that is to be displayed through AR is referred to in the database of nD information, and the daily cost input is represented in the S-curve and histogram, as shown in Fig. 10.
Moreover, the VR-based 5D simulation and AR-based 5D simulation are simultaneously performed to compare the virtual activity to that synthesized from field data to examine actual schedule progress.
Fig. 10. AR and 5D Simulation Based on Real-time Images
5.5 AR Functions for Simulating Subsequent Activities Figure 11 shows an AR implementation example of verifying constructability and checking the subsequent processes, considering real work conditions.
The progress of processes, such as the plan schedule and physical work progress can be represented according to a random period
of time through 4D simulation, as shown in Fig. 11. Such process information can be confirmed by field web camera images through the AR-based 4D simulation. In other words, the status of work progress in the field can be identified based on real-time images obtained by the web camera on the present date of September 15th, 2011, and the intended activities on the same
Fig. 11. AR Simulation of Subsequent Activities in 4D CAD System
Fig. 12. Application of AR Object for Managing Schedule Progress of Railway Infrastructure Project
date can be confirmed through the VR-based 4D simulation.
Subsequently, this process is implemented through AR to facilitate visual process comparison between the construction process of the structure until the present period and the VR-based process object.
However, when an expected work progress status is monitored through the AR-based 4D simulation from the current date (September 15th, 2011) to a random date (September 21st, 2013), as shown in the middle part of Fig. 11, certain virtual objects are overlaid with the structures completely established, thereby making it difficult for the users to visually distinguish them. To solve this problem, the system is designed to implement the subsequent activities through the AR-based 4D simulation for a random period. As shown in the left part of Fig. 11, when only the activities that are performed between September 15th, 2011 and September 21st, 2013 are implemented through the AR- based 4D simulation, a 3D object is not overlaid with the well foundation established. Moreover, as only the 3D objects of pier and tower activities appear, as they are subsequent to well foundation construction, the activities remaining to be executed can be easily identified.
6. Application of AR Objects for a Railway Infra- structure Project
As BIM technology is applied for civil works, the application of AR objects will also be increased for construction projects.
The AR objects can be used for both bridge structures with nonlinear-type operations and railway structures with linear type operations. If AR objects are integrated with an earthwork plan, it is easy for a construction manager to track the construction schedule and work status by comparing a planned-roadbed and a current real-roadbed. In case of the railway infrastructure project with tunnels longer than 10 km, it is hard to monitor the work status from the outside of tunnels. The visualization of real- operations and virtual operations based on AR objects can provide a manager with an effective decision making tool for the progress control and construction management. Fig. 12 shows the application of AR objects for the railway infrastructure project.
The AR objects can be linked with the structure parts and the progress control information based on the Work Breakdown Structure (WBS) codes of a railway project as shown in Fig. 12.
The application technology of nD simulation and AR objects will be implemented as a real system for railway infrastructure projects in a continuous research. The railway works consist of linear processes based on the X-axis position of all processes.
This type of work is a distinctive feature compared to the building work. Therefore, the schedule simulation by 4D CAD is required to be able to grasp the position in the project work area of the simulated activity together with the construction schedule, rather than simply simulating the construction schedule like the building work. For this reason, when AR technology is applied to railway infrastructure construction, it is required to enhance
the technology to enable the position information of the X axis to be grasped when augmenting the future schedule in the image at the present time.
In order to use AR technology more actively in the construction site, the matching technique between real image and virtual image should be simplified and the accuracy of matching should be improved. In other words, it is required to develop a methodology that can be more easily matched when connecting future process images to actual finished images. Especially, railway infrastructure construction is performed on a wide range of construction sites. Therefore, the change of completion level in a certain period of time can be expressed in conjunction with actual image of present time, and the utilization of 4D CAD in railway infrastructure construction can be further increased.
7. Conclusions
In this study, the AR-based 4D CAD system has been proposed to increase the understanding of BIM-based schedule information, as well as to provide a sense of realism and reduce the differences between the virtual construction models and the real world models.
First, the method for connecting 4D-based schedule information with AR objects has been presented to provide a means of indicating various types of schedule-related information such as schedule, progress, and cost management through AR. Using this technique, the system developed in this study can instantly reflect the constantly changing schedule information in AR instead of performing one-time implementation of 3D and 4D simulations through AR. As the AR module uses the 4D engine, different types of information analyzed through the built-in schedule analysis module can be easily displayed in AR.
This paper also proposes connecting the telepresence technique with real-time field images to implement AR. As various types of schedule information analyzed based on 4D models can be implemented in the real-time field images through AR in this method, actual field conditions can be reflected. This method can help long distance managers become more accustomed to VR simulation. The AR-based 4D CAD system has been established based on the discussed methods and applied to the case project.
Moreover, the ability to implement 4D and 5D simulations based on real-time field images through AR has been developed to effectively use schedule information. The ability to prevent an AR object from being overlaid with completed actual structures has also been developed to allow for efficient examination of subsequent activities.The AR-based 4D CAD system allows the users to directly examine the progress of schedule in the field at a distance, whereas it previously had to be analyzed based on the field work reports and digital photos. Thus, it is anticipated that this system can be applied as a tool to monitor fieldwork and support cooperation and decision making between long distance managers and field workers.
Further studies should be performed to solve the limitations of obtaining the field images required for AR. These difficulties are
due to the limitations of the photography angle of web cameras during AR operation and the number of web cameras being installed.
Acknowledgements
A part of this research was supported by a grant (16RTRP- B104237-02) from railway research program funded by Ministry of Land, Infrastructure and Transport of Korean government.
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