## 4 System Architecture  
From a systematical perspective, the iAPR architecture is designed by adopting a standard web-app model including three major components: iAPR browser-based client, web service server, and central database; see Figure 3. In the iAPR system architecture, the server and database are implemented in data center on the cloud, while the client is locally installed on user devices. Accordingly, the connection between the server and database is internal. Meanwhile, the client and server communicate with each other via an external networking infrastructure (e.g., Internet).  
In the iAPR system, the web service server plays a key role and performs four core functions, namely virtual photoreality scenario, 3D anatomy of objects, system management, and user management. In particular, the VP scenario management function provides digital image rendering and projection to reconstruct the captured real environment on the screen. By using multiple deterministic anchors on the surface of the projected sphere, multimedia contents and inter-scene linking objects can be integrated to generate an interactive environment for human sightseeing behavior. On the other hand, the 3D anatomy object management function enables users to perform object projection configuration, multi-level anatomization, and integration on the VP scenario. By default, these functions are activated by user requests through interaction with the corresponding objects inside the VP scenes. In addition, typical system and user management functions are developed to monitor the system operations (e.g., backup, logging, and reporting) and user access (e.g., access control, role assignment, and time&date and location), respectively.  
On the iAPR database server side, three databases are designed to manage visualization, construction topics, and user information data. Among these databases, the visualization volume contains VP scenarios, multimedia contents, and 3D object components. The construction topics volume stores the learning data such as safety, error inspection, construction regulation, standard, and case studies. Lastly, the user information volume manages user ID, name, education major, subject score and feedback, etc.  
These iAPR servers are located on the cloud to ensure high service availability, and they are accessed from user devices through a networking infrastructure [46]. It is worth noting that the web-based application is designed to work on the Internet protocol (IP) stack; therefore, the iAPR system is able to adapt to almost all popular networking technologies such as cellular and WiFi access networks [47; 48]. In the user devices, the iAPR client mainly operates using the built-in web browser to access the iAPR system. When a VP scene is loaded, its integrated 3D objects are temporally cached  
Figure 3: System architecture.  
on the dedicated memory. The 3D objects are called by using popup interaction in the web browser to initiate a 3D anatomy window.  
The iAPR system architecture design is advantageous from several perspectives as follows:  
* The adoption of Web-app model allows the iAPR application to provide services to a variety of user device classes regardless of their current operation systems (iOS, Android, Windows, and Linux) and processing capability.
* The Central server deployment and module-based function development enables easy maintenance and upgrade of system functions and contents.
* The IP-based service implementation facilitates either local or remote user access technologies via heterogeneous networking infrastructures.