## 2 Research Background
### BIM Lightweight Visualization Methods  
Nowadays, commercial software companies such as Autodesk, Tekla, and Bentley have already developed mature BIM software to visualize BIM models. However, such software is all desktop and requires hardware with adequate performance. As a neutral file format, IFC has been widely supported by commercial software and open-source software. Therefore, our research focuses on the visualization of the IFC model on the Web side. In order to improve the display performance, some researchers employed several methods, such as visual culling [44; 45], spatial index [46], scene graph [47], LOD [48], cache design, and component redundancy removal. For instance, [PERSON] et al. [49] analyzed the modeling effort required at different LODs and the impact of the LOD on modeling effort and clash detection. [PERSON] et al. [50] presented a comparable content-based compression algorithm to reduce the redundant information in IFC files through an iterative compression procedure based on an IFC model's tree structure. [PERSON] et al. [51] presented a prototype of a lightweight and real-time Web3D visualization system for large-scale BIM scenes, striving to increase the data accessing rate and achieve better data management. [PERSON] et al. [52] performed geometric optimization for BIM models in mechanical, electrical, and plumbing (MEP) projects by improving algorithms and techniques for storage, transmission, and display. [PERSON] et al. [17] developed an online BIM visualization system, WebBIM, based on IFC and WebGL by rendering decompressed triangular BIM data. In their study, WebBIM first converts the raw IFC geometry data into triangles, which can be directly rendered in WebGL. Then, it removes the redundancy in the geometry information by sharing the geometry data among the object instances generated from the same facility components. [PERSON] et al. [16] provided an implementation plan for data integration and information interaction between BIM and a geographic information system (GIS) andimproved the ability of network transmission and browser rendering. IFC.js specifically excels in the dynamic loading and rendering of IFC models, which is crucial for managing the typically large datasets involved in BIM. It enables more efficient and interactive viewing experiences, thus improving system responsiveness and reducing computational burdens. However, its performance may suffer when dealing with large and complex steel structural models, which contain a large number of members, plates, bolts, welds, and so on. Because of the limitations of JavaScript and WebGL, the rendering of complex steel structural models may cause the browser to respond slowly or even temporarily freeze. With respect to steel structural models, taking the IFC file exported from Tekla software (2022) as an example, the main components of the file include beams, columns, bolts, and gusset plates. Although the employment of LOD technology can improve the performance of the BIM system, the LOD of different entity categories is not clearly defined in the IFC format. It is important to clarify that the LOD discussed herein pertains specifically to graphical detail management rather than the informational depth traditionally associated with BIM stages. In computer graphics, LOD technology dynamically adjusts the complexity of a 3D model's geometry based on viewing distance or computational constraints, aiming to optimize processing loads and maintain visual fidelity other than the granular information modeling required at different stages of a BIM project. If the components are simply divided by instance categories, their specific sizes will greatly influence the performances of LOD models. For example, the size of gusset plates may be large or small, and removing all of them will reduce the display accuracy of the model. In addition, since no wall, slab, or other component blocks the view in the construction models of steel structures, the spatial index method cannot improve the performance.