Lightweight construction and display method of LOD-based BIM model

Abstract

The application relates to a building information model visualization technology, and relates to a LOD-based BIM model lightweight construction and display method and system. The method comprises the following steps: generating a BIM model according to a LOD rule, dividing building component elements required for rendering and display into multiple LOD levels, and setting the levels according to the distance from a viewpoint to the edge of a BIM model bounding box; converting the data of the BIM model in each level into a binary data format, and compressing the BIM model; defining a configuration file, combining the binary data of the BIM model, and generating a layered binary data model; and rendering a three-dimensional model in combination with the screen space error SSE of a current node and a loading mode. The technical problems of performance waste caused by the fact that the organization of three-dimensional data tiles in the prior art does not conform to the actual scene requirements during webpage rendering, and poor webpage decompression and rendering performance after tile compression are solved.

Description

Technical Field

[0001] This invention relates to building information modeling visualization technology, specifically a method and system for lightweight construction and display of BIM models based on LOD (Level of Detail). Background Technology

[0002] Building Information Modeling (BIM) is a digital representation of the physical and functional characteristics of a construction project and its facilities. It serves as a shared resource for project-related information, enabling management throughout the project's design, construction, and operation lifecycle. With the development of the web and mobile internet, the demand for displaying BIM models—previously reliant on desktop computers—on web browsers is increasing.

[0003] Traditional BIM data formats such as RVT, DNG, and LBIM possess systematic component libraries and attribute information, serving as the data foundation for achieving full lifecycle management of building information. However, because they are designed for offline use and are not optimized for download speeds or runtime loading, they cannot be efficiently displayed on web pages. 3D data formats designed for efficient web page rendering, such as GLTF, primarily store 3D data in binary format (bin), allowing direct loading into the GPU buffer without additional parsing, achieving efficient transmission and loading. However, to suit GPU rendering, GLTF sacrifices the component hierarchy; the directly disassembled vertex information cannot be reassembled into a specific component within the BIM system, thus making it impossible to manage the model's component hierarchy after page rendering. The 3D Tiles data format released by CesiumGS builds upon GLTF, using a Hierarchical Level of Detail (HLOD) method to segment 3D data, enabling streaming transmission and rendering of massive amounts of 3D geographic data. The tile data in the 3D Tiles dataset is organized in a tree structure (e.g., Figure 1 Similar to tile organization in 2D maps, WebGL can load and render data in a hierarchical and tile-based manner. Because it conforms to the logic of graphics rendering APIs and organizes data in the form of vertex textures, WebGL can directly render the 3D model data it organizes without conversion.

[0004] Popular 3D Tiles data conversion tools include the officially recommended Cesium Ion and the domestically developed Cesiumlab, which can convert between gltf, obj, fbx, and other formats and 3D Tiles data formats. Tile data segmentation is based on data volume; for example, grid splitting is performed when a single b3dm exceeds a certain threshold, or CMPT merging is performed when the ratio of a single b3dm to i3dm is less than a certain threshold. The draco algorithm is used for general vertex compression.

[0005] However, loading individual models differs from loading maps. Even if a single model is divided into hierarchical tiles, the entire data still needs to be loaded and rendered. Since the organization of tile data is based on maximizing computer rendering performance rather than on the component system of the BIM model, in real-world scenarios, generating a complete model from broken tiles still takes a long time and results in a poor user experience when rendering the model in blocks. When the map zoom level is small, users cannot see the interior of the model, making rendering the internal components an unnecessary waste of performance. Furthermore, the model compressed by the Draco algorithm requires decompression using Wasm during page rendering. While this optimizes the amount of data requested, it reduces rendering performance, and can even cause application crashes when the model data volume is large. Summary of the Invention

[0006] This invention provides a lightweight construction and display method for BIM models based on LOD, in order to solve the technical problems in the prior art where the organization of 3D data tiles does not meet the actual scene requirements of web page rendering, resulting in performance waste, and poor web page decompression and rendering performance after tile compression.

[0007] The method of this invention is implemented using the following technical solution: a lightweight construction and display method for LOD-based BIM models, comprising the following steps:

[0008] S1. Generate a BIM model based on LOD rules;

[0009] The BIM modeling rules are as follows: the building component elements to be rendered and displayed are divided into multiple LOD levels, and the level is set based on the distance from the viewpoint to the edge of the BIM model bounding box; under each level, the display settings of the corresponding building component elements and structural component elements are set according to the building component elements and structural component elements that can be clearly seen at the viewpoint location.

[0010] S2. Convert the data of the BIM model in each level into binary data format and compress the BIM model;

[0011] S3. Define the configuration file, combine the binary data of the BIM model, and generate a hierarchical binary data model;

[0012] S4. Render the 3D model by combining the current node's screen space error (SSE) and loading method.

[0013] In a preferred embodiment, step S1 divides the architectural component elements to be rendered and displayed into four LOD levels, namely the first level, the second level, the third level, and the fourth level.

[0014] At the first level, only outdoor building components and outdoor structural components are set. Indoor components that are not within the field of view are not set. No texture mapping is applied to the components. Only the pick color of the corresponding texture is set for the components.

[0015] In the second level, add details to the exterior outline of the house and electromechanical components, and add interior floors, walls and doors to show the interior outline. Do not apply texture mapping to the components, only set the pick color of the corresponding texture to the components.

[0016] In the third level, the untextured components of the first and second levels are replaced with textured components, and textured architectural component elements are added, including interior furniture, stairs, and mechanical equipment;

[0017] At the fourth level, all building component elements, structural component elements, outline component elements, and MEP component elements are added to display the most detailed BIM model.

[0018] In a preferred embodiment, when performing step S4, if the current node's SSE is less than the threshold, or if the current node's SSE exceeds the threshold and the hierarchical relationship of its child nodes is replacement, then the current node is unloaded; if the current node's SSE exceeds the threshold and the hierarchical relationship of its child nodes is increment, then the current node is loaded.

[0019] Further, step S4 includes:

[0020] S41. Does the recursive marker node SSE exceed the threshold?

[0021] The node is recursively checked, starting from the root node and moving towards the leaf node. The node is marked as SSE if it exceeds the threshold. If it does not exceed the threshold, the current node is unloaded and the current recursion is terminated. If it exceeds the threshold, the current node is marked as "exceeds the threshold" and the judgment on the replacement relationship is added to the asynchronous task queue. The synchronous task ends after the recursion reaches the leaf node.

[0022] S42. Load or unload nodes based on parent-child node replacement relationships.

[0023] Asynchronous tasks are executed from leaf nodes to root nodes. The loading or unloading of the current node is determined based on the replacement relationship between parent and child nodes. First, it is determined whether the number of child nodes of the current node exceeds the threshold. If the number of child nodes at this level exceeds the threshold and the hierarchical relationship of the child node is a replacement relationship, it means that the child node has been loaded and replaced the parent node, so the current node is unloaded. If the hierarchical relationship of the child node is an incremental relationship, it means that the child node is loaded incrementally based on the parent node, so the current node is loaded. If the number of child nodes at this level does not exceed the threshold, that is, the current node is the finest node at this LOD level, then the current node is loaded.

[0024] The system of this invention is implemented using the following technical solution: a lightweight BIM model construction and display system based on LOD, comprising the following modules:

[0025] The model generation module is used to generate BIM models according to LOD rules. The BIM modeling rules are as follows: the building component elements to be rendered and displayed are divided into multiple LOD levels, and the level is set based on the distance from the viewpoint to the edge of the BIM model bounding box. Under each level, the display settings of the corresponding building component elements and structural component elements are set according to the building component elements and structural component elements that can be seen from the viewpoint.

[0026] The model compression module is used to convert the data of the BIM model at each level into binary data format and compress the BIM model.

[0027] The layered data generation module is used to define configuration files, combine binary data from the BIM model, and generate a layered binary data model.

[0028] The rendering module is used to render 3D models by combining the screen space error (SSE) of the current node and the loading method.

[0029] Compared with the prior art, the technical effects achieved by the present invention include:

[0030] 1. The hierarchical establishment of the BIM modeling rules of this invention is based on the distance d from the viewpoint (i.e., the camera) to the bounding box of the BIM model, thereby enabling LOD hierarchical display of the BIM model based on the user's perspective. In addition, the organization of BIM model tile data is optimized to achieve lightweight hierarchical models and improve rendering performance. Overall, it solves the technical problems in the prior art where the organization of 3D data tiles does not meet the actual scene requirements of web page rendering, resulting in performance waste, and poor web page decompression rendering performance after tile compression.

[0031] 2. This invention converts BIM model data at each level into lightweight binary 3D data and performs compression processing through the meshopt pipeline. By compressing the BIM model tile data, the efficiency of model rendering and loading is improved. Attached Figure Description

[0032] Figure 1 A schematic diagram illustrating the tree structure organization of tile data in existing 3D Tiles datasets;

[0033] Figure 2 This is a schematic diagram illustrating the process of BIM model construction and display in an embodiment of the present invention;

[0034] Figure 3 This is a schematic diagram of the incremental switching method for switching model components in an embodiment of the present invention, where Figure a is a rendering diagram before incremental switching and Figure b is a rendering diagram after incremental refinement.

[0035] Figure 4This is a schematic diagram of switching model components by replacement in an embodiment of the present invention, wherein Figure a is a rendering diagram before replacement and Figure b is a rendering diagram after replacement and refinement.

[0036] Figure 5 This is a schematic diagram illustrating the loading methods for different LOD layers in an embodiment of the present invention;

[0037] Figure 6 This is a schematic diagram illustrating the calculation of screen space error (SSE) in an embodiment of the present invention.

[0038] Figure 7 This is a flowchart illustrating the process of determining whether to load or unload the current node when rendering a 3D model in an embodiment of the present invention.

[0039] Figure 8 This is one of the schematic diagrams showing the display effect of the first-level rendering model;

[0040] Figure 9 This is the second illustration showing the rendering effect of the first-level model.

[0041] Figure 10 This is one of the schematic diagrams showing the display effect of the second-level rendering model;

[0042] Figure 11 This is the second illustration showing the rendering effect of the second-level model.

[0043] Figure 12 This is one of the schematic diagrams showing the rendering effect of the third-level model;

[0044] Figure 13 This is the second illustration showing the rendering effect of the third-level model.

[0045] Figure 14 This is a schematic diagram showing the rendering effect of the fourth-level model. Detailed Implementation

[0046] The embodiments of the present invention will be described in further detail below with reference to the examples and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0047] Example 1

[0048] This embodiment provides a lightweight construction and display method for BIM models based on LOD (Level of Detail). See [link to documentation]. Figure 2 Specifically, it includes the following steps:

[0049] S1. Generate BIM model according to LOD rules

[0050] In this embodiment, the BIM modeling rules based on LOD are shown in Table 1 below. The building component elements to be rendered and displayed are divided into four levels, namely the first level LOD1, the second level LOD2, the third level LOD3, and the fourth level LOD4.

[0051] Table 1

[0052]

[0053]

[0054] The hierarchical structure of BIM modeling rules is based on the distance d from the viewpoint (i.e., the camera) to the edge of the BIM model bounding box (i.e., the distance from the tile to the camera in Table 1). At each level, the display settings for the corresponding building and structural components are configured according to the visible building and structural elements at the viewpoint's location.

[0055] At the first level, LOD1 (300m < d ≤ 3000m), users can only see the external outline of the house. Interior components will not appear in the field of view; only the surface color of the components can be seen. Therefore, this level only sets outdoor architectural components (doors, roofs, floors, windows, railings, walls) and outdoor structural components (beams, slabs, columns). Interior components that are not in the field of view are not set, and no texture mapping is applied to the components. Only the pick color of the corresponding texture is set for the components.

[0056] At the second level LOD2 (100m<d≤300m), users can see the entire exterior of the house and part of the interior outline through the windows, but cannot see clear texture details. Therefore, the exterior outline details of the house (decorative panels, decorative lines, etc.) and electromechanical components (piping equipment, etc.) are added, and interior floors, walls and doors are added to show the interior outline, but the components are still not texture-mapped, only the corresponding texture colors are set for the components.

[0057] At the third level LOD3 (0m<d≤100m), users can see all outdoor components and textures. They can see indoor furniture and stairs through the windows. Therefore, at this level, the untextured components of the first level LOD1 and the second level LOD2 should be replaced with textured components, and textured indoor furniture, stairs, mechanical equipment and other architectural components should be added.

[0058] At the fourth level, LOD4 (d=0m), users enter the interior and can move around to observe all the components inside. Therefore, this level displays the most detailed BIM model, adding all architectural component elements, structural component elements, outline component elements, and MEP component elements.

[0059] The distance d between the tile and the camera is determined by the distance from the viewpoint to the edge of the BIM model bounding box. Because distance is used for determination, the map level in the modeling rules is an estimated value, not an exact value.

[0060] In this embodiment, Revit software is used to generate the BIM model, so the generated BIM model is also called the RVT model, and the FBX file is exported as the output of the model.

[0061] S2. Convert the data of the BIM model in each level into binary data format and compress the BIM model.

[0062] The four-level BIM model data is converted into lightweight binary 3D data and compressed using the meshopt pipeline.

[0063] The meshopt pipeline uses algorithms to add indexes to BIM models, optimize vertex buffers, vectorize vertices, compress vertex caches, and compress point clouds. It can compress the model size to one-quarter of its original size without affecting the visual appearance. During rendering and decompression, only the unpacker serialized in the string needs to be used, without the need for network requests.

[0064] In this embodiment, the BIM model data is an fbx file. This step converts the fbx file into a gltf file, compresses the gltf file and exports it as a glb format file, and then converts the glb format into B3DM tile format data.

[0065] S3. Define the configuration file, combine the binary data of the BIM model, and generate a hierarchical binary data model.

[0066] By defining properties in the configuration file, binary data from different levels is combined as tiles to generate a hierarchical binary data model. The relevant properties defined in the configuration file include:

[0067] The transformation matrix of the model (including rotation, translation, and scaling) is used to set the angle, position, and size of the model in the scene when the scene is loaded. Among them, the translation value of the model in the scene needs to be set according to the coordinate system of the loaded map. For example, if the map is a 4978 spatial rectangular coordinate system, then the coordinates of the corresponding position in the 4978 coordinate system need to be set as the translation value.

[0068] Geometric error (GE) is defined in the 3D Tiles standard as a metric of the error between a computer-generated approximate geometric model and an ideal mathematical model, measured in meters. In this embodiment, GE is used to determine the LOD level of the BIM model during rendering. It is used together with the tile distance from the camera (d), screen height, and camera status to calculate the screen space error (SSE). If the screen space error exceeds a set threshold, the node is rendered; if the screen space error is less than the set threshold, the node is hidden.

[0069] Node switching methods are divided into incremental switching and replacement switching; among them, incremental switching (add) means that when loading the next level, the model components of the previous level are retained, such as... Figure 3 Figure a shows the rendering diagram before incremental refinement, and Figure b shows the rendering diagram after incremental refinement. The replacement switching method (replace) means that when loading the next level, the previous level model component is directly replaced, such as... Figure 4 Figure a shows the rendering diagram before replacement, and Figure b shows the rendering diagram after replacement and refinement. This embodiment sets the corresponding loading method according to the LOD rule level, as follows: Figure 5 It is used to switch model components at different LOD levels. The first level LOD1, the second level LOD2, and the fourth level LOD4 are all incremental switching methods, while the third level LOD3 is a replacement switching method.

[0070] The bounding box of a node model records the x, y, and z coordinates of its center point, as well as the model's length, width, and height. During rendering, the bounding box is used to calculate the distance d between the model and the camera (i.e., the viewpoint), which is used to determine the LOD level at that point.

[0071] This step imports the corresponding binary data file by using four levels of child node parameters, including the child node's transformation matrix, geometric error, node switching method, and pointing to the tile data of the corresponding level.

[0072] In this embodiment, a tileset.json configuration file is defined, and b3dm tile data is combined to generate 3D Tiles files.

[0073] S4. Render the 3D model by combining the current node's screen space error (SSE) and loading method.

[0074] Screen Space Error (SSE) is a metric, measured in pixels, representing the final representation of geometric errors on the screen after processing by the 3D rendering pipeline. For example... Figure 6 As shown, the formula for calculating the screen space error (SSE) is as follows:

[0075]

[0076] Among them, e s Screen Space Error (SSE); e g θ represents the geometric error GE; H is the rendering window height; d is the distance between the viewpoint and the BIM model bounding box; d′ is the distance between the viewpoint and the screen; θ fov This represents the size of the field of view.

[0077] Computer-generated BIM models are raster data composed of pixels. When the distance *d* from the viewpoint to the bounding box of the BIM model decreases, the rasterization effect of the BIM model will amplify the gap between the approximate model and the ideal mathematical model with more pixels, specifically manifesting as an increase in the Screen Space Error (SSE). When the SSE exceeds a set threshold, the original coarse model is switched to a more refined BIM model.

[0078] If the current node's SSE is less than the threshold, or if the current node's SSE exceeds the threshold and the hierarchical relationship of its child nodes is replacement, then the current node is unloaded; if the current node's SSE exceeds the threshold and the hierarchical relationship of its child nodes is incremental, then the current node is loaded. See [link to logic for loading / unloading the current node] for details. Figure 7 Specifically, it includes the following steps:

[0079] S41. Does the recursive marker node SSE exceed the threshold?

[0080] The process recursively checks each node, starting from the root node and moving towards the leaf nodes, marking whether the node's SSE (Separation and Replacement) exceeds a threshold. If it does not exceed the threshold, the current node is unloaded, and the recursion ends. If it exceeds the threshold, the current node is marked as "exceeding the threshold," and the determination of the replacement relationship is added to the asynchronous task queue. The synchronous task ends after recursion reaches the leaf node.

[0081] S42. Load or unload nodes based on parent-child node replacement relationships.

[0082] Asynchronous tasks are executed from leaf nodes to root nodes. In these asynchronous tasks, the SSE (Self-Skilled Node) of the current node is marked as exceeding a threshold. At this point, the loading or unloading of the current node is determined based on the replacement relationship between parent and child nodes. First, it is checked whether the child nodes of the current node exceed the threshold. If the child nodes at this level exceed the threshold, and the hierarchical relationship of the child node is a replacement relationship, it means that the child node has been loaded and will replace the parent node, so the current node needs to be unloaded. If the hierarchical relationship of the child node is an incremental relationship, it means that the child node is loaded incrementally based on the parent node, so the current node is loaded. If the child nodes at this level do not exceed the threshold, meaning the current node is the finest node at this LOD level, then the current node is loaded.

[0083] See the rendering models for each level from LOD1 to LOD4. Figure 8-14, Figure 8-14 The display effect is based on a 1920*1080 resolution.

[0084] Example 2

[0085] Based on the same inventive concept as Embodiment 1, this embodiment is a lightweight BIM model construction and display system based on LOD, specifically including the following modules:

[0086] The model generation module is used to implement step S1 of embodiment 1, which generates a BIM model according to the LOD rules. The BIM modeling rules are as follows: the building component elements to be rendered and displayed are divided into multiple LOD levels, and the level is set based on the distance from the viewpoint to the edge of the BIM model bounding box. Under each level, the corresponding building component elements and structural component elements are set according to the building component elements and structural component elements that can be seen at the viewpoint.

[0087] The model compression module is used to implement step S2 of embodiment 1, which converts the data of the BIM model in each level into binary data format and compresses the BIM model.

[0088] The layered data generation module is used to implement step S3 of embodiment 1, define the configuration file, combine the binary data of the BIM model, and generate a layered binary data model.

[0089] The rendering module is used to implement step S4 of embodiment 1, which renders the 3D model by combining the screen space error SSE of the current node and the loading method.

[0090] In this embodiment, the model generation module divides the building component elements to be rendered and displayed into four LOD levels, namely the first level, the second level, the third level, and the fourth level.

[0091] At the first level, only outdoor building components and outdoor structural components are set. Indoor components that are not within the field of view are not set. No texture mapping is applied to the components. Only the pick color of the corresponding texture is set for the components.

[0092] In the second level, add details to the exterior outline of the house and electromechanical components, and add interior floors, walls and doors to show the interior outline. Do not apply texture mapping to the components, only set the pick color of the corresponding texture to the components.

[0093] In the third level, the untextured components of the first and second levels are replaced with textured components, and textured architectural component elements are added, including interior furniture, stairs, and mechanical equipment;

[0094] At the fourth level, all building component elements, structural component elements, outline component elements, and MEP component elements are added to display the most detailed BIM model.

[0095] The rendering module handles the current node as follows: if the screen space error (SSE) of the current node is less than the threshold, or if the screen space error (SSE) of the current node exceeds the threshold and the hierarchical relationship of the child nodes is replacement, then the current node is unloaded; if the screen space error (SSE) of the current node exceeds the threshold and the hierarchical relationship of the child nodes is incremental, then the current node is loaded.

[0096] The technical means adopted in this embodiment and the effects achieved therein correspond to those in Embodiment 1, and will not be repeated here.

[0097] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A lightweight construction and display method for BIM models based on LOD, characterized in that, Includes the following steps: S1. Generate a BIM model based on LOD rules; The BIM modeling rules are as follows: the building component elements to be rendered and displayed are divided into multiple LOD levels, and the level is set based on the distance from the viewpoint to the edge of the BIM model bounding box; under each level, the display settings of the corresponding building component elements and structural component elements are set according to the building component elements and structural component elements that can be clearly seen at the viewpoint location. S2. Convert the data of the BIM model in each level into binary data format and compress the BIM model; S3. Define the configuration file, combine the binary data of the BIM model, and generate a hierarchical binary data model; S4. Render the 3D model by combining the screen space error (SSE) of the current node and the loading method; Step S2 compresses the BIM model through the meshopt pipeline, including adding an index to the BIM model, optimizing the vertex buffer, vectorizing the vertex, compressing the vertex cache, and compressing the point cloud. Step S3 combines the binary data of each level in the form of tiles by defining the attributes of the configuration file, generating a hierarchical binary data model. The relevant properties of the configuration file include: The transformation matrix of the model is used to set the model's angle, position, and size in the scene when the scene is loaded; Geometric error is used to calculate screen space error together with tile distance from camera d, screen size, and pixels. If the screen space error exceeds a set threshold, the node is rendered; if the screen space error is less than the set threshold, the node is hidden. The tile distance from camera d is determined by the distance from the viewpoint to the edge of the BIM model bounding box. Node switching methods are divided into incremental switching and replacement switching. Incremental switching means that the previous level model components are retained when loading the next level; replacement switching means that the previous level model components are directly replaced when loading the next level. The bounding box of the node model is used to record the x, y, and z coordinates of the center point of the bounding box, as well as the length, width, and height dimensions of the model. During rendering, the bounding box is used to calculate the distance d between the BIM model and the viewpoint, which is used to determine the LOD level at this time. In step S4, if the screen space error SSE of the current node is less than the threshold, or if the screen space error SSE of the current node exceeds the threshold and the hierarchical relationship of the child nodes is replacement, then the current node is unloaded; if the screen space error SSE of the current node exceeds the threshold and the hierarchical relationship of the child nodes is incremental, then the current node is loaded. Step S4 includes: S41. Does the recursive marker node SSE exceed the threshold? The node is recursively checked, starting from the root node and moving towards the leaf node. The node's SSE is marked as exceeding the threshold. If it does not exceed the threshold, the current node is unloaded and the current recursion is terminated. If it exceeds the threshold, the current node is marked as "exceeding the threshold", and the judgment on the replacement relationship is added to the asynchronous task queue. The synchronous task ends after the recursion reaches the leaf node. S42. Load or unload nodes based on parent-child node replacement relationships. Asynchronous tasks are executed from leaf nodes to root nodes. The loading or unloading of the current node is determined based on the replacement relationship between parent and child nodes. First, it is determined whether the child nodes of the current node exceed the SSE threshold. If the child nodes of the current node exceed the SSE threshold and the hierarchical relationship of the child node is a replacement relationship, it means that the child node has been loaded and replaced the parent node, so the current node is unloaded. If the hierarchical relationship of the child node is an incremental relationship, it means that the child node is loaded incrementally based on the parent node, so the current node is loaded. If the child nodes of the current node do not exceed the SSE threshold, that is, the current node is the finest node under the corresponding LOD level, then the current node is loaded.

2. The lightweight construction and display method for BIM models according to claim 1, characterized in that, Step S1 divides the architectural component elements to be rendered and displayed into four LOD levels: the first level, the second level, the third level, and the fourth level. At the first level, only outdoor building components and outdoor structural components are set. Indoor components that are not within the field of view are not set. No texture mapping is applied to the components. Only the pick color of the corresponding texture is set for the components. In the second level, add details to the exterior outline of the house and electromechanical components, and add interior floors, walls and doors to show the interior outline. Do not apply texture mapping to the components, only set the pick color of the corresponding texture to the components. In the third level, the untextured components of the first and second levels are replaced with textured components, and textured architectural component elements are added, including interior furniture, stairs, and mechanical equipment; At the fourth level, all building component elements, structural component elements, outline component elements, and MEP component elements are added to display the most detailed BIM model.

3. The lightweight construction and display method for BIM models according to claim 2, characterized in that, In the first level, outdoor building components include doors, roofs, floors, windows, railings, and walls, while outdoor structural components include beams, slabs, and columns.

4. A lightweight BIM model construction and display system based on LOD, characterized in that, The system, implemented according to any one of claims 1-3, comprises the following modules: The model generation module is used to generate BIM models according to LOD rules. The BIM modeling rules are as follows: the building component elements to be rendered and displayed are divided into multiple LOD levels, and the level is set based on the distance from the viewpoint to the edge of the BIM model bounding box. Under each level, the display settings of the corresponding building component elements and structural component elements are set according to the building component elements and structural component elements that can be seen from the viewpoint. The model compression module is used to convert the data of the BIM model at each level into binary data format and compress the BIM model. The layered data generation module is used to define configuration files, combine binary data from the BIM model, and generate a layered binary data model. The rendering module is used to render 3D models by combining the screen space error (SSE) of the current node and the loading method.

5. The lightweight BIM model construction and display system according to claim 4, characterized in that, The model generation module divides the architectural components to be rendered and displayed into four LOD levels: the first level, the second level, the third level, and the fourth level. At the first level, only outdoor building components and outdoor structural components are set. Indoor components that are not within the field of view are not set. No texture mapping is applied to the components. Only the pick color of the corresponding texture is set for the components. In the second level, add details to the exterior outline of the house and electromechanical components, and add interior floors, walls and doors to show the interior outline. Do not apply texture mapping to the components, only set the pick color of the corresponding texture to the components. In the third level, the untextured components of the first and second levels are replaced with textured components, and textured architectural component elements are added, including interior furniture, stairs, and mechanical equipment; At the fourth level, all building component elements, structural component elements, outline component elements, and MEP component elements are added to display the most detailed BIM model.

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