WO2023124842A1 - 基于lod的bim模型轻量化构建与显示方法 - Google Patents

基于lod的bim模型轻量化构建与显示方法 Download PDF

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WO2023124842A1
WO2023124842A1 PCT/CN2022/137043 CN2022137043W WO2023124842A1 WO 2023124842 A1 WO2023124842 A1 WO 2023124842A1 CN 2022137043 W CN2022137043 W CN 2022137043W WO 2023124842 A1 WO2023124842 A1 WO 2023124842A1
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model
node
bim model
current node
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陈明花
陈顺清
邓明亮
蔡金怡
黎旦阳
谢权
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奥格科技股份有限公司
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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F16/00—Information retrieval; Database structures therefor; File system structures therefor
    • G06F16/90—Details of database functions independent of the retrieved data types
    • G06F16/904—Browsing; Visualisation therefor
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F30/00—Computer-aided design [CAD]
    • G06F30/10—Geometric CAD
    • G06F30/13—Architectural design, e.g. computer-aided architectural design [CAAD] related to design of buildings, bridges, landscapes, production plants or roads
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T15/00—3D [Three Dimensional] image rendering
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T15/00—3D [Three Dimensional] image rendering
    • G06T15/04—Texture mapping
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T17/00—Three dimensional [3D] modelling, e.g. data description of 3D objects
    • G06T17/10—Constructive solid geometry [CSG] using solid primitives, e.g. cylinders, cubes

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  • the invention relates to a building information model visualization technology, in particular to a BIM model lightweight construction and display method and system based on LOD (level of detail).
  • BIM Building Information Modeling
  • Traditional BIM data formats such as rvt, dng, and lbim, have systematic component libraries and attribute information, and are the data basis for realizing the full life cycle management of building information. However, because they are designed for offline use, there is no guarantee for download speed or runtime. Loading is optimized and cannot be displayed efficiently on the web page.
  • the three-dimensional data format designed for efficient rendering of web pages such as gltf, stores three-dimensional data in the form of bin binary, so that it can be directly loaded into the GPU buffer without additional parsing, and can achieve efficient transmission and loading.
  • the 3D Tiles data format released by CesiumGS is based on gltf, and based on the hierarchical level of detail (HLOD) method, the 3D data is segmented to realize the streaming and rendering of massive 3D geographic data.
  • the tile data in the 3D Tiles dataset is organized in a tree structure (as shown in Figure 1), which is similar to the tile organization in a two-dimensional map, and can be loaded and rendered hierarchically and block by block. Since it fits the logic of the graphics rendering API and organizes data in the form of vertex textures, WebGL can directly render the 3D model data organized within it without conversion.
  • the existing popular 3D Tiles data conversion tools include the officially recommended Cesium Ion and domestic cesiumlab, which can convert gltf, obj, fbx and other formats and 3D Tiles data formats.
  • the segmentation of tile data is based on the size of the data, such as in a single b3dm Grid segmentation is performed when a certain threshold is exceeded, or cmpt merge is performed when a single b3dm/i3dm is less than a certain threshold.
  • Common vertex compression uses the draco algorithm.
  • the loading of a single model is different from that of a map. Even if a single model is divided into blocks, it still needs to load and render the entire data. Because the organization of tile data is based on the maximization of computer rendering performance, not based on the component system of the BIM model, In the actual scene, when the model is rendered in blocks, it still takes a long time to generate a complete model from broken tiles and the user experience is not good. When the map zoom level is small, the user cannot see the interior of the model, and rendering internal components is an unnecessary waste of performance. In addition, the model compressed by the Draco algorithm needs to be decompressed with wasm when the page is rendered and decompressed. Although the amount of requested data is optimized, the rendering performance is reduced, and the application may crash when the amount of model data is large.
  • the present invention provides a LOD-based BIM model lightweight construction and display method to solve the problems in the prior art that the organization of three-dimensional data tiles does not meet the actual scene requirements during web page rendering, resulting in performance waste and poor web page decompression rendering performance after tile compression technical problems.
  • the method of the present invention adopts following technical scheme to realize: the BIM model lightweight construction and display method based on LOD comprises the following steps:
  • the BIM modeling rules are as follows: Divide the building component elements to be rendered and displayed into multiple LOD levels, and the level setting is based on the distance from the viewpoint to the edge of the bounding box of the BIM model; Clear architectural elements and structural component elements, and display and set the corresponding architectural component elements and structural component elements;
  • 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;
  • the outdoor architectural component elements and outdoor structural component elements are set, and the indoor components that are not within the field of view are not set, and the texture map is not applied to the components, and only the picking color of the corresponding texture is set for the components;
  • step S4 when step S4 is executed, if the current node SSE is less than the threshold, or the current node SSE exceeds the threshold and the hierarchical relationship of the sub-node is replacement, then the current node is uninstalled; the current node SSE exceeds the threshold and the sub-node's If the layer relationship is incremental, the current node is loaded.
  • step S4 includes:
  • the LOD-based BIM model lightweight construction and display system includes the following modules:
  • the model generation module is used to generate a BIM model according to the LOD rules; where the BIM modeling rule is: divide the building component elements required to be rendered and displayed into multiple LOD levels, and the level is set up with the distance from the viewpoint to the edge of the BIM model bounding box as Basis; at each level, according to the architectural elements and structural element elements that can be seen clearly at the position of the viewpoint, display and set the corresponding architectural element elements and structural element elements;
  • the model compression module is used to convert the data of the BIM model in each level into a binary data format, and compress the BIM model;
  • a layered data generation module for defining configuration files, combining binary data of BIM models, and generating layered binary data models
  • the rendering module is used to render the 3D model in combination with the screen space error SSE of the current node and the loading method.
  • the hierarchical establishment of the BIM modeling rules of the present invention is based on the distance d from the viewpoint (that is, the camera) to the bounding box of the BIM model, so that the LOD hierarchical display of the BIM model can be performed based on the user's perspective, and the BIM model tiles are also optimized
  • the organizational form of the data realizes the lightweight of the hierarchical model and improves the rendering performance; on the whole, it solves the problem of performance waste caused by the organization of 3D data tiles in the prior art that does not meet the actual scene requirements of web page rendering, and the decompression and rendering of web pages after tile compression Technical issues with poor performance.
  • the present invention converts the BIM model data at each level into lightweight binary 3D data, and performs compression processing through the meshopt pipeline, and improves the model rendering and loading efficiency by compressing the BIM model tile data.
  • Fig. 1 is the tree structure organization schematic diagram of the tile data of 3D Tiles dataset in the prior art
  • Fig. 2 is a schematic flow chart of BIM model construction and display in the embodiment of the present invention.
  • Fig. 3 is a schematic diagram of implementing model component switching in an incremental switching manner in an embodiment of the present invention, where a is a schematic rendering before the increment, and b is a schematic rendering after the incremental refinement;
  • Fig. 4 is the schematic diagram that realizes model component switching in the switching mode of replacement in the embodiment of the present invention, and wherein a diagram is the rendering diagram before replacing, and b diagram is the rendering diagram after replacing refinement;
  • Fig. 5 is a schematic diagram of the loading mode of different levels of LOD layer settings in the embodiment of the present invention.
  • FIG. 6 is a schematic diagram of calculating a screen space error SSE in an embodiment of the present invention.
  • FIG. 7 is a flow chart for judging whether to load or unload the current node when rendering a 3D model in an embodiment of the present invention
  • FIG. 8 is one of the schematic diagrams showing the rendering effect of the first-level rendering model
  • FIG. 9 is the second schematic diagram of the display effect of the first-level rendering model
  • FIG. 10 is one of the schematic diagrams showing the rendering effect of the second-level rendering model
  • Fig. 11 is the second schematic diagram of the display effect of the rendering model of the second level
  • Fig. 12 is one of the schematic diagrams showing the rendering effect of the third-level rendering model
  • Fig. 13 is the second schematic diagram of the display effect of the third-level rendering model
  • FIG. 14 is a schematic diagram of the display effect of the rendering model of the fourth level.
  • This embodiment provides a method for lightweight construction and display of a BIM model based on LOD, as shown in FIG. 2 , which specifically includes the following steps:
  • the LOD-based BIM modeling rules are specifically shown in Table 1 below, and the architectural component elements required 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.
  • the hierarchy of BIM modeling rules is established based on the distance d from the viewpoint (that is, the camera) to the edge of the bounding box of the BIM model (that is, the distance between the tiles and the camera in Table 1).
  • the viewpoint that is, the camera
  • the edge of the bounding box of the BIM model that is, the distance between the tiles and the camera in Table 1.
  • the user can only see the exterior outline of the house, and the indoor components will not appear in the field of vision, only the color of the surface of the components can be seen, so this level only sets the outdoor Architectural component elements (doors, roofs, floors, windows, railings, walls) and outdoor structural component elements (beams, slabs, columns) are not set for indoor components that are not within the field of view, and texture mapping is not performed on components. Just set the pick color of the corresponding map to the component.
  • the user can see the whole exterior of the house clearly, and can see part of the interior outline through the window, but cannot see clear texture details, so increase the exterior contour details of the house ( Decorative panels, decorative lines, etc.) and electromechanical components (pipeline equipment, etc.), add interior floors, walls and doors to show the internal outline, but still do not texture the components, only set the color of the corresponding texture for the components.
  • the user can clearly see all outdoor components and textures, and can see the indoor furniture and stairs through the windows, so the first level LOD1 and the second level
  • the non-textured components of level LOD2 are replaced with textured components, and textured interior furniture, stairs and mechanical equipment and other architectural component elements are added.
  • the distance d between the tile and the camera is judged by the distance from the viewpoint to the edge of the bounding box of the BIM model. Due to the distance judgment, the map level in the modeling rules is an estimated value, not an accurate value.
  • the 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.
  • the meshopt pipeline adds indexes to the BIM model through algorithms, optimizes vertex buffers, vertex vectorization, compresses vertex buffers, compresses point clouds and other compression processes, which can compress the model size to a quarter of the original without affecting the visual appearance, rendering When decompressing, just use the decompressor unpack serialized in the string, no network request is required.
  • the BIM model data is an fbx file.
  • the fbx file is converted into a gltf file
  • the gltf file is compressed and exported as a glb format file
  • the glb format is converted into b3dm tile format data.
  • the binary data of each level is combined in the form of tiles to generate a hierarchical binary data model.
  • the relevant properties of the definition configuration file include:
  • 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 loaded map coordinate system , if the map is in the 4978 space Cartesian coordinate system, you need to set the coordinates of the corresponding position in the 4978 coordinate system as the translation value.
  • Geometric Error (GE for short), is defined in the 3D Tiles standard as a measurement error between the approximate geometric model drawn by the computer and the ideal mathematical model, in meters.
  • the geometric error GE is used as the LOD level judgment of the BIM model during rendering, and it is used to calculate the screen space error (Screen Space Error/SSE) together with the tile distance d from the camera, the screen height, and the camera state.
  • the screen space error exceeds If the threshold is set, the node will be rendered, and if the screen space error is smaller than the threshold, the node will be hidden.
  • Node switching methods are divided into incremental switching methods and replacement switching methods; among them, the incremental switching method (add) means that the model components of the previous level are retained when loading the next level, as shown in Figure 3, where a is the incremental The previous rendering schematic diagram, b is the rendering schematic diagram after incremental refinement; the replacement switching method (replace) means to directly replace the upper-level model components when loading the next level, as shown in Figure 4, where a is the rendering before replacement Schematic diagram, Figure b is the rendered schematic diagram after replacement and refinement.
  • the corresponding loading method is set as shown in Figure 5, which is used to realize the switching of model components under different LOD levels.
  • the first level LOD1, the second level LOD2, and the fourth level LOD4 are all incremental switching modes
  • the third level LOD3 is an alternative switching method.
  • 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.
  • the bounding box is used to calculate the distance d between the model and the camera (namely, the viewpoint), and is used to judge the LOD level at this time.
  • the corresponding binary data files are imported through four levels of child node parameters, including the transformation matrix of the child node, geometric error, node switching mode, and tile data pointing to the corresponding level.
  • the tileset.json configuration file is defined, and the b3dm tile data is combined to generate a 3D Tiles file.
  • Screen Space Error (Screen Space Error, SSE for short) is the measurement error that the geometric error is finally presented on the screen after the three-dimensional rendering pipeline is processed, and the unit is pixel. As shown in Figure 6, the calculation formula of the screen space error SSE is as follows:
  • the BIM model drawn by the computer is raster data composed of pixels.
  • the rasterization effect of the BIM model will enlarge the gap between the approximate model and the ideal mathematical model with more pixels. , which is specifically presented as an increase in the screen space error SSE.
  • the screen space error SSE exceeds the set threshold, the original rough model is switched to a finer BIM model.
  • the current node SSE is less than the threshold, or the current node SSE exceeds the threshold and the hierarchical relationship of the child nodes is replaced, then the current node is unloaded; the current node SSE exceeds the threshold and the child node’s hierarchical relationship is incremental, then the current node is loaded.
  • Figure 7 The logical judgment of loading or unloading the current node is shown in Figure 7, which specifically includes the following steps:
  • the child node 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 will replace the parent node, and the current node needs to be uninstalled; if The hierarchical relationship of the child node is an incremental relationship, which means that the child node is incrementally loaded on the basis of the parent node, and the current node is loaded; if the child node at this level does not exceed the threshold, that is, the current node is the latest node at the LOD level. For finer nodes, load the current node.
  • this embodiment is a LOD-based BIM model lightweight construction and display system, specifically including the following modules:
  • the model generation module is used to implement step S1 of Embodiment 1, and generates a BIM model according to the LOD rule; wherein the BIM modeling rule is: divide the building component elements required to be rendered and displayed into multiple LOD levels, and the levels are set up from viewpoint to The distance from the edge of the bounding box of the BIM model is used as the basis; at each level, according to the architectural elements and structural element elements that can be seen clearly at the position of the viewpoint, the corresponding architectural element elements and structural element elements are displayed and set;
  • the model compression module is used to realize the step S2 of embodiment 1, converts the data of the BIM model in each level into a binary data format, and compresses the BIM model;
  • a layered data generation module used to implement step S3 of Embodiment 1, define a configuration file, combine binary data of a BIM model, and generate a layered binary data model;
  • the rendering module is used to implement step S4 of Embodiment 1, and renders the 3D model in combination with the screen space error SSE of the current node and the loading method.
  • the model generation module divides the architectural component elements required to be rendered and displayed into four LOD levels, namely the first level, the second level, the third level and the fourth level;
  • the outdoor architectural component elements and outdoor structural component elements are set, and the indoor components that are not within the field of view are not set, and the texture map is not applied to the components, and only the picking color of the corresponding texture is set for the components;
  • the processing method of the rendering module for the current node is: the screen space error SSE of the current node is less than the threshold, or 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 uninstalled; the screen space error of the current node If the error SSE exceeds the threshold and the hierarchical relationship of the child nodes is incremental, the current node is loaded.

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Abstract

本发明涉及建筑信息模型可视化技术，为基于LOD的BIM模型轻量化构建与显示方法、系统，其方法包括：根据LOD规则生成BIM模型，将所需渲染显示的建筑构件元素划分为多个LOD层级，且层级设立以视点至BIM模型包围盒边缘的距离为依据；将各层级中BIM模型的数据转成二进制数据格式，并对BIM模型进行压缩；定义配置文件，组合BIM模型的二进制数据，生成分层二进制数据模型；结合当前节点的屏幕空间误差SSE及加载方式渲染三维模型。解决了现有技术中三维数据瓦片的组织不符合网页渲染时的实际场景需求而造成性能浪费、瓦片压缩后网页解压渲染性能差的技术问题。

Description

基于LOD的BIM模型轻量化构建与显示方法 技术领域
本发明涉及建筑信息模型可视化技术，具体为基于LOD(多细节层次)的BIM模型轻量化构建与显示方法、系统。
背景技术
建筑信息模型(BIM)是建设工程及其设施的物理和功能特性的数字化表达，作为工程项目相关信息的共享资源，依此对项目设计、施工、运营的全生命周期中进行管理。随着web互联网和移动互联网的发展，以往依赖桌面端的BIM模型在网页端显示的需求日益增多。
传统的BIM数据格式如rvt、dng、lbim，其具有体系化的构件库及属性信息，是实现建筑信息全生命周期管理数据基础，但由于其设计为离线使用，没有针对下载速度或运行时的加载进行优化，无法在网页端高效显示。为web页面高效渲染而设计的三维数据格式如gltf，其主体将三维数据以bin二进制的形式存储，使其可以直接加载到GPU缓冲区中，无需额外解析，能实现高效传输和加载。但为了适用于GPU渲染，gltf牺牲了构件的体系，直接拆散的顶点信息无法重组回BIM体系中的某一个构件，因此在页面渲染后无法对模型的构件体系进行管理。CesiumGS发布的3D Tiles数据格式是在gltf的基础上，基于分级细节层次(HLOD)的方法，将三维数据切分，实现海量三维地理数据的流式传输和渲染。3D Tiles数据集中的瓦片数据以树结构组织(如图1)，与二维地图中的瓦片组织相似，能分级分块加载渲染。由于贴合图形渲染API的逻辑，以顶点纹理等形式组织数据，WebGL对其内部组织起来的三维模型数据，不需要转换，可以直接渲染。
现有流行的3D Tiles数据转换工具包括官方推荐的Cesium Ion和国产cesiumlab，能实现gltf、obj、fbx等格式与3D Tiles数据格式转换，瓦片数据的切分依据数据量大小，如在单一b3dm超过某一阈值时进行格网分割，或在单一b3dm/i3dm小于某阈值时进行cmpt合并。通用的顶点压缩使用draco算法。
然而单体模型的加载与地图不同，单个模型即使分级分块，依然需要将整个数据加载渲染出来，由于瓦片数据的组织是基于计算机渲染性能的最大化，而非依据BIM模型的构件体系，在实际场景中模型分块渲染时从破碎的瓦片生成完整的模型用时依然较长且用户体验不好。当地图缩放级别较小时，用户无法看到模型内部，此时渲染内部构件是不必要的性能浪费。此外，Draco算法压缩后的模型在页面渲染解压时需要使用wasm解压，虽然优化了请求的数据量，但降低渲染性能，模型数据量较大时还会出现应用崩溃的情况。
发明内容
本发明提供基于LOD的BIM模型轻量化构建与显示方法，以解决现有技术中三维数据瓦片的组织不符合网页渲染时的实际场景需求而造成性能浪费、瓦片压缩后网页解压渲染性能差的技术问题。
本发明的方法采用如下技术方案来实现：基于LOD的BIM模型轻量化构建与显示方法，包括以下步骤：
S1、根据LOD规则生成BIM模型；
其中BIM建模规则为：将所需渲染显示的建筑构件元素划分为多个LOD层级，且层级设立以视点至BIM模型包围盒边缘的距离为依据；在各层级下，根据视点所在位置能看清的建筑构元素和结构构件元素，对相应的建筑构件元素及结构构件元素进行显示设置；
S2、将各层级中BIM模型的数据转成二进制数据格式，并对BIM模型进行压缩；
S3、定义配置文件，组合BIM模型的二进制数据，生成分层二进制数据模型；
S4、结合当前节点的屏幕空间误差SSE及加载方式渲染三维模型。
在优选的实施例中，步骤S1将所需渲染显示的建筑构件元素划分为四个LOD层级，即第一层级、第二层级、第三层级和第四层级；
在第一层级，只设置室外的建筑构件元素以及室外的结构构件元素，对不在视野范围内的室内构件不设置，不对构件进行纹理贴图，只对构件设置相应贴图的拾取颜色；
在第二层级，增加房屋的外部轮廓细节以及机电构件，增加室内楼板、墙和门以显示内部轮廓，不对构件进行纹理贴图，只对构件设置相应贴图的拾取颜色；
在第三层级，将第一层级和第二层级的无纹理构件替换成带纹理的构件，并增加带纹理的建筑构件元素，包括室内家具、楼梯和机械设备；
在第四层级，增加所有建筑构件元素、结构构件元素、轮廓构件元素以及机电构件元素，以显示最精细的BIM模型。
在一个优选的实施例中，执行步骤S4时，当前节点SSE小于阈值，或当前节点SSE超过阈值且子节点的分层关系为替换，则卸载当前节点；当前节点SSE超过阈值且子节点的分层关系为增量，则加载当前节点。
进一步地，步骤S4包括：
S41、递归标记节点SSE是否超过阈值
对节点进行递归判断，从根节点向叶子节点递归，标记节点SSE是否超过阈值；若不超过阈值，则卸载当前节点，跳出当前递归；若超过阈值则对当前节点标记“超过阈值”，且将对替换关系的判断加入异步任务队列，递归至叶子节点后同步任务结束；
S42、根据父子节点替换关系加载或卸载节点
从叶子节点向根节点执行异步任务，根据父子节点的替换关系判断加载或卸载当前节点；首先判断当前节点的子节点是否超过阈值，如果该级别下子节点超过阈值，且该子节点的分层关系是替换关系，表示该子节点已加载且替换父节点，则卸载当前节点；如果该子节点的分层关系是增量关系，表示该子节点在父节点的基础上增量加载，则加载当前节点；如果该级别下子节点不超过阈值，即当前节点为该LOD级别下的最精细节点，则加载当前节点。
本发明的系统采用如下技术方案来实现：基于LOD的BIM模型轻量化构建与显示系统，包括以下模块：
模型生成模块，用于根据LOD规则生成BIM模型；其中BIM建模规则为：将所需渲染显示的建筑构件元素划分为多个LOD层级，且层级设立以视点至BIM模型包围盒边缘的距离为依据；在各层级下，根据视点所在位置能看清的建筑构元素和结构构件元素，对相应的建筑构件元素及结构构件元素进行显示设置；
模型压缩模块，用于将各层级中BIM模型的数据转成二进制数据格式，并对BIM模型进行压缩；
分层数据生成模块，用于定义配置文件，组合BIM模型的二进制数据，生成分层二进制数据模型；
渲染模块，用于结合当前节点的屏幕空间误差SSE及加载方式渲染三维模型。
与现有技术相比，本发明取得的技术效果包括：
1、本发明的BIM建模规则的层级设立以视点(即相机)至BIM模型包围盒的距离d为依据，从而能够基于用户视角对BIM模型进行LOD分级显示，此外还优化了BIM模型瓦片数据的组织形式，实现层级模型轻量化，提升渲染性能；整体上解决了现有技术中三维数据瓦片的组织不符合网页渲染时的实际场景需求而造成性能浪费、瓦片压缩后网页解压渲染性能差的技术问题。
2、本发明将各层级的BIM模型数据转换为轻量化的二进制三维数据，并通过meshopt管线进行压缩处理操，通过压缩BIM模型瓦片数据，提升了模型渲染加载效率。
附图说明
图1为现有技术中3D Tiles数据集的瓦片数据的树结构组织示意图；
图2为本发明实施例中BIM模型构建与显示的流程示意图；
图3为本发明实施例中以增量的切换方式实现模型构件切换的示意图，其中a图为增量之前的渲染示意图，b图为增量细化之后的渲染示意图；
图4为本发明实施例中以替换的切换方式实现模型构件切换的示意图，其中a图为替换之 前的渲染示意图，b图为替换细化之后的渲染示意图；
图5为本发明实施例中不同层级LOD层设置的加载方式示意图；
图6为本发明实施例中屏幕空间误差SSE的计算示意图；
图7为本发明实施例中渲染三维模型时加载或卸载当前节点的判断流程图；
图8为第一层级的渲染模型显示效果示意图之一；
图9为第一层级的渲染模型显示效果示意图之二；
图10为第二层级的渲染模型显示效果示意图之一；
图11为第二层级的渲染模型显示效果示意图之二；
图12为第三层级的渲染模型显示效果示意图之一；
图13为第三层级的渲染模型显示效果示意图之二；
图14为第四层级的渲染模型显示效果示意图。
具体实施方式
下面结合实施例及附图对本发明的实施方式做进一步详细的描述，但本发明的实施方式并不限于此。
实施例1
本实施例提供基于LOD的BIM模型轻量化构建与显示方法，参见图2，具体包括以下步骤：
S1、根据LOD规则生成BIM模型
本实施例中，基于LOD的BIM建模规则具体如下表一所示，将所需渲染显示的建筑构件元素划分为四个层级，即第一层级LOD1、第二层级LOD2、第三层级LOD3和第四层级LOD4。
表一
Figure PCTCN2022137043-appb-000001
Figure PCTCN2022137043-appb-000002
BIM建模规则的层级设立以视点(即相机)至BIM模型包围盒边缘的距离d(即表一中的瓦片距相机距离)为依据。在各层级下，根据视点所在位置能看清的建筑构元素和结构构件元素，对相应的建筑构件元素及结构构件元素进行显示设置。
在第一层级LOD1的级别(300m＜d≤3000m)，用户只能看清房屋的外部轮廓，室内构件不会出现在视野范围内，只能看到构件表面的颜色，因此该级别只设置室外的建筑构件元素(门、屋顶、楼板、窗、栏杆、墙)以及室外的结构构件元素(梁、板、柱)，对不在视野范围内的室内构件则不设置，且不对构件进行纹理贴图，只需对构件设置相应贴图的拾取颜色。
在第二层级LOD2的级别(100m＜d≤300m)，用户可看清房屋的外部全貌，能通过窗户看到部分室内轮廓，但无法看到清晰的纹理细节，因此增加房屋的外部轮廓细节(装饰板、装饰线条等)以及机电构件(管道设备等)，增加室内楼板、墙和门以显示内部轮廓，但仍不对构件进行纹理贴图，只对构件设置相应贴图的颜色。
在第三层级LOD3的级别(0m＜d≤100m)，用户可以看清所有室外构件以及贴图纹理，通过窗户能看到室内的家具、楼梯，因此该级别下要将第一层级LOD1和第二层级LOD2的无纹理构件替换成带纹理的构件，并增加带纹理的室内家具、楼梯和机械设备等建筑构件元素。
在第四层级LOD4的级别(d＝0m)，用户进入室内，可移动观察室内所有的构件，因此该级别显示最精细的BIM模型，增加所有建筑构件元素、结构构件元素、轮廓构件元素以及机电构件元素。
其中瓦片距相机距离d由视点至BIM模型包围盒边缘的距离判断，由于使用距离判断，建模规则中的地图级别为估算值，并非精确值。
本实施例中，采用Revit软件生成BIM模型，因此所生成的BIM模型也称为rvt模型，并导出fbx文件作为模型的输出。
S2、将各层级中BIM模型的数据转成二进制数据格式，并对BIM模型进行压缩。
将四个层级的BIM模型数据转换为轻量化的二进制三维数据，并通过meshopt管线进行压缩处理操作。
meshopt管线通过算法对BIM模型添加索引、优化顶点缓冲区、顶点向量化、压缩顶点缓存、压缩点云等压缩处理，可将模型大小压缩至原来的四分之一，且不影响视觉外观，渲染解压时只需使用序列化在字符串中的解压器unpack即可，无需网络请求。
本实施例中，BIM模型数据即为fbx文件，本步骤将fbx文件转化为gltf文件，压缩gltf文件并导出为glb格式文件，将glb格式转为b3dm瓦片格式数据。
S3、定义配置文件，组合BIM模型的二进制数据，生成分层二进制数据模型。
通过定义配置文件的属性，将各个层级的二进制数据以瓦片的形式组合，生成一个分层分级的二进制数据模型。其中，定义配置文件的相关属性包括：
模型的变换矩阵(包括旋转、平移、缩放)，用于在场景加载时，设置模型在场景中的角度、位置和大小；其中，模型在场景中的平移值需要根据所加载的地图坐标系设置，如地图为4978空间直角坐标系，则需要将4978坐标系下相应位置的坐标设为平移值。
几何误差，(Geometric Error，简称GE)，在3D Tiles标准中定义为计算机绘制的近似几何模型与理想数学模型之间的一种度量误差，以米为单位。本实施例中将几何误差GE作为渲染时BIM模型的LOD等级判断，用于与瓦片距相机距离d、屏幕高度、相机状态一起计算屏幕空间误差(Screen Space Error/SSE)，屏幕空间误差超过设定阈值则渲染该节点，屏幕空间误差小于设定阈值则隐藏节点。
节点切换方式，分为增量的切换方式和替换的切换方式；其中，增量的切换方式(add)表示加载下一级时保留上一级模型构件，如图3，其中a图为增量之前的渲染示意图，b图为增量细化之后的渲染示意图；替换的切换方式(replace)表示加载下一级时直接替换上一级模型构件，如图4，其中a图为替换之前的渲染示意图，b图为替换细化之后的渲染示意图。本实施例根据LOD规则对应级别设置相应的加载方式如图5，用于实现不同LOD级别下模型构件的切换，第一层级LOD1、第二层级LOD2、第四层级LOD4均为增量的切换方式，第三层级LOD3为替换的切换方式。
节点模型的包围盒，用于记录包围盒中心点的x、y、z坐标，以及模型的长宽高尺寸。渲染时，包围盒用于计算模型距相机(即视点)的距离d，用于判断此时的LOD级别。
本步骤通过四个级别的子节点参数，包括子节点的变换矩阵、几何误差、节点切换方式，以及指向对应级别的瓦片数据，将对应二进制数据文件引入。
本实施例中，定义tileset.json配置文件，组合b3dm瓦片数据，生成3D Tiles文件。
S4、结合当前节点的屏幕空间误差SSE及加载方式渲染三维模型
屏幕空间误差(Screen Space Error，简称SSE)是几何误差在三维渲染管线处理后最终呈现在屏幕上的度量误差，以像素为单位。如图6所示，屏幕空间误差SSE的计算公式如下：
Figure PCTCN2022137043-appb-000003
其中，e s为屏幕空间误差SSE；e g为几何误差GE；H为渲染窗口高度；d为视点与BIM模型包围盒的距离；d′为视点与屏幕的距离；θ fov为视场角大小。
计算机所绘制的BIM模型是由像素组成的栅格数据，视点距BIM模型包围盒距离d减小时，BIM模型的光栅化效果将会以更多的像素放大近似模型与理想数学模型之间的差距，具体呈现为屏幕空间误差SSE增大。当屏幕空间误差SSE超过设定的阈值时，将原来的粗略模型切换为更精细的BIM模型。
当前节点SSE小于阈值，或当前节点SSE超过阈值且子节点的分层关系为替换，则卸载当前节点；当前节点SSE超过阈值且子节点的分层关系为增量，则加载当前节点。加载或卸载当前节点的逻辑判断见图7，具体包括以下步骤：
S41、递归标记节点SSE是否超过阈值
对节点进行递归判断，从根节点向叶子节点递归，标记节点SSE是否超过阈值。若不超过阈值，则卸载当前节点，跳出当前递归；若超过阈值则对当前节点标记“超过阈值”，且将对替换关系的判断加入异步任务队列，递归至叶子节点后同步任务结束。
S42、根据父子节点替换关系加载或卸载节点
从叶子节点向根节点执行异步任务，异步任务中当前节点的SSE都是标记超过阈值的，此时根据父子节点的替换关系判断加载或卸载当前节点。首先判断当前节点的子节点是否超过阈值，如果该级别下子节点超过阈值，且该子节点的分层关系是替换关系，表示该子节点已加载且会替换父节点，则需要卸载当前节点；如果该子节点的分层关系是增量关系，表示该子节点在父节点的基础上增量加载，则加载当前节点；如果该级别下子节点不超过阈值，即当前节点为该LOD级别下的最精细节点，则加载当前节点。
LOD1-LOD4各层级渲染模型见图8-14，图8-14显示效果基于1920*1080分辨率。
实施例2
与实施例1基于相同的发明构思，本实施例为基于LOD的BIM模型轻量化构建与显示系统，具体包括以下模块：
模型生成模块，用于实现实施例1的步骤S1，根据LOD规则生成BIM模型；其中BIM建模规则为：将所需渲染显示的建筑构件元素划分为多个LOD层级，且层级设立以视点至BIM模型包围盒边缘的距离为依据；在各层级下，根据视点所在位置能看清的建筑构元素和结构构件元素，对相应的建筑构件元素及结构构件元素进行显示设置；
模型压缩模块，用于实现实施例1的步骤S2，将各层级中BIM模型的数据转成二进制数据格式，并对BIM模型进行压缩；
分层数据生成模块，用于实现实施例1的步骤S3，定义配置文件，组合BIM模型的二进制数据，生成分层二进制数据模型；
渲染模块，用于实现实施例1的步骤S4，结合当前节点的屏幕空间误差SSE及加载方式渲染三维模型。
在本实施例中，模型生成模块将所需渲染显示的建筑构件元素划分为四个LOD层级，即第一层级、第二层级、第三层级和第四层级；
在第一层级，只设置室外的建筑构件元素以及室外的结构构件元素，对不在视野范围内的室内构件不设置，不对构件进行纹理贴图，只对构件设置相应贴图的拾取颜色；
在第二层级，增加房屋的外部轮廓细节以及机电构件，增加室内楼板、墙和门以显示内部轮廓，不对构件进行纹理贴图，只对构件设置相应贴图的拾取颜色；
在第三层级，将第一层级和第二层级的无纹理构件替换成带纹理的构件，并增加带纹理的建筑构件元素，包括室内家具、楼梯和机械设备；
在第四层级，增加所有建筑构件元素、结构构件元素、轮廓构件元素以及机电构件元素，以显示最精细的BIM模型；
渲染模块对当前节点的处理方式为：当前节点的屏幕空间误差SSE小于阈值，或当前节点的屏幕空间误差SSE超过阈值且子节点的分层关系为替换，则卸载当前节点；当前节点的屏幕空间误差SSE超过阈值且子节点的分层关系为增量，则加载当前节点。
本实施例所采取的各技术手段及其所取得的效果与实施例1为对应关系，在此不赘述。
上述实施例为本发明较佳的实施方式，但本发明的实施方式并不受上述实施例的限制，其他的任何未背离本发明的精神实质与原理下所作的改变、修饰、替代、组合、简化，均应为等效的置换方式，都包含在本发明的保护范围之内。

Claims (10)

  1. 基于LOD的BIM模型轻量化构建与显示方法，其特征在于，包括以下步骤：
    S1、根据LOD规则生成BIM模型；
    其中BIM建模规则为：将所需渲染显示的建筑构件元素划分为多个LOD层级，且层级设立以视点至BIM模型包围盒边缘的距离为依据；在各层级下，根据视点所在位置能看清的建筑构元素和结构构件元素，对相应的建筑构件元素及结构构件元素进行显示设置；
    S2、将各层级中BIM模型的数据转成二进制数据格式，并对BIM模型进行压缩；
    S3、定义配置文件，组合BIM模型的二进制数据，生成分层二进制数据模型；
    S4、结合当前节点的屏幕空间误差SSE及加载方式渲染三维模型。
  2. 根据权利要求1所述的BIM模型轻量化构建与显示方法，其特征在于，步骤S1将所需渲染显示的建筑构件元素划分为四个LOD层级，即第一层级、第二层级、第三层级和第四层级；
    在第一层级，只设置室外的建筑构件元素以及室外的结构构件元素，对不在视野范围内的室内构件不设置，不对构件进行纹理贴图，只对构件设置相应贴图的拾取颜色；
    在第二层级，增加房屋的外部轮廓细节以及机电构件，增加室内楼板、墙和门以显示内部轮廓，不对构件进行纹理贴图，只对构件设置相应贴图的拾取颜色；
    在第三层级，将第一层级和第二层级的无纹理构件替换成带纹理的构件，并增加带纹理的建筑构件元素，包括室内家具、楼梯和机械设备；
    在第四层级，增加所有建筑构件元素、结构构件元素、轮廓构件元素以及机电构件元素，以显示最精细的BIM模型。
  3. 根据权利要求2所述的BIM模型轻量化构建与显示方法，其特征在于，第一层级中，室外的建筑构件元素包括门、屋顶、楼板、窗、栏杆、墙，室外的结构构件元素包括梁、板、柱。
  4. 根据权利要求1所述的BIM模型轻量化构建与显示方法，其特征在于，步骤S2通过meshopt管线对BIM模型进行压缩处理，包括对BIM模型添加索引、优化顶点缓冲区、顶点向量化、压缩顶点缓存和压缩点云。
  5. 根据权利要求1所述的BIM模型轻量化构建与显示方法，其特征在于，步骤S3通过定义配置文件的属性，将各个层级的二进制数据以瓦片的形式组合，生成一个分层分级的二进制数据模型。
  6. 根据权利要求5所述的BIM模型轻量化构建与显示方法，其特征在于，定义配置文件的相关属性包括：
    模型的变换矩阵，用于在场景加载时，设置模型在场景中的角度、位置和大小；
    几何误差，用于与瓦片距相机距离d、屏幕大小、像素一起计算屏幕空间误差，屏幕空间误差超过设定阈值则渲染该节点，屏幕空间误差小于设定阈值则隐藏节点；其中，瓦片距相机距离d由视点至BIM模型包围盒边缘的距离判断；
    节点切换方式，分为增量的切换方式和替换的切换方式；其中，增量的切换方式表示加载下一级时保留上一级模型构件；替换的切换方式表示加载下一级时直接替换上一级模型构件；
    节点模型的包围盒，用于记录包围盒中心点的x、y、z坐标，以及模型的长宽高尺寸；渲染时，包围盒用于计算BIM模型距视点的距离d，用于判断此时的LOD级别。
  7. 根据权利要求1所述的BIM模型轻量化构建与显示方法，其特征在于，步骤S4中，当前节点的屏幕空间误差SSE小于阈值，或当前节点的屏幕空间误差SSE超过阈值且子节点的分层关系为替换，则卸载当前节点；当前节点的屏幕空间误差SSE超过阈值且子节点的分层关系为增量，则加载当前节点。
  8. 根据权利要求7所述的BIM模型轻量化构建与显示方法，其特征在于，步骤S4包括：
    S41、递归标记节点SSE是否超过阈值
    对节点进行递归判断，从根节点向叶子节点递归，标记节点SSE是否超过阈值；若不超过阈值，则卸载当前节点，跳出当前递归；若超过阈值则对当前节点标记“超过阈值”，且将对替换关系的判断加入异步任务队列，递归至叶子节点后同步任务结束；
    S42、根据父子节点替换关系加载或卸载节点
    从叶子节点向根节点执行异步任务，根据父子节点的替换关系判断加载或卸载当前节点；首先判断当前节点的子节点是否超过阈值，如果该级别下子节点超过阈值，且该子节点的分层关系是替换关系，表示该子节点已加载且替换父节点，则卸载当前节点；如果该子节点的分层关系是增量关系，表示该子节点在父节点的基础上增量加载，则加载当前节点；如果该级别下子节点不超过阈值，即当前节点为该LOD级别下的最精细节点，则加载当前节点。
  9. 基于LOD的BIM模型轻量化构建与显示系统，其特征在于，包括以下模块：
    模型生成模块，用于根据LOD规则生成BIM模型；其中BIM建模规则为：将所需渲染显示的建筑构件元素划分为多个LOD层级，且层级设立以视点至BIM模型包围盒边缘的距离为依据；在各层级下，根据视点所在位置能看清的建筑构元素和结构构件元素，对相应的建筑构件元素及结构构件元素进行显示设置；
    模型压缩模块，用于将各层级中BIM模型的数据转成二进制数据格式，并对BIM模型进行压缩；
    分层数据生成模块，用于定义配置文件，组合BIM模型的二进制数据，生成分层二进制 数据模型；
    渲染模块，用于结合当前节点的屏幕空间误差SSE及加载方式渲染三维模型。
  10. 根据权利要求9所述的BIM模型轻量化构建与显示系统，其特征在于，模型生成模块将所需渲染显示的建筑构件元素划分为四个LOD层级，即第一层级、第二层级、第三层级和第四层级；
    在第一层级，只设置室外的建筑构件元素以及室外的结构构件元素，对不在视野范围内的室内构件不设置，不对构件进行纹理贴图，只对构件设置相应贴图的拾取颜色；
    在第二层级，增加房屋的外部轮廓细节以及机电构件，增加室内楼板、墙和门以显示内部轮廓，不对构件进行纹理贴图，只对构件设置相应贴图的拾取颜色；
    在第三层级，将第一层级和第二层级的无纹理构件替换成带纹理的构件，并增加带纹理的建筑构件元素，包括室内家具、楼梯和机械设备；
    在第四层级，增加所有建筑构件元素、结构构件元素、轮廓构件元素以及机电构件元素，以显示最精细的BIM模型；
    渲染模块对当前节点的处理方式为：当前节点的屏幕空间误差SSE小于阈值，或当前节点的屏幕空间误差SSE超过阈值且子节点的分层关系为替换，则卸载当前节点；当前节点的屏幕空间误差SSE超过阈值且子节点的分层关系为增量，则加载当前节点。
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