Research And Implementation Of BIM Model Lightweighting Basd On WebG

With the advent of "Internet+" era,computer technology has gradually penetrated into all walks of life,the concept of building information modeling and its related technologies are also gradually applied and promoted in the railroad construction industry,communication engineering,signal engineering,electric power engineering and electrification engineering are called the four railroad electrical engineering,and the high precision BIM model has become an important source of data for the four electric railroad models.Due to the advantages of convenience and speed,the Web integration of BIM models has become the focus of attention from all walks of life.Among them,3D Tiles,as the main standard format for Cesium loading on the Web side,has differences between its model format and BIM’s Rvt building model in terms of spatial features,semantic information and multi detail levels.Therefore,model conversion is required to load BIM models efficiently on the Web side.Moreover,with the increasing volume and fineness of the model,the rendering by 3D drawing protocol technology on the Web side will cause lagging and incomplete loading of the model,which will seriously affect the effect and the user’s experience.And the four electricity BIM model components are complex and the amount of data is huge.The existing LOD technology based on the model simplification algorithm to simplify the model into different levels of refinement in the application of the four electric BIM models,the display of the model material and details loaded effect is poor.Therefore,the purpose of this paper is to lighten the railroad BIM model,and to improve the efficiency of realtime rendering and optimize the loading effect under the Cesium framework.For the current problems of large data volume and poor realtime rendering of building information model,this paper proposes to convert the railroad four electric BIM model into3 DTiles data format that can be displayed on the Web platform,and studies the LOD display of the model on the Web terminal.The main research work is as follows:(1)Research on model lightweighting method.Firstly,the model format problem is solved.Since the Rvt format model and IFC format model cannot be read and displayed directly on the Web side,OBJ is chosen as the intermediate format of the research process in this paper.Based on Revit secondary development,the conversion of Rvt format to OBJ format is completed through IExport Context interface in Revit API.Then the QEM based edge folding algorithm is improved so that it can better simplify the railroad four electric BIM model in OBJ format.In the case of higher degree of simplification,the complete model details can be ensured while simplifying the materials,so as to achieve the mutual correspondence between material information and model components.Finally,the duplicated geometric data are removed in the simplification process,and the size of the model file is reduced as needed.(2)To improve the efficiency of model rendering,this paper carries out IFC model data extraction and LOD display on the basis of research content(1).First,the IFC model data extraction method is studied,combined with the BIMServer platform to filter the component set by IFC component class and perform data extraction and JSON attribute information extraction.Then,study the LOD algorithm suitable for railroad four electric BIM model,this paper adopts two methods for LOD display of four electric model: one is based on IFC model,screening components to divide the layers,controlling the phase distance by mouse wheel and loading different layers;the other is based on OBJ model,dividing layers according to the overall simplification degree,loading models with different simplification degree by different phase distance,calculating screen Spatial error is calculated to determine the tile rendering level.In order to convert the model data into 3D Tiles format that can be loaded quickly on the Web side,an octree spatial index structure is designed to improve the rendering speed.Finally,the spatial organization is designed according to the set tile division rules;at the same time,the view cone rejection technique is introduced to determine the complexity of loading 3D Tiles by the distance between the model and the camera viewpoint.(3)Implement WebGL based model dynamic interaction.Research the Web GIS visualization system based on Cesium to realize functions including component highlighting,model attribute display,model flight roaming,spatial measurement,etc.Publish the railroad four electric model to Cesium platform for LOD display and realize the fast rendering of the model.