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ncr-399 edited this page Mar 11, 2024 · 2 revisions

Xbim Geometry Exploration

Case Study IXbimGeometryEngine Interface

How the IXbimGeometryEngine Interface is defined

  1. The IXbimGeometryEngine Interface is defined in Xbim.Essentials.Ifc4 Module
  2. IXbimGeometryEngine Interface is Implemented Xbim.Geometry.Engine.Interop > XbimGeometryEngine.cs Project as a Wrapper for the Native Code
  3. XbimGeometryEngine class defined in XbimGeometryEngine.cs internally calls the Functions defined in Xbim.Geometry.Engine (Native C++ Project) which uses OCCT Code
  4. Both XbimGeometryEngine classes defined in C# and C++ derive from the IXbimGeometryEngine interface defined in Xbim.Essentials.Ifc4 Module.

How XbimGeometryEngine calls the Native C++ Code

  1. The XbimGeometryEngine's Constructor loads the Xbim.GeometryEngine32.dll or Xbim.GeometryEngine64.dll depending upon the platform architecture (x86 or x64)
  2. It grabs the function XbimGeometryEngine class defined in native code and then creates an instance of it
  3. To call the methods defined in the XbimGeometry's instance created loaded from the dll it has to be cast as IXbimGeometryEngine
  4. This is why both XbimGeometryEngine classes defined in C# and C++ are derived from a Common Interface. If they don't implement the same interface the instance created at runtime cannot access the methods.
  5. All the calls made to XbimGeometryEngine C# code then call the native methods defined in C++ through the instance loaded through the dll.

How We Have Added a Custom Functionality to Extract Edges

  1. Add an Interface called IXbimEdgeGeometry in Xbim.Essentials.Ifc4 Module
  2. Build the Xbim.Essentials Project with the new IXbimEdgeGeometry interface and create custom packages
  3. Import the Custom Packages in Xbim.Geometry Project
  4. Add a class called XbimEdgeGeometry in Xbim.Geometry.Engine.Interop Project which implements Xbim.Essentials.Ifc4.IXbimEdgeGeometry interface
  5. The XbimEdgeGeometry class has a member variable of type IXbimEdgeGeometry which will be used to access native code loaded from the dll at runtime
  6. Add a class called XbimEdgeGeometry in Xbim.Geometry.Engine which extends from Xbim.Essentials.Ifc4.IXbimEdgeGeometry and implement all the methods defined in the interface. Use OCCT to extract edges.
  7. The Xbim.Geometry.Engine.Interop.XbimEdgeGeometry class then loads the Xbim.Geometry.Engine.XbimEdgeGeometry class from dll at runtime to access its functionality.
  8. The Xbim.Geometry.Engine.Interop.XbimEdgeGeometry is just a wrapper class that forwards the calls to native code using OCCT.
  9. Build the XbimGeometry and XbimEssentials Solutions and generate new packages
  10. Install the newly generated packages into another project to use the new functionality.

How Xbim.Geometry.Engine computes Geometry and Tessellation

Overview

  1. Consider a Simple Wall having IfcShapeRepresentationType as IfcExtrudedAreaSolid
  2. All processes related to Geometry Generation and Triangulation are carried out in Xbim.ModelGeometry.Scene.Xbim3DModelContext class's CreateContext method
  3. Xbim.Geometry.Engine Module and Xbim.Tessellator Modules are used for generating Geometry and Triangulation
  4. The Tesselation is either generated by Xbim.Tessellator or Xbim.Geometry.Engine Module depending upon the type of the module (See Table Below)
  5. For ShapeRepresentation like IfcExtrudedArea where the shape is not defined the Xbim.Geometry.Engine first generates a Geometry Model and based on the model, the tessellation is computed.
  6. Xbim.Geometry.Engine uses OCCT to generate Geometry Model and Triangulation.
  7. In the case of simple shape representation types like IfcFacetedBrep the tesselation is computed by Xbim.Tesselator module.
Shape Representation Module Used
IfcFacetedBrep Xbim.Tesselator
IIfcFaceBasedSurfaceModel Xbim.Tesselator
IIfcShellBasedSurfaceModel Xbim.Tesselator
IIfcConnectedFaceSet Xbim.Tesselator
IIfcTessellatedFaceSet Xbim.Tesselator
IfcExtrudedArea Xbim.Geometry.Engine
All Other Xbim.Geometry.Engine

Geometry and Triangulation Computation Case Study for a Simple Wall (IfcExtrudedAreaSolid)

  1. Load the Model using Xbim SDK, and create an instance of Xbim.ModelGeometry.Scene class using the model and call the CreateContext() method.
  2. Inside CreateContext() method, first the Shape is queried using ShapeId and stored as IIfcGeometricRepresentationItem
  3. Using this Shape of type IIfcGeometricRepresentationItem we check if its triangulation can be computed using Xbim.Tesselator method using Xbim.Tesselator.CanMesh() method.
  4. In this case, since the type of IIfcGeometricReprsentationItem is of type IfcExtrudedArea it cannot be computed using Xbim.Tesselator module.
  5. Next, we use Xbim.Geometry.Engine module to generate a Geometry Model for the ShapeRepresentation IfcExtrudedArea.
  6. For ShapeRepresentation of type IfcExtrudedArea, to create a geometry model we need a Base Shape and a Direction.
  7. Xbim.Geometry.Engine creates an XbimSolid the ShapeRepresentation using OCCT. Using the SweptArea property of the ShapeRepresentation creates an object of type XbimFace which can be used as a Base Shape along with the ExtrudeDirection property of ShapeRepresentation to create a geometry model. Thsese two parameters are then passed to OCCT: BRepPrimAPI_MakePrism api which returns a Shape using the inputs.
  8. XbimSolid is derived from TopDS_Shape so it can be used with Meshing Algorithms
  9. At this point, we have a geometry model which we can use to generate Triangulation. We use the XbimSolid created in the previous step and pass it Xbim.Geometry.Engine.CreateShapeGeometry method to compute tesselation and return a buffer containing the tesselated data.
  10. Internally, Xbim.Geometry.Engine.CreateShapeGeometry calls Xbim.Geometry.Engine.WriteTriangulation method to compute the tesselation using OCCT and writes it back into a buffer. This buffer is then stored in Xbim.Model.GeometryStor

Adding New Classes to Xbim.Geometry.Engine Project

  1. Right-click on the Xbim.Geometry.Engine Project in Visual Studio
  2. Select Add from the Dropdown Menu
  3. Create a .cpp and .h file
  4. Right Click on the .cpp file and select properties
  5. Go to C/C++ Section
  6. Make Sure the Common Language Runtime Support Option is set to Common Language Runtime Support (/clr) otherwise the build will throw fatal errors if any C# Interop Code is Used in the File.

Building XbimEssentials Project

  1. Clone the XbimEssentials repo from XbimEssentials
  2. Switch to Branch issues/issue465
  3. Open the Solution using Visual Studio
  4. Build the Solution (It will build 9 Projects)

Packaging the XbimEssentials Project

  1. After Successfully building the projects, right-click on the Project and Select the Pack option from the dropdown menu
  2. Selecting the Pack option will generate a .nuget package with version 5.1.341 e.g (Xbim.Common.5.1.341.nupkg)
  3. Do this for all the Projects (Ignore the Test Projects)
  4. Make Sure to Build the Packages in Release Mode
  5. Go to Project Root Folder (C:\workspace\Xbim\XbimEssentials)
  6. Search for .nupkg in the Search Bar. It will return all the packages
  7. Copy all the Packages and place them in a new Folder called packages

Building XbimGeometry Project

  1. Clone the XbimGeometry repo from XbimGeometry
  2. Switch to Branch issues/issue465
  3. Open the Solution using Visual Studio
  4. It will restore the required packages from nuget.org instead of our custom packages
  5. Go Project > Manage Nuget Packages
  6. Click on the Settings Icon on the Right Most Corner next to Package Sources. It will open up the menu to configure package sources
  7. Click on the + Sign to Add a new Package Source and Set the Path to the new Package Folder created while building XbimEssential packages (See Packaging XbimEssentials Project Step 7)
  8. Uncheck the nuget.org as Package Source in the Config Menu.
  9. Go to C:\Users\{UserName}\.nuget\packages and remove all the xbim packages
  10. Restart Visual Studio
  11. New Custom Packages are restored from the local source created in Building XbimGeometry Project > Step 7
  12. Build the Xbim.Geometry.Engine.Interop Project First and then build the rest of the projects
  13. Directly Building the Solution May Fail and Throw a Lot of Errors

Packaging XbimGeometry Project

  1. Once all the Projects are Built, Right Click on the C# Projects and Click on Pack from the dropdown menu to generate the nuget packages (Ignore the Regression and Tests Projects). It will generate a nupkg file with version 5.1.437
  2. Place the generated packages into the local package folder.

Testing, Debugging, and Profiling Geometry Engine Projects

  1. To Test, Debug, or Profile the Xbim.Geometry.Engine and Xbim.Geometry.Engine.Interop projects use the Xbim.Regression Project
  2. Modify the **Xbim.Regression > Program. cs and add custom logic to test and debug the changes

Using the Generated Packages in Another Project

  1. Follow the Steps mentioned in Building Xbim Geometry Project to add a Custom Package Source for nuget packages.
  2. Install the Custom Packages from the Local folder to use the custom functionality in another project

Risks

  1. Difficult to Maintain the Custom Packages
  2. Need to Push the Packages on some remote sources to reduce a few steps while building the projects.
  3. The Current Solution is based on XbimGeometry 5.1.437 and XbimEssentials 5.1.341 which no longer contain the latest changes. These Projects have been migrated to netcore version 6.0

Additional Resources and References

  1. Xbim Ticket
  2. Quick-C-CLI-Learn-C-CLI-in-less-than-10-minutes
  3. How Geometry is Created with OCC
  4. Xbim Toolkit Overview
  5. Open Cascade CAD Assistant Viewer
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