Read cad geometry and assemblies with OCCT

March 29, 2024 4 minutes

Open CASCADE Technology (OCCT) is used to get data from geometry files(such as .stp or .iges files), in CAD/CAE applications. The retrieved geometry data could be converted to be VTK data, which is rendered afterwards in VTK render window.

Read geometry files with neutral formats

OCCT has specific readers for stp and iges geometry files, IGESControl_Reader and STEPControl_Reader. With those readers, stp geometry files will be converted to TopoDS_Shape, an OCCT defined object. In the converting progress, you can choose output one shape or a group of shapes when the geometry file is an assembly. Here let’s look at the one shape transport.

STEPControl_Reader reader;
TopoDS_Shape shape;
IFSelect_ReturnStatus status = reader.ReadFile(file);
if (status != IFSelect_RetDone){
    ///Failed to read file
}else{
    IFSelect_PrintCount count = IFSelect_ListByItem;
    reader.PrintCheckLoad(Standard_False,count);
    reader.TransferRoots();
    //Import the geometry as a whole part
    shape = reader.OneShape();
}

When we get the shape object, then we could convert it to vtk data.

IVtkTools_ShapeDataSource* source = IVtkTools_ShapeDataSource::New();
source->SetShape(new IVtkOCC_Shape(shape));

Analyze assembly of the geometry files

If the geometry file is an assembly, looks like this

---Assembly
-----Component 1
----------Part 1
----------Part 2
-----Component 2
----------Part 3
----------Part 4

We must convert the geometry data to TDocStd_Document first, then retrieve shapes from it with XCAFDoc_ShapeTool. At the same time, XCAFDoc_ShapeTool also supports identification of the type of the shape, assembly or component or just a simple part without children, e.g. IsAssembly, IsComponent, IsSimpleShape.

STEPControl_Reader reader;
TopoDS_Shape shape;
IFSelect_ReturnStatus status = reader.ReadFile(file);
if (status != IFSelect_RetDone){
    ///Failed to read file
}
//Initialize doc
Handle(TDocStd_Document) doc;
Handle(XCAFApp_Application) anApp = XCAFApp_Application::GetApplication();
anApp->NewDocument("MDTV-XCAF", doc);
if(!reader.Transfer(doc)){
    ///Failed to initialize doc
}
//Initialize assembly
Handle(XCAFDoc_ShapeTool) assembly = XCAFDoc_DocumentTool::ShapeTool(doc->Main());
TDF_LabelSequence labels;
assembly->GetShapes(labels);
TDF_Label label;
TDataStd_Name::GetID();

if(labels.Length() <= 1){
    /// The geometry itself is a simple shape
}

for (int i = 1; i <= labels.Length(); i++) {
    label = labels.Value(i);
    Handle(TDataStd_Name) nameAttr;
    if (label.FindAttribute(TDataStd_Name::GetID(), nameAttr)) {
        // Convert the retrieved name to QString for use in Qt
        TCollection_ExtendedString name = nameAttr->Get();

        if(i == 1) {
            ///It's the top level of the assembly
        }

        TopoDS_Shape shape = assembly->GetShape(label);

        if(assembly->IsSimpleShape(label)){
            //save the shape as a part
        }
        else if(assembly->IsComponent(label)){
            //save the shape as a component
        }
    }
}

There is another point we need to care about: OCCT usually merges the same geometrical parts into one part for memory consideration, e.g., a line of circles. If we want to separate those parts, we need to do other operations on the geometry. The code below separates the shape into TopAbs_SOLID parts.

if(assembly->IsSimpleShape(label)){
    TopExp_Explorer topExp;
    for (topExp.Init(shape, TopAbs_SOLID); topExp.More(); topExp.Next())
    {
        TopoDS_Shape individual_solid = topExp.Current();
        TopoDS_Shape* TopoDS_Shape_ = new TopoDS_Shape(individual_solid);
        //save each part
    }
}

Reference

Opencascade website

CAD