How I could get the actual toposhape data in OpenCascade?

Question

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have file from CAD (SW) in STEP format and was able to read it via Python OCC binding:

    importer = aocxchange.step.StepImporter(fname)
    shapes = importer.shapes

    shape = shapes[0]

    # promote up
    if (shape.ShapeType() == OCC.TopAbs.TopAbs_SOLID):
        sol =  OCC.TopoDS.topods.Solid(shape)

I could display it, poke at it, check flags etc

t = OCC.BRepCheck.BRepCheck_Analyzer(sol)
print(t.IsValid())
print(sol.Checked())
print(sol.Closed())
print(sol.Convex())
print(sol.Free())
print(sol.Infinite())

So far so good. It really looks like small tube bent along some complex path.

Question: how I could extract geometry features from what I have? I really need tube parameters and path it follows. Any good example in Python and/or C++ would be great

What do you mean by "tube parameters" and "path it follows" ? Are you interested on the underlying geometry (ex.: a bspline curve) ? — Fernando, Jul 18 '17 at 20:34

score 6 · Accepted Answer · answered Jul 19 '17 at 22:04

In OpenCASCADE there's a separation between topology and geometry. So, usually your first contact will be the topological entities (i.e.: TopoDS_Wire or a TopoDS_Edge), that can give you access to the geometry (take a look here for more details).

In your case, after reading the STEP file you ended up with a TopoDS_Shape. This is the highest level topological entity and most probably is formed by one or more sub-shapes.

Assuming that your shape is formed by a bspline curve (it seems to be!), you could explore the shape, looking for TopoDS_Edge objects (they are the topological entities that map to geometric curves):

TopExp_Explorer myEdgeExplorer(shape, TopAbs_EDGE);
while (myEdgeExplorer.More())
{
    double u0, u1;
    auto edge = TopoDS::Edge(myEdgeExplorer.Current());
    auto curve = BRep_Tool::Curve(edge, u0, u1);

    // now you have access to the curve ...
    // to get a point lying on it, check
    // the method curve->Value(u);

    myEdgeExplorer.Next();
}

How I could get the actual toposhape data in OpenCascade?

1 Answers1