Extracting boundary from TopoDS_Shape

Hello Sunayana,

if you have a TopoDS_Shape, you can extract the vertices by exploring the vertices of the shape. This function for example takes the shape, extracts the vertices into gp_Pnt - s and returns them in an array.

 22 Handle_TColgp_HArray1OfPnt StlPointsExtractor::extractVerticesFromTopoDSShape(const TopoDS_Shape& shape)
 23 {
 24         TopTools_IndexedMapOfShape vertices;                                                                                                                                                              
 25         TopExp::MapShapes(shape,TopAbs_VERTEX,vertices);
 26
 27         Handle_TColgp_HArray1OfPnt result = new TColgp_HArray1OfPnt(1,vertices.Extent());
 28
 29         for(Standard_Integer i = 1;i<=vertices.Extent();i++)
 30         {
 31                 TopoDS_Vertex vertex = TopoDS::Vertex(vertices.FindKey(i));
 32                 gp_Pnt currentGeometricPoint = BRep_Tool::Pnt(vertex);
 33                 result->SetValue(i,currentGeometricPoint);
 34         }
 35
 36         return result;
 37
 38 }

I hope this helps.

László

If you are interested, I have a small example here:

https://github.com/lvk88/OccTutorial/blob/master/OtherExamples/runners/extractStlPoints.cpp

This program takes an stl file, extracts the points and writes them out in a text file.

Let me know if it works out!

Hi Sunayana,

Is it a non-closed STL surface? I mean, in STL all the vertices kind of lie on thw boundary of the shape, don't they?

Can you upload a small figure of what you would like to achieve? Maybe I could help you further with that.

László

Hello Sunayana,

Alright, I understand what you would like to do. Here is how I would do it:

The STL file consists of faces, each bounded by edges. You can check for each edge if it has one or two parent faces. If the edge is connected to two faces, it is an internal edge (assuming that the STL file is a manifold mesh). If the edge is connected to only one face (one triangle), then it lies on the boundary of the mesh.

To do this, you could use TopExp::MapShapesAndAncestors, which would create you a map, where the key of each element are the edges in your mesh, and the value is a list of faces, the parents of a given edge.

I created you an example that you can check at the following link:

https://github.com/lvk88/OccTutorial/blob/master/OtherExamples/runners/extractStlManifoldBoundaries.cpp

This creates a curved plate with a hole in the middle and generates an STL file out of it. Then reads it in again and finds the boundary edges of the mesh. It is pretty slow, but I guess I was not using the most optimal containers, and not in the most optimal way.

Have a look on the attached pictures also!

Best regards

László

Hey,

Could you tell me how did you read an STL file into a TopoDs_Shape??

Thanks