A Shape and a Plane intersection polygon

[Vahid44]

Hello everyone, I got the intersection between a shape and a plane using "BRepAlgoAPI.BRepAlgoAPI_Section" function. Is it possible to extract a polygon from the intersection? (Or the points of the intersection)

This is my code to extract the intersection. and the image is the result.

def IfcObj_to_TopoDsObj (IfcObj):
    settings = ifcopenshell.geom.settings()
    settings.set(settings.USE_PYTHON_OPENCASCADE, True)
    product = ifcopenshell.geom.create_shape(settings, IfcObj)
    aShape = OCC.Core.TopoDS.TopoDS_Shape(product.geometry)
    return aShape

product = ifc_file.by_guid('1YaheuayH6kBI6D9$zc5YL')

shape = IfcObj_to_TopoDsObj(product)

plan = gp_Pln(gp_Pnt(0,0,4), gp_Dir(0,0,1))

section_face = OCC.Core.BRepBuilderAPI.BRepBuilderAPI_MakeFace(plan, -10, 10, -10, 10).Face()

section = OCC.Core.BRepAlgoAPI.BRepAlgoAPI_Section(section_face, shape).Shape()

Sorry if it is a trivial question, but I did not find anything to solve my problem. Best regards.

[OkanPinar]

TopologyExplorer can be used to obtain vertices and edges.

[Vahid44]

Thank you very much, it totally solved my problem. I first extract the edges. And then from the edges construct a polygon. Is there any comprehensive documentation for pythonocc? Since I searched for a while but could not find anything.

[OkanPinar]

Spending some time on @tpaviot's utils (https://github.com/tpaviot/pythonocc-utils) repository and opencascade forum(https://old.opencascade.com/forums) can be good for obtain main ideas.

[OkanPinar]

The following site is great to study some code related with opencascade.

http://www.creativecadtechnology.com/OCC

[Vahid44]

Thank you very much, dear OkanPinar.

[AnganMitra]

@Vahid44 can you share your solution snippet to first extract the edges and then the polygon.? I was trying with the following snippet but with no luck

` section = OCC.Core.BRepAlgoAPI.BRepAlgoAPI_Section(section_face, shape).Shape()

section_edges = list(OCCUtils.Topo(section).edges()) ec += len(section_edges) print ("ec----------",ec) if len(section_edges) > 0:

          edges = OCC.Core.TopTools.TopTools_HSequenceOfShape()
          for edge in section_edges: edges.Append(edge)
          edges_handle = OCC.Core.TopTools.TopTools_HSequenceOfShape(edges)
          wires = OCC.Core.TopTools.TopTools_HSequenceOfShape()
          wires_handle = OCC.Core.TopTools.TopTools_HSequenceOfShape(wires)
          OCC.Core.ShapeAnalysis.ShapeAnalysis_FreeBounds_ConnectEdgesToWires(edges_handle, 1e-5, True, wires_handle)
          # wires = wires_handle.GetObject()
          wires = wires_handle
          for i in range(wires.Length()):
              wire_shape = wires.Value(i+1)
              wire = topods.Wire(wire_shape)` 

[Vahid44]

Hello @AnganMitra, and sorry for the late reply. The following code extracts the edges of the intersection of a plane and a shape. It extracts the edges and the points of the edges, but the polygon extraction part behaves correctly just for simple shapes.

def Space_Section (IfcSpace, Elevation):      # Intersects a plane and a space in a certain elevation and returns the result as a List of ordered points
    if IfcSpace.is_a('IfcSpace'):
        aShape = IfcObj_to_TopoDsObj(IfcSpace)
        
        ## middel of the Space coordination (zMidP), type: number
        xMidP = get_aligned_boundingbox(aShape)[0].Coord()[0]
        yMidP = get_aligned_boundingbox(aShape)[0].Coord()[1]
        
        # defining a (Horizontal) plane to intersect with the space shape. (z = "Elevation")
        plane = gp_Pln(gp_Pnt(xMidP,yMidP,Elevation), gp_Dir(0,0,1))

        # intersect the plane and the space:
        section_face = OCC.Core.BRepBuilderAPI.BRepBuilderAPI_MakeFace(plane, -150, 150, -150, 150).Face()
        section = OCC.Core.BRepAlgoAPI.BRepAlgoAPI_Section(section_face, aShape).Shape()

        # get the edges and vertices of the intersection:
        Unordered_Edge_List = []
        Ordered_Vertex_List = []

        edges = list(TopologyExplorer(section).edges())
        for i in range(len(edges)):
            firstVert = OCC.Core.TopExp.topexp.FirstVertex(edges[i])
            p1_edge = BRep_Tool.Pnt(firstVert).Coord()
            p1_edge = tuple(map(lambda x: isinstance(x, float) and round(x, 3) or x, p1_edge))
            lastVert = OCC.Core.TopExp.topexp.LastVertex(edges[i])
            p2_edge = BRep_Tool.Pnt(lastVert).Coord()
            p2_edge = tuple(map(lambda x: isinstance(x, float) and round(x, 3) or x, p2_edge))
            TheEdge = [p1_edge, p2_edge]
            Unordered_Edge_List.append(TheEdge)

        if Unordered_Edge_List != []:
            Ordered_Vertex_List.append(Unordered_Edge_List[0][1])

        # order the points (to extract a polygon from ordered points)
        while Unordered_Edge_List:
            for x in Unordered_Edge_List:
                if Ordered_Vertex_List[-1] == x[0]:
                    Ordered_Vertex_List.append(x[1])
                    Unordered_Edge_List.remove(x)

         #Space_polygon = Polygon(Ordered_Vertex_List)
        return Ordered_Vertex_List

    else:
        return ('It is not an IfcSpace!')

Feel free if you had any questions or suggestions ;)