CGAL users discussion list

[Chris Theis <>]

Hello,

I've been trying to do some boolean ops on polyhedra and encountered a
number of spurious asserts which I could trace back to problems in my
polyhedron definition. However, I can't really see the problem and maybe
there is some fundamental misunderstanding on my side.

I'm trying to create for example a simple triangulated box. For this purpose
I have written a builder function:

---------------------------------------------------------------------------------------------------------
  void operator()( HDS& HalfEdgeDS ) {
    CGAL::Polyhedron_incremental_builder_3<HDS> BuilderObj( HalfEdgeDS,
true);

    assert( !(m_Vertices.size() % 3) );      // m_Vertices = vector keeping
all vertices of the polyhedron

    for( size_t i = 0; i < m_Vertices.size(); i += 3 ) {
      BuilderObj.begin_surface(  3, 1 );
      
        BuilderObj.add_vertex( Point( m_Vertices[i].X, m_Vertices[i].Y,
m_Vertices[i].Z ) );
        BuilderObj.add_vertex( Point( m_Vertices[i+1].X, m_Vertices[i+1].Y,
m_Vertices[i+1].Z ) );
        BuilderObj.add_vertex( Point( m_Vertices[i+2].X, m_Vertices[i+2].Y,
m_Vertices[i+2].Z ) );

        BuilderObj.begin_facet();
          BuilderObj.add_vertex_to_facet( 0 );
          BuilderObj.add_vertex_to_facet( 1 );
          BuilderObj.add_vertex_to_facet( 2 );
        BuilderObj.end_facet();
      
      BuilderObj.end_surface();
    }

  }
---------------------------------------------------------------------------------------------------------

When I then create a box I can export it to an OFF file and view it without
any problem. However, if I call the is_valid() function I'm told that my
structure is not valid. It contains 72 half-edges (I would have expected 6
faces * 2 triangles = 36 half-edges) and every second HE is reported as a
border half edge. Consequently, the solid does not classify as closed and
boolean ops with nef-polyhedra fail.

As a test I tried to build the solid with the make_triangle function like
this:
---------------------------------------------------------------------------------------------------------
  Poly.make_triangle( CGPoint( 0.0, 0.0, 0.0 ), CGPoint( 0.0, 1.0, 0.0 ),
CGPoint( 1.0, 0.0, 0.0 ) );
  Poly.make_triangle( CGPoint( 1.0, 0.0, 0.0 ), CGPoint( 0.0, 1.0, 0.0 ),
CGPoint( 1.0, 1.0, 0.0 ) );

  Poly.make_triangle( CGPoint( 0.0, 0.0, 0.0 ), CGPoint( 1.0, 0.0, 0.0 ),
CGPoint( 0.0, 0.0, 1.0 ) );
  Poly.make_triangle( CGPoint( 0.0, 0.0, 1.0 ), CGPoint( 1.0, 0.0, 0.0 ),
CGPoint( 1.0, 0.0, 1.0 ) );

  Poly.make_triangle( CGPoint( 0.0, 0.0, 0.0 ), CGPoint( 0.0, 0.0, 1.0 ),
CGPoint( 0.0, 1.0, 0.0 ) );
  Poly.make_triangle( CGPoint( 0.0, 1.0, 0.0 ), CGPoint( 0.0, 0.0, 1.0 ),
CGPoint( 0.0, 1.0, 1.0 ) );

  Poly.make_triangle( CGPoint( 0.0, 0.0, 1.0 ), CGPoint( 1.0, 0.0, 1.0 ),
CGPoint( 0.0, 1.0, 1.0 ) );
  Poly.make_triangle( CGPoint( 0.0, 1.0, 1.0 ), CGPoint( 1.0, 0.0, 1.0 ),
CGPoint( 1.0, 1.0, 1.0 ) );

  Poly.make_triangle( CGPoint( 1.0, 0.0, 0.0 ), CGPoint( 1.0, 1.0, 0.0 ),
CGPoint( 1.0, 0.0, 1.0 ) );
  Poly.make_triangle( CGPoint( 1.0, 0.0, 1.0 ), CGPoint( 1.0, 1.0, 0.0 ),
CGPoint( 1.0, 1.0, 1.0 ) );

  Poly.make_triangle( CGPoint( 0.0, 1.0, 0.0 ), CGPoint( 0.0, 1.0, 1.0 ),
CGPoint( 1.0, 1.0, 0.0 ) );
  Poly.make_triangle( CGPoint( 1.0, 1.0, 0.0 ), CGPoint( 0.0, 1.0, 1.0 ),
CGPoint( 1.0, 1.0, 1.0 ) );


---------------------------------------------------------------------------------------------------------

However, the result was exactly the same.

I would appreciate if somebody could shed some light on this issue because I
do not really understand where the border half-edges come from. In my
current understanding border half edges are adjacent to holes and thus, have
no face connected to them. However, I do not have any holes in my polyhedron
or do I?


Thanks a lot
Chris





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