CGAL users discussion list

The most important information here might be the fact that "the set of 
maintained faces is topologically equivalent to a two-dimensional 
triangulated sphere." See the user manual (Section 1 - definition, in 
particular 26.1.2 The Set of Faces and Vertices, that also mentions the 
lower dimensional cases).

best
        Monique Teillaud

Louis wrote:
> Thanks for the information Pedro. Initially I didn't set the
> neighborhood information because I don't know what are done
> automatically in by the data structure itself or what the user has to
> specify. When I insert a vertex or face, calling vertex.is_valid() or
> face.is_valid() all returned true. But the triangulation data
> structure remained invalid. Now after I set the neighborhood
> information, the triangulation still says it's invalid. I'm also not
> sure about boundary faces where 1 or 2 neighbor faces are not defined,
> do I need to do anything special in this case?
>
> What's frustrating is the lack of information in the documentation on
> what are the necessary steps to build a valid triangulation using this
> data structure. There are many weird behaviors in this data structure
> that need answers. Why does the class initialize with 0 vertices, 1
> face, 1 edge? Calling tds.clear() made sure they became 0, but when
> you add the first vertex, somehow it's not counted
> (number_of_vertices() returns 0).
>
> I wonder if anyone has a concrete example or maybe skeleton code that
> shows how to build a triangulation data structure from vertices and
> faces.
>
> Thanks,
> Louis
>
>
> On Tue, Mar 25, 2008 at 7:27 AM, Pedro Machado Manhães de Castro
> <>
>  wrote:
>
> > I meant stuffs like:
> >
> > fHandle->set_neighbor(0, some_face_handle_1);
> > fHandle->set_neighbor(1, some_face_handle_2);
> > fHandle->set_neighbor(2, some_face_handle_3);
> > sry (to not confuse even more),
> > Pedro
> >
> >
> >
> > On Tue, Mar 25, 2008 at 3:25 PM, Pedro Machado Manhães de Castro
> > <>
> >  wrote:
> >
> > > Hello,
> > >
> > > Do you define the neighborhood of your face inside your 2nd for iteration?
> > >
> > > Stuffs like:
> > > fHandle->set_neighbor(0, some_face_handle_1);
> > > fHandle->set_neighbor(0, some_face_handle_2);
> > > fHandle->set_neighbor(0, some_face_handle_3);
> > > inside
> > >
> > > ?
> > >
> > > This constructor here is easier to use:
> > > create_face(Face_handle f1, int i1,
> > >                   Face_handle f2, int i2,
> > >                   Face_handle f3, int i3)
> > >
> > > You define a face by its 3 neighbors, i1, i2, i3 are any sequence defined
> >
> > by the indices 0, 1, 2.
> >
> > > Hope it could help,
> > > Best Regard,
> > > Pedro Machado
> > >
> > >
> > >
> > >
> > >
> > > On Tue, Mar 25, 2008 at 11:43 AM, Louis Feng 
> > > <>
> >
> > wrote:
> >
> > > > Hello,
> > > >
> > > > I'm trying to construct a 2D triangle mesh from a list of vertices and
> > > > faces. I didn't want to use Polyhedron because I needed some properties
> >
> > from
> >
> > > > the 2D triangulation class. I'm using Triangulation_2 and
> > > > Triangulation_data_structure_2. Basically I use T.tds() to get a
> >
> > reference
> >
> > > > to the internal data structure. Then I add vertices and faces to it. But
> > > > this has become very confusing and I couldn't find examples on this.
> > > >
> > > > An outline of my code:
> > > >
> > > > triDS.set_dimension(2);
> > > > for(...) {
> > > > VertexHandle vHandle = triDS.create_vertex();
> > > > vHandle->set_point(Point2(...));
> > > > }
> > > > for(...) {
> > > > FaceHandle fHandle = triDS.create_face(vHandle1, vHandle2, vHandle3); }
> > > >
> > > > When I check triDS.is_valid() at the end, it returns false. More
> > > > interestingly, before I add anything to triDS, when I check the
> >
> > vertices,
> >
> > > > edge, etc. I get this
> > > >
> > > > Triangulation vertices: 0
> > > > Triangulation faces: 1
> > > > Triangulation full faces: 1
> > > > Triangulation edges: 1
> > > >
> > > > So am I suppose to clear this before I add something to it? Looking at
> >
> > the
> >
> > > > template file for the data structure, it check this:
> > > >
> > > > case 2:
> > > >   result = result &&  edge_count == 3*face_count/2 ;
> > > >   CGAL_triangulation_assertion(edge_count == 3*face_count/2);
> > > >   break;
> > > >
> > > > I'm not sure what the expected behavior is. Could someone tell me what
> >
> > I'm
> >
> > > > doing wrong and how this class suppose to be used? Thanks!
> > > >
> > > > Louis
> > > >
> > > > --
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