CGAL users discussion list

[Anthony Truchet <>]

Hello,

Thanks for whose taking some moment to read this information request and 
please does not hesitate to tell me if I break unwillingly some courtesy 
of this mailing list.
I'm working with terrain data : basically level curves (polylines whose 
points all have the same elevation).
I thus use constrained Delaunay triangulation in the spirit of exemple 
[37.5.2] to be able to evaluate the estimated elevation of any point. I 
would like to draw the 2D projection in Qt thanks to CGAL_Qt4.

The code below works fine if I use pure 2D triangulations ::

typedef CGAL::Exact_predicates_inexact_constructions_kernel       K;
//typedef CGAL::Projection_traits_xy_3<K>                         Gt;
typedef K  Gt; // XXX Dummy typedef to stay in pure 2D
typedef CGAL::Triangulation_vertex_base_2<Gt>                      Vb;
typedef CGAL::Triangulation_face_base_with_info_2<FaceInfo, Gt>    Fbb;
typedef CGAL::Constrained_triangulation_face_base_2<Gt, Fbb>       Fb;
typedef CGAL::Triangulation_data_structure_2<Vb, Fb>               TDS;
typedef CGAL::Exact_predicates_tag                                Itag;
typedef CGAL::Constrained_Delaunay_triangulation_2<Gt, TDS, Itag> CDT;
typedef CDT::Point                                                
CGAL_Point;
typedef CGAL::Polygon_2<Gt>                                       
CGAL_Polygon;

   [...]
    CGAL::Qt::TriangulationGraphicsItem<CDT>* p_cdt_gitem =
        new CGAL::Qt::TriangulationGraphicsItem<CDT>(&cdt);
   [...]
    CGAL::Qt::PolygonGraphicsItem<CGAL_Polygon>* p_gi =
            new CGAL::Qt::PolygonGraphicsItem<CGAL_Polygon>(p_p);

But when I define the Geometric Traits Gt to be 
CGAL::Projection_traits_xy_3<K>, then the compilatioon fails deep inside 
CGAL::Qt functions in way I don't understand.

I've tried blindly to use "pure 2D" variants of CDT, Point, Polygon 
together with the projected one but it of course failed because I was 
just trying blindly :-(

So here are the questions :
* is there an known way to get CGAL::Qt::TriangulationGraphicsItem and 
the like worh with CGAL::Projection_traits_xy_3<K>
* if not, is there a way to build a pure 2D triangulation from a 
"projected 2D" triangulation, so that I could visualize the former to 
debug the latter (I don't mind for perf consideration as this will be a 
debug only visualization)
* what are the right entry points to understand what's going on and 
contribute a working exemple back ?

Thanks for your attention and have a nice day / evening / night :-)
--
Anthony Truchet
Logilab S.A.

PS : I'm using latest CGAL (4.1) compiled from sources on Debian Linux 
stable with GCC 4.4.5

[37.5.2] 
http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Triangulation_2/Chapter_main.html#Subsection_37.5.2