CGAL users discussion list

[Andreas Fabri <>]

Hi Tyver,

You cannot permute them in the triangulation itself. I mean maybe
you can, but this doesn't really make sense.

What you know is that vertex(i) is followed in counterclockwise order
by vertex((i+1) % 3

best regards,

andreas





Tyver Ray wrote:
> Thanks Andreas, but do CGAL provides a function to permute the vertices 
> of a triangle without changing the Delaunay construction?
>
>
> 2007/6/29, Patrick Hétroy < 
>
>  
> <mailto:>>:
>
>     Ok so Tyver should permute manually the vertices of the triangles ?
>
>
>     2007/6/28, Andreas Fabri < 
>
>     
> <mailto:>>:
>
>
>         The face doesn't know that you walk from top to bottom.
>         Imagine if at the bottom left was a triangle like this
>
>
>              *
>              *
>             u**
>              *  *
>              *    *
>              *  f   *
>              **************
>             v        w
>
>         and you first walk down, and then from left to right along
>         the lower edge. You can't expect that f->vertex(0) is once u
>         and the next time v, only because you do another walk.
>
>
>         The vertices can be in a cyclic permutation, they are always
>         in counterclockwise order, so that the interior is to the left.
>
>
>         andreas
>
>
>
>         Patrick Hétroy wrote:
>         >  I don't have any idea to resolve your problem but I think that :
>         >
>         >  The Delaunay triangulation doesn't care about the order of
>         point insertion.
>         >  But, the function line_walk depends on the order of the
>         insertion...
>         >
>         >  If you can change your points order :p ^^
>         >
>         >  Patrick
>         >
>         >
>         >  2007/6/28, Tyver Ray < 
>
>         
> <mailto:>
>  <mailto: 
>
>         
> <mailto:>>>:
>         >
>         >
>         >
>         >     ---------- Forwarded message ----------
>         >     From: *Tyver Ray* < 
>
>         
> <mailto:>
>  <mailto: 
>
>         
> <mailto:>>>
>         >     Date: 28 juin 2007 10:22
>         >     Subject: Problem with Line_walk
>         >     To: 
>
>         
> <mailto:>
>         >     <mailto: 
>
>         
> <mailto:>>
>         >
>         >     Hello!
>         >
>         >     I'm trying to get all the vertices on the left side of my
>         square
>         >     surface with line_walk but it seems to be a problem...
>         >
>         >     I use Delaunay transformation with these points :
>         >         int step = 25;
>         >         dt.insert(Point3(0*step, 0, 3));
>         >         dt.insert(Point3(1*step, 0, 6));
>         >         dt.insert (Point3(2*step, 0, 13));
>         >         dt.insert(Point3(3*step, 0, 9));
>         >         dt.insert(Point3(4*step, 0, 6));
>         >         dt.insert(Point3(0*step, 1*step, 3));
>         >         dt.insert(Point3(1*step, 1*step, 7));
>         >         dt.insert(Point3(2*step, 1*step, 8));
>         >         dt.insert(Point3(4*step, 2*step, 8));
>         >         dt.insert(Point3(0*step, 3*step, 13));
>         >         dt.insert(Point3(1*step, 3*step, 17));
>         >         dt.insert(Point3(2*step, 3*step, 13));
>         >         dt.insert(Point3(3*step, 3*step, 18));
>         >         dt.insert(Point3(4*step, 3*step, 18));
>         >         dt.insert(Point3(0*step, 4*step, 13));
>         >         dt.insert(Point3(1*step, 4*step, 7));
>         >         dt.insert(Point3(2*step, 4*step, 3));
>         >         dt.insert(Point3(3*step, 4*step, 1));
>         >         dt.insert(Point3(4*step, 4*step, 1));
>         >         dt.insert (Point3(12.5, 12.25, 7));
>         >         dt.insert(Point3(62.5, 12.25, 7));
>         >         dt.insert(Point3(90, 25, 1));
>         >         dt.insert(Point3(87.5, 87.5, 1));
>         >
>         >     After I use the line_walk function to get the vertices
>         which are in
>         >     the up to down direction of my squre surface with:
>         >     (the bottom left of the squre is the point (0, 0) and the
>         up right
>         >     is (100, 100))
>         >
>         >         const Point3& start = Point3(0, 100, 0);
>         >         const Point3& end = Point3(0, 0, 0);
>         >
>         >         Line_face_circulator lfc = dt.line_walk(start, end),
>         done(lfc);
>         >         if (lfc != (CGAL_NULL_TYPE) NULL)
>         >         {
>         >             do
>         >             {
>         >                 //Show the coordinates of all triangles on he
>         left sife
>         >     of my square
>         >                 if( !dt.is_infinite(lfc) )
>         >                 {
>         >                     showCoordinates(lfc, 0);
>         >                     showCoordinates(lfc, 1);
>         >                     showCoordinates(lfc, 2);
>         >
>         >                 }
>         >             }
>         >             while (++lfc != done);
>         >         }
>         >
>         >     The result are :
>         >
>         >     //Triangle 1
>         >     Vertex Point : 0
>         >     x: 25
>         >     y: 75
>         >     z: 17
>         >     Vertex Point : 1
>         >     x: 0
>         >     y: 100
>         >     z: 13
>         >     Vertex Point : 2
>         >     x: 0
>         >     y: 75
>         >     z: 13
>         >
>         >     //Triangle 2
>         >     Vertex Point : 0
>         >     x: 25
>         >     y: 25
>         >     z: 7
>         >     Vertex Point : 1
>         >     x: 0
>         >     y: 75
>         >     z: 13
>         >     Vertex Point : 2
>         >     x: 0
>         >     y: 25
>         >     z: 3
>         >
>         >     //Triangle 3
>         >     Vertex Point : 0
>         >     x: 0
>         >     y: 0
>         >     z: 3
>         >     Vertex Point : 1
>         >     x: 12.5
>         >     y: 12.25
>         >     z: 7
>         >     Vertex Point : 2
>         >     x: 0
>         >     y: 25
>         >     z: 3
>         >
>         >
>         >     The triangle 3 has not the same count of the vertices. It
>         doesn't
>         >     display first the vertex opposed to the edge which is on the
>         >     direction up to down as the Triangle 1 and 2.
>         >     I would like the same order of display for each point.
>         >     How can I fix the problem?
>         >
>         >
>
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