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["avanindra" <>]

Hi Sebastian,

Thanks for the reply. Actually, I was talking about 3D convex hulls. The
Dynamic convex hull construction given in cgal solved my problem.

I need one more solution regarding convex hull only. Now suppose that I pick
any point(say P) on the Delaunay triangulation, which is a point(finite) of
infinite cell. I need to calculate average of outward normals of all the
hull faces meeting at this particular point(P).

The cells are oriented, but it still I believe, does not give outward normal
in all the cases because cells are oriented with respect to their adjacent
cells. Many time just taking cross product of sides in the orientation gives
normal inward the hull. (I am saying this, because I displayed the
triangulation of implicit surface mesher by calculating normals by taking
cross product of sides of triangle in the relative orientation and I found a
large chunk of displayed surface was having inward normals).

If anyhow I can get a unit vector or Ray in the direction of the infinite
point of cell with respect to the finite points of cell, It would resolve my
problem. I am still searching out Delaunay triangulation reference manual to
accomplish it.

Please throw light on it.

Thanks
Avanindra

  

-----Original Message-----
From: Sebastien Loriot 
[mailto:]
 
Sent: Wednesday, February 04, 2009 4:30 PM
To: 

Subject: Re: [cgal-discuss] A point not on convex hull

On Wednesday 04 February 2009 11:38:11 am 

 wrote:
> Hi All,
>
> I need to find a particular point from a sample of points of which I made
> convex hull, is on the convex hull or not. I am not finding any particular
> function for it in manual.
>
> Please give right direction, how to do it. I created ch_object of n
points,
> now I don't know which points were involved in making the hull and which
> were excluded.
>
> Thanks
> Avanindra

Hello,


From the manual page
http://www.cgal.org/Manual/3.4/doc_html/cgal_manual/Convex_hull_2/Chapter_ma
in.html#Section_11.3
You can find one example of how to construct the convex hull of a set of 
points. Instead of using the standard input and output like in the example, 
you can use lists or another container.
Indeed, following the link behind CGAL::ch_graham_andrew, we can see that
the 
third parameter of CGAL::ch_graham_andrew is an OutputIterator which gives 
you the list of points on the convex hull. A simple equality test then
solves 
your problem.


Another solution is to use the triangulation_2 and the vertices incident to 
the infinite vertex.


Sebastien.
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