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[Juan Carlos Lopez Alfonso <>]

Hi Sebastien:

First of all thank you again for your answer and your comments. I have done my homework but I cant see any result when I use alpha_shape. I have a set of points that you can see in mi file PTV_all.off wich is a triangulation with matlab. I have tried to use Alpha_Shape to obtain a triangulation of my set of point where the shape of these domain of points were adjusted, but without lucky. I have read the documentation, but there not examples of how I can handle the result of these alpha_shape.

http://dl.dropbox.com/u/2516160/MAIN.cpp

http://dl.dropbox.com/u/2516160/PTV_all.off

When I run my code, I obtain a .off or vtk file with a similar triamgulation of matlab, but i want to obtain a triangulation of the shape of these points not of the convex hull of them.

Could you see my code and give me any suggestion or way to do this?

On the other hand, in all examples of the documentation they use "Exact_predicates_inexact_constructions_kernel" why do you say me that use "Exact_predicates_exact_constructions_kernel" instead?

Best regards and waiting for your help

Juan Carlos

On Mon, Nov 14, 2011 at 2:57 PM, Sebastien Loriot (GeometryFactory) <> wrote:

Juan Carlos Lopez Alfonso wrote:

Hi Sebastien:

First of all thank you for your answer and your comments.
In these links you can find my file of set of points and the code of my example, which is simple and exactly as the example "42.5.2   Building Basic Alpha Shapes for Many Points" on the Alpha_Shape charapter of Cgal documentation.

http://dl.dropbox.com/u/2516160/MAIN.cpp
http://dl.dropbox.com/u/2516160/Brainstem_all.txt

When replacing Exact_predicates_inexact_constructions_kernel
by Exact_predicates_exact_constructions_kernel, the assertion is
gone. Explanation is here: http://www.cgal.org/FAQ.html#__inexact_NT
Alpha_shapes_3 require constructions so to guarantee the result to be
correct you need to use a kernel with exact constructions.

About my first question, I beleived that with alpha_shape was obtained an approximated surface of my set of points. In my problem I have a set of points and I need that my resultant triangulation has the exactly same set of points, in the surface and in the interior (I want to create a triangulation of a head with their interior organs).

I don't get your point. Are you talking about missing singular vertices
because of the REGULARIZED mode you used in alpha_shape? If so, use
Alpha_shape_3::GENERAL instead.

http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Alpha_shapes_3_ref/Class_Alpha_shape_3.html

Sebastien.

Best regards and waiting for your help
Juan carlos

On Mon, Nov 14, 2011 at 2:24 PM, Sebastien Loriot (GeometryFactory) < <mailto:>> wrote:

   Juan Carlos Lopez Alfonso wrote:

       Hi all,

       There are any form to use 3D Alpha_Shapes with a general
       triangulation like Triangulation_3? because Delaunay
       triangulation or Regular triangulation move my points in order
       to obtain a linear approximation of the original shape and I
       need to have a triangulation from a fix set of points.


   Nor Delaunay, nor Regular triangulation moves your points. It's building
   a triangulation on top of your point set, so what do you mean?



       On the other hand, I have created a Delaunay Triangulation
       (Delaunay  dt;) from a set of points and when I try to create an
       alpha_shape (Alpha_shape_3 as(dt);) I have the following error:
        CGAL::Assertion_exception !!!


   Did you read this?

   http://www.cgal.org/FAQ.html#__inexact_NT

   <http://www.cgal.org/FAQ.html#inexact_NT>

   If yes, showing some code might help giving you an answer.

   Sebastien.



       Best regrads and thank in advance
       Juan Carlos



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