CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

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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