CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

Juan Carlos Lopez Alfonso wrote:
> Hi Sebastien:
>
> I have a question that I can try to solve reading the documentation but 
> is imposible for my. When I have done my alpha_shape how Can I iterate 
> over the cells and vertices that are on it? You say me:
>
> The second problem is that in your output functions, you consider all
> simplices where as you should only consider non-exterior simplices
> (and probably use the REGULARIZED version to avoid having components
> of dimension less than 3).
>
> And I understood your point, but how can I iterate over non-exterior 
> simplices?
> All examples only compute de alpha_shape but after dont do anything with it.

Iterate over all simplices like you did and then use the classify member 
function documented here:
http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Alpha_shapes_3_ref/Class_Alpha_shape_3.html

Sebastien.


> Best regard and thank you a lot for your help 
> Juan Carlos
>
> On Mon, Nov 14, 2011 at 7:04 PM, Sebastien Loriot (GeometryFactory) 
> < <mailto:>> wrote:
>
>     Juan Carlos Lopez Alfonso wrote:
>
>         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/MAIN.cpp>
>
>
>     There are several problems in your code:
>     First, you set alpha to 0, so you will get only all the points.
>     Try calling find_optimal_alpha(1) and then set_alpha to the result
>     for example.
>
>     The second problem is that in your output functions, you consider all
>     simplices where as you should only consider non-exterior simplices
>     (and probably use the REGULARIZED version to avoid having components
>     of dimension less than 3).
>
>     See the Alpha_shape_3 class documentation here:
>     
> http://www.cgal.org/Manual/__latest/doc_html/cgal_manual/__Alpha_shapes_3_ref/Class___Alpha_shape_3.html
>     
> <http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Alpha_shapes_3_ref/Class_Alpha_shape_3.html>
>
>     See here the classification of simplices:
>     
> http://www.cgal.org/Manual/__latest/doc_html/cgal_manual/__Alpha_shapes_3/Chapter_main.__html#Subsection_42.2.3
>     
> <http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Alpha_shapes_3/Chapter_main.html#Subsection_42.2.3>
>
>         http://dl.dropbox.com/u/__2516160/PTV_all.off
>         <http://dl.dropbox.com/u/2516160/PTV_all.off>
>
>
>     Looking at your input, I have a question, are trying to reconstruct a
>     shape from polygonal contours in parallel planes?
>
>
>
>         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?
>
>
>     Examples are written with an inexact construction kernel because when
>     the data is not to nasty there is usually no problem and the computation
>     is faster. In you case you probably have bad elements (almost flat
>     tetrahedra for example) and the squared circum-radii cannot be computed
>     using double.
>
>     Buried here:
>     
> http://www.cgal.org/Manual/__latest/doc_html/cgal_manual/__Alpha_shapes_3/Chapter_main.__html#Section_42.4
>     
> <http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Alpha_shapes_3/Chapter_main.html#Section_42.4>
>     You can read:
>     "the CGAL::Alpha_shape_3<Dt> requires a kernel with exact predicates
>     and exact constructions"
>
>     We have something under review to address this problem with an inexact
>     construction kernel.
>
>
>     Sebastien.
>
>         Best regards and waiting for your help
>         Juan Carlos
>
>
>         On Mon, Nov 14, 2011 at 2:57 PM, Sebastien Loriot
>         (GeometryFactory) 
> <
>         
> <mailto:>
>  
> <mailto:
>         
> <mailto:>>>
>  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/MAIN.cpp>
>                <http://dl.dropbox.com/u/__2516160/MAIN.cpp
>         <http://dl.dropbox.com/u/2516160/MAIN.cpp>>
>                http://dl.dropbox.com/u/____2516160/Brainstem_all.txt
>         <http://dl.dropbox.com/u/__2516160/Brainstem_all.txt>
>                <http://dl.dropbox.com/u/__2516160/Brainstem_all.txt
>         <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
>         <http://www.cgal.org/FAQ.html#____inexact_NT>
>
>            <http://www.cgal.org/FAQ.html#____inexact_NT
>         <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
>         
> <http://www.cgal.org/Manual/__latest/doc_html/cgal_manual/__Alpha_shapes_3_ref/Class___Alpha_shape_3.html>
>
>          
>          <http://www.cgal.org/Manual/__latest/doc_html/cgal_manual/__Alpha_shapes_3_ref/Class___Alpha_shape_3.html
>         
> <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:>
>                
> <mailto:
>         
> <mailto:>>
>  
> <mailto:
>         
> <mailto:>
>
>                
> <mailto:
>         
> <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>
>                <http://www.cgal.org/FAQ.html#____inexact_NT
>         <http://www.cgal.org/FAQ.html#__inexact_NT>>
>
>
>                   <http://www.cgal.org/FAQ.html#____inexact_NT
>         <http://www.cgal.org/FAQ.html#__inexact_NT>
>                <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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