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[Benoît Presles <>]

Hello Everybody,

In my last email, I submitted you 2 problems I had with the  
"Alpha_shape_2" class. I still have the first problem but after some  
tests I realised that the results I obtained about the second problem  
were OK.
However, I still get some wrong results for some list of points (cf.  
attached file):
First test:
- I compute the alpha shape (GENERAL) with the optimal alpha value  
found with the function "find_optimal_alpha(1)" and I display on the  
screen the alpha edges. With such a value of alpha I should get a  
closed boundary, but instead I get this result:
Reading 13 points from file
Alpha Shape computed
Optimal alpha: 0.01
Nb of solid components: 1
0.3 0.2 0.2 0.2
0 0 0.1 0
0 0.2 0 0.1
0 0.1 0 0
0.2 0.2 0.1 0.2
0.1 0.2 0 0.2
0.1 0 0.1 0.1
0.4 0.1 0.4 0.2
0.4 0 0.4 0.1
0.3 0.1 0.3 0
0.4 0.2 0.3 0.2
0.3 0 0.4 0

Second test:
- I compute the alpha shape (GENERAL) with alpha=0.0025 and I display  
on the screen the alpha edges.  With such a value of alpha I should  
get much more edges but instead I get this result:
Reading 13 points from file
Alpha Shape computed
Optimal alpha: 0.01
Nb of solid components: 0
0.3 0.2 0.2 0.2

Third test:
- I compute the alpha shape (GENERAL) with alpha=0.0026 and I display  
on the screen the alpha edges.  With such a value of alpha I get too  
much edges:
Reading 13 points from file
Alpha Shape computed
Optimal alpha: 0.01
Nb of solid components: 0
0.3 0.2 0.2 0.2
0.3 0.1 0.3 0.2
0 0 0.1 0
0 0.2 0 0.1
0 0.1 0 0
0.2 0.2 0.1 0.2
0.1 0.2 0 0.2
0.1 0.1 0 0.1
0.1 0.1 0.1 0.2
0.1 0 0.1 0.1
0.4 0.1 0.4 0.2
0.4 0 0.4 0.1
0.3 0.1 0.3 0
0.4 0.2 0.3 0.2
0.4 0.1 0.3 0.1
0.3 0 0.4 0


Thank you for your help,
Best Regards,
Ben_P



Le 11 août 10 à 19:07, Benoît Presles a écrit :

> Hello Everybody,
>
> I use the latest patched version of CGAL and I think some bugs could  
> be present in the "Alpha_shape_2" class (all problems can be  
> reproduced by using the list of points "fin" attached in this email).
>
> First problem:
> - First, I use the function "find_optimal_alpha" to find the best  
> alpha to get 1 connected component.
> - Second, I set this alpha.
> - Third, I use the function "number_of_solid_components" to get the  
> number of solid components of A.
> - Fourth, I compute the alpha edges which I display on the screen.
>
> When I do this last step, I get a "CGAL::Assertion_exception":
> Reading 6 points from file
> Alpha Shape computed
> Optimal alpha: 380.5
> Nb of solid components: 1
> terminate called after throwing an instance of  
> 'CGAL::Assertion_exception'
>  what():  CGAL ERROR: assertion violation!
> Expr: (classify((*edge_alpha_it).second.first,  
> (*edge_alpha_it).second.second) == REGULAR)
> File: /appli/include/CGAL/Alpha_shape_2.h
> Line: 1170
> Aborted
>
> Second problem:
> - First, I compute the alpha shape with alpha=50.
> - Second, I compute the alpha edges which I display on the screen.
>
> With such a value of alpha I should get the convex hull of the  
> points, but instead I get this result:
> Reading 6 points from file
> 0 0 1 0
> 1 0 2.5 0
> 2.5 0 4.5 0
> 4.5 0 7 0
>
>
> Attached, you can fine the c++ code (basically the same as in the  
> example) and a list of points for which the program doesn't work as  
> expected (both problems).
>
>
>
> Thank you for your help,
> Best Regards,
> Ben_P
>
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> <fin.txt><alphaShape.cpp>

Attachment: alphaShape.cpp
Description: Binary data

Attachment: fin
Description: Binary data