CGAL users discussion list

[Mael <>]

Hello,

What is your version of CGAL? There was a bug in AS_2 till not so long ago where it failed to use weights sometimes. As far as I remember the bug fix came with CGAL 4.12. With master, and the following test code, I get the following output:

Alpha Shape computed
4 alpha shape edges
Optimal alpha: 11.7899

Faces...
    face:
        2.5 5 4
        3 3 0
        4 5.7 16
    squared radius directly from the kernel: 6.06026
    compute with AS, sq_r: 6.06026
   

    face:
        3 3 0
        8 3 0
        4 5.7 16
    squared radius directly from the kernel: 11.7899
    AS sq_r: 11.7899

Alpha values
    value: 0.57046
    value: 1.79264
    value: 3.82369
    value: 6.06026
    value: 11.7899

which seem to be what you expect.

Mael

On 06/04/2018 05:03 PM, Kathi Oe wrote:

Hello,

I want to compute Alpha shapes of weighted points in 2D (and later 3D).
When I compare my own implementation with CGAL, I get very different results for the alpha values of the simplices.

I tried this basic example:
point 0: 4 5.7 weight 16
point 1: 2.5 5 weight 4
point 2: 3 3
point 3: 6 4 weight -9
point 4: 8 3

We get the triangles 012 and 024, which is fine.
They should have squared radius 6.06 and 11.79, but CGAL gives me 2.15 and 6.62. I don't know where these numbers come from.

Just to be clear, my definition for the weighted Alpha shape is the following:
A simplex is contained in the Alpha shape of radius r if the smallest sphere orthogonal to all its points (AB orthogonal if d(A,B)^2-weight_A-weight_B=0) has weight <= r^2.

For triangle 012 I compute
solve (4 - x)^2 + (5.7 - y)^2 - 16 - w = 0, (2.5 - x)^2 + (5 - y)^2 -  4 - w = 0, (3 - x)^2 + (3 - y)^2 - 0 - w = 0
to get the center (x,y) and weight/radius^2 w of this weighted circumsphere, which indeed gives w=6.06...
Why is the alpha value in CGAL 2.15? The correct value does not even appear in the list of alpha values (which are 0.685 1.0625 2.15052 5.8225 6.25 6.6212).

Did I understand something wrong about how to use alpha shapes in CGAL?

Thanks for your help,
Kathi