CGAL users discussion list

[Marco Feder <>]

I’m sorry but I’m not quite sure I understood what you meant. Are you saying that the std::vector<Point_3> return type doesn’t cover the case when the intersection is a polygon?

Thanks,

Marco

On 28 Jul 2022, at 08:27, Sebastien Loriot <> wrote:

For sure but if you get let say a quad you need to add its area.

Do you seen what I mean?

Best,

Sebastien.

On 7/26/22 18:08, Marco Feder ( via cgal-discuss Mailing List) wrote:

Dear Sebastien,
That is precisely what I did in the GitHub gist I linked in my post, isn’t it ?
Best,
Marco

On 26 Jul 2022, at 17:08, Sebastien Loriot <> wrote:

The intersecton of a triangle with a tet is not always a triangle, it is
a polygon,

Doc says:
|Type1  | Type2 |Return Type: T...|
|Triangle_3 Tetrahedron_3 | Point_3, or Segment_3, or Triangle_3, or std::vector < Point_3 > |

https://doc.cgal.org/latest/Kernel_23/group__intersection__linear__grp.html


So it is most likely you miss some polygons.

Best,

Sebastien.

On 7/20/22 10:44, Marco Feder ( via cgal-discuss Mailing List) wrote:

Dear all,
I have two cubes (they’re coming from two cells of a 3D finite element mesh) and a square (a 2D object) between the two, intersecting both of them. I triangulated each cube with a Triangulation_3, and the square with a Delaunay_triangulation_2.
To compute the intersection of the square with the first cube, I do the following: I loop over all tets of the triangulated cube, and for each Tetrahedron I compute the intersection with a Triangle_3 coming from the triangulated 2D square. Oftentimes I have correct results, but in some scenarios I observe inaccurate results.
For instance: for each cube, I computed the area of the part of square inside it. The sum of these two areas should be precisely equal to the area of the square (it is exactly *0.136900*), but it’s not. In particular, the first area is *0.0237738*, and the second *0.0948123. *Then the sum is *not 0.136900.*
The following GitHub gist shows this: https://gist.github.com/fdrmrc/49f397a5f72a7a00b1dfb992ab7babe1 <https://gist.github.com/fdrmrc/49f397a5f72a7a00b1dfb992ab7babe1> . The output is the area of the intersection of the square with the second cube. To get the first area, change at line 52 /pts_second_hex /with /pts_first_hex./
I can’t spot what is going wrong here.
Best,
Marco
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