CGAL users discussion list

[Marco Feder <>]

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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