CGAL users discussion list

[Thiago Milanetto Schlittler <>]

> On 05 Oct 2015, at 21:31, Olivier Devillers 
> <>
>  wrote:
> 
> Le 05/10/15 16:59, Thiago Milanetto Schlittler a écrit :
>> Hello!
>> 
>>    I'm having a really weird bug when trying to intersect two triangles 
>> inside a 3D space while using the 
>> `CGAL::Exact_predicates_inexact_constructions_kernel` kernel. Essentially, 
>> I have two triangles that should not intersect. The function 
>> `CGAL::do_intersect` returns always `false` when testing them, but the 
>> function `CGAL::intersection` builds an intersection, depending on the 
>> order of the vertices of the triangles. 
>> 
>>    The bug disappears when I use the 
>> `CGAL::Exact_predicates_exact_constructions_kernel` kernel, but I can't 
>> afford to use it in the real case scenario.
> So, it does not look like a bug.
> If you use inexact constructions, constructions are inexact....

While this is true, the triangles are sufficiently far away from each other 
to do not create an intersection due to an rounding number error associated 
to the inexact constructions (or at least I think so, correct me if I’m wrong 
…). I’ve attached a figure and a gmsh .geo file with both triangles. The 
triangle A is formed by vertices 1, 2 and 3, B is formed by vertices 11, 12, 
13, and the “intersection" found is formed by the segment 21 - 22 (which is 
co-planar to the triangle A).

Thanks in advance,
Thiago Milanetto Schlittler

Attachment: triangle_intersection.geo
Description: Binary data