CGAL users discussion list

[cdoucet <>]

Thank you, that's a good idea!

Do you think it will work even in special cases, such as as when the ray lies inside a facet shared by two of incident cells to q?

Best,

Cédric

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De: "Sebastien Loriot (GeometryFactory) [via cgal-discuss]" <[hidden email]>
À: "cdoucet" <[hidden email]>
Envoyé: Lundi 28 Juillet 2014 11:26:01
Objet: Re: Intersection between a ray and a cell

I think for each cell you only need to consider the triangle facet that
is opposite to q in all your intersection queries.

Sebastien.

On 07/28/2014 11:17 AM, Cedric Doucet wrote:

>
> Hello Sébastien,
>
> thank you very much for your answer.
>
> My problem is the following: given two points p and q, find the cells which are
> 1. incident to q,
> 2. intersected by ray (p,q)
> 3. NOT intersected by segment [p,q].
>
> I do not exactly how to do that efficiently.
>
> My (naive) algorithm is the following:
> 1. create a ray (p,q) and find cells incident to q which are intersected by (p,q)
> 2. create a segment [p,q] and find cells incident to q which are intersected by [p,q]
> 3. compute the topological intersection of sets from 1 and 2.
>
> I managed to find cells which are incident to q using incident_cells function from CGAL.
> I also managed to construct a ray and a segment from p and q.
>
> I now have to find which cells are intersected by (p,q) and not [p,q].
> My naive algorithm consist in
> 1. intersectedCells = std::vector(number of incident cells to q);
> 2. for each incidentCellToQ do
> 3.   for vertexIndex = 0 to 3 do
> 4.     Triangle t = triangle(incidentCellToQ,vertexIndex);
> 5.     if intersection(segment,t).empty() do
> 6.       if ! intersection(segment,t).empty() do
> 7.         intersectedCells.pushback(incidentCellToQ)
> 8.       end if
> 9.     end if
> 10.  end for
> 11. end for
>
> The above algorithm is not very well written (and maybe false) because I have not finished to implement it and test it.
>
> However, I feel that I will compute to many things.
> Having a function to compute intersection between a ray and a cell could be useful here.
>
> Do you think there exist a more simple solution in CGAL?
>
> Best,
>
> Cédric
>
>
>
>
> ----- Mail original -----
>> De: "Sebastien Loriot (GeometryFactory)" <[hidden email]>
>> À: [hidden email]
>> Envoyé: Lundi 28 Juillet 2014 10:42:27
>> Objet: Re: [cgal-discuss] Intersection between a ray and a cell
>>
>> It's not clear to me what you are looking for (what you say about nodes
>> confuses me).
>>
>> Is the ray arbitrary and do you want to know all the finite cells
>> intersected by that ray?
>>
>> Sebastien.
>>
>> On 07/28/2014 10:22 AM, cdoucet wrote:
>>> Hello,
>>>
>>> In a 3D Delaunay triangulation, I would like to know which tetraedra
>>> (indicent to the same node) are intersected by a given ray (passing through
>>> this node).
>>>
>>> I found in the documentation that intersections between rays and other
>>> cells
>>> can be computed:
>>> http://doc.cgal.org/latest/Kernel_23/group__intersection__linear__grp.html
>>> However, it does not seem possible to call intersection function for a ray
>>> and a tetrahedron (or a Cell_handle).
>>>
>>> How could I perform such an intersection with CGAL?
>>>
>>> Thank you very much for your help.
>>>
>>> Cédric Doucet
>>> Inria Paris-Rocquencourt
>>>
>>>
>>>
>>> --
>>> View this message in context:
>>> http://cgal-discuss.949826.n4.nabble.com/Intersection-between-a-ray-and-a-cell-tp4659619.html
>>> Sent from the cgal-discuss mailing list archive at Nabble.com.
>>>
>>
>>
>> --
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>>
>>
>

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