[Dune] same ID for several interfaces!

[Benjamin Faigle]

Hello Dune,

I maybe stumbled over some strange behaviour in dune (stable 2.0), and I 
was told to send it to the list:

After some work on refinement, I had the situation where I got the same 
ID for completely different interfaces when simulating in 2D with UGgrid 
(numbers are CellIndice). I encountered a situation like this:

+-------+
|       |
|   7   |
|       |
+---+---+
| 39| 38|
+---+---+
| 36| 37|
+---+---+
|       |
|   6   |
|       |
+-------+

If I regard the Interface (IF) between 38 and 7 from Element 38, I 
should get an unique ID with
	grid.globalIdSet().subId(*is.inside(), is.indexInOutside(), 1);
Works most often.

With the right geometries, after many refinement steps, I had now the 
situation where this method gave the ID 798 - the same ID as for the IF 
between 36 and 6 (as seen from 36)!

Of course I could fix this by using (inside, indexInInside,1), but I 
nevertheless thought that IDs should be unique.

In case you are interested, tell me what information you need,
best regards,

Benjamin


On 05/16/2011 12:04 PM, Jö Fahlke wrote:
> Am Mon, 16. May 2011, 11:27:09 +0200 schrieb Benjamin Faigle:
>> if i understood it right, the method
>> 	grid.localIdSet().id(const EntityType&e)
>> should return a unique ID for every sort of entities, be it
>> elements, vertices or intersections. It works out pretty well with
>> elements, as shown for example in the Dune Grid Howto
>> 	IdType idf = idset.id (* ep );
>>
>> However, if I try the same with an *intersection, I get the error message:
>> ...dune-grid/dune/grid/common/indexidset.hh:394:43: error:
>> ‘codimension’ is not a member of ‘Dune::IntersectionIterator<const
>> Dune::UGGrid<2>, Dune::UGGridLeafIntersectionIterator,
>> Dune::UGGridLeafIntersection>’
>>
>> I am using dune 2.0.
>> I am sorry if it is just an incredibly silly question, and many
>> thanks for your help,
>
> Only entities have ids.  Intersections are not entities!
>
> All grids have entities of codimension dim (vertices) and codimension 0
> (elements), and all have intersections.  Some grids have entities of other
> codimensions as well.  Even if a grid has entities of codimension 1, these are
> not the same as entities.  Most obvious differences:
>
>   * Intersections know the neighbouring cells, codim 1 entities don't.
>
>   * Intersections have an orientation, codim 1 entities don't.
>
>   * Codim 1 entities have indices and ids and can be used to obtain array
>     indices from a mapper (even if the grid does not support codim 1 entity
>     objects!)  For intersections, indices, id's and maps are not provided by
>     core Dune, though it is possibly to build such things yourself.
>
>   * In non-conforming grids you may have the following situation:
>
>       +-------+---+---+
>       |       | b | d |
>       |   a   +---+---+
>       |       | c | e |
>       +-------+---+---+
>
>     Elements b, c, d and e all have four codim 1 subentities and four
>     corresponding intersections.  Element a also has four codim 1 subentities,
>     but it has five intersections.  The codim 1 entity to the right of b is the
>     same as the codim 1 entity to the left of d.  You have three codim 1
>     entities to the right of a however: on covering the whole of a's right
>     border, and two smaller one each covering the upper and the lower half of
>     a's border, respectively.
>
> If you know you have a conforming grid, you have a correspondance between
> intersections and codim 1 entities (ignoring the orientation of the
> intersections).  In that case you can usually get away by using the id of the
> corresponding codim 1 entity as the id of the intersection (even if the grid
> does not support codim 1 entities as independent objects.  Use
>
>    grid.globalIdSet().subId(e, i.indexInInside(), 1);
>
> where i is the intersection and e is the codim 0 entity i was obtained from.
> If you don't have e readily available, you can use
>
>    grid.globalIdSet().subId(*i.inside(), i.indexInInside(), 1);
>
> instead.
>
> Bye,
> Jö.
>