[Dune] Re: ug intersection methods

[Christian Engwer]

Hi Oliver,

> Since I've mostly worked with UG I tend to think in conforming meshes.
> Therefore, I implemented intersection...Local() such that the returned
> geometry corresponds to the corresponding subface of the element.
> In particular, this means the return value of intersectionSelfLocal()
> is oriented differently from the return value of 
> intersectionNeighborLocal().  That is why you don't get the same
> world coordinate for the local coordinate 0.25.  You would get the
> same if you chose 0.25 on one side and 0.75 on the other.

perhaps the Documentation in this case is a little bit inaccurate, but
the from the fact that we assume the intersection to be a dim-1 object
it is quite clear that it has a uniq, not predefined,
orientation. That means

intersection local coord -> neighbor local coord -> global coord
intersection local coord -> self local coord -> global coord
and
intersection local coord -> global coord

must always return the same result.

The behaviour you have implemented is quite useless, as you can't know
how the elements are oriented towards each other and therefor can't
know thet 0.25 coresponds to 0.75.

I think also taht Peters "What is a grid" article has a definition of
an intersection. If not this should be added.

Cheers Christian :-)