[Dune] Projection direction in dune-grid-glue with psurface

[Miguel de Benito Delgado]

Hi Oliver,

On 14 May, 2013, at 16:05, Oliver Sander <sander at igpm.rwth-aachen.de> wrote:

> please update dune-grid-glue and try again.  

Now, that's what I call fast support! :)

> I implemented the outerNormal method
> for 1d and 2d intersections in 2d and 3d worlds, respectively.  The unit tests for
> these methods in dune-grid-glue are very rudimentary, so there may still be bugs.
> Patches for the tests and for the code are welcome.

Thanks a lot, I'll let you know in a couple of days if I find any problems.

> Beware: the details here are a bit tricky.
> 
> a) the outerNormal method gives you face normals
> b) When the paper says that the outer normal is used for projection,
>   the following is actually meant: psurface averages the face normals
>   to obtain vertex normals.  These are then interpolated linearly
>   across the faces, giving you a continuous direction field of
>   approximate(!) normals (and hence a continuous projection).
> 
> Consequently, these two vectors fields are not exactly the same.
> If you want to obtain b) through the interface you have to use
> 
> intersection->geometryInOutside().global(pos) - intersection->geometry().global(pos)

I thought I was doing a quick-hack approximation with exactly this and it turns out it's what I was looking for. The face outer normal will nevertheless come in handy too. Thanks!

Best,
--
Miguel de Benito.