[Carpet] reductions

[Erik Schnetter]

On Mar 20, 2007, at 12:56:38, Luca Baiotti wrote:

> Hi,
>
> are the buffer points included or excluded while doing reductions?

Outer buffer zones are excluded, inner buffer zones are included.   
Including buffer zones is wrong, since the information is  
interpolated from the next coarser grid, so one should instead use  
the information from that coarse grid.  However, the error is  
probably small, except when the reduction involves derivatives (such  
as looking at the norm of a constraint or a Weyl scalar).

In both cases it is guaranteed that the total volume of all grid  
points which contribute adds up exactly to the total volume.  That  
is, if buffer zones are used, the corresponding coarse grid points  
are omitted.

The weight function in CarpetReduce specifies which grid points are  
used and how much they contribute.  You can output it and look at it  
in a 3D viewer.  (Looking at it on a symmetry plane can be confusing;  
the 3D picture should make things clear.)

-erik

-- 
Erik Schnetter <schnetter at cct.lsu.edu>

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