[vtk-developers] ghost levels

[Amy Henderson]

In my e-mail yesterday, I didn't really go into a discussion of what's 
contained in the array that keeps track of ghost levels.  We added the 
array GhostLevels to vtkDataSetAttributes.  We chose this implementation 
because it allows you the option of using ghost levels or not, and if you 
are using them, the array gets carried along automatically.  If the entry 
for a particular cell in the GhostLevels array have a value of 0, then that 
cell is part of the piece you have specified.  If the value is 1, then the 
cell is a neighbor to the cells in this piece.  If the value is 2, the cell 
neighbors a cell in level 1, etc.

Our concern right now is what the best name for this ghost level array 
is.  Do you have any suggestions?  So far, we've come up with 
PieceBoundaryLevels and PieceOverlapLevels as two more possibilities.

We have checked in an example tcl script that makes demonstrates using 
ghost cells.  We have two spheres, each divided into two pieces and then 
run through the vtkPolyDataNormals filter.  In one sphere, we have used 
ghost cells, and in the other we have not.  For the sphere that uses ghost 
cells, the only artifact is at the poles (because we did not handle ghost 
cells correctly there).  For the other sphere, a ridge is visible where the 
two pieces come together.  The tcl script, called NormalSpheres.tcl, is 
located in vtk/graphics/examplesTcl.  We chose to implement ghost cells for 
the sphere source because we needed an example of a source that uses ghost 
cells.  We do not intend to make changes to all the sources to supply ghost 
cells.  Instead, we are in the process of writing a filter that will take 
vtkPolyData as input, break it into a specified number of pieces, and 
return the requested piece including the requested number of ghost levels.

Amy