[Paraview] vtkDistributedDataFilter and ghost cells

Wed Nov 7 07:22:18 EST 2012

Hmmm. I thought global point IDs were always created. Maybe it only happens when ghost cells are created. Try getting that working first.

As far as global cell IDs are concerned, you are out of luck. vtkDistributedDataFilter does not create them. But they are not all that useful anyway since, unlike points, once you remove the out cells there is no overlap. Usually you can get all the information you want from the ghost level.

If you really need global cell IDs, I'm sure you would have no trouble making them yourself. I would do it before running vtkDistributedDataFilter. Assuming when you first read in your data you have no duplicate cells, just label those on process 0 as 0..N_0, those on process 1 as N_0+1..N_1, and so on.

-Ken

Sent from my iPad so blame autocorrect.

On Nov 7, 2012, at 2:38 AM, "Andrew Parker" <andy.john.parker at googlemail.com<mailto:andy.john.parker at googlemail.com>> wrote:

So that's working now in terms of the cast, forgot GetOutput() inside the cast operator!  The returned vtkUGrid is now fully populated and contains sensible information.

However, both getPointData()->GetGlobalIds()  and  getCellData()->GetGlobalIds() return null pointers.  Any thoughts?

Also, should I be using CellData since I want the cell global to local mapping for cells not the nodes, at the moment at least?

On 6 November 2012 19:05, Moreland, Kenneth <kmorel at sandia.gov<mailto:kmorel at sandia.gov>> wrote:
Perhaps it is outputting a composite data set of some type?  Try running GetClassName() to see what type of data object it really is.

-Ken

From: Andrew Parker <andy.john.parker at googlemail.com<mailto:andy.john.parker at googlemail.com>>
Date: Tuesday, November 6, 2012 9:50 AM

To: Kenneth Moreland <kmorel at sandia.gov<mailto:kmorel at sandia.gov>>
Cc: "vtkusers at vtk.org<mailto:vtkusers at vtk.org>" <vtkusers at vtk.org<mailto:vtkusers at vtk.org>>, "paraview at paraview.org<mailto:paraview at paraview.org>" <paraview at paraview.org<mailto:paraview at paraview.org>>
Subject: [EXTERNAL] Re: [Paraview] vtkDistributedDataFilter and ghost cells

Thanks on both accounts.  Any thoughts why the downcast called after dd->Update() on distributedDataFilter is a null pointer?  As in, dd is working perfectly properly, but I don't seem to be able to extract a valid unstructuredgrid.  For a follow up question I assume getPointData()->GetGlobalIds() gives the local to global for the mesh nodes, and I should use getCellData()->GetGlobalIds() to get the local to global for the cells?  Once I get a valid pointer that is....

Cheers again,
Andy

On 6 November 2012 16:40, Moreland, Kenneth <kmorel at sandia.gov<mailto:kmorel at sandia.gov>> wrote:
You should be able to do a vtkUnstructuredGrid::SafeDownCast() to the data to get the unstructured mesh (and access to the point data).

-Ken

From: Andrew Parker <andy.john.parker at googlemail.com<mailto:andy.john.parker at googlemail.com>>
Date: Tuesday, November 6, 2012 9:32 AM
To: Kenneth Moreland <kmorel at sandia.gov<mailto:kmorel at sandia.gov>>
Cc: "vtkusers at vtk.org<mailto:vtkusers at vtk.org>" <vtkusers at vtk.org<mailto:vtkusers at vtk.org>>, "paraview at paraview.org<mailto:paraview at paraview.org>" <paraview at paraview.org<mailto:paraview at paraview.org>>
Subject: [EXTERNAL] Re: [Paraview] vtkDistributedDataFilter and ghost cells

Another question which I'd forgotten, how do I get to a vtkUnstructuredGrid per processor from the vtkDistributedDataFilter.

For instance, dd->GetOutput()->GetPointData()->GetGlobalIds() doesn't work as it's a vtkDataObject

Stupid question I'm sure but the doxy notes say this type returns an unstructured mesh, but I can't seem to get it out?

Also, why exactly do I need the vtkPieceScalars and vtkDataSetSurfaceFilter again? If the above can be made to work and return the mapping, what are they adding in terms of information?

Thanks again,
Andy

On 6 November 2012 16:00, Moreland, Kenneth <kmorel at sandia.gov<mailto:kmorel at sandia.gov>> wrote:
I believe vtkDistributedDataFilter will always return with global point ids (a mapping from local point ids to global point ids), although it might pass them if they already exist.  So dd->GetOutput()->GetPointData()->GetGlobalIds() should return the array that gives this mapping.

Ghost cells are only created on demand, and this is usually done by pipeline convention.  If you have a filter that needs a layer of ghost cells, it should override the RequestUpdateExtent method to increment the value of UPDATE_NUMBER_OF_GHOST_LEVELS from the output information to the input information.  This method would look something like this.

int vtkDistributedDataFilter::RequestUpdateExtent(
  vtkInformation *vtkNotUsed(request),
  vtkInformationVector **inputVector,
  vtkInformationVector *outputVector)
{
  // get the info objects
  vtkInformation *inInfo = inputVector[0]->GetInformationObject(0);
  vtkInformation *outInfo = outputVector->GetInformationObject(0);

  int piece, numPieces, ghostLevels;

  // We require an extra layer of ghost cells from upstream.

  piece = outInfo->Get(vtkStreamingDemandDrivenPipeline::UPDATE_PIECE_NUMBER());
  numPieces =
    outInfo->Get(vtkStreamingDemandDrivenPipeline::UPDATE_NUMBER_OF_PIECES());
  ghostLevels =
    outInfo->Get(vtkStreamingDemandDrivenPipeline::UPDATE_NUMBER_OF_GHOST_LEVELS());

  inInfo->Set(vtkStreamingDemandDrivenPipeline::UPDATE_PIECE_NUMBER(), piece);
  inInfo->Set(vtkStreamingDemandDrivenPipeline::UPDATE_NUMBER_OF_PIECES(),
              numPieces);
  inInfo->Set(vtkStreamingDemandDrivenPipeline::UPDATE_NUMBER_OF_GHOST_LEVELS(),
              ghostLevels);

  return 1;
}

The operation of the RequestData method should also strip off this layer of ghost cells.  It might be possible to request a layer of ghost cells by setting UPDATE_NUMBER_OF_GHOST_LEVELS at the bottom of the pipeline, but I'm not totally sure how to make that work.  It's probably easier (or at least cleaner) to do it from within a filter.

-Ken

From: Andrew Parker <andy.john.parker at googlemail.com<mailto:andy.john.parker at googlemail.com>>
Date: Tuesday, November 6, 2012 8:25 AM
To: "vtkusers at vtk.org<mailto:vtkusers at vtk.org>" <vtkusers at vtk.org<mailto:vtkusers at vtk.org>>, "paraview at paraview.org<mailto:paraview at paraview.org>" <paraview at paraview.org<mailto:paraview at paraview.org>>
Subject: [EXTERNAL] [Paraview] vtkDistributedDataFilter and ghost cells

Hi,

Hope you can help.  I have some code running in parallel, that by other means I have constructed nprocs worth of vtkRectilinearGrids, one per process.  Each of which is a valid nprocs-worth of the whole serial mesh, I've check this and I am happy with that i.e. it's partitioned properly and nothing is missing.  I need the following information to process my data in parallel:

1) I would like the local -> global cell mapping between the local rgrid and the corresponding global single mesh.
2) I would like to know which cells are on processor boundaries for parallel exchange purposes.
3) I would like all the double arrays per processor to be "expanded" by the amount of (1 level of) ghost cells such that I can properly do the computations I want with the ability to exchange only those additional cells given the local to global mapping.

I have tried from the examples to use the following code, which I call on every process, each of which has it's own local rgrid as I said.  I do the following:

 vtkSmartPointer<vtkDistributedDataFilter> dd = vtkSmartPointer<vtkDistributedDataFilter>::New();
 dd->SetInput(rgrid);

 dd->SetController(getVtkController());
 dd->SetBoundaryModeToSplitBoundaryCells();
 //dd->SetBoundaryModeToAssignToOneRegion();
 //dd->SetBoundaryModeToAssignToAllIntersectingRegions();
 dd->UseMinimalMemoryOff();
 dd->Update();
 vtkPieceScalars *ps = vtkPieceScalars::New();
 ps->SetInputConnection(dd->GetOutputPort());
 ps->SetScalarModeToCellData();
 vtkDataSetSurfaceFilter *dss = vtkDataSetSurfaceFilter::New();
 dss->SetInputConnection(ps->GetOutputPort());

The dd object works fine and writing its contents out on each processor gives nprocs worth of meshes, each of which look slightly different to the way I've partitioned them up, but sum to the same serial mesh so I am happy with that working correctly. But I can't for the life of me figure out how to obtain local to global cell mappings, allocate ghost cells, or work out how to exchange data given the above partition info and comms....

Note I have not provided any additional information to "dd" regarding global cells as per the doxy notes so I assume it went away and computed it.  I can't figure out how to extract it however.  I also have no idea how to modify each local processor rgrid to include the ghost cells for that processor.  Finally given that info, I could exchange between processors to write to each local processors ghost cells the corresponding "real" cell data from the neighbouring meshes and continue the code.

Any help really appreciated!

Cheers,
Andy

--

__________________________________

   Dr Andrew Parker

   Em at il<mailto:Em at il>:  andrew.parker at cantab.net<mailto:andrew.parker at cantab.net>

--

__________________________________

   Dr Andrew Parker

   Em at il<mailto:Em at il>:  andrew.parker at cantab.net<mailto:andrew.parker at cantab.net>

--

__________________________________

   Dr Andrew Parker

   Em at il<mailto:Em at il>:  andrew.parker at cantab.net<mailto:andrew.parker at cantab.net>

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