VTK/Streaming

Streaming pieces

Streaming a number of pieces is pretty straightforward. Here is an example

<source lang="cpp"> alg->UpdateInformation(); vtkStreamingDemandDrivenPipeline* exec = vtkStreamingDemandDrivenPipeline::SafeDownCast(alg->GetExecutive()); if (!exec || exec->GetMaximumNumberOfPieces() < 10)

 {
 die();
 }

for (int i=0; i<10; i++)

 {
 exec->SetUpdateExtent(0, i, 10, 0);
 alg->Update();
 // do something with alg->GetOutputDataObject(0)
 }

</source>

This example streams a pipeline that ends with the algorithm alg in 10 pieces. The best example of streaming pieces is vtkPolyDataMapper:

<source lang="cpp"> void vtkPolyDataMapper::Render(vtkRenderer *ren, vtkActor *act) {

 // ...

 nPieces = this->NumberOfPieces * this->NumberOfSubPieces;

 for(int i=0; i<this->NumberOfSubPieces; i++)
   {
   // If more than one pieces, render in loop.
   currentPiece = this->NumberOfSubPieces * this->Piece + i;
   input->SetUpdateExtent(currentPiece, nPieces, this->GhostLevel);
   this->RenderPiece(ren, act);
   }

} </source>

For a good example of streaming extents, see vtkMemoryLimitImageDataStreamer and its superclass vtkImageDataStreamer.

Streaming time steps

This example demonstrates how a filter can stream multiple time steps. The example we will use to demonstrate this functionality is vtkTemporalStatistics.

Streaming happens over multiple executions of the filter. Therefore, we need a way of keeping track of which time step we are currently processing. For this purpose, vtkTemporalStatistics uses an data member called CurrentTimeIndex. CurrentTimeIndex is initialized to 0 in the constructor.

<source lang="cpp"> vtkTemporalStatistics::vtkTemporalStatistics() {

 this->CurrentTimeIndex = 0;

} </source>

In RequestUpdateExtent(), we request the current time index. This is initially set to 0 but will be incremented after the streaming starts.

<source lang="cpp"> int vtkTemporalStatistics::RequestUpdateExtent(

                                vtkInformation *vtkNotUsed(request),
                                vtkInformationVector **inputVector,
                                vtkInformationVector *vtkNotUsed(outputVector))

{

 vtkInformation *inInfo = inputVector[0]->GetInformationObject(0);

 double *inTimes = inInfo->Get(vtkStreamingDemandDrivenPipeline::TIME_STEPS());
 if (inTimes)
   {
   inInfo->Set(vtkStreamingDemandDrivenPipeline::UPDATE_TIME_STEPS(),
               &inTimes[this->CurrentTimeIndex], 1);
   }

 return 1;

} </source>

In RequestData(), we first process the current time step. We then increment the current time index and set CONTINUE_EXECUTING if there are more time steps to process. This tells the executive to perform another pass of the REQUEST_UPDATE_EXTENT and REQUEST_DATA. This will lead to another call to RequestUpdateExtent() and RequestData() with the next time index. This will continue until the filter removes CONTINUE_EXECUTING from the request.

<source lang="cpp"> int vtkTemporalStatistics::RequestData(vtkInformation *request,

                                      vtkInformationVector **inputVector,
                                      vtkInformationVector *outputVector)

{

 vtkInformation *inInfo = inputVector[0]->GetInformationObject(0);
 vtkInformation *outInfo = outputVector->GetInformationObject(0);

 vtkDataObject *input = vtkDataObject::GetData(inInfo);
 vtkDataObject *output = vtkDataObject::GetData(outInfo);

 if (this->CurrentTimeIndex == 0)
   {
   // First execution, initialize arrays.
   this->InitializeStatistics(input, output);
   }
 else
   {
   // Subsequent execution, accumulate new data.
   this->AccumulateStatistics(input, output);
   }

 this->CurrentTimeIndex++;

 if (  this->CurrentTimeIndex
     < inInfo->Length(vtkStreamingDemandDrivenPipeline::TIME_STEPS()))
   {
   // There is still more to do.
   request->Set(vtkStreamingDemandDrivenPipeline::CONTINUE_EXECUTING(), 1);
   }
 else
   {
   // We are done.  Finish up.
   this->PostExecute(input, output);
   request->Remove(vtkStreamingDemandDrivenPipeline::CONTINUE_EXECUTING());
   this->CurrentTimeIndex = 0;
   }

 return 1;

} </source>