vtkPolyDataAlgorithm是一个方便的类，使编写算法变得更容易。它还旨在帮助将旧算法转换为新的管道体系结构。这个类有一些假设和默认值，你应该知道。

• 这个类的默认设置是，过滤器将有一个输入端口和一个输出端口。如果不是这样，只需使用SetNumberOfInputPorts等更改它。请参阅此类构造函数以获取默认值。
• 该类还提供了一个FillInputPortInfo方法，默认情况下，该方法表示所有输入都将是PolyData。如果不是这样，请在子类中重写此方法。

1.类图结构

vtkPolyDataAlgorithm类图

vtkPolyDataAlgorithm写作图：

vtkPolyDataAlgorithm Class Reference

2.代码实现

2.1 ProcessRequest()

vtkTypeBool ProcessRequest(
    vtkInformation*, vtkInformationVector**, vtkInformationVector*) override;

该方法是对基类vtkAlgorithm::ProcessRequest()方法的重写。

//------------------------------------------------------------------------------
vtkTypeBool vtkPolyDataAlgorithm::ProcessRequest(
  vtkInformation* request, vtkInformationVector** inputVector, vtkInformationVector* outputVector)
{
  // generate the data
  if (request->Has(vtkDemandDrivenPipeline::REQUEST_DATA()))
  {
    return this->RequestData(request, inputVector, outputVector);
  }

  if (request->Has(vtkStreamingDemandDrivenPipeline::REQUEST_UPDATE_EXTENT()))
  {
    return this->RequestUpdateExtent(request, inputVector, outputVector);
  }

  // execute information
  if (request->Has(vtkDemandDrivenPipeline::REQUEST_INFORMATION()))
  {
    return this->RequestInformation(request, inputVector, outputVector);
  }

  return this->Superclass::ProcessRequest(request, inputVector, outputVector);
}

2.1.1 RequestData

//------------------------------------------------------------------------------
// This is the superclasses style of Execute method.  Convert it into
// an imaging style Execute method.
//这是Execute方法的超类风格。将其转换为图像样式的执行方法
int vtkPolyDataAlgorithm::RequestData(vtkInformation* vtkNotUsed(request),
  vtkInformationVector** vtkNotUsed(inputVector), vtkInformationVector* vtkNotUsed(outputVector))
{
  return 1;
}

vtkPolyDataAlgorithm类RequestData方法是个虚方法，实际场景调用vtkPolyDataAlgorithm子类的RequestData（）方法。
例如绘制箭头时，就应当调用子类vtkArrowGlyphFilter的RequestData()方法。

int vtkArrowGlyphFilter::RequestData(
  vtkInformation* request, vtkInformationVector** inputVector, vtkInformationVector* outputVector)
{
  vtkInformation* inInfo = inputVector[0]->GetInformationObject(0);
  vtkDataObject* input = inInfo->Get(vtkDataObject::DATA_OBJECT());
  vtkDataSet* dsInput = vtkDataSet::SafeDownCast(inInfo->Get(vtkDataObject::DATA_OBJECT()));

  if (!dsInput)
  {
    if (input)
    {
      vtkErrorMacro("This filter cannot process input of type: " << input->GetClassName());
    }
    return 0;
  }

  // Glyph a subset.
  vtkIdType maxNumPts = this->MaximumNumberOfPoints;
  vtkIdType numPts = dsInput->GetNumberOfPoints();
  vtkIdType totalNumPts = this->GatherTotalNumberOfPoints(numPts);

  // What fraction of the points will this processes get allocated?
  maxNumPts = (vtkIdType)((double)(maxNumPts) * (double)(numPts) / (double)(totalNumPts));

  maxNumPts = (maxNumPts < 1) ? 1 : maxNumPts;

  vtkInformationVector* inputVs[2];

  vtkInformationVector* inputV = inputVector[0];
  inputVs[0] = vtkInformationVector::New();
  inputVs[0]->SetNumberOfInformationObjects(1);
  vtkInformation* newInInfo = vtkInformation::New();
  newInInfo->Copy(inputV->GetInformationObject(0));
  inputVs[0]->SetInformationObject(0, newInInfo);
  newInInfo->Delete();
  inputVs[1] = inputVector[1];

  int retVal = this->MaskAndExecute(numPts, maxNumPts, dsInput, request, inputVs, outputVector);

  inputVs[0]->Delete();
  return retVal;
}

2.1.2 RequestUpdateExtent

//------------------------------------------------------------------------------
int vtkPolyDataAlgorithm::RequestUpdateExtent(vtkInformation* vtkNotUsed(request),
  vtkInformationVector** inputVector, vtkInformationVector* vtkNotUsed(outputVector))
{
  int numInputPorts = this->GetNumberOfInputPorts();
  for (int i = 0; i < numInputPorts; i++)
  {
    int numInputConnections = this->GetNumberOfInputConnections(i);
    for (int j = 0; j < numInputConnections; j++)
    {
      vtkInformation* inputInfo = inputVector[i]->GetInformationObject(j);
      inputInfo->Set(vtkStreamingDemandDrivenPipeline::EXACT_EXTENT(), 1);
    }
  }
  return 1;
}

2.1.3 RequestInformation

//------------------------------------------------------------------------------
int vtkPolyDataAlgorithm::RequestInformation(vtkInformation* vtkNotUsed(request),
  vtkInformationVector** vtkNotUsed(inputVector), vtkInformationVector* vtkNotUsed(outputVector))
{
  // do nothing let subclasses handle it
  return 1;
}

vtkArrowGlyphFilter子类没有RequestInformation（）方法。