[vtkusers] programmable filter pipeline

[Dean Inglis]

Im inheriting from vtkProgrammableFilter to encapsulate pipeline
segments for doing curved planar reformats (simulated dental x-ray 
panoramic ).
The first filter works perfectly: 2D polydata line input generates
3D polydata vertex sampling grid.  The second filter sets up vtkProbeFilter
with the grid as input, and a 3D image as source.  The trouble begins
when I pass the output as a 2D vtkImageData to a 
vtkImageMapToWindowLevelColors
/ vtkImageActor downstream for visualization.  I keep getting pixel index
out of range /not within extent errors.   Within the filter's user supplied
execute method, I retrieve the filter's generated output which is a copy
of the input 3D vtkImageData by default,  I then set it up to take
flattened geometry from the grid and the scalars generated by the probe 
filter:

vtkImageData* image = this->GetOutput();

double origin[3] = {0,0,0};
image->Initialize();
image->SetScalarType(this->Image->GetScalarType());  // the type of the 
input image
image->SetExtent(this->OutputExtent);                              // extent 
defined by the grid
image->SetNumberOfScalarComponents(1);
image->SetOrigin(origin);
image->SetSpacing(this->OutputSpacing);                        // spacing 
defined by the grid
image->AllocateScalars();
image->GetPointData()->GetScalars()->FillComponent(0,0);  // just in case!

this->Probe->Update();
image->GetPointData()->GetScalars()->DeepCopy(
  this->Probe->GetOutput()->GetPointData()->GetScalars());
image->Modified();

Since the upstream pipeline information seems to be based on the input
3D image, would I modify a vtkInformation object here to correctly prepare
the downstream filters' requests?  Anyone know how to do this?  There
is, AFAIK, no substantial/detailed documentation on the nuts and bolts of 
the VTK
pipeline architecture.  I think I need similar workings from 
vtkImageChangeInformation
to accomplish this.

Dean