[vtkusers] How to get the input to a filter as algorithm output?

[David Gobbi]

Where to start... there are a couple important items here.

First, there's nothing wrong with directly accessing data objects when
you are writing a VTK filter.  Obviously the filter must access the
data.  That's completely different from accessing the data objects
outside of the filters, and whenever I said "don't mess with the data
objects", I really meant "if you want to mess with the data, then
write a filter".

Second item: In VTK it isn't safe to write a VTK filter that executes
other VTK filters inside of it.  Which is really unfortunate, because
it sure is a useful thing to be able to do.  But if you look through
all the filters in VTK (i.e. vtkAlgorithm derived objects), you will
not find a single one that does this.

Each vtkAlgorithm has an executive that deals with the vtkInformation
and sends Requests to the vtkAlgorithm.  If, internally, you grab the
algorithm's input and feed it into your little "internal pipeline",
then that internal pipeline will propagate its own requests to the
input's producer.  As a result a RequestData in the main (external)
pipeline can cause the internal pipeline to update, which causes all
sorts of requests to be sent up both the internal and external
pipeline before the RequestData completes.  This is a violation of the
usual, careful way that the pipeline is supposed to step its way
through the various requests.  It may result in undefined behavior of
the pipeline.

It would be nice if someone devised a recipe for how to safely use a
mini-pipeline inside a VTK filter, by using deep copies of the data
objects or whatnot, but so far I haven't seen one.  About the closest
thing that I've done is use ITK from inside a VTK filter.

   David


On Tue, Dec 15, 2009 at 1:18 AM, Jérôme <jerome.velut at gmail.com> wrote:
> Hi David D.,
>
> If I understood well, you write VTK pipeline inside a VTK filter. If so, I
> also do that *very* often. My way avoids to get the input data, just
> connects the input:
> blend->AddInputconnection( this->GetInputConnection( ) ); // 'this' being
> your vtkAlgorithm-derived filter.
>
> Secondly, I read with interest the thread in which David G. taught about VTK
> pipelining. And I don't think he would have your head for using the
> old-fashioned SetInput method. It is just like... old-fashioned, but you
> don't "touch" at the data. It is unfair because you add some -maybe-
> unuseful static cast, but the integrity of the pipeline is still preserved.
>
> David G. can you confirm my feelings?
>
> Jerome
>
> 2009/12/15 David Doria <daviddoria+vtk at gmail.com>
>>
>> The first thing I typically do (and I got this by looking at existing
>> filters) at the beginning of a RequestData() is:
>>
>>  vtkInformation *inInfo = inputVector[0]->GetInformationObject(0);
>>  vtkImageData *input = vtkImageData::SafeDownCast(
>>      inInfo->Get(vtkDataObject::DATA_OBJECT()));
>>
>> However, now I have an actual object, so I have to do things like the
>> following:
>>
>>  vtkSmartPointer<vtkImageBlend> blend =
>> vtkSmartPointer<vtkImageBlend>::New();
>>  //blend->AddInputConnection(input->GetOutputPort()); //can't do this
>> because 'input' is not an algorithm output
>>  blend->AddInput(input);
>>
>> David G. would have my head for this! How should this be done instead?
>>
>> Thanks,
>>
>> David D.
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