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

Tue Dec 15 12:00:07 EST 2009

It's not the wrong way in itk. You'll see in the itk snippet I posted
that the first filter in the mini-pipeline does set it's input to the
filter's input directly.

On Tue, Dec 15, 2009 at 11:53 AM, David Gobbi <david.gobbi at gmail.com> wrote:
> Not sure if this "Graft" is really any different from the ShallowCopy
> technique that Berk described.  If there's a simple way do what's
> needed with ShallowCopy and/or DeepCopy then no changes to VTK are
> required, just better documentation.
>
> The main concern that I have is that the first thing that any beginner
> will attempt to do is connect the filter's input directly to their
> internal mini-pipeline, and that is the wrong way to do it.
>
>   David
>
>
> On Tue, Dec 15, 2009 at 9:44 AM, Bill Lorensen <bill.lorensen at gmail.com> wrote:
>> ITK has mechanisms to support minipipelines and they are used
>> frequently in filters. The main facility is a Graft method that each
>> DataObject implements. Filters implement GraftOutput() methods that
>> Graft() a data object to a filter's output. Here is the documentation
>> for an image's Graft method:
>>  /** Graft the data and information from one image to another. This
>>   * is a convenience method to setup a second image with all the meta
>>   * information of another image and use the same pixel
>>   * container. Note that this method is different than just using two
>>   * SmartPointers to the same image since separate DataObjects are
>>   * still maintained. This method is similar to
>>   * ImageSource::GraftOutput(). The implementation in ImageBase
>>   * simply calls CopyInformation() and copies the region ivars.
>>   * Subclasses of ImageBase are responsible for copying the pixel
>>   * container. */
>>
>> It may be an interesting future project to see if such a mechanism can
>> be added to VTK.
>>
>> Bill
>>
>>
>> On Tue, Dec 15, 2009 at 9:41 AM, David Gobbi <david.gobbi at gmail.com> wrote:
>>> 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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>>>>
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>