[vtkusers] How to get the input to a filter as algorithm output?
David Gobbi
david.gobbi at gmail.com
Tue Dec 15 12:07:17 EST 2009
But in VTK, especially with streaming, it is definitely wrong. For
example, the algorithm itself might request one UpdateExtent, and then
the when RequestData is called, the mini-pipeline might request a
different update extent, and everything upstream will re-execute.
David
On Tue, Dec 15, 2009 at 10:00 AM, Bill Lorensen <bill.lorensen at gmail.com> wrote:
> 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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