[Insight-developers] RequestedRegion are modified when calling itk filters (2)

[Karthik Krishnan]

Karthik Krishnan wrote:

> What I meant was: what if I want to put the filter in a pipeline and 
> just have my application run on an interactive user chosen VOI. I 
> would need to use crop filters etc ?
> It could run the separable gaussian filters on the requested region 
> instead of the whole region.
>
> If I did want it to run on the whole image, I could always say: 
> UpdateLargestPossibleRegion().  But there is no way I can have it 
> update on just a defined image region.
>
Correcting myself:
I guess I could say :

filter->GetInput()->SetLargestPossibleRegion()->( voiRegion );
filter->Update();

and have it work.

sorry for the noise
-karthik

> -karthik
>
> Luis Ibanez wrote:
>
>>
>> Hi Arnaud,
>>
>> The Recursive Gaussian image filter needs the entire image as
>> input because it is implemented using IIR separable filters.
>>
>> IIR filters have infinite support and therefore cannot be
>> computed using pieces of the image.  A better understanding
>> of this implementation can be gained by looking at how it
>> is expressed in the Z transform space.
>>
>>
>> You may want to read the report by Rachid Deriche:
>>
>> "Recursively Implementing The Gaussian and Its Derivatives",
>> INRIA, 1993,
>> ftp://ftp.inria.fr/INRIA/tech-reports/RR/RR-1893.ps.gz
>>
>>
>> Where these filters were originally proposed.
>>
>>
>>
>>
>>    Regards,
>>
>>
>>
>>        Luis
>>
>>
>> ------------------------
>> Karthik Krishnan wrote:
>>
>>>
>>>
>>> Arnaud Garcia wrote:
>>>
>>>> Hi itk developers,
>>>>
>>>> It seems that most itk filters change the RequestedRegion when 
>>>> calling them ...
>>>> Some filters overide  the GenerateInputRequestedRegion with  
>>>> something like:
>>>>
>>>> SetRequestedRegion( input->GetLargestPossibleRegion() );
>>>>
>>>>
>>>>
>>>> => So if you set your requested region and call such filters you 
>>>> have to set it again after calling theses filters ...
>>>>
>>>> is it right ?
>>>>
>>> Yes. This is the expected behaviour since if you use such a filter 
>>> in a pipeline and the filter needs the whole input to process, it 
>>> must make such a request to those filters on its upstream. That 
>>> said, such a request will not be propagated downstream. For instance 
>>> if you use
>>>
>>> A --> B --> C
>>> and B needs the whole input to process.
>>>
>>> C->SetRequestedRegion( ... )
>>> C->Update();
>>>
>>> This should set the requested regions of A's input and B's input to 
>>> the whole image, but C's input's requested region should stay 
>>> unchanged.
>>>
>>> That said, there are a good number of filters that require the whole 
>>> input to process, either because they do, or in some cases the 
>>> filter is not well written to handle them.
>>>
>>> For instance RecursiveSeparableGaussianFilter: I am not sure why the 
>>> filter must make an assumption that it requires the entire input. It 
>>> doesn't sound reasonable. Maybe the others have something to say 
>>> about it.
>>>
>>> -karthik
>>>
>>>> thanks for answer,
>>>>
>>>> Arnaud
>>>>
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>>>> Insight-developers at itk.org
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>>>>
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>>>
>>
>>
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