[Insight-developers] Confused about persistance of filter outputs

[Hans Johnson]

Luis,

Thanks for your response.  After further review, I noticed that I forgot 
to call myGrad->Update().

I have been looking at the software guide, and believe that there are 
two things missing.

1) An in-depth discussion of how the pipeline works.
     -) what does GetOutput() return,
     -) why you don't have to call filter->Update when chaining many 
filters together
     -) When memory is allocated, and when is it released (this is 
already described, and may not need any more)


2) A common pitfalls appendix with examples
      -) All of the images in MyNewImagesList will have a smart pointer 
to the same image.

         std::list<itk::Image<>> MyImagesList;
         std::list<itk::Image<>> MyNewImagesList;
         itk::FilterX X=itk::FilterX::New()
         for( currImage in MyImagesList )
         {
             X.SetInput(currImage);
             X.Update();
             MyNewImagesList.push_back(X.GetOutput);
          }

       -) This will cause errors because MyNewImage was never allocated.

         itk::Image<> MyImage;
         itk::Image<> MyNewImage;
         {
            itk::FilterX X=itk::FilterX::New()
            X.SetInput(MyImage);
            //NOTE: No UpdateCommand
            MyNewImage=X.GetOutput();
          }
          itk::FilterY Y=itk::FilterY::New();
          Y.SetInput(MyNewImage);
          Y.Update();


Both of these issues are closely related to not fully understanding the 
pipeline, and making improper assumptions about it's behavior.

Again, Thanks for all your help.

Hans




Luis Ibanez wrote:

> 
> Hi Hans,
> 
> Your code looks good.
> 
> By creating the STL list of SmartPointer to images
> before the for() loop, you should be safely assuring
> the survival of the images.
> 
> The destruction of the gradient filters will only
> decrement the reference count of the output images.
> 
> 
> Luis
> 
> 
> ----------------------------
> 
> Hans J. Johnson wrote:
> 
>> Filter and Pipeline experts:
>>
>> I am trying to create a list of gradient images from a list of images. 
>> Does the following code produce a list of SmartPointers to gradient
>> images that persist outside the for loop, or are all the SmartPointers
>> invalid outside the for loop?
>>  
>>
>> std::list<GradImageType::Pointer > ImageCache;
>> ...read in images...
>>
>> std::list<GradImageType::Pointer > GradientsCache;
>> for (std::list<itk::Image<unsigned char,3>::Pointer >::const_iterator it
>> = ImageCache.begin();
>> it != ImageCache.end();   it++)
>> {
>>         typedef itk::GradientImageFilter<itk::Image<unsigned char,3> >
>> myGradType;
>>         myGradType::Pointer myGrad=myGradType::New();
>>         myGrad->SetInput(*it);
>>         GradientsCache.push_back(myGrad->GetOutput());
>> }
>>
>> Thanks,
>> Hans
>>
>>
> 
> 
> 
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-- 
===================================================================
Hans J. Johnson                              W294 GH
hans-johnson@uiowa.edu                       Dept. of Psychiatry
http://www.psychiatry.uiowa.edu/~hjohnson    The University of Iowa
(319) 353-8587                               Iowa City, IA 52242
===================================================================