[Insight-users] native function returning an itk::Image

[Luis Ibanez]

Hi Namgyun,


As long as you have changed the return type of your function
to be a SmartPointer to the image, then there shouldn't be
any difference between using (1) or (2), except for the fact
that in (2) you create an extra intermediary Smart Pointer,
and that can be a slow operation.

On the other hand, the method (2) gives you the opportunity
to perform error checking, in case the "brain.dcm" doesn't
exist. Of course, you may already be managing errors via
Exceptions...in which case, this drawback is not relevant.



    Regards,


        Luis



------------------------------------
namgyun Lee wrote:
> I have a further question about this.
> 
> Let me describe the situation briefly.
> 
> I have defined all types in the header file of class and also defined a 
> private member variable "list" as std::vector<ImageType::Pointer>.
> And I return the ImageType::Pointer to the "list" using two different 
> methods,
> 
> 1) list.push_back(this->readImg("brain.dcm"));
> 2)ImageType::Pointer myImage = this->readImg("brain.dcm");
>   list.push_back(myImage);
> 
> My question is what is the difference between the methods describe above?
> Could you explain the difference emphasizing the scope and reference 
> counting perspective?
> 
> 
> Thank you so much.
> 
> Sincerely,
> Namgyun Lee
> 
> 
> 
> On Sun, Aug 31, 2008 at 7:56 AM, Luis Ibanez <luis.ibanez at kitware.com 
> <mailto:luis.ibanez at kitware.com>> wrote:
> 
> 
>     Hi Protein,
> 
> 
>     In order to keep an ITK image alive when returning it from
>     a function, you *MUST* return it as a SmartPointer.
> 
>     Instead of your current signature:
> 
>         ImageType* readImg(char* imageFileName);
> 
>     You should use
> 
>        ImageType::Pointer readImg(char* imageFileName);
> 
> 
>     In that way the smart pointer will be copied in the stack when
>     the function is returning, and that temporary copy wil keep the
>     image alive long enough to survive the destruction of data in
>     the local scope of the function.
> 
>     Make sure that the returned variable is assigned to a
>     SmartPointer as well.
> 
> 
>     That is, when you call the function do something like:
> 
>       ImageType::Pointer myImage = readImg("brain.dcm");
> 
> 
>     NOTE: You *do not* need, and you *should not* modify the
>          reference counting of ITK images. SmartPointers
>          are supposed to do that for you.
> 
> 
> 
>      Regards,
> 
> 
>          Luis
> 
> 
>     ---------------
> 
>     protein wrote:
> 
>         Hey all,
> 
>         This might be a duplicated question but I really didn't find a
>         answer..
> 
>         Basically my scenario could be described as to write a native
>         function
>         to read a .png file, using an instantiated
>         itk::ImageFileReader<unsigned char>,
>         and return an itk::Image<unsigned char>. How to keep that
>         itk::Image alive
>         after returning from the native function?
> 
>         So in the header file I defined:
> 
>           const    unsigned int    Dimension = 2;
>           typedef  float           PixelType;
>           typedef itk::Image< PixelType, Dimension >  ImageType;
>           typedef itk::ImageFileReader< ImageType  > ImageReaderType;
> 
>         then I declear a function, which takes
>         char* imageFileName as input
>         and
>         output an ImageType*
> 
>            ImageType* readImg(char* imageFileName);
> 
>         the definition readImg function:
>            ImageReaderType reader = ImageReaderType::New();
>            reader->SetFileName(imageFileName);
>            reader->Update();
>            return reader->GetOutput();
> 
> 
>         But when calling this function in main(), by:
>            ImageType::Pointer img = readImg("test.png");
> 
>         I know the reader is destructed when going back to main, thus the
>         there will be error.
> 
>         So I increased the reference count of reader before returning,
>         to keep it alive even after returned from the readImg function,
>         I added:
>            reader->SetReferenceCount(1+reader->GetReferenceCount());
>         before "return reader->GetOutput();"
> 
>         Then in the main, I could use the img.
> 
>         However this seems not that nice to me since reader won't get
>         released....
> 
> 
>         Could anyone tell me in general how to deal with such kind of
>         problems?
>         I really appreciate that!
> 
>         Thanks,
>         Yi
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