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

[Michael Jackson]

// Produce a 'Read Only' copy to be used as Input
inputCopy->ShallowCopy(input);
// Add the Input to the 'internal' pipeline
blend->AddInput(inputCopy);
// Update the 'internal' pipeline
blend->Update();

/* Now, blend produced an entirely brand new 'output' object after the
* update method was called. The 'input' to the blend filter should have
* never been modified by the blend filter. Because the output is a brand
* new object you do not need to deep copy it.
*/
output->DeepCopy(blend->GetOutput());
/* If you want to 'save' the output for use later you might want to
* call 'output->Register(NULL);' which will increase the reference
* count of the object so when the 'blend' filter goes out of scope
* and is destroyed the pointer that represents 'output' will NOT be
* deleted. Another way to accomplish the same goal would be to again
* shallow copy the output into another variable.
*/

Basically the difference between a shallow copy and a deep copy is this.
The Shallow copy only makes a copy of the pointer to each of the  
internal
data structures and increases the reference count on each of those  
objects.
So on a 32 bit system you are copying a single 32 bit value (which  
happens
to be a pointer) into a new location. Thus you may have 2 objects  
(input and
inputcopy) BUT they share the same exact data structures internally.  
So changing
the data in one will effect the other.
   A deep copy will make a copy of the actual DATA that the pointer  
points to. Thus you
end up with 2 versions of the data. One in "input" and one in  
"inputcopy". Changing
the data in one of those will NOT effect the other object.

   To use an abstract example. If you have a vtk object that wraps 1  
GigaByte of data and perform
a shallow copy then your memory requirements only go up maybe 100  
bytes (all the
overhead of the vtk object) or so. If you do a DEEP copy then your  
memory requirements
jumps from 1 GigaByte to 2 Gigabytes because all the actual data is  
copied.

  Does that help?
_________________________________________________________
Mike Jackson                  mike.jackson at bluequartz.net

On Dec 15, 2009, at 12:13 PM, Jérôme wrote:

> Oops... it seems that I understand now some weird "bad extent"  
> happening... I will keep this thread bookmarked, since it teaches a  
> lot about VTK pipelining!
>
> This made me look at the documentation of ShallowCopy/DeepCopy.  
> Finally, I found this class:
> http://www.vtk.org/doc/nightly/html/classvtkCell.html#a573202e8c9c3b5367b35fe7ad3fdfb43
>
> where it is said that ShallowCopy is for read-only copy... Is the  
> code snippet right? ShallowCopying the output of the internal filter  
> should then produce a read-only output for the main filter!
>
> Do I mistake if I rather propose:
> inputCopy->ShallowCopy(input);
> blend->AddInput(inputCopy);
> blend->Update();
> output->DeepCopy(blend->GetOutput());
>
> Jerome
>
> 2009/12/15 Michael Jackson <mike.jackson at bluequartz.net>
>
>
> On Dec 15, 2009, at 11:53 AM, Bill Lorensen wrote:
>
> After seeing Berk's reply it looks as though his
> vtkImageData* inputCopy = input->NewInstance();
> inputCopy->ShallowCopy(input);
> blend->AddInput(inputCopy);
> blend->Update();
> output->ShallowCopy(blend->GetOutput());
>
> would be:
>
> blend->AddInput(input);
> blend->Update()
> this->GraftOutput(blend->GetOutput());
>
> Bill,
>  As, Berk explained, you _really_ need to have a shallow copy to  
> decouple the internal and external pipelines. If this is not done  
> the all sorts of "Bad" will happen.
>
>  Maybe what is needed is this:
>
> /* Get a Shallow copy from the input */
> vtkImageData* inputCopy = input->NewShallowCopyInstance();
>
> blend->AddInput(inputCopy);
> blend->Update();
> output->ShallowCopy(blend->GetOutput());
> /* Don't Forget to memory clean up somewhere.*/
>
> _________________________________________________________
> Mike Jackson                  mike.jackson at bluequartz.net
> BlueQuartz Software                    www.bluequartz.net
> Principal Software Engineer                  Dayton, Ohio
>
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