[Insight-developers] "itkSyNImageRegistrationMethod" cannot return its metric's transform properly

[Nicholas Tustison]

Okay, I figured it was just something I wasn't getting (particularly since
I was attempting an answer late last night).  I thought more about this
and I think I might have an idea of what the issue might be.  

The SyNImageRegistrationMethod is used to optimize an itk
DisplacementFieldTransform since SyN doesn't fit in the generic 
optimization framework.  The way optimization works is that we
initially create two additional displacement field transforms (in 
addition to m_OutputTransform) called something like

fixedToMiddleTransform
movingToMiddleTransform

The first describes the forward and inverse transform from the fixed
image to the middle space.  The second describes the forward and
inverse transform from the moving image to the middle space.  During
the optimization, we strictly optimize just these two transforms.  
m_OutputTransform only gets updated at the end of optimization by 
composing the correct forward and inverse fields of the 
fixed/movingToMiddleTransform(s).  We could potentially do this 
composition at each iteration (i.e. update m_OutputTransform) but 
that would be unnecessary computation (at least in terms of just getting
the final solution).

Nick


On Dec 2, 2012, at 11:34 AM, "Johnson, Hans J" <hans-johnson at uiowa.edu> wrote:

> Ali,
> 
> You original e-mail is vague in its description of how you deterred that the displacement field has all zero's at every iteration.
> -- Where you using a debugger?
> -- How many zero values were listed?  
> 
> Why do you need the const_cast in this code?  Could that change the behavior?  
> 
> The use of const_cast is an indication that you are tyring to do something that is not supported.  Either ITK is deficient in not providing a const version of that member function, or you are trying to do something that has been explicitly prohibited.
> 
> Due to the ambiguity in your original posting, it is hard to figure out what your problem is.  You know that the the overall program produces a good result at the end, but you are trying to interrogate the process at every iteration, and for that you need to get the moving transform at each iteration.
> 
> ***  I think that the problem you are having is exposing the current transform at each iteration to the observer.  ***
> 
> The response from Nick does not seem to address your real problem.
> 
> Please make a more thorough and very clearly stated problem, and then post enough information so that an external developer can replicate your error on their machine QUICKLY and EASILY. 
> 
> Keep these facts in mind when requesting help from the ITK developer community:
> 1) We are almost all volunteers.  We help out of kindness not financial gain.
> 2)  If we can not replicate your error on my computer, we can not fix it.
> 3) Our time is valuable, if we need to guess, work, hard, or decipher code to determine how to replicate your problem, we will not have the time or energy even to get started
> 4) Provide both a) A description of the problem, b) description of how to replicate the problem, *** c) a description of how you believe the code should behave ***, and d) a proposed solution
> 
> 
> Hans
> 
> PS: Nick Thanks for trying to address this request.  The original request was not very clear, and your effort showed that the end result is good (that was never in doubt).  The problem is interrogating for the current transform at every iteration.
> 
> 
> From: Nick Tustison <ntustison at gmail.com>
> Date: Saturday, December 1, 2012 8:24 PM
> To: Ali Ghayoor <ali-ghayoor at uiowa.edu>
> Cc: ITK <insight-developers at itk.org>
> Subject: Re: [Insight-developers] "itkSyNImageRegistrationMethod" cannot return its metric's transform properly
> 
> Hi Ali,
> 
> I don't think that there's a deficiency.  I just ran the itkSyNImageRegistrationTest
> and added the following snippet to write the displacement field to disk starting
> at line 275.
> 
>   typedef itk::ImageFileWriter<DisplacementFieldType> WriterType2;
>   typename WriterType2::Pointer writer2 = WriterType2::New();
>   writer2->SetFileName( "/Users/ntustison/Desktop/testDisplacementField.nii.gz" );
>   writer2->SetInput( outputTransform->GetDisplacementField() );
>   writer2->Update();
> 
> The result was a non-zero displacement field that looked reasonable based
> on the parameters specified by the test.
> 
> Nick
> 
> 
> On Dec 1, 2012, at 8:30 PM, "Ghayoor, Ali" <ali-ghayoor at uiowa.edu> wrote:
> 
>> Hello Developers,
>> 
>> In a registration framework, at each iteration, I get the current moving transform from the metric of the "itkSyNImageRegistrationMethod" filter; however, the returned moving transform has a displacement field with all zero values at every iteration. 
>> 
>> To inspect this deficiency in ITK, I used "itkSyNImageRegistrationTest.cxx", and made some modifications in the test code for this purpose.
>> I added an iteration observer to the SyN registration stage:
>> 
>>     typedef CommandIterationUpdate<DisplacementFieldRegistrationType> DisplacementFieldCommandType; 
>>     typename DisplacementFieldCommandType::Pointer DisplacementFieldObserver = DisplacementFieldCommandType::New(); 
>>     displacementFieldRegistration->AddObserver( itk::IterationEvent(), DisplacementFieldObserver ); 
>> 
>> Then, inside the observer class, I asked for the parameters of the moving transform at the current iteration:
>> 
>>    std::cout << " parameters: " << const_cast<TFilter *>( filter )->GetMetric()->GetMovingTransform()->GetParameters() << std::endl; 
>>    std::cout << " fixed parameters " << const_cast<TFilter *>( filter )->GetMetric()->GetMovingTransform()->GetFixedParameters() << std::endl;
>> 
>> However, the Parameters of the moving transform is an all zero valued displacement field at each iteration.
>> Do you know why the registration filter does not return the right moving transform? Could you please help about this problem.
>> 
>> Thanks,
>> Ali
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> 
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