[Insight-users] Re: seg fault

[Luis Ibanez]

Hi Christos,

Thanks for letting us know about the details of
your changes.

Since you are replacing the transform, but are
using the same transform for the entire set of
pyramid levels, the problem should be that you
missed to initialize some of the arrays needed
for interacting with the transform.

For example the "scales" array that your are now
filling with 12 elements instead of the previous 7...
This is an itk::Array type, whose size is initialized
in two possible ways:

A) At construction time
B) At assignment time


I thinks you should look in to the other
"small alterations". It is likely that arrays
are still being initialized to a length 7 instead
of the 12 required by the affine transform.


  Regards,


    Luis



---------------------------
Christos Panagiotou wrote:

> 
> 
> Luis Ibanez wrote:
> 
>>
>> Hi Christos,
>>
>> Just to double check...
>>
>> What you want to do is to use the Multi-Resolution framework
>> and change the transfrom when you change resolution levels.
>> Is that correct ?
> 
> 
> 
> -----
> Dear Luis
> i am just trying to add an observer... to return the metric values
> so i can check how it converges so i can later on use this to
> optimize the learning rates and the steplength...
> nothing more or nothing less. What I ve done is i just added
> the CommandIteratationUpdate class as seen in the 
> /examples/registration/MultiResImageRegistration1.cxx
> and i am using it by adding:
> 
>  CommandIterationUpdate::Pointer observer = CommandIterationUpdate::New();
>  m_Optimizer->AddObserver( itk::IterationEvent(), m_Observer );
> 
> 
>>
>> -- 
>>
>> Here is a guess of what may be happening:
>>
>> The Optimizers communicate with the CostFunction (in this case
>> the ImageMetric) by passing an array of parameters. This array
>> is declared to be of type:   itk::Array< double >
>>
>> This is a dynamic array whose size is defined at run time.
>> When you run an optimizer with a particular cost function
>> the optimizer prepares a number of internal variables to
>> deal with the number of elements of this parameters array.
>> If you change the cost function at run-time, care has to
>> be taken to ensure that the optimizer resizes all the
>> variables that rely on the size of the parameters array.
>>
>> In the case of registration, the number of parameters of the
>> cost function is directly the array of Transform parameters.
>>
>> When you change the Transform at run time, you are probably
>> changing the number of parameters that the optimizer is
>> receiving. The segmentation faults that you observe may be
>> the consequence of not giving the optimizer a chance to
>> reinitialize the variables that deal with the size of the
>> parameters array.
>>
>> The changes in the settings for learning rates if probably
>> unrelated to your problem. It is unlikely that changing the
>> learning rate may lead to a seg. fault.
>>
>> ....
>>
>> A couple of questions:
>>
>> 1) What Transforms are you using at every level of the pyramid ?
> 
> 
> 
>    it is an affine transform...
> 
>>
>> 2) When you get the seg. fault. What are the transforms being
>>    used in the previous and next level of the pyramid ?
> 
> 
> 
>    affine all the way
> 
>>
>> 3) Where in the code are you replacing the Transform ?
>>    (file, line number, and please: ITK Version !)
> 
> 
> +---
>    I dont alter the transform i just initialize it as you have suggested 
> in previous emails
>    and i assign it to the registration method using:
> 
>    registration->SetTransform(     transform     );
> 
>    i dont interfere with the transform after that... i let the optimizer 
> do it (and it worked
>    until i added the observer!)
> 
>   i initaly alter the transform type from rigid body to affine in 
> MIMRegistrator.h
>   and modified the MIMRegistrator.txx to work with the affine (for example
>   i modified the loop that applies the translation scaling from rigid body:
> 
>  for ( int j = 4; j < 7; j++ )
>    {
>    scales[j] = m_TranslationScale;
>    }
> 
> to affine (the last three parameters)
> 
>  for ( int j = 9; j < 12; j++ )
>    {
>    scales[j] = m_TranslationScale;
>    }
> 
> and other small alterations
> 
> +---
>    my ITK version is 1.6.0
> +---
>    everything takes place in the MultiResMIRegistration application in 
> files:
>    MIMRegistrator.txx, .h and I have also added a .h file where i define 
> the commandIterationUpdate class which
>    i then include and use in the MIMRegistrator.h and use it in 
> MIMRegistrator.txx
>  
> 
>>
>> 4) How are you restarting the Optimizer once that you have
>>    replaced the transform ?
> 
> 
>    as i said i dont alter the transform during the optimization
> 
> 
> finally Two more questions:
> a.    I tried to replace the MI image to image metric with the 
> normalized histogram
>    (before i added the observer - when the first metric was giving me 
> results (not the best but it was working) )
>    when i used the normalized histogram MI metric i was still getting 
> the overlap exception..
>    i thought the normalized histogram MI metric is supposed to be 
> INVARIANT from selections in learning rates
>    etc. am i wrong?
> b.   is there an optimizer in ITK that uses line search? is it the 
> regular step optimizer? i haven't really read their implementations
> 
>    
> sorry for the long reading
> thanks very much for your fast reply
> christos
> 
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