[Insight-users] VersorRigid3dTransform

[Karthik Krishnan]

Hi Martin,

Yes the transforms are indeed different.

transform->GetParameters() does not tell you the whole story though. It 
prints out the translation components and the versor, but not the 
center. You can try transform->Print(std::cout ) to see the center. In 
fact, if you place the following line

  std::cout << transform->TransformPoint( point ) << std::endl;

before and after you set the center, you will see that the transformed 
point is indeed different.

It does bring up one minor point concerning transforms like the 
VersorRigidTransform and other transforms that contain the center (they 
derive from the MatrixOffSetTransformBase). If you were to copy the 
transform parameters over into another transform, you will also need to 
explicitly copy the center otherwise your new transform will be 
incorrect.  ie....

  TransformType::Pointer finalTransform = TransformType::New();
  copiedTransform->SetCenter( originalTransform->GetCenter() );
  copiedTransform->SetParameters( originalTransform->GetParameters() );

Maybe this info should be put in the Doxygen ...... ? ?

Thanks
Regards
Karthik
 

Martin Urschler wrote:

> hi all,
>
> Can anyone explain to me why the following code does what it does?
>
>   typedef itk::VersorRigid3DTransform< double > TransformType;
>   TransformType::Pointer transform = TransformType::New();
>   TransformType::OutputVectorType translation;
>   translation[0] = 16.0;
>   translation[1] = -12.0;
>   translation[2] = 11.5;
>   transform->SetTranslation( translation );
>
>   typedef TransformType::VersorType  VersorType;
>   typedef VersorType::VectorType     VectorType;
>
>   VersorType     rotation;
>   VectorType     axis;
>
>   axis[0] = 0.0;
>   axis[1] = 0.0;
>   axis[2] = 1.0;
>
>   const double angle = 3.14 / 4.0;
>   rotation.Set(  axis, angle  );
>   transform->SetRotation( rotation );
>
>   std::cout << transform << std::endl;
>   std::cout << transform->GetParameters() << std::endl;
>
>   TransformType::InputPointType center;
>   center[0] = 64 * fixedImage->GetSpacing()[0];
>   center[1] = 64 * fixedImage->GetSpacing()[1];
>   center[2] = 64 * fixedImage->GetSpacing()[2];
>
>   transform->SetCenter( center );
>
>   std::cout << transform << std::endl;
>   std::cout << transform->GetParameters() << std::endl;
>
>
> In both cases I get the same values for the transform->GetParameters 
> call. Despite the fact that two different rotation centers are used. 
> I'm not sure but I think a different rotation center should have an 
> effect on the transform parameters, or did I misunderstand something? 
> It should provide us with a different point to start in the 
> optimization of the registration.
>
> I read the chapters of the software guide that deal with these 
> things... I guess I lack some understanding...
>
> thanks,
> Martin
>
>
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