[Insight-users] Finding the inverse of a Transformation

[Luis Ibanez]

Hi Neilson,


The problem is that the "Inverse()" method is not defined in
the CenteredRigid2DTransform.

Could you please log a bug repport on the bug tracker ?
The link is:

                http://www.itk.org/Bug/

You just need to create an account using your email address.


-------


In the meantime, what you can do is to pass the rotation matrix
and offset from this transform into an itkAffineTransform, and
then ask the affinne transform to compute the inverse.  The resulting 
transform should still be a rigid transform (but stored in the form
of an affine transform).



The code will be something like



   typedef itk::AffineTransform< 2, double >  AffineType;

   AffineType::Pointer affine = AffineType::New();

   finalTransform->SetParameters( parameters );

   affine->SetMatrix( finalTransform->GetRotationMatrix() );
   affine->SetOffset( finalTransform->GetOffset() );

   AffineType::Pointer invTransform = affine->Inverse();

   AffineType::MatrixType matrix = invTransform->GetMatrix();
   AffineType::OffsetType offset = invTransform->GetOffset();




Regards,


    Luis


-----------------------------
N.E. Mackenzie Mackay wrote:

> Hi,
> 
>     I have been able to register two images together successfully.  
> Right now I am trying to find the inverse of the Transformation between 
> the two but having problems.  I am using CenteredRigid2DTansform as my 
> transformation.
> 
>     Two questions:
> 
>         First, how do I get the inverse of the transform.  I tried this:
> 
>             TransformType::Pointer finalTransform = TransformType::New();
>             finalTransform->SetParameters ( finalParameters );
> 
>             TransformType::Pointer finalTrasnformInvers = 
> TransformType::New();
>             finalTransformInverse = finalTransform->Inverse();
> 
>         When I compile this code I get a compile error :
>             binary '=' operator defined which takes a right-hand operand 
> of type 'class itk::SmartPointer<class itk::Rigid3DTransform<double> >' 
> (or there is no acceptable conversion)
> 
>         I tried changing my code to:
> 
>             TransformType::Pointer finalTransform = TransformType::New();
>             finalTransform->SetParameters ( finalParameters );
>        
>             finalTransform->Inverse();
> 
>             TransformType::Pointer finalTrasnformInvers = 
> TransformType::New();
>             finalTransformInverse = finalTransform;
> 
>         This compiled but didn't give the actual inverse.  I gave the 
> original one.
> 
>     My second question is (and probably answered before) how do I output 
> the transform as a 4x4 matrix.  My goal is to eventually use it in vtk.
> 
> Thanks for all the help,
> Neilson
> 
>     
> 
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