[Insight-users] VersorRigid3dTransform

[Karthik Krishnan]

Hi Martin,

As you've pointed out, the documentation of MatrixOffsetTransformBase says:
"    * WARNING: When using the Center, we strongly recommend only 
changing the
   * matrix and translation to define a transform.   Changing a transform's
   * center, changes the mapping between spaces - specifically, 
translation is
   * not changed with respect to that new center, and so the offset is 
updated
   * to * maintain the consistency with translation.   If a center is 
not used,
   * or is set before the matrix and the offset, then it is safe to 
change the
   * offset directly. "

The GetInverse method is one of those methods that changes the matrix 
and the offset, since it has to compute the inverse matrix.

The correct way to invert a transformation and retain the option of 
defining your own center is to do it in the following order:

transform2->SetCenter( transform1->GetCenter() );
transform1->GetInverse( transform2 );

transform2 will now contain the inverse of transform1 and will contain 
transform1's center. Flipping the two statements will produce an 
incorrect transform.
 
I'll put this stuff in the doxygen.

Thanks
regards
karthik

Martin Urschler wrote:

> Hi Karthik,
>
> Karthik Krishnan wrote:
>
>>
>> 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 ...... ? ?
>
>
>
> Coming back to this issue we discussed a while ago. If I use the 
> GetInverse method of a class derived from MatrixOffsetTransformBase 
> then the inverse transform is returned but the center of rotation is 
> set to 0 in the returned transformation.
>
> I.e. in
>
> Rigid3DTransformType::Pointer initialTransform =
>    someTransformFromARegistration();
>
> Rigid3DTransformType::Pointer inverseInitialTransform =
>    Rigid3DTransformType::New();
> initialTransform->GetInverse( inverseInitialTransform );
>
>
> the center of rotation of inverseInitialTransform is [0,0,0] no matter 
> what the center in initialTransform is
>
> This is consistent with the documentation which states that the center 
> of rotation of the inverse is centered at the origin.
>
>
> My question:
> Is it now sufficient to simply apply
> inverseInitialTransform->SetCenter( initialTransform->GetCenter() );
>
> to update the center? Or do I have to call another member function 
> that updates the internal matrix and offset accordingly?
>
> regards,
> Martin
>