[Insight-users] question ? [ITK]

[Luis Ibanez]

Hi Agata,

..

> Is it any possibility to return matrix of TPS transform ?
>
>
Not quite.

The TPS is more complex than an Affine transform,
and cannot be fully expressed just with a nxn Matrix.

One could however, "approximate" it with an Affine transform
and in this way reduce it to a Matrix, if you are willing to accept
the loss of deformation representation that it will imply.

The TPS is representing the equivalent of taking a thin metallic bar
and bending it to force it to pass through several points in space.
That deformation can not be represented by an affine matrix only.


The same for example in rigid transformation (itkVersorRigid3DTransform)
> I could return matrix  4x4 rotation & translation
>
>
Yes, this is because a Rigid transform can be fully represented
by a Matrix. This is possible for the family of Affine Transforms
(of which Rigid 3D is a member)


But I don't know. What is it in itkThinPlateSplineKernelTransform ?
>
>
The TPS transform can not be fully represented by a matrix.

You could however get a gross approximation of a TPS using matrices,

You can find these pieces in

http://www.itk.org/Doxygen/html/classitk_1_1KernelTransform.html#a8819c174198218c4054511df7affeb94
http://www.itk.org/Doxygen/html/classitk_1_1KernelTransform.html#a7d4a5477c9df969edd0d91aae2cca05a


I am trying to save tps transform  with  :TransformFileWriter
> I can't receive good results.
> I receive only parameters sources and targets. Something like this :
> #Insight Transform File V1.0
> # Transform 0
> Transform: ThinPlateSplineKernelTransform_double_3_3
> Parameters: -81.29 -31.07 -770.58 -83.11 -21.26 -822.64 -93.45 -32.44
> -858.72 -68.08 -126.89 -813.07 -61.04 75.74 -808.36
> FixedParameters: 140.6 230.7 -30.5 140.2 231.7 -71.1 144.8 235.9 -116.1
> 45.8 220.2 -66.7 231.6 211.3 -66.1
>
>

Yes,
This is normal...

Since these are the key points of
information that fully define the Transform.

4x4 Matrices can only represent Affine transforms
(rotation, shearing, scaling and translation).

4x4 Matrices can not represent deformations such as
the ones managed by the family of KernelTransforms
and the BSplineDeformableTransforms.


  HTH


     Luis
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.itk.org/pipermail/insight-users/attachments/20120819/6b9c3429/attachment.htm>