[Insight-users] Question on BSplineDecompositionImageFilter

[Jochen Cammin]

Hi,

Upon responding to another question on this forum, I looked at examples
using the class BSplineDecompositionImageFilter. DeformableRegistration6
and DeformableRegistration15 both make use of this filter.

Here is my confusion: from the documentation of that class (and
logically) I understand that the input is an image and the output are
the b-spline coefficients. However, in the two examples a b-spline
coefficient image is the input which is then decomposed into b-spline
coefficients. There is a comment section in the example codes that says:

    //  Software Guide : BeginLatex
    //
    //  Now we need to initialize the BSpline coefficients of the higher
    //  resolution transform. This is done by first computing the actual
    //  deformation field at the higher resolution from the lower
    //  resolution BSpline coefficients. Then a BSpline decomposition is
    //  done to obtain the BSpline coefficient of the higher resolution
    //  transform.
    //
    //  Software Guide : EndLatex

 From this comment I would expect the input to the decomposition filter
to be the components of the deformation field. However, I can't see any
code that would calculate the deformation vectors. Further down the code
the input to the decomposition filter is then given as

...
upsampler->SetInput( transformLow->GetCoefficientImage()[k] );
...
decomposition->SetInput( upsampler->GetOutput() );
...

So clearly, the input are b-spline coefficients and not the deformation
field. What am I missing or misunderstanding?

Thank you,
Jochen