[ITK] [ITK-users] Compose Displacement Field Order using Syn regsitration

[Matt McCormick]

Hi Emma,

Yes, inverse affine lm_separate followed by the forward displacement
fields should be the order to compose the new transform.

A CompositeTransform is probably best for this job.

As we can see from the ComposeDisplacementFieldsImageFilter source
[1], the Warping displacement field follows the input
DisplacementField in the output.

Note that a more general solution for creating a summary displacement
field is the TransformToDisplacementFieldFilter [2], which could use
CompositeTransform's that have AffineTransform's.

Hope this helps,
Matt

[1] http://itk.org/gitweb?p=ITK.git;a=blob;f=Modules/Filtering/DisplacementField/include/itkComposeDisplacementFieldsImageFilter.hxx;h=e06b71729e53a057a7504c5dc4d6d388355958b2;hb=HEAD#l118

[2] http://www.itk.org/Doxygen/html/classitk_1_1TransformToDisplacementFieldFilter.html

On Mon, Apr 28, 2014 at 11:51 AM, Emma Saunders
<emmasaunders123 at gmail.com> wrote:
> Hi everyone
>
> I have a moving image sequence with a reference  or fixed image called
> Im_fixed.
>
>  I have used Syn registration on multiple moving images and therefore have a
> sequence of forward (takes fixed to moving) and backward (takes moving to
> fixed) displacement fields.
>
> Assuming I perform an affine registration of Im_fixed to a separate image
> called Im_seperate, not contained within the sequence, how can I then
> proceed to put the Syn registrations into the reference frame of Im_seperate
> such that the forward and backward fields move Im_seperate with the same
> motion as the moving image sequence
>
> I am  using  the Compose Displacement Field Image filter but not clear on
> the order of composition.
>
> What is the warping field and how does this differ from the displacement
> field? Would using the inverse of the affine with the forward displacement
> fields give me fields that move Im_separate in the manner desired?  If so
> which order do I compose them in?
>
> Thanks
>
> Emma
>
>   typedef itk::ComposeDisplacementFieldsImageFilter<DisplacementFieldType,
>    DisplacementFieldType> ComposerType;
>    ComposerType::Pointer composer = ComposerType::New();
>
>   composer->SetDisplacementField( inputfield1);
>   composer->SetWarpingField( inputfield2);
>   composer->Update();
>   combinedfield=composer->GetOutput();
>
>
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