[ITK] [ITK-users] CompositeTransform forward backward order

[Matt McCormick]

Hi Emma,

When adding transforms to a CompositeTransform, the order matters--
transforms are added to a queue and applied sequentially.  A point is
transformed with the transform in the order given.

The inverse therefore needs to go in the inverse order.  Note that
CompositeTransform has a ::GetInverse implemented, and it makes sure
to also reverse the order [1].

Drawing a picture of the two images and the two transforms can help to
conceptually clarify the process.

Hope this helps,
Matt

[1] http://itk.org/gitweb?p=ITK.git;a=blob;f=Modules/Core/Transform/include/itkCompositeTransform.hxx;h=b390d2706fc50eab140de091dc9b90ca640c61cb;hb=HEAD#l623

On Wed, Apr 2, 2014 at 3:57 PM, Emma Saunders <emmasaunders123 at gmail.com> wrote:
> Hi
>
> could someone please clarify the order when considering composite
> transforms.
>
> When warping the moved image back to the fixed this order is required:
>
>   CompositeTransform->AddTransform(affine);
>   CompositeTransform->AddTransform(DisplacementFieldTransform);
>
> When propogating the fixed to the moving using the inverse affine this order
> is required:
>
>   CompositeTransform->AddTransform(DisplacementFieldTransform);
>   CompositeTransform->AddTransform(Inverseaffine);
>
> Could someone clarify the process involved here or perhaps point me to some
> documentation that does?
>
> Many thanks
>
> Emma
>
>
>
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