Single transform from composition of linear transform ?

Beginner Questions
1

Dear All,
I have some linear transforms that are packed into a composite transform. Is there a way of calculating a single affine transform that would be the result of the composition ? I haven’t found any function to do this…
Thanks for your help,
Yann

2

With linear transforms, you could go through all of them using composite->GetNthTransform, get their 4x4 matrices, and multiply them together. If you started with an identity 4x4, you will end up with you composite transform in a single 4x4 matrix. Some typecasting to affine transform might be involved.

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3

There is no function to do this directly in ITK, but a while ago I wrote a tool to handle some simple operations with ITK transforms (available here). You can compile it and call compose if you have only 2 affine transforms to combine, or call concatenate for N transforms (this option also handles non-rigid transforms and generates a displacement field (or deformation field?). You can read the code to see the different options or run the program without any option.

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4

Thanks both ! I explored a bit further, and found the a compose() function in itk::MatrixOffsetTransformBase.

So I wrote this simple code that I share in case other people need it:

#include "itkAffineTransform.h"
#include "itkCompositeTransform.h"

// ... //

//compoTform

typedef itk::AffineTransform< Type, Dimension > AffineType;

AffineType::Pointer tformResult = AffineType::New();
tformTest->SetIdentity();
size_t nTform = compoTform->GetNumberOfTransforms();
for (size_t i = nTform; i-- > 0 ; ) {
    tformResult->Compose( static_cast< const AffineType * >( compoTform->GetNthTransformConstPointer(i) ) );
}
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