[vtkusers] using itk transform in vtk

[Karl]

Hi,
I am trying to use an itk transform in vtk.  Here is a high level overview
of what I do:

1) In itk use itk::ImageRegistrationMethod with itk::AffineTransform to find
a transform.

2) I use itk::ResampleImageFilter to create an image transformed using the
itk::AffineTransform.

3) I use the GetLastTransformParameters() function to get the affine
transformation matrix to use in vtk as.
[param0 param1 param2 param9]
[param3 param4 param5 param10]
[param6 param7 param8 param11]
[0 0 0 1]

4) Now in vtk I try to display both the resampled image and the original
image with the transform applied.  They should show as exactly the same if I
can apply the transform in vtk the same as it is applied in itk.

5) I load both images in vtk.  The images are binary so I use marching cubes
to create meshes of them.  I display the transformed image polydata as is.

6) I apply a vtkTransform to the original image polydata.  I do this by
concatenating 3 vtkMatrix4x4. Two to adjust for different origins in itk and
vtk and one for the itk transform.
Translate_Back * itkTransform * Translate_itk_origin_to_vtk_origin

Where
itkTransform is as shown in 3 above.

Translate_itk_origin_to_vtk_origin is
[1 0 0 -originX]
[0 1 0 -originY]
[0 0 1 -originZ]
[0 0 0 1]
Where the origin is as found in the dicom header for the original image

Translate_Back
[1 0 0 originX]
[0 1 0 originY]
[0 0 1 originZ]
[0 0 0 1]

7) Set vtkTransform inverse flag and transform and display the original
image.

8) The two objects show up at different places in the scene instead of at
the same location.

If I understand this correctly the itk transform is to be applied about the
physical origin.  Not the location of the corner pixel or the center of the
image.

The vtk transform uses an origin at the corner of the image.

By translating by the origin defined in the dicom header I move the physical
origin to the corner pixel of the image.  This corrects for the
discrepancies between the point with itk and vtk apply their transforms.

Also the itk transform goes from output to input, so the inverse transform
needs to be applied in vtk.

Can anyone see what I am missing?
Thanks