[ITK-users] Extracting image coordinate transform matrix

[Yaniv, Ziv Rafael (NIH/NLM/LHC) [C]]

Hello Ramana,

Curious what your use case is, as I can’t think of the reason to do the computation the way you want to.
In any case, the code below does what you want:

image_direction = image.GetDirection()
image_spacing = image.GetSpacing()
image_origin = image.GetOrigin()

dim = 3
matrix = [m*s for m,s in zip(image_direction, image_spacing*dim)]

index_2_physical = sitk.AffineTransform(dim)
index_2_physical.SetMatrix(matrix)
index_2_physical.SetTranslation(image_origin)

physical_2_continous_index = index_2_physical.GetInverse()


If you need the indexes as integers you will need to transform the point, round the result (np.rint) and then astype(int):
np.rint(physical_2_continous_index.TransformPoint([-82.76, 33.32, 101.5])).astype(int)

        hope this helps
                  Ziv
 

On 8/1/17, 8:22 AM, "Ramana C" <rc1515 at ic.ac.uk> wrote:

    Hi All,
    
    I am using SimpleITK in Python (but help for any ITK/SimpleITK version would
    be appreciated too!). 
    
    I am looking for a way to extract (or construct) the 4 by 4 transformation
    matrix from an image that lets me transform physical space coordinates to
    voxel coordinates of the image grid, and vice-versa.
    
    I know that the image class has methods such as
    image.TransformIndexToPhysicalPoint, but I was hoping that it would be
    possible to extract the transformation matrix itself.
    
    Any help with this would be greatly appreciated.
    
    -Ramana
    
    
    
    
    
    
    
    
    --
    View this message in context: http://itk-users.7.n7.nabble.com/Extracting-image-coordinate-transform-matrix-tp38537.html
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