# Understanding CompositeTransform discrepancies

**URL:** https://discourse.itk.org/t/understanding-compositetransform-discrepancies/4474
**Category:** Beginner Questions
**Tags:** python, simpleitk
**Created:** [October 5, 2021, 1:58pm UTC](https://discourse.itk.org/t/understanding-compositetransform-discrepancies/4474 "2021-10-05T13:58:04Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![Theoretialperson](https://discourse.itk.org/letter_avatar_proxy/v4/letter/t/c5a1d2/32.png) [@Theoretialperson](https://discourse.itk.org/u/Theoretialperson)
#### Post date: [October 5, 2021, 1:58pm UTC](https://discourse.itk.org/t/understanding-compositetransform-discrepancies/4474/1 "2021-10-05T13:58:04Z")

</div>

I’m getting different output from the two code blocks below. The only difference is that in the second I wrap the rotation with a CompositeTransform. I would expect that this transform would just apply the rotation, but there is something weird going on that I don’t understand. Am I missing something?

 ![Screenshot 2021-10-05 at 15.51.56](https://discourse.itk.org/uploads/default/original/2X/e/e396812719d75ab4de0784a8247adece74c4aece.jpeg)  
 ![Screenshot 2021-10-05 at 15.52.06](https://discourse.itk.org/uploads/default/original/2X/5/530f0c68c2106ff37997d6e7f37e150c6b04c57e.jpeg)

```python
import SimpleITK as sitk
import matplotlib.pyplot as plt
import numpy as np

def myshow(img, title=None, margin=0.05, dpi=80):
    nda = sitk.GetArrayViewFromImage(img)[:,:,62]
    spacing = img.GetSpacing()
        
    ysize = nda.shape[0]
    xsize = nda.shape[1]
      
    figsize = ((1 + margin) * ysize / dpi, (1 + margin) * xsize / dpi)

    fig = plt.figure(title, figsize=figsize, dpi=dpi)
    ax = fig.add_axes([margin, margin, 1 - 2*margin, 1 - 2*margin])
    
    extent = (0, xsize*spacing[1], 0, ysize*spacing[0])

    t = ax.imshow(nda,
            extent=extent,
            interpolation='none',
            cmap='gray',
            origin='lower',
            vmin=0.0, vmax=1.0)
    
    if(title):
        plt.title(title)

    plt.show()

if __name__ == ' __main__':
    scale = 1.0
    spacing = 0.2
    image_size = np.array([128, 250, 250])
    patch_size = np.array([64, 125, 125])
    sampling = sitk.sitkBSplineResamplerOrder3

    grid_array = ((np.indices(image_size) // 4).sum(axis=0) % 2).astype(float)
    grid = sitk.GetImageFromArray(grid_array)
    grid.SetSpacing(np.ones(3) * spacing)
    grid.SetOrigin((30.9, 10.2, 28.4))

    patch_border = 30
    patch_size_augmented = patch_size + 2*patch_border
    patch_origin = ((0.5 * image_size - 0.5 * patch_size_augmented) * np.ones(3)).astype(int)
    patch = grid_array[
        patch_origin[0]:patch_origin[0]+patch_size_augmented[0],
        patch_origin[1]:patch_origin[1]+patch_size_augmented[1],
        patch_origin[2]:patch_origin[2]+patch_size_augmented[2]
    ]
    patch_image = sitk.GetImageFromArray(patch)

    rotation = sitk.VersorTransform(np.array([1.0, 0.0, 0.0]), 0.15, 0.5 * np.array(patch.shape)[::-1])

    resampled_patch_image = sitk.Resample(patch_image, rotation, sampling, 255)
    resampled2 = sitk.GetArrayViewFromImage(resampled_patch_image)
    resampled2 = resampled2[
        patch_border:patch_border+patch_size[0],
        patch_border:patch_border+patch_size[1],
        patch_border:patch_border+patch_size[2]
    ]
    resampled2 = sitk.GetImageFromArray(resampled2)

    myshow(resampled2, 'Resampled')

    transform = []
    transform.append(rotation)
    transform = sitk.CompositeTransform(transform)
    resampled_patch_image = sitk.Resample(patch_image, transform, sampling, 255)
    resampled2 = sitk.GetArrayViewFromImage(resampled_patch_image)
    resampled2 = resampled2[
        patch_border:patch_border+patch_size[0],
        patch_border:patch_border+patch_size[1],
        patch_border:patch_border+patch_size[2]
    ]
    resampled2 = sitk.GetImageFromArray(resampled2)

    myshow(resampled2, 'Resampled Composite')

```

---

<div class="post-metadata">

### Author: ![zivy](https://discourse.itk.org/user_avatar/discourse.itk.org/zivy/32/1726_2.png) [@zivy](https://discourse.itk.org/u/zivy)
#### Post date: [October 5, 2021, 6:25pm UTC](https://discourse.itk.org/t/understanding-compositetransform-discrepancies/4474/2 "2021-10-05T18:25:43Z")

</div>

Hello @Theoretialperson,

You have uncovered a bug. The rotation is not cloned correctly inside the composite transform, the `offset` portion of the transformation is not updated.

Temporary solution: Use `VersorRigid3DTransform` with zero translation.

Minimal working example showing bug and temporary workaround:

```auto
    import SimpleITK as sitk
    
    axis = [1.0,0.0,0.0]
    angle = 0.15
    fixed_center = [92.5, 92.5, 62]

    # rotation is not copied correctly inside the composite transform
    # offset parameter is not updated
    rotation = sitk.VersorTransform(axis, angle, fixed_center)
    print(rotation)
    print('*'*30)
    print(sitk.CompositeTransform([rotation]))

    print('*'*30)

    # workaround: use a versor based rigid transformation with zero translation
    rigid = sitk.VersorRigid3DTransform(axis, angle, [0,0,0], fixed_center)
    print(rigid)
    print(sitk.CompositeTransform([rigid]))

```

Edit: bug [reported here](https://github.com/SimpleITK/SimpleITK/issues/1467).