# BSplineTransform object has no attribute SetCoefficientImages

**URL:** <https://discourse.itk.org/t/bsplinetransform-object-has-no-attribute-setcoefficientimages/4136>\
**Category:** Algorithms\
**Tags:** simpleitk, registration, python\
**Created:** [May 18, 2021, 12:55pm UTC](https://discourse.itk.org/t/bsplinetransform-object-has-no-attribute-setcoefficientimages/4136 "2021-05-18T12:55:17Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![ctorti](https://discourse.itk.org/user_avatar/discourse.itk.org/ctorti/32/1782_2.png) [@ctorti](https://discourse.itk.org/u/ctorti)\
**Post date:** [May 18, 2021, 12:55pm UTC](https://discourse.itk.org/t/bsplinetransform-object-has-no-attribute-setcoefficientimages/4136/1 "2021-05-18T12:55:17Z")

</div>

I used SimpleITK v1.2 to perform a BSpline image registation. I used `BSplineTransformInitializer` to initialize the registration:

```
MeshSize = [NumControlPts] * MovIm.GetDimension()

InitialTx = sitk.BSplineTransformInitializer(FixIm, MeshSize)

```

I set the metric, optimizer, interpolator, etc. then get the final transform:

`FinalTx = RegMethod.Execute(FixIm, MovIm)`

The result looks fine. I want to store the transform parameters and deformation field in a [DICOM registration object (DRO)](http://dicom.nema.org/medical/dicom/current/output/chtml/part03/sect_C.20.3.html). The idea being that the DRO will be stored and when the same pair of `FixIm` and `MovIm` come about, I can avoid repeating the time-consuming deformable registration by parsing the DRO for the required parameters, create a BSpline transformation, apply the parsed parameters, and resample `MovIm`.

I have access to the grid size, spacing, origin and direction already but need the coefficient images. It seemed reasonable to get it from the final transform:

`CoeffImX, CoeffImY, CoeffImZ = FinalTx.GetCoefficientImages()`

but I got the error:

> ‘Transform’ object has no attribute ‘GetCoefficientImages’

I tried using the initial transform instead:

`CoeffImX, CoeffImY, CoeffImZ = InitialTx.GetCoefficientImages()`

which worked. I was puzzled by this result since both `InitialTx` and `FinalTx` are `BSplineTransform`s:

`print(InitialTx)`

> itk::simple::BSplineTransform  
> BSplineTransform (00000267BA4D7330)  
> RTTI typeinfo: class itk::BSplineTransform\<double,3,3\>  
> Reference Count: 3  
> Modified Time: 19499183  
> Debug: Off  
> Object Name:  
> Observers:  
> none  
> CoefficientImage: [00000267B2E2FEA0, 00000267B2E2F3A0, 00000267B8BA8430]  
> TransformDomainOrigin: [-220.461, -137.269, -102.056]  
> TransformDomainPhysicalDimensions: [260.253, 260.253, 181.5]  
> TransformDomainDirection: 0.998543 -0.0277357 -0.046297  
> 0.0277059 0.999615 -0.00128451  
> 0.0463148 0 0.998927
> 
> GridSize: [11, 11, 11]  
> GridOrigin: [-250.992, -170.661, -126.226]  
> GridSpacing: [32.5316, 32.5316, 22.6875]  
> GridDirection: 0.998543 -0.0277357 -0.046297  
> 0.0277059 0.999615 -0.00128451  
> 0.0463148 0 0.998927

`print(FinalTx)`

> itk::simple::Transform  
> BSplineTransform (00000267BA4D7330)  
> RTTI typeinfo: class itk::BSplineTransform\<double,3,3\>  
> Reference Count: 3  
> Modified Time: 19499183  
> Debug: Off  
> Object Name:  
> Observers:  
> none  
> CoefficientImage: [00000267B2E2FEA0, 00000267B2E2F3A0, 00000267B8BA8430]  
> TransformDomainOrigin: [-220.461, -137.269, -102.056]  
> TransformDomainPhysicalDimensions: [260.253, 260.253, 181.5]  
> TransformDomainDirection: 0.998543 -0.0277357 -0.046297  
> 0.0277059 0.999615 -0.00128451  
> 0.0463148 0 0.998927
> 
> GridSize: [11, 11, 11]  
> GridOrigin: [-250.992, -170.661, -126.226]  
> GridSpacing: [32.5316, 32.5316, 22.6875]  
> GridDirection: 0.998543 -0.0277357 -0.046297  
> 0.0277059 0.999615 -0.00128451  
> 0.0463148 0 0.998927

I acknowledge that the first lines are different, i.e. “itk::simple::BSplineTransform” for `InitialTx` and “itk::simple::Transform” for `FinalTx` - perhaps suggesting that they are of different classes. Yet “BSplineTransform” is on the next line for both. I’m not sure how to interpret these differences.

I’d like to know why I was unable to get the coefficient images from `FinalTx`, and whether it even makes sense to be getting it from the pre-registered transform `InitialTx`. In any case I thought I would proceed with what I have and see what it gets me.

I created a new BSpline transform, `NewBsplineTx`, using `BSplineTransformInitializer` as before. Then I tried to set the coefficient images:

`NewBsplineTx.SetCoefficientImages((CoeffsImX_new, CoeffsImY_new, CoeffsImZ_new))`

but got the error:

> ‘BSplineTransform’ object has no attribute ‘SetCoefficientImages’

After some searching I realised that `SetCoefficientImages` only [became possible in SimpleITK v2.0](https://discourse.itk.org/t/simpleitk-2-0-release/3482) (although that release note contains “SetCoefficientImage”, i.e. missing “s”, which I assume to be a typo).

I created a new virtual environment and installed SimpleITK v2.0.2 yet to my surprise, I get the exact same error as before (I also tried `SetCoefficientImage` just to be sure but got the same error).

Any ideas on where I’m going wrong?

Thanks.

---

<div class="post-metadata">

**Author:** ![blowekamp](https://discourse.itk.org/user_avatar/discourse.itk.org/blowekamp/32/79_2.png) [@blowekamp](https://discourse.itk.org/u/blowekamp)\
**Post date:** [May 18, 2021, 1:19pm UTC](https://discourse.itk.org/t/bsplinetransform-object-has-no-attribute-setcoefficientimages/4136/2 "2021-05-18T13:19:17Z")

</div>

> [@ctorti](#):
>
> `print(FinalTx)`
> 
> > itk::simple::Transform

Check the results of `type(FinalTx)`. The return type is `sitk::Transform` it needs to be converted to `sitk::BSplineTransform`:

```auto
FinalTx = sitk.BSplineTransform(FinalTx)

```

---

<div class="post-metadata">

**Author:** ![ctorti](https://discourse.itk.org/user_avatar/discourse.itk.org/ctorti/32/1782_2.png) [@ctorti](https://discourse.itk.org/u/ctorti)\
**Post date:** [May 18, 2021, 5:08pm UTC](https://discourse.itk.org/t/bsplinetransform-object-has-no-attribute-setcoefficientimages/4136/3 "2021-05-18T17:08:29Z")

</div>

@blowekamp : Thanks for the tip on converting from `sitk::Transform` to `sitk::BSplineTransform`.

Interestingly, there was no difference between the arrays from the coefficient images obtained from `InitialTx` and `FinalTx` (after converting `FinalTx` as above).

Anyhow, what still remains is the question of why I get the error:

> ‘BSplineTransform’ object has no attribute ‘SetCoefficientImages’

when I try to set the coefficient images that I got from `FinalTx` to a new BSpline transform:

```
NewBsplineTx = sitk.BSplineTransformInitializer(FixIm, MeshSize)

```

that I can confirm is of type `sitk::BSplineTransform`.

My take on [this](https://simpleitk.org/doxygen/latest/html/classitk_1_1simple_1_1BSplineTransform.html#ab834697fc42e7aea3a15a6d74d07603f) and [this](https://simpleitk.org/doxygen/latest/html/sitkBSplineTransform_8h_source.html) is that it should be possible.

I tried both:

`NewBsplineTx.SetCoefficientImages((CoeffImX, CoeffImY, CoeffImZ))`

and

`NewBsplineTx.m_pfSetCoefficientImages((CoeffImX, CoeffImY, CoeffImZ))`

(not understanding what the “m\_pf” is meant to mean).

Any clues on what I’m getting wrong?

---

<div class="post-metadata">

**Author:** ![blowekamp](https://discourse.itk.org/user_avatar/discourse.itk.org/blowekamp/32/79_2.png) [@blowekamp](https://discourse.itk.org/u/blowekamp)\
**Post date:** [May 18, 2021, 5:27pm UTC](https://discourse.itk.org/t/bsplinetransform-object-has-no-attribute-setcoefficientimages/4136/4 "2021-05-18T17:27:16Z")

</div>

> [@ctorti](#):
>
> Anyhow, what still remains is the question of why I get the error:
> 
> > ‘BSplineTransform’ object has no attribute ‘SetCoefficientImages’

This can be done with the constructor:

```auto
sitk.BSplineTransform([CoeffImX, CoeffImY, CoeffImZ])

```

---

<div class="post-metadata">

**Author:** ![ctorti](https://discourse.itk.org/user_avatar/discourse.itk.org/ctorti/32/1782_2.png) [@ctorti](https://discourse.itk.org/u/ctorti)\
**Post date:** [May 19, 2021, 7:13am UTC](https://discourse.itk.org/t/bsplinetransform-object-has-no-attribute-setcoefficientimages/4136/5 "2021-05-19T07:13:07Z")

</div>

@blowekamp: Thanks very much.

I was getting tripped up on trying `SetCoefficientImages`, meanwhile the answer was there all along - ironically at the same two places I linked above: [BSplineTransform](https://simpleitk.org/doxygen/latest/html/classitk_1_1simple_1_1BSplineTransform.html)( const std::vector &coefficientImages, unsigned int order=3 ) 🤦‍♂️