` that performs optimized computations.
Current tests have been expanded to exercise the added features.
- Performance test:
Using [this image](https://github.com/mkrcah/bone-segmentation/blob/master/sample-volumes/001-CT.nii), and [this example](https://github.com/InsightSoftwareConsortium/ITKBoneEnhancement/tree/eab6df404c5b5c25464ded41d1ab9002e21b6e0d/example) --modifying the SymmetricEigenAnalysis there accordingly–
with the command
```bash
for i in {1..10}; do ./BoneEnhancementExamples ~/Data/BoneEnhancement/001-CT.nii ~/Data/BoneEnhancement/out_preprocessed.nrrd ~/Data/BoneEnhancement/out_measure.nrrd 0 0 3 2 2 2; done
```
Results:
```auto
Eigen FixedDimension: 40.19
Old -const netlib-: 48.12
Eigen Dynamic: 57.10
```
Which is around `16%` faster when using the fixed dimension.
- Classes that have been modified to use the faster SymmetricEigenAnalysisFixedDimension:
```auto
Modules/Core/Common/include/itkSymmetricSecondRankTensor.h
Modules/Filtering/ImageFeature/include/itkHessian3DToVesselnessMeasureImageFilter.h
Modules/Filtering/ImageFeature/include/itkHessianToObjectnessMeasureImageFilter.hxx
Modules/Registration/Common/include/itkLandmarkBasedTransformInitializer.hxx
```
Beyond the shown usage for computing eigenvalues, Eigen3 would allow ITK to reach and bridge with more existing algorithms.
---
### Author:  [@matt.mccormick](https://discourse.itk.org/u/matt.mccormick)
#### Post date: [November 20, 2018, 4:59pm UTC](https://discourse.itk.org/t/add-internal-third-party-module-eigen3/1429/2 "2018-11-20T16:59:54Z")
@phcerdan congratulations on this monumental contribution! 🏗
This will enable us to gradually transition to Eigen from VNL.
---
### Author:  [@spinicist](https://discourse.itk.org/u/spinicist)
#### Post date: [December 6, 2018, 11:17am UTC](https://discourse.itk.org/t/add-internal-third-party-module-eigen3/1429/3 "2018-12-06T11:17:06Z")
Fantastic news! I just spotted this when updating my master branch.
Is there any plan for transitioning to Eigen throughout ITK? I know this has been discussed on-and-off for a long time. For my own code, it will be a big help if the underlying type of `itk::VectorImage` is an `Eigen::ArrayXd` instead of a `vnl_vector`, but I am aware that’s a pretty fundamental change.
If I can find time to help with this transition, I will gladly do so.
---
### Author:  [@dzenanz](https://discourse.itk.org/u/dzenanz)
#### Post date: [December 6, 2018, 6:08pm UTC](https://discourse.itk.org/t/add-internal-third-party-module-eigen3/1429/4 "2018-12-06T18:08:25Z")
There is a 2-step plan:
1. Encapsulate `vnl_vector` and friends behind ITK type aliases such as `NumericVector` or `InternalVector` or something similar. Make VNL an internal dependency not visible to users of ITK.
2. Swap out Eigen in place of VNL. Possibly gradually, as time and resources allow.
---
### Author:  [@phcerdan](https://discourse.itk.org/u/phcerdan)
#### Post date: [December 6, 2018, 7:40pm UTC](https://discourse.itk.org/t/add-internal-third-party-module-eigen3/1429/5 "2018-12-06T19:40:26Z")
I think it won’t be that hard to change the internal data, however it will span the whole library. But with the awesome test coverage of ITK, it shoudn’t be a problem.
Related with @dzenanz suggestion, the current name to access the vnl data is GetVnlVector, GetVnlMatrix, etc… which is unfortunate for this kind of internal refactoring.
Maybe 5.0 is a good opportunity to deprecate `GetVnlVector`, in favor of an equivalent new method: `GetInternalVector` or similar, so changes will be less painful for third parties in the future.
---
### Author:  [@matt.mccormick](https://discourse.itk.org/u/matt.mccormick)
#### Post date: [December 7, 2018, 7:03pm UTC](https://discourse.itk.org/t/add-internal-third-party-module-eigen3/1429/6 "2018-12-07T19:03:37Z")
As we come across small, bite-size tasks to facilitate the migration from VNL to Eigen, I think it will be helpful to create issues in GitHub. After we have a sufficient number of issues, we could collect them in a GitHub Project. 🐙 This should help facilitate forward progress! 🦉
---
### Author:  [@spinicist](https://discourse.itk.org/u/spinicist)
#### Post date: [December 10, 2018, 2:12pm UTC](https://discourse.itk.org/t/add-internal-third-party-module-eigen3/1429/7 "2018-12-10T14:12:52Z")
Great idea. I’ve started “Watching” the github repo - but I may stop that as on something the size of ITK there tend to be a _lot_ of notifications. Having this in a project I can keep an eye on would be fantastic.
---
### Author:  [@matt.mccormick](https://discourse.itk.org/u/matt.mccormick)
#### Post date: [December 10, 2018, 3:07pm UTC](https://discourse.itk.org/t/add-internal-third-party-module-eigen3/1429/8 "2018-12-10T15:07:17Z")
After we have enough issues collected, perhaps we should hold a hackathon to begin tackling them. 🏈