DOC: adjusted to fit more general case n != m

[romangrothausmann]

To my understanding eigenvalues and PCA are the special case for a m×n matrix with n = m, i.e. a square matrix. However, the first order derivative (Jacobian) can be a non-square matrix, i.e. m ≠ n, in case the image dimension does not equal the number of vector components (channels).
The PR is meant as a discussion initiation, as I am not an expert in this field and stumbled upon this while trying to understand the math behind this filter.

[romangrothausmann]

Also, the output is a scalar image no matter how many channels the input has (which is the reason for the filter and the "magnitude" in its name), see e.g. the example WatershedSegmentation1.cxx

[romangrothausmann]

How to see in the ci/circleci page ("Details" links) what caused the failure? I only see that ctest -V -S "${DASHBOARD_BRANCH_DIRECTORY}/circleci.cmake" is marked red but it is not clear to me from the logs what is the actual error/problem.

[thewtex]

@romangrothausmann thanks for contributing this fix!

We have now officially migrated to GitHub!

Could you please rebase this patch with something like:

cd ITK
git checkout master
git pull origin master
git checkout VectorGM-doc
git rebase master
./Utilities/SetupForDevelopment.sh
# Follow prompt
git review-push --force

?

[dzenanz]

Thanks for the contribution. As this is a purely documentation change, there should be no test CI failures. As Matt said, rebasing on current master and re-pushing will re-trigger all the checks, which should now all be OK.

[thewtex]

Thanks for the branch update @romangrothausmann , and the documentation correction 👍 Looks good.

The Windows failure is a false positive.

[romangrothausmann]

Many thanks @thewtex and @dzenanz for taking a look and approving, and also for making contribution to ITK via GH possible. I find it much easier this way, especially the barrier to propose a contribute is much lower with GH PRs and the GH front-end allows many more ways to investigating the project and its code.

[dzenanz]

Ease of contribution was a major point of the move to GitHub!