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RF: Simplify high-pass filtering in algorithms.confounds #3651

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Legendre and cosine detrending are implemented almost identically, although with several minor variations. Here I separate regressor creation from detrending to unify the implementations.

This now uses np.linalg.pinv(X) to estimate the betas in both cases, rather than using np.linalg.lstsq in the cosine filter. lstsq uses SVD and can thus fail to converge in rare cases. Under no circumstances should (X.T @ X) be singular, so the pseudoinverse is unique and precisely what we want.

Issue raised in https://neurostars.org/t/fmriprep-numpy-linalg-linalg-linalgerror-svd-did-not-converge/29525.

Legendre and cosine detrending are implemented almost identically,
although with several minor variations. Here I separate regressor
creation from detrending to unify the implementations.

This now uses `np.linalg.pinv(X)` to estimate the betas in both cases,
rather than using `np.linalg.lstsq` in the cosine filter. lstsq uses SVD
and can thus fail to converge in rare cases. Under no circumstances
should (X.T @ X) be singular, so the pseudoinverse is unique and
precisely what we want.
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@jhlegarreta I wonder if I could bug you for a review. I suspect this would be a quick one for you, but let me know if it's not.

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codecov bot commented May 30, 2024

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 70.47%. Comparing base (4d1352a) to head (4dde564).

Current head 4dde564 differs from pull request most recent head 17bac08

Please upload reports for the commit 17bac08 to get more accurate results.

Files Patch % Lines
nipype/algorithms/confounds.py 85.71% 2 Missing ⚠️
Additional details and impacted files
@@            Coverage Diff             @@
##           master    #3651      +/-   ##
==========================================
- Coverage   70.83%   70.47%   -0.36%     
==========================================
  Files        1276     1276              
  Lines       59314    59305       -9     
  Branches     9824     9822       -2     
==========================================
- Hits        42013    41797     -216     
- Misses      16125    16353     +228     
+ Partials     1176     1155      -21     

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For Legendre regressors, the ith column is the ith-order polynomial, so
the constant column is 0. For the cosine regressors, a constant column
was appended to the end, in contradiction of the docstring. This brings
both into alignment so columns are sorted from lowest to highest
frequency and aligns the DCT behavior with its docstring.
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Take my review with some care: I am not familiar with fMRI data processing.

Changes look sensible; the methods being changed are not tested, though 😬. Documenting the methods would help 📖.

Thanks for addressing the reported issue so fast.

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2 participants