docs(frangi_math): correct frangi() polarity docstring — bright vessels on dark#255
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…ls on dark, λ₂/λ₃ ≤ 0 The ``frangi()`` docstring had the eigenvalue polarity reversed on both halves: - Said "Bright structures (positive λ₂, and positive λ₃ in 3D) are zeroed" — but bright structures have NEGATIVE λ₂/λ₃ (the image curves downward outward from a bright center, so the second derivative perpendicular to a bright tubular structure is negative). The code zeros where λ₂/λ₃ are POSITIVE, which is the dark-on-bright case. - Said "Frangi assumes vessels are darker than background" — but this implementation is tuned for bright vessels on dark background, matching nellie's typical use case (fluorescence microscopy of bright mitochondria on dark cytoplasm). Code is correct; only the docstring was misleading. Updated attribution: dark structures (positive λ₂/λ₃) are zeroed; bright structures (λ₂ ≤ 0, and λ₃ ≤ 0 in 3D) are kept. Added a one-line explanation of the second-derivative reasoning so a future reader doesn't have to re-derive it. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
Docstring-only fix. The
frangi()function innellie/segmentation/frangi_math.pyhad the eigenvalue polarity reversed on both halves of its returns block:Code is correct. Only the docstring text changed; one-line reasoning added so a future reader doesn't have to re-derive the second-derivative convention.
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