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thinr (development version)

Correctness fixes from the figureextract ecosystem review (2026-07-03). All are boundary conditions the published algorithms assume away; see the new vignette("correctness-properties") for the guarantees they restore.

  • thin() now skeletonises shapes that touch the matrix border correctly. The kernels inspect an 8-neighbourhood and so never deleted pixels in the outermost row or column, leaving edge-touching shapes two to three pixels thick; thin() now pads the image with a one-pixel background margin and crops it back, so a shape thins identically whether or not it touches the frame. Applies to all seven methods (#F012).

  • thin(method = "opta") now keeps two-pixel-wide strokes connected. The SPTA kernel evaluated all four contour directions against the pre-cycle snapshot and deleted them in one batch, so both sides of a 2px stroke were removed together and the skeleton fragmented (a 2×9 bar collapsed to its four corner pixels). Deletion is now sequential per direction, as Naccache & Shinghal's two-scan formulation requires (#F011).

  • distance_transform(metric = "manhattan" | "chessboard") now returns Inf for a fully-foreground image (no background pixel exists), matching the "euclidean" metric and EBImage::distmap(). The two-pass metrics previously seeded foreground with a finite sentinel that leaked out as a plausible-looking but meaningless finite distance (#F015).

  • thin(), distance_transform(), and medial_axis() now reject NA input with a clear error at the coercion boundary instead of silently turning it into NA_integer_ (INT_MIN) in the C++ kernels (#F016).

  • thin(method = "hilditch") now thins junctions to the published parallel form. The look-ahead conditions 3 and 4 skip deleting a pixel only when a cardinal neighbour has crossing number A == 1 on the current image; the kernel had compared the look-ahead crossing number (computed with the centre already removed) against 1, which is strictly stronger and also spared junction neighbours where A >= 2, leaving a redundant pixel beside the junction. Skeletons are now equal to or thinner than before, never thicker (verified against a reference implementation of the published form over random images) (#F014).

  • Added a connectivity-preservation property test across all seven methods and tightened the Holt straight-line test (#F013).

  • Documented and pinned the isolated-2×2-block behaviour: the default zhang_suen erases an isolated 2×2 block entirely while guo_hall keeps one pixel. See vignette("choosing-a-method") for guidance on small-blob masks (#F017).

  • Removed thinImage(). Use thin() (Zhang-Suen is the default method).

thinr 0.2.0

Initial CRAN release.