A native PrusaSlicer plugin companion to printfins.com, targeting the 3.0 plugin API (project.plugin 1.0.0). Bundle com.printfins.support-fins/ is one flat directory with a single menu entry:

Support Fins → Add a Fin (add_fin.lua) — drops one breakaway support fin onto the plate: a thin blade on a flared foot, necking to a thin tip, with a comb of horizontal tines up the gripping face (denser near the base, per Slant3D's spec) — the same fin the website bakes in. You position it by hand with PrusaSlicer's own move tool: turn it so the tine face (the combed +X side) sits against your part's overhanging face, foot on the plate, and nudge it until the tines just touch. Then bend the fin off — the horizontal tines fuse in one layer line and snap clean. Params: fin height, length, wall, foot, and a Gripping Tines toggle (off = a plain breakaway wall for a flat overhang).

Why placement is by hand — read before filming

The website is the real product: load any STL → rotate it into a stronger orientation → it finds the overhangs and contours breakaway fins to the part's underside → export an STL that prints support-free in any slicer. All of that is mesh analysis: it reads the model's triangles.

The 3.0 plugin sandbox cannot read a mesh. Verbatim from the API notes: "a mesh's geometry cannot be read back — Mesh exposes bounds() and translate(), not triangles," and there is no getter for the user's loaded object's geometry anywhere in ProjectApi.cpp. The API is generative (build parametric objects from make_cube/make_cylinder/…). So a plugin can't auto-detect an overhang, contour a fin to it, or snap onto a surface — it can only generate the fin and let you place it. Auto-fitting stays on the web. The honest on-camera line: "Prusa shipped plugins mid-build, so I brought the fin into the slicer — it can't see your model, so it can't auto-fit like the site, but it drops the real fin in and you set it under your overhang."

Install

Dev (no signing) — the fast loop: copy the bundle directory into PrusaSlicer's data dir:

cp -R .../plugin/com.printfins.support-fins "<data dir>/lua/"

<data dir> is the folder holding PrusaSlicer.ini. On the 3.0 alphas it is not the plain name — a 3.0.0-alpha11 Windows build used %APPDATA%\PrusaSlicer3-dev\; on macOS look under ~/Library/Application Support/. Don't guess; find the folder with PrusaSlicer.ini, or launch with --datadir. Create the lua/ subfolder if needed, then restart the slicer; the entry appears under Support Fins → Add a Fin.

Release (signed ZIP import): the convenient install path requires a signature:

PrusaSlicer plugin keygen -P author.private.pem -p author.public.pem
PrusaSlicer plugin sign   -P author.private.pem com.printfins.support-fins

The ZIP must hold the bundle's files at its root (manifest.json at the top level, not inside a com.printfins.support-fins/ folder), and importers need authorized_authors/matthewtrahan.pem (the author field names the key file).

Verify (must be done in a running 3.0 slicer — it can't be unit-tested)

Support Fins → Add a Fin appears; the dialog shows four floats + the Tines toggle.
Generate with defaults → a thin blade on a T-foot, a necked tip, and a comb of ~7 tine ribs up the lower part of one face. (Preview of the intended shape: ~/Downloads/support-fin-plugin-preview.png.)
Move it so the tine face meets a part's overhanging face, foot on the plate; nudge until the tines just kiss the face.
It must be ONE connected fin, not scattered boxes. All pieces are built in one flat object space via shapes.builder() (vendored from leotrax3d, MIT) — the same pattern the shipped box_generator/tolerance_test use, which anchors every volume to the first and never assumes a primitive's origin. Corner-origin make_cube (base at z = 0) is confirmed by those working plugins. The earlier hand-rolled version centred the attachments at y = 0 while the wall spanned y[0,length], flinging the foot/tip/tines ~half the length off the blade — that bug is what the builder + the new overlap assertions in the test now rule out.
Slice, print, bend the fin off — tines should snap clean and leave faint marks.

Lint / local checks: run ./run-tests.sh from the plugin/ dir (needs lua/luac). It covers syntax (luac -p), the manifest JSON, the slicer's scan pass on a bare engine, and the fin arithmetic against a mock api (tests/add_fin_test.lua) — whose mock now models the real semantics (corner-origin cubes; other_volumes translates relative to the main mesh) and asserts every piece actually overlaps the wall, the check that catches the scatter. The fatal, silent trap the scan guards is any require/api call at file scope — the scan runs the whole file just to read info, on an engine with neither, so a hit there produces no menu entry and no error. Both files keep all api/require use inside functions (execute() for the plugin, method bodies for the module); keep it that way.

Notes / possible polish

Chamfered foot. The foot is currently a flat flange; a bevel up to the wall would read even more like the breakaway.py profile. Skipped for now — it needs rotated Negative cuts whose result can't be verified headlessly; add it with an in-slicer look.
Distribution: optionally PR to leotrax3d/prusaslicer-plugins-unofficial for reach + its CI and signing/release workflow, keeping the canonical copy here.
The earlier self-contained Overhang Test and Combined Support Demo entries were removed (git history keeps them) — the single Add-a-Fin is the product.