git:20261019.9f93139 to git:20261019.cc0fa84

2 added, 0 removed. Audit A to A.

---

name: mechanism-draft

description: Technical-drawing films of a mechanism from published evidence, drawn as a vector Paper sheet (section views, hatching, flat material colours, no lighting). Every number in the spec is tagged SPEC, MEASURED, DERIVED or ESTIMATED and the gate refuses a spec whose numbers do not close. Use for a section or drafting-sheet explainer of how a mechanism works, rendered to MP4 or PNG. Triggers on "technical drawing of X", "section view of X", "draft X", "/mechanism-draft".

---

# Mechanism Draft

A standalone part of Mechanism: the Paper engine, the evidence gate and the film pipeline, without the 3D Studio film or the Workflow engine. Run every command **from this folder** (cloned as a skill: `~/.claude/skills/mechanism-draft`). Work in the user's project, never inside this folder: renders and new objects go in a work folder such as `./mechanism-work`.

Needs Node 22+, Edge or Chrome, and `ffmpeg` (films only). No install step, no network, no API key.

## Quick start

1. **Check the install:** `node scripts/skill-doctor.mjs` (every file present and unchanged, the optional mesh pack, the tools).

2. **The example:** `node scripts/check-spec.mjs examples/lock/spec.js` (the gate), then `node scripts/preflight.mjs examples/lock/index.html --method paper --length short --aspect 9:16`, then a still and a film:

`node scripts/render.mjs examples/lock/index.html --method paper --background ivory --length short --aspect 9:16 --still 9 --out <work>/lock.mp4` and the same without `--still`.

3. **Your object:** `node scripts/new.mjs <name> --dir <work> --title "..." --fig "Fig N"` creates `<work>/<name>/` with a draft `spec.js` that fails the gate until every TODO is replaced by researched, tagged numbers. Gate it, preflight it, render it as above.

## What it draws

- **Piece drawings from declared 2D geometry** (no 3D model, no three.js, no drawing code): parts of `kind: 'drawing'` whose `p` declares the geometry in mm (an outline: rect, circle or polygon; holes with stable ids; polar and linear patterns; centre lines and arcs; an optional section with hatching) and `paperScene: { kind: 'geometry' }`. The gate checks the geometry (closed contours, holes inside the material, no overlap, net area) and Paper draws each part with dimensions taken from it. Examples: `examples/demo-plate`, `demo-flange`, `demo-section` (synthetic, ESTIMATED). The schema and what a dimension needs to count as compared: `references/scenes.md`.

- **Vector scenes** (no 3D model, no three.js): a spec with `paperScene: { kind: 'pin-section', ... }` is drawn as an animated section from the spec's own numbers (see `examples/lock`). Read `references/scenes.md` before writing one.

- **Mesh sheets** (optional): any other spec is drawn as front/top/side orthographic sheets projected from its parts. That needs the optional mesh pack (three.js); without it the page stops with a load error naming the missing file. `skill-doctor` says whether it is installed.

+ **SVG drawings of the 2D parts** (Draft 0.3, no browser): `node scripts/export-svg.mjs <spec.js> --mode geometry-1to1|sheet-fit --out <work>/svg` writes one SVG per `drawing` part. `geometry-1to1`: 1 user unit = 1 mm of the part (a hole declared Ø6.6 is a circle of radius 3.3). `sheet-fit`: an A4 sheet with a title block, the geometry scaled and the scale written as not 1:1. Circles and arcs stay exact; every dimension is a value of the geometry, coloured by its evidence; the spec must pass the gate; a layout too dense to be legible stops the export and names the labels. The file says it is not a manufacturing drawing (no tolerances, fits or finish). Details: `references/scenes.md`.

## What is checked, and what is not

The gate checks the numbers against their published sources; the preflight checks text and layout frame by frame. With a vector scene the mesh area check cannot run and is reported as `NOT RUN GEOMETRY_AREA` with its reason: that is not a pass. A passing gate means the numbers agree with their sources, not that the sources are right, that the drawing is to scale (it is not) or that the user approved it. A piece drawing that passes is internally consistent: its dimensions count as compared only where `params` ties them to a sourced claim, and the sheet says how many are declared, not verified. A flat part is never given an invented side view or a 3D model; asking Studio for one is an error. Details: `references/checks.md`.

Paper's look is fixed: strokes, hatching and flat fills; no gradients, shadows, filters or lighting.

## Report

End with: what was modelled and from which sources; what the gate compared; what is ESTIMATED (drawn dashed); what the preflight reported, NOT RUN lines included; the files made; anything not checked.