Become arthur: a standalone suite, not an Animator Pro front-end

The test renderer turned out to be the product. Everything that decides how the
work looks - stabilisation, reduction, timing, frame removal, palette - already
happens here, and the flat indexed output already reads the way it should.

The reason to leave is in the original design's own rule: never make a timing
decision that requires a full render to evaluate. Honouring that moved every
judgement out of Animator Pro, which left the host doing nothing but writing a
file, in exchange for modal UI, minutes-long renders, one-level undo, FLX delta
invariants, a single tween state and a cel singleton.

What does NOT change is the constraint. 320x200, indexed palette, flat fills,
no antialiasing - inherited, but load-bearing rather than accidental. The
rasteriser writes palette indices and expands to RGBA only at the end precisely
so nothing can soften an edge. Modern conveniences belong in the workflow.

Adds docs/design.md: the principles, carried over without the Poco/FLX/cel
machinery, plus architecture and an honest list of what is missing - the
largest gap being that plates still have nowhere to be drawn.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/design.md

@@ -0,0 +1,219 @@
+# arthur — design
+
+A small animation suite for turning live-action video into work that reads as
+hand-authored, in the idiom of *Another World* (Éric Chahi, 1991): flat shapes,
+a tiny palette, hard edges, motion carried by silhouette.
+
+## The aesthetic is a representation, not a filter
+
+Chahi did not process video. He shot reference footage and hand-traced polygons
+over it in a custom editor; the engine stored and replayed polygon lists, never
+bitmaps. Three properties follow, and all three are load-bearing:
+
+1. **Temporal identity.** The same shape, with the same vertex count and vertex
+   order, *edited* across frames. A shape re-detected independently each frame
+   produces a new contour every frame. That boils, and boiling reads as "filter"
+   within half a second no matter how good the individual shapes are.
+2. **Stylised timing.** Poses held, and for most parts a hard cut between them
+   rather than an interpolation. A distinct pose on every frame of 24fps footage
+   looks like video even when every pose is a polygon.
+3. **Authored colour.** A small fixed ramp with two or three tones per part,
+   chosen by a person. Sampling colour from the source produces a pixel-art
+   filter immediately and irrecoverably.
+
+None of those are computer-vision problems. That is the whole argument about
+where CV belongs.
+
+## The constraint is the point
+
+320×200, indexed palette, flat fills, no antialiasing. That is inherited from
+Animator Pro, where this work started, but it is not an accident to be
+modernised away — it is why the output looks right. The rasteriser writes palette
+indices into a byte buffer and expands to RGBA only at the end, precisely so no
+canvas antialiasing can soften an edge.
+
+Modern conveniences belong in the *workflow* — instant feedback, audio, real
+undo, scrubbing, layers. Not in the output.
+
+## CV tracks; it does not draw
+
+- **Its job.** Where the rigid features of the face are, what the head's
+  transform is, where the lip contour runs, which frame is a motion extreme.
+- **Its non-job.** Deciding what the shapes are. A face mesh offers 468 points;
+  an animator's mouth is eight. **That decimation ratio is the style.** It is a
+  decision encoded in the tool, not a measurement extracted from footage.
+
+Every number crossing from analysis into rendering is a *parameter* or a
+*correspondence*. Every number determining how something looks comes from the
+artist or a fixed authored table.
+
+## Two kinds of part
+
+| Kind | Source | Vocabulary | Interp |
+| --- | --- | --- | --- |
+| **Plate** — head, hair, body | Hand-drawn | Closed: a few drawings per character | hold |
+| **Feature** — mouth, lids | Rotoscoped from landmarks | Open: derived from this take | hold |
+| **Interior** — mouth interior, teeth | Image content within a feature | Open | hold |
+| **Primitive** — iris | Landmark centroid as a disc | Quantised | hold |
+
+The asymmetry is deliberate, and it is the opposite choice in each case.
+
+A **closed vocabulary is wrong for the mouth.** Pre-authored mouth shapes are
+never *this performance's* shapes, and that genericness is what the project
+exists to avoid. So the mouth gets an open vocabulary derived from footage, and
+the stylisation applies to its **timing** instead.
+
+A **closed vocabulary is right for the head.** The head carries structure, not
+performance. A handful of hand-drawn angles is better, because you drew them —
+and it means the head never needs segmentation, contour tracking, or boil
+avoidance.
+
+## Two kinds of sparseness
+
+Conflating these was the original design error. **Aesthetic** sparseness is set
+by the extraction rate: pick 12fps and the timing is already chosen. **Labour**
+sparseness is a human drawing each one, and it binds only on the plate.
+
+So the mouth keeps **every** frame — it is traced, and therefore free. In limited
+animation lip sync is routinely the densest element, on 1s, while heads hold on
+2s and 3s.
+
+The other half is frame removal: which frames need their own plate drawing.
+Everything starts kept; delete what you don't want. Automatic suggestion runs
+error-tolerance decimation over head pose as a starting point, then you
+hand-correct.
+
+## Stabilisation
+
+A rotoscoped mouth is only reusable if expressed independently of where the head
+was. That inverts the obvious composition:
+
+```text
+mouth_local(t) = anchor(t)⁻¹ · mouth_world(t)
+```
+
+- **Rigid landmarks only** for the fit: eye corners, nose bridge, nose tip.
+  Including a feature that moves bleeds performance into the stabilisation.
+- **Similarity, not affine or homography.** The extra degrees of freedom absorb
+  head rotation as distortion and smear it into the mouth. Four DOF removes
+  exactly translation, roll and depth scale, leaving yaw and pitch as a
+  measurable residual.
+- **Reference is the mean** configuration over the shot, not frame zero.
+- **Smooth the transform, never the contour** — with one bounded exception,
+  below.
+- Landmarks convert to an **isotropic** space first (unit = one image height).
+  MediaPipe normalises x by width and y by height, so its space is stretched; a
+  "similarity" fitted there is not one.
+
+Out-of-plane rotation cannot be removed by any 2D transform. The answer is not a
+better transform: it is to draw the head at each angle and let each drawing
+declare where its mouth sits. Foreshortening becomes authored metadata.
+
+## Hold versus interpolate
+
+Another World had no inbetweening engine: polygon sets played back frame by
+frame at a low rate. For parts that should read as hand-animated snaps — mouths
+above all — **cut between keys, do not interpolate.** A tweened mouth is rubbery
+and reads as puppet software immediately.
+
+**Fixed topology is non-negotiable for cut parts.** Vertex *meanings* must be
+stable across every key, so a closed mouth and a wide-open mouth are the same
+polygon at different positions. Interpolation partially hides ordering drift;
+cutting exposes it completely, as static.
+
+## Deriving shapes from pixels without boiling
+
+Some things have no landmarks — teeth, tongue, anything inside the lips. The
+hazard is vertex correspondence: a traced contour reorders between frames.
+
+Two escapes, both used:
+
+- **Extract a scalar, not a shape**, where the shape can be derived from
+  geometry you already trust.
+- **Radial sampling** where a real contour is needed. March outward from the
+  blob's centroid along N fixed directions; vertex *k* is then always "the extent
+  in direction *k*". Correspondence holds by construction, the count is fixed,
+  temporal smoothing is well defined, and the star-shaped result suits flat
+  colour.
+
+## The bounded smoothing exception
+
+*Smooth the transform, never the contour* held while keys were sparse: sampling
+at velocity minima rejected per-frame detector noise for free. With a key on
+every frame it does not, so a short contour average is justified — the window
+must stay **shorter than the shortest articulation worth keeping**. At 12fps,
+mouth movement spans 3–6 frames and detector noise is per-frame, so ±1 separates
+them and ±3 starts eating speech. At 24fps, double it.
+
+## Performance tracks may lead the clock
+
+A centred moving average has no phase lag, but it blurs onsets, so the visually
+salient moment of a mouth opening moves later even though the mean does not.
+Animators also draw mouth shapes a frame or two ahead of the sound as standard
+practice. Only performance tracks shift; the head stays with the audio, because
+it is the mouth that should anticipate.
+
+## Colour discipline
+
+- A small ramp. Another World ran 16 colours at 320×200.
+- Two or three tones per part: base, shadow, occasionally a rim.
+- No dithering, no antialiasing anywhere.
+- Never sample colour from the source. Analysis may report *which* tone a region
+  should be; it must never report an RGB value.
+- Plate art and generated parts share one palette, authored together.
+
+## Three artifacts, not two
+
+| Artifact | Cost | Regenerated when |
+| --- | --- | --- |
+| Dense track — landmarks and fitted anchor per frame | Minutes, once per shot | Re-shoot, or a model change |
+| Take — parts, keys, kept frames | Instant | Every knob change |
+| Render | — | Continuous |
+
+Keep the dense track: re-keying is then a regenerate, never a re-trace. And
+because the take is disposable, **hand corrections must not live in it** — they
+belong in an override layer keyed by `(part, frame)`, applied on top at render
+time.
+
+## Architecture
+
+The organising idea is an **indexed raster core**, a **part/track model**, and
+**pluggable sources** feeding it. Sources so far are rotoscope and image-derived
+interior; hand-drawn and procedural are the obvious next ones. Everything else —
+stabilisation, key selection, frame removal, palette — operates on the track
+model and is source-agnostic.
+
+Current modules:
+
+| Module | Role |
+| --- | --- |
+| `landmarks.js` | Index tables. Ring arrays are ordered traversals: slot position *is* vertex identity. |
+| `mathutil.js` | Similarity fit, Procrustes mean, temporal smoothing. |
+| `pipeline.js` | Stabilise → subsample → key-select → frame-removal. |
+| `interior.js` | Teeth from image content: Otsu, morphology, components, radial contour. |
+| `underlay.js` | Registered photo reference and palette posterisation. |
+| `raster.js` | Indexed scanline fill. No antialiasing, by construction. |
+| `take.js` | Take-file writer. |
+| `synth.js` | Synthetic landmarks, so everything below detection is testable without a video. |
+| `selftest.js` | Assertions, including source-level wiring checks. |
+
+## Not built yet
+
+- **A paint surface.** The plates have nowhere to be drawn. This is the largest
+  gap between "tool" and "suite": a pixel paint canvas with onion skin, palette
+  constraint, and the registered underlay behind it.
+- Eyes, irises, brows as parts.
+- Plate libraries with per-plate mouth slots.
+- Real performer→character calibration (currently identity).
+- The override layer.
+- Export beyond the take file — FLI/FLC would connect back to the lineage and is
+  a simple format.
+
+## Lineage
+
+This began as an analysis front-end for Animator Pro, whose render path was the
+intended destination. `Animator-Pro-SDL/docs/roto-puppet.md` documents that
+design and what the Poco/FLX machinery can and cannot do. The reason for leaving
+is in that document's own rule — *never make a timing decision that requires a
+full render to evaluate* — which, once honoured, left the host with nothing to do
+but write the file.