Paper cut-out (stop-motion paper) engine skill

Paper cut-out (stop-motion paper) engine

Every frame is a pure function of time t. studio.html loads src/kit.js (shared ReelMimic runtime: timeline, motion helpers, camera, render hooks), src/cutout.js (this look: materials, cut/torn pieces, shadows, set dressing, props, transitions, captions, the photographed finish), src/cast.js (the puppets) and one scene file per shot. render.mjs drives headless Chrome for contact sheets, strips, crops and the final frames.

Workflow

1. Scaffold — bash .claude/skills/paper-cutout/scripts/new_project.sh <project-dir> (add --keep-demo to keep the example). Then read GUIDE.md (in this skill folder) in full, and look at docs/demo-sheet.jpg, docs/expressions.jpg (13 expressions × both puppets) and docs/poses.jpg.
2. Storyboard with reads (what the viewer must understand, when) — the same discipline as painted-animation's ANIMATION_GUIDE.md (timing, one read at a time, anticipation → action → follow-through, transitions at every seam).
3. Build one IIFE file per shot in src/scenes/, each ending with shots([[t0, fn]]); add its <script> to studio.html after cast.js. Set duration in src/config.js. Start shots on multiples of 1/12 s.
4. Look — node render.mjs --sheet=... --cols=4 --w=480 --out=out/check/a.jpg, --strip=a:b for motion, --crop=x,y,w,h / --crop-at= for faces and hands. Open every image with Read. Fix, re-render.
5. Render — node render.mjs --frames --workers=4 && node render.mjs --encode --out=out/video.mp4 [--audio=...].

API in one screen (all angles in DEGREES, sizes in px, time via sm(lt))

paperCam(lt, [[0,[cx,cy,zoom]], [2,[cx,cy,zoom]]], { stepped })   // camBegin with a (smooth | stepped) move
backdrop(PC.sky, { to })            layer(.5, () => ...)            // screen-fixed sheet; parallax depth plane
hills(x,y,w,h,col,{seed,torn})  grass(x,y,w,h,col,seed)  tree(x,y,s,{fruit,sway}) → {fruit:[[x,y]..]}  bush  flower
cloud(x,y,w,{hang:'string'|'stick'})  sun(x,y,r)  house(x,y,s)  bunting(x0,y0,x1,y1,n,cols)  stripes  confetti(lt,x,y,{t0})
piece(SH.xxx(...), { x,y,rot,sx,sy, col, mat, depth, torn, seed, alpha, jit })   // ONE cut-out; the base of everything
SH.ellipse/circle/rect/round/poly/smooth/capsule/blob/cloud/leaf/heart/star/drop/tri/band/zig/hills ; clipHalf ; xform
mat: 'paper' | 'card' | 'kraft' | 'felt' | 'tissue' | 'corr' | 'wood'       brad(x,y,r)  thread(pts)  popUp(k,x,y,fn)
PROPS.apple|balloon|flower|star|gift|letter(x, y, ang, s)
// transitions — call AFTER camEnd(), in the INCOMING shot, k = 0→1 on stepped time:
trBook(t,k)  trTear(t,k)  trFlip(t,k)  trSlide(t,k,{dir})  sheetIn(k)  sheetOut(k)
captionTrack([[t0, t1, '文字', { style: 'label' | 'letters', y, size }]])

// puppets (cast.js) — (x, y) = ground point, u = size unit
pom(x, y, u, opts)  lulu(x, y, u, opts)  → joints { hand:[L,R], elbow, shoulder, foot, head, top, mouth }
opts: face, view:'front'|'3q', flip, look, blink, armL/armR:[shoulder,elbow], footL/footR:[dx,dy], sq, dy, lean, tilt,
      holdL/holdR:'apple'|{prop,s,dy}|fn, jit, dry (measure only)
moods(lt, [[0,'sad'],[2,'surprised']])  walkCycle(lt,{stride,rate}) + walkSpeed()  jumpPose(lt,t0,t1,h)  emote(kind,x,y,s,age)
EXPR: neutral happy laugh sad cry angry surprised scared love sleepy wink determined shy

Rules that matter most

1. Everything is paper. Draw only through piece() / the helpers (never raw fillRect/arc in scenes). Pick a material per object; torn edges for landscapes and labels, scissor edges for characters and props.
2. Depth = shadow. Background layers depth .3–.8, set pieces ~1, puppets 1–1.3, things floating/held up 1.5–3. Parallax with layer(k). Never fake 3D.
3. Stop-motion timing. Drive every puppet/prop pose with sm(lt) (12 steps/s). Puppets jitter a hair per step by themselves; set pieces stay glued (jit: 0). Camera moves may be smooth (motion control) or stepped.
4. Replacement faces. Change expressions with moods() (blink hides the swap), not by flipping face per frame.
5. Transitions from the kit of four (book fold, torn reveal, flip card, sheet slide) at every seam; the previous shot must NOT draw its own out-transition when the next shot uses one of them (they re-render it live).
6. Legible at phone size: puppets big (pom u ≥ 55, lulu u ≥ 36 in a medium shot), captions ≥ 70 px.
7. Captions only via captionTrack (overlay, CJK font "Chiron GoRound TC" → "Zen Maru Gothic" → "Noto Sans TC").

Engine gotchas

• Pure function of t: no Math.random(), no state between frames. Use hash/rnd/sway(key).
• piece() caches the cut outline by shape key + seed. Give each distinct piece its own seed, or two identical shapes get identical wobble (fine for confetti, odd for a pair of trees).
• The cut outline depends on the shape's size, so an animated size re-cuts every frame (edges shimmer). Scale with sx/sy or the camera instead of animating the shape's dimensions.
• prevFrame() renders the previous shot at the CURRENT time (it keeps moving under a transition) and swaps G; scenes must use G, not a captured context.
• Props held by a flipped puppet are un-mirrored automatically (text/asymmetric props stay readable).
• Speed: ~5–30 ms draw per frame, ~130 ms/frame effective with 4 workers incl. JPEG encode at 1920×1080.