Juice as Choreography

What it is. Feel is not decoration sprinkled on a sim — it’s a choreography the sim authors and the view performs. The sim resolves an event instantly and returns a script (what happened, when); the view replays it as particles, shake, pops, and sound. FUN.md law 6, the cosmetic-view rule: views must be deletable without changing a single sim bit. Juice is loud and free of gameplay risk because it never touches the hash.

Player fantasy. “That hit felt like it mattered.” Weight, punch, satisfaction — the difference between clicking a spreadsheet and landing a blow. The game answering your input like it heard you.

Why it works

The 2-senses feedback contract. Every meaningful event answers on ≥ 2 senses within the frame — sfx + particles, or sfx + flash + shake. One channel reads as cheap; two reads as professional (JUICE.md Part 2). This is the single highest-leverage feel rule.
Choreography-not-state keeps determinism sacred. Because the view reads sim events, not state it feeds back, a replay reproduces the exact same show and deleting the whole view changes not one game bit (FUN.md law 6). You can lean all the way into juice at zero risk to the proofs.
Timing is sim time. Impact, beat, cascade timing is world.time / the fixed dt, never wall-clock — so the choreography is reproducible and the hash holds (JUICE.md Part 1).

Levers

Lever	The Hayao primitive	Envelope (JUICE.md Part 2)
Screen shake	`Shaker` (camera’s parent)	land ≈ 0.12, death ≈ 0.5; quadratic falloff, decay ≈ 3/s
Hit-stop	freeze frames on impact	sells weight; > ~12 frames reads as a hitch
Impact particles	`Particles` + `PARTICLE_PRESETS`	answer an event, life 0.2–0.6s, then vanish
Pop numbers / pickups	`FloatingText` + `FLOAT_PRESETS`	one pop per event; don’t stack to soup
Atmosphere	`AmbientField` + `AMBIENT_PRESETS`	persistent field (snow/ash), not a burst stream
Depth	`ParallaxLayer`	0 = far/pinned, 1 = foreground
Procedural SFX	`audio.tone/blip/play/success`	pitch-rising combo; the second sense, cheaply

(Grep docs/API.md before citing — all of the above are real; examples/small-flame wires this kit and passes its feel gates.)

Applied across genres

Genre	The signature choreography
Physics arcade (`genre-physics-arcade`, `anchor-peggle`)	Pitch-rising ricochet chain, big burst on multipliers — the juice is the product
Action-adventure (`genre-action-adventure`)	Hit-stop + flash + i-frame shimmer sell the blow (FUN.md §4)
Horde survival (`genre-horde-survival`)	Screen full of pops on a level-up; the “number goes up” feast
Match-3 (`genre-match3`)	The purest law 6 — the deterministic sim returns the cascade script, the view animates it (FUN.md §13)
Deckbuilder (`anchor-balatro`)	The score-multiplier crescendo; each joker its own beat
Precision platformer (`genre-precision-platformer`)	Dust on land, shake on death, snappy landing — feel that rewards clean movement
Rhythm (`genre-rhythm`)	Feedback on the beat — sim time drives both the hit and its flourish (FUN.md §18)

Overdone when…

Juice smothers the read. Constant shake and particle spam bury the avatar and the threat — feel that fights pattern-readability. Reserve the big choreography for the big events (the 2-senses contract, not the 5-senses assault).
Nausea shake. Trauma that doesn’t decay fast reads as motion sickness; the feedback gate caps it (JUICE.md Part 2).
Hitch hit-stop. Freezes > ~12 frames read as a bug, not weight.
Juice bound to state, not events. If the view reads back state it influences, determinism breaks and the replay diverges — law 6 violated. Wire to events.
Seizure-bright / frantic. The judge flags it as a high-severity juice-restraint failure (JUDGE.md rubric 4).

Verify / feel-gate link

Feel is made checkable — this is the whole point of Channel 4:

Declare a FeedbackContract, then gate it (JUICE.md Part 3): feedbackIssues(FEEDBACK, [...]).length === 0 asserts every listed event answers on ≥ 2 senses with bounded juice. A silent one-channel event is now a failing test.
Camera smoothness. cameraIssues(camSamples).length === 0 — no snaps or jerk (the pattern’s timing rail; JUICE.md Part 4 lists the frame-1 snap gotchas).
Determinism (FUN.md law 6). Run headless, hash; run with view, hash; assert equal. Proves the choreography is cosmetic — the view is deletable.
The vision judge (JUDGE.md rubric 4 & 5). Juice restraint and motion clarity are scored from the actual pixels — feel gates can pass while the image still reads as noise; the judge closes that mile.

Worked micro-example

“A sword swing that feels like it lands.” The sim resolves the hit instantly and returns a script: { hit: true, at: pos, hitstop: 5, knockback: dir }. The view performs it — Shaker.addTrauma(0.12), a PARTICLE_PRESETS.hit() burst at pos, a 5-frame freeze that buffers the next input through it, an audio.tone that pitches up on a combo, and a FloatingText damage pop. That’s the 2-senses contract met several times over (sfx + particles + flash + shake), all cosmetic, all driven off the event not read-back state. Delete the whole view and the sim hash is identical (FUN.md law 6). Prove it: run headless, hash; run with view, hash; assert equal — and feedbackIssues(FEEDBACK, ['hit']).length === 0 guards the ≥2-senses rule.

Composes with

pattern-readability — juice must serve the read, never bury it; the two are in constant tension and readability wins ties.
pattern-pacing-and-tension — reserve the loudest choreography for the peaks so it stays meaningful.
pattern-anti-frustration — a death that feels clean (readable cause, quick reset) is half feel, half forgiveness.
system-combat-model — hit-stop and i-frames are where the shape of a hit and its choreography meet.