game-feel

# Game feel (juice)

The difference between a mechanic that *works* and one that feels *good* is feedback: the
layered, slightly-exaggerated response an action provokes. This skill covers the engine-
neutral techniques — screen shake, hit-stop, easing, squash & stretch, knockback, and stacked
feedback — and tells you how to apply them without burying the underlying simulation. It
**adds polish on top of** an existing mechanic; it does not implement the mechanic.

## When to use

- Use when an action (hit, jump, dash, pickup, death, button press) is mechanically correct
  but feels weak, weightless, or unsatisfying, and you want it to feel responsive and punchy.
- Use to add screen shake, hit-stop/freeze frames, eased motion, squash & stretch, knockback,
  flashes, or to layer multiple feedback channels onto one event.
- Use to decide *how much* juice is enough and where it crosses into noise.

**When *not* to use:** for the raw controller math (jump height, coyote time) use the
`platformer` genre and the engine movement skill. For camera *follow/deadzone/orbit* framing
use `camera-systems` (this skill only triggers the shake). For mixing, ducking, and adaptive
music use `audio-design`. For shader-based dissolves/flashes use `shader-programming` and the
engine shader skill. For the concrete tween/particle node APIs, use the engine animation skill
(`godot-animation`, `unity-animation`).

## Core principle: feedback is layered and exaggerated

One satisfying hit is usually **5–8 tiny responses firing together** within ~100 ms: a sound,
a particle burst, a brief hit-stop, a flash, a knockback, a small screen shake, and a number
popping up. Each is cheap; stacked, they read as "impact". Two rules keep it from becoming a
mess: **(1)** exaggerate *briefly* and return to rest (juice is transient, not a new resting
state); **(2)** scale juice to event importance — a footstep is not a boss death.

## Core workflow

1. **Confirm the event hooks exist.** Juice attaches to discrete events: `on_hit`, `on_land`,
   `on_pickup`, `on_death`, `on_fire`. If the mechanic doesn't emit these, add them first.
2. **Pick feedback channels per event** from the menu (sound, particles, shake, hit-stop,
   flash, knockback, tween, number pop). Start with 2–3; add until it reads, then stop.
3. **Make motion eased, not linear.** Route scale/position/UI changes through a tween with an
   ease (overshoot for "pop", ease-out for "settle"). Linear motion feels robotic.
4. **Reserve hit-stop and shake for impact.** They are the strongest, most abusable tools —
   short durations, scaled to importance, and never on routine actions.
5. **Keep feedback off the critical simulation.** Shake moves the *camera/visual*, not the
   body; hit-stop uses time scale or a real-time pause, not a gameplay-logic stall.
6. **Tune by importance tiers.** Define small/medium/large feedback presets and assign events
   to a tier, so the whole game's juice stays consistent and proportional.
7. **Verify by playing and watching.** Trigger the event repeatedly; confirm the feedback
   fires, returns to rest, and is not nauseating or input-blocking. Report what you observed
   (does shake decay? does input still register during hit-stop?).

## Patterns

### 1. Screen shake by decaying "trauma" (smooth, not a random jitter)

```gdscript
# Godot 4.7. Store trauma 0..1; shake = trauma^2 so small hits barely move, big hits punch.
# Drives a Camera2D OFFSET (the visual), never the player body. Decays every frame.
@export var decay := 1.2          # trauma lost per second
@export var max_offset := Vector2(12, 8)
@export var max_roll := 0.1       # radians
var trauma := 0.0
var _t := 0.0

func add_trauma(amount: float) -> void:
    trauma = clampf(trauma + amount, 0.0, 1.0)   # hits ADD; they don't reset

func _process(dt: float) -> void:
    if trauma <= 0.0: return
    trauma = maxf(trauma - decay * dt, 0.0)
    var shake := trauma * trauma                  # quadratic: gentle low, sharp high
    _t += dt * 30.0
    # Smooth pseudo-random via sampled noise/sin, NOT rand each frame (that buzzes).
    offset = Vector2(max_offset.x * shake * sin(_t * 1.7),
                     max_offset.y * shake * sin(_t * 2.3))
    rotation = max_roll * shake * sin(_t * 1.1)
# Unity 6.3 LTS: identical model on a CinemachineCamera via CinemachineBasicMultiChannelPerlin
# (set AmplitudeGain/FrequencyGain from trauma^2) — see camera-systems.
```

### 2. Hit-stop / freeze frame (sell impact by briefly stopping time)

```gdscript
# Godot 4.7. Drop time scale, then restore after a REAL-TIME delay (unaffected by time_scale).
func hit_stop(duration := 0.08, scale := 0.05) -> void:
    Engine.time_scale = scale
    # 4th arg ignore_time_scale=true → the timer still fires while the game is frozen.
    await get_tree().create_timer(duration, true, false, true).timeout
    Engine.time_scale = 1.0
```

```csharp
// Unity 6.3 LTS (C#). WaitForSecondsRealtime ignores Time.timeScale, so the timer still elapses.
IEnumerator HitStop(float duration = 0.08f, float scale = 0.05f) {
    Time.timeScale = scale;
    yield return new WaitForSecondsRealtime(duration);
    Time.timeScale = 1f;            // RIGHT: real-time wait. WRONG: WaitForSeconds (never resumes at scale 0)
}
```

### 3. Squash & stretch + overshoot via an eased tween (the "pop")

```gdscript
# Godot 4.7. Conserve volume: stretch one axis, squash the other, then spring back with overshoot.
func pop(node: Node2D) -> void:
    node.scale = Vector2(1.3, 0.7)                       # instant squash on the event
    var tw := create_tween()
    tw.tween_property(node, "scale", Vector2.ONE, 0.18) \
      .set_trans(Tween.TRANS_BACK).set_ease(Tween.EASE_OUT)   # BACK = overshoots past 1, settles
# RIGHT: ease back (TRANS_BACK/ELASTIC) for life. WRONG: linear tween → mechanical, dead.
```

### 4. A feedback bundle scaled by importance (keep juice proportional)

```gdscript
# One call per event; the tier decides intensity so the whole