game-feel - SKILL.md Agent Skill

name: game-feel description: "Deep game-feel and juice reference — input forgiveness windows, movement curves, hit stop, trauma-based screen shake, squash & stretch, camera kick, audio feel — with concrete numbers from the canonical sources (Swink, Vlambeer, Celeste, Smash). Use when tuning how a game FEELS: 'the controls feel floaty/sluggish/slippery', 'jumping feels bad', 'hits don't land', 'make combat feel weighty', 'add juice', 'coyote time', 'input buffering', 'screen shake feels wrong', or any movement/impact tuning pass. The game-playbook craft checklist covers the basics; this is the deep module with the tuning values."

Game feel: the numbers

Game feel = real-time control + simulated space + polish (Swink). The polish amplifies the simulation but never lies about it: if the screen shakes, something must have hit. Small tweaks compound — Vlambeer's flat shooter becomes Nuclear Throne via ~30 individually-invisible tricks. Apply them in passes, then play it: feel is tuned in motion, never designed on paper.

All frame counts assume 60fps — which means physics must be delta-time-based. Per-frame integration (x += vx) runs 2× fast on a 120Hz display; this is the single most common feel bug in browser games. Multiply all motion by dt, and convert the frame windows below to ms when your loop isn't fixed-step.

Input forgiveness (the invisible half of "tight controls")

Fudge everything slightly in the player's favor — players read forgiveness as their own skill (Maddy Thorson, "Celeste & Forgiveness").

Input buffer 4–10 frames: a jump/attack pressed slightly early fires on the first legal frame. Celeste ~4, Smash 10. Start at 5 for precise games, 8–10 for forgiving ones. Track a bufferedAt timestamp per action; consume in update().
Coyote time 5–8 frames: jump still works after walking off a ledge (Celeste ~5–6). Track lastGroundedAt.
Corner correction: a jump that clips a ceiling corner by a few px gets nudged sideways; a dash that clips a ledge pops up onto it. Celeste's wall-jump reaches ~2px off the wall at 320×180 — scale to your resolution.
Never demand frame-perfection. Any 1-frame window gets widened.
Touch: enlarge hitboxes, not visuals — tappable region > sprite.
Respond to core verbs within ~100ms total (Swink's correction cycle); never add startup delay to movement inputs.
Continuous/tracking inputs (pointer, analog stick, webcam/hand): the forgiveness knob is smoothing-vs-latency. Lerp the controlled object toward the input (factor ~0.1–0.3 per 60fps frame) to kill jitter, but keep total lag under the ~100ms cycle; add a small dead zone, and fudge the interaction hitboxes (paddle/catch zones) larger than the visuals.

Movement curves

Reach max run speed in ~6 frames, stop in ~3 (Celeste). Stopping faster than starting reads "responsive"; the reverse reads "slippery".
Variable jump height: releasing jump early cuts upward velocity.
Apex float: while jump is held, halve gravity near the arc's peak (Celeste) — gives air-control where players need it.
Fall faster than you rise: heavier gravity on descent + a fall-speed cap. Symmetric arcs feel floaty.
Keep platform momentum for a few frames after leaving a moving platform.
Phaser: implement as manual velocity writes in update() — Arcade drag alone can't express asymmetric accel or apex gravity.

Impact

Hit stop: freeze attacker AND victim 2–5 frames on light hits, 6–12 on kills, hard cap ~20–30 (Smash ≈ damage × 0.65 + 6 frames). Scale with importance. Pause tweens/anims for the parties, not the whole world, when multiple fights overlap. In non-Phaser loops (rAF/useFrame), implement as a freeze timer that skips integration for the frozen entities while rendering continues.
Knockback both ways: victim recoils from hits; shooter kicks back 1–2px on fire. Suspend AI steering ~150ms so the shove reads.
Hit flash: solid white tint 1–3 frames.
Particles at every contact (muzzle, impact, dust on land), lifetimes 0.2–0.5s.
Permanence: corpses, craters, shell casings stay (Nijman's highest-leverage trick). Stamp into a texture or cheap static images, not live bodies.
Bigger projectiles, lower accuracy, higher fire rate — perfect accuracy reads sterile.

Animation principles (Disney, applied)

Squash & stretch, volume conserved: scale Y 0.7 on landing → scale X ~1.3. Stretch along velocity at speed; squash on impact. Typical: jump stretch 1.1–1.25, land squash 1.2–1.4, recover over 100–200ms ease-out.
Anticipation on big actions only: 2–6 frame wind-up before a boss leap or charged shot. Never on core movement — that's input lag.
Overshoot and settle: Back.easeOut/Elastic.easeOut on everything that appears or changes; stagger group entrances 20–50ms apart.
Nothing pops: scale-in on spawn, shrink-and-fade on death.

Camera

Lerp-follow with look-ahead in the facing/aim direction. Phaser: startFollow(player, true, 0.08–0.15) + manual look-ahead offset.
Trauma shake (Eiserloh, GDC 2016): events add 0.2–0.5 to a 0–1 trauma value; decay to 0 over 0.5–1s; offset = maxOffset × trauma², sampled from smooth noise per axis. Add rotational shake (max ~5–10°) — cheapest way to make 2D shake feel violent. Maxima ~5–10% of screen size. Trauma stacks naturally where fixed-magnitude camera.shake() calls fight each other.
Directional camera kick: punch a few px opposite the shot / into the hit, ease back. Distinct from omni shake.
Keep shakes short (<0.5s). Offer reduce-motion for sustained shakers.

Audio feel

Every interaction sounds — fire, hit, kill, land, pickup, UI. Audio is half of juice and the cheapest half.
Randomize pitch ±5–15% per play: sound.play({ rate: 0.9 + Math.random() * 0.2 }).
Lower pitch = heavier. Fatten kills/explosions with a bass layer; put a distinct confirm sound on kills above the routine hit sound.
Duck or sting music on death/level-clear; silence after noise is itself an effect.
Browser: unlock audio on first gesture; pool instances to avoid GC hitches.

Tuning session

Numbers above are starting points, not answers. Wire the 5–6 feel constants (accel frames, buffer, coyote, hit-stop, shake trauma) to a debug panel or query params, play 2 minutes, adjust, repeat. If you can't decide between two values, the larger forgiveness / smaller effect is usually right.

Sources

Swink, Game Feel — the theory: real-time control, ~100ms cycle.
Jonasson & Purho, "Juice It or Lose It" (GDC 2012) — youtube.com/watch?v=Fy0aCDmgnxg
Nijman (Vlambeer), "The Art of Screenshake" — youtube.com/watch?v=AJdEqssNZ-U
Thorson, "Celeste & Forgiveness" — maddymakesgames.com/articles/celeste_and_forgiveness/index.html
Eiserloh, "Juicing Your Cameras With Math" (GDC 2016) — youtube.com/watch?v=tu-Qe66AvtY
Sakurai on hitstop — ssbwiki.com/Hitlag
GMTK Platformer Toolkit (feel the variables yourself) — gmtk.itch.io/platformer-toolkit

Related skills: game-playbook (build order + basic craft pass), phaser (engine APIs), onboarding (difficulty feel), design-lenses (is it fun at all).