Building Games

Build a playable, correct browser game — not a static screenshot. A game is just a React route with a <canvas> (or <Canvas> for R3F) plus DOM overlay UI. Style the overlay (start screen, HUD, menus) with the design-ui skill; this skill owns the gameplay loop and world.

Controls / inverted A/D: open .grok/skills/controls/SKILL.md before writing WASD, steering, or flight input. Vehicle/flight demos often ship with A/D flipped if you only read this file or a single genre playbook.

Scope note — single-player, bots, or small P2P co-op:

• Ship single-player or single-player + AI/bots by default.
• 2–8 player co-op / casual realtime (shared cursors, party games, casual action among friends) is supported — use the multiplayer-p2p skill (WebRTC mesh, signaled at /api/rtc). Read its trust model first.
• P2P is the only supported multiplayer right now. Do not half-build sockets that can’t connect.

References (load on demand):

• controls skill (../controls/) — required for movement/steer/flight: player-visible A/D, inverted-steer anti-pattern, flight ailerons, mandatory self-test + window.__controlsTest. Not optional for vehicles/planes.
• references/threejs-foundational.md — the deep 3D/loop/perf reference. Read for 3D.
• references/3d-libs.md — three.js + @react-three/fiber + drei + rapier usage.
• threejs skill (../threejs/) — official full Three.js + TSL API dump (llms-full.txt). Load for advanced materials/shaders/WebGPU/loaders; not for simple 2D canvas games.
• references/babylon.md — Babylon.js, the batteries-included 3D engine alternative.
• references/phaser.md — Phaser 3, the default engine for 2D games.
• references/ecs-architecture.md — entity-component-system structure for larger games.
• references/genres/*.md — per-genre playbooks (fps, platformer-2d, racing-kart, puzzle-match3-tetris, voxel-minecraft, endless-runner, topdown-twin-stick, tower-defense, board-card-chess). Genre files do not replace controls.
• references/game-feel-juice.md, input.md, audio.md, collision-physics.md, save-persistence.md, procedural-generation.md, ai-pathfinding.md.
• game-asset-core (+ game-animation-frames / game-tilesets / game-character-consistency / game-ui-icons) — engine-ready 2D art defaults and verification when generating sprites, sheets, tiles, or UI (see §6).

Pick the specific genre/topic reference for the build; this file is the universal core for loop/world. Input signs → controls. 2D game art → game-asset-core.

1. Game loop & timing (the #1 correctness issue)

• Drive the loop with the engine's RAF loop (renderer.setAnimationLoop, R3F useFrame, or requestAnimationFrame for 2D canvas). Never setInterval/ setTimeout/Date.now() for game timing.
• Scale ALL movement/animation by delta time (seconds) so speed is frame-rate independent (60Hz vs 144Hz). Compute delta once per frame and reuse it.
  • three.js: use THREE.Timer (not Clock — Clock.getDelta() returns ~0 on a second call in the same frame, a classic freeze bug).
• Cap delta (min(delta, 0.1)) so a backgrounded tab doesn't teleport things.
• Fixed timestep for physics/gameplay: accumulate delta and step simulation at a fixed rate (e.g. 1/60) while rendering at display rate — prevents tunneling and non-determinism.

2. Controls (delegate to the controls skill)

Open .grok/skills/controls/SKILL.md before implementing any WASD / steer / flight code. That skill is the source of truth for:

• Player-visible A = left / D = right (chase cam, while moving forward)
• Why KeyA → steer− + yaw += steer * +rate inverts (most common bug)
• Vehicle vs FPS (strafe ≠ steer), fixed-wing ailerons, heli/drone notes
• Mandatory self-test + window.__controlsTest probe

Do not treat genres/racing-kart.md as the only place steer signs live — planes, jetskis, and mechs never open it.

Short reminder (full detail in controls):

// Vehicle yaw body (chase cam): A must increase yaw with this basis
forward = (-sin(yaw), 0, -cos(yaw))
// KeyA → steer = +1;  yaw += steer * turnRate * speedFactor * dt
// WRONG (ships inverted): KeyA → steer = -1; yaw += steer * turnRate * dt

• Pointer lock: mouse-look only — implement WASD yourself; click-to-play overlay; dismiss on lock.
• Track keys with held state + dt; unify devices via references/input.md.
• Finish: run the controls skill checklist. Screenshot-only is insufficient for vehicles/flight.

3. 3D orientation & world objects (the "sideways/backwards" bugs)

• three.js is right-handed, +Y up: +X right, +Y up, +Z toward viewer. Meshes face +Z; cameras look −Z (the classic "camera backwards" gotcha).
• Primitives like Cone/Cylinder point +Y by default → rotate to align a tip with forward (geo.rotateX(Math.PI/2)).
• Orienting a mesh to face forward (meshes face +Z): simplest correct way is mesh.lookAt(mesh.position.clone().add(forward)). To build the basis by hand, set the +Z column to forward and choose the x-axis that keeps it a proper right-handed rotation (det = +1): xAxis = normalize(cross(up, forward)), then makeBasis(xAxis, up, forward). Note this is cross(up, forward) — not the movement right = cross(forward, up) from §2. Targeting +Z (instead of a camera's −Z) flips the x-axis sign so that xAxis × up = forward; the frame stays right-handed (no mirroring). Do not use makeBasis(xAxis, up, -forward) for a mesh — that targets −forward (the camera convention) so the mesh faces backwards.
• Orienting a camera to look along forward (cameras look −Z): set the +Z column to -forward — xAxis = normalize(cross(forward, up)), then makeBasis(xAxis, up, -forward) (or just camera.lookAt(target)).
• Keep a consistent world up so objects stay upright; only rotate flat primitives to stand up.
• Verify glTF import orientation; debug with AxesHelper/ArrowHelper. Upright self-test: characters stand on the ground plane, not lying/sunk.

4. Camera must agree with movement

• Keep a dedicated moveForward/moveRight for movement, computed once and never mutated by camera code (aliasing a shared temp vector makes camera and movement disagree — a real repro bug).
• Third-person follow: desired = playerPos + up*height + moveForward*(-followDist); lerp the camera toward it (use exp-based smoothing, delta-scaled), lookAt(player).
• Isolate-the-layer debug order: (1) keys register → (2) movement signs correct → (3) camera agrees. Fix in that order.

5. Performance

• Minimize draw calls (renderer.info.render.calls, target <100): share materials, InstancedMesh/BatchedMesh for repeated objects, atlases.
• Dispose GPU resources yourself (geometry/material/texture.dispose()) on level change — three.js does not GC them. Pool bullets/enemies/particles.
• No per-frame allocations (reuse temp vectors). Compress textures; LOD for distance.

6. Assets (avoid the generated-photo trap)

• Interactive 3D elements (weapon viewmodels, characters, props, projectiles) → build from 3D geometry / glTF, not a generated image. A flat photorealistic JPG of a gun-in-hands used as an FPS viewmodel looks wrong, can't animate, and (JPG has no alpha) renders as a black box. Parent a real 3D viewmodel to the camera as an overlay render layer.
• Reserve image generation for flat 2D assets only (textures, sky/menu backgrounds, 2D sprites, UI art). Never use a generated photo as a 3D mesh, viewmodel, or character substitute — build those in 3D geometry / glTF. Set crossOrigin="anonymous" on images drawn to canvas/textures. See the imagine skill (2D only — image tools cannot produce real 3D).
• Engine-ready game art doctrine → open game-asset-core (../game-asset-core/) for defaults + blind verify + retry discipline, then the matching specialist: game-animation-frames (loop / motion laws), game-tilesets, game-character-consistency, game-ui-icons. These are QC/doctrine, not the export pipeline. Do not ship stick-figure placeholders when real art is expected.
• 2D game sprites / animation sheets → run generate2dsprite (.grok/skills/generate2dsprite/SKILL.md): solid #FF00FF magenta image_gen sheets + chroma postprocess scripts (magenta is required for the processor). Wire transparent PNGs/GIFs into Canvas/Phaser. Still apply game-asset-core (+ animation/character specialists when relevant).
• 2D maps / levels / prop packs → open generate2dmap (.grok/skills/generate2dmap/SKILL.md). Prefer foundation-only base + separate props/collision for playable maps. Browser default: raw_canvas / Phaser. Tileable ground/walls → also game-tilesets for 2×2 seam checks.
• Optional denser locomotion → run video2dsprite (Grok image_to_video + sandbox scripts; magenta base). Prefer generate2dsprite for crisp production heroes. Use game-animation-frames for loop/flip-test laws; prefer video2dsprite over ad-hoc ffmpeg-only harvest in this sandbox.

7. Audio, save, feel

• Audio: unlock AudioContext on the first user gesture (tap-to-start) or iOS is silent; re-resume on visibilitychange. (references/audio.md)
• Save: localStorage/IndexedDB with a version field + migrations.
• Juice: screen shake, hit-stop, eased tweens, particles — cheap, huge perceived-quality lift. Keep presentation separate from simulation.

8. Mobile

• Distinguish canvas buffer size from CSS size; respect devicePixelRatio.
• touch-action: none, letterbox-fit to a base resolution, handle orientation.
• Touch controls (virtual joystick + action buttons), ≥44px targets.

Stack / engine choice

• 3D → three.js, ideally via @react-three/fiber + drei (fits the React app; drei gives pointer-lock/controls/loaders) + @react-three/rapier for physics/character controllers. See references/3d-libs.md.
• 2D → native Canvas 2D is enough for snake/tetris/flappy/platformer; reach for Phaser only when the genre needs it.
• These game deps are not preinstalled — npm install them (and make sure they land in package.json so the Vercel build has them).

Finish criteria (before "done")

• Loads with no console errors; visible gameplay (not a blank canvas).
• controls skill self-test passed (A = left / D = right from chase cam while moving forward; flip one sign if inverted). Not screenshot-only.
• 3D upright & camera-agrees self-tests pass (§3, §4).
• Runs on mobile viewport with touch controls.
• Production build (npm run build) renders the built output, not just dev.