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20 — Physics

Bullet physics (Ammo.js WASM) in a web worker, talking to the main thread through a shared matrix buffer. Forks: ammo.js#jel/master, three-ammo#jel/master, ammo-debug-drawer.

Architecture (src/systems/physics-system.js)

  • The worker (three-ammo's AmmoWorker) owns the Bullet world: gravity (0, −9.8, 0), ~90 Hz step, MAX_BODIES = 512, debug wireframe drawer available.
  • Main-thread ↔ worker transform sync via one ArrayBuffer viewed as Float32Array/Int32Array of per-body matrices — zero-copy-ish, ownership transferred back and forth (message types INIT / BODY_READY / SHAPES_READY / TRANSFER_DATA).
  • bodyUuidToData maps bodies to entities; ready/reset callbacks per body.

Declaring physics on entities

  • body-helper — body type (dynamic/kinematic/static), mass, collisionFilterGroup/Mask.
  • shape-helper — box/sphere/cylinder/cone/plane/hull shapes; media auto-generate box/hull.
  • heightfield — terrain chunks (17).
  • Voxel models get shapes computed from voxel data by VoxSystem.

Collision layers (src/constants.js:1)

Bitmask groups: INTERACTABLES 1, ENVIRONMENT 2, AVATAR 4, HANDS 8, PROJECTILES 16, BURSTS 32. Composites: DEFAULT_INTERACTABLE = 31 (everything but bursts), UNOWNED_INTERACTABLE = 25, ENVIRONMENTAL_VOX = 17, DEFAULT_SPAWNER = 9.

Ownership & kinematics

Only the network owner simulates an object dynamically; everyone else's copy is kinematic (set-unowned-body-kinematic) and follows NAF lerps. Grabbing = take ownership + hand↔body constraint (constraints-system.js); release restores floaty-object behavior (low/zero gravity float, auto-lock).

Consumers

  • Character controller — grounding raycasts, jump ballistics, walkable surfaces (incl. GLB walkability via floor/wall colliders, one of the final features).
  • Cursor/interaction — obstacle checks; note the late optimization "stop computing bounding box for every raycast".
  • Projectiles (emoji launcher) — PROJECTILES/BURSTS layers, projectile-system.js.
  • Transform system — ground detection for snap/stack.

Physics debug: ammo-debug-drawer renders wireframes when enabled (helpers-system/debug flags).