Forge Universe

Sim bounce (Usb)

Hash: Usb · Demo: /demos/sim-bounce.html

WebGL rigid-body bounce driven by assets/scenes/sim-bounce.json. Five solids — two spheres and three polyhedra — fall under gravity, collide with the ground, and bounce with sphere/AABB contact resolution.

Scene

Item Value
Fixture assets/scenes/sim-bounce.json
Nodes ball-a, ball-b, cube-a, oct, tet
Sim bodies b1–b5 linked via nodeId
Ground y: 0, restitution: 0.35
Gravity [0, -9.81, 0]

See scene fixtures — sim-bounce.json for per-node detail.

Behaviour

  1. Load — UniverseSimGl.mount fetches the scene, calls UniverseWorld.loadScene and UniverseSim.createSim.
  2. Meshes — one Three.js MeshStandardMaterial mesh per node (colours match other WebGL demos).
  3. Play — each animation frame runs UniverseSim.step(sim) and copies poses into meshes.
  4. Reset — UniverseSim.reset(sim) restores initial positions and velocities from the create-time snapshot.
  5. Ground — visual ground plane at Y=0; physics ground uses sim.ground.y.

When prefers-reduced-motion: reduce is active, Play sim becomes Step once (one step per click, no continuous loop).

Controls

Control data-action Effect
Play sim / Pause play-sim Toggle data-playing on [data-ks-hash="Usb"]; step each frame while playing
Step once play-sim Under reduced motion only — single step
Reset reset-sim UniverseSim.reset, resync meshes, pause playback

Script load order

<div id="mount-sim" class="fu-demo-mount"></div>
<script src="/js/vendor/three.min.js"></script>
<script src="/js/universe-world.js"></script>
<script src="/js/universe-sim.js"></script>
<script src="/js/universe-fps-hud.js"></script>
<script src="/js/universe-sim-gl.js"></script>
<script src="/js/universe-demo-boot.js"></script>

universe-demo-boot.js detects #mount-sim and calls:

UniverseSimGl.mount(sim, '/assets/scenes/sim-bounce.json');

API

Module Entry
UniverseSim createSim, step, reset, maxSpeed — see Simulation
UniverseSimGl mount(container, sceneUrl, opts) — see JavaScript API

Oracle

docs/design/universe/oracles/Usb.json — Playwright scenarios on [data-ks-hash="Usb"]:

Scenario Checks
usb-dom-present Root mount exists
usb-play-toggle Click play sets data-playing="true"

Run with the full verifier: Verification.

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