why the fixed body position is updated?

[keman5]

I have a box on the plane

1. the box mass is 1, no force on it.
2. call world.fixedStep(), the box position will be updated every tick
3. after a few minites, the box will move out the plane.

If the box mass = 0, the position.x will not be updated, it's fine.

    const world = new CANNON.World({
      gravity: new CANNON.Vec3(0, -9.81, 0),
    });

    // box
    const boxBody = new CANNON.Body({
      mass: 1,
      shape: new CANNON.Box(new CANNON.Vec3(2, 2, 2)),
    });

    boxBody.position.set(2, 0, -2);
    world.addBody(boxBody);

    // ground
    const groundBody = new CANNON.Body({
      shape: new CANNON.Plane(new CANNON.Vec3(10, 10)),
    });

    groundBody.quaternion.setFromEuler(-Math.PI / 2, 0, 0); // 绕x轴旋转90度
    world.addBody(groundBody);

    const tick = () => {
      world.fixedStep();
      console.log(`boxBody.x: ${boxBody.position.x}`);
      cannonDebugger.update();
      window.requestAnimationFrame(tick);
    };

    tick();

how to fix it?

CleanShot_2023-10-30_3.37.22.mp4

[isaac-mason]

The workaround for this is "body sleeping".

You can configure cannon-es so bodies moving slower than a configurable speed for a configurable time go to "sleep", and do not move until they are touched by another body, or manually woken up.

See this demo:
https://pmndrs.github.io/cannon-es/examples/sleep
https://github.com/pmndrs/cannon-es/blob/master/examples/sleep.html

[keman5]

Thanks, can I use body.applyLocalImpulse(new CANNON.Vec3(5, 0, 0), new CANNON.Vec3()) to wake up body when I press keyboard?

[isaac-mason]

Yep, applyLocalImpulse will wake up a sleeping body 🙂