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Centre of Gravity

Description

You can simulate pointing gravity towards a 3D point by cancelling out the world gravity and applying a force towards the point where you want the new simulated gravity to be.

This example creates many spheres all attracted to the point 0,0,0. In the example, each body is applied a force opposite to its own world position relative to 0,0,0.

Note that in this example, world gravity is set to 0,-1,0 rather than cancelling it out completely to 0,0,0. This is so that bodies will still react to surface friction and the spheres will appear to rotate as they roll over or smash against each other. If you do set world gravity to 0,0,0, then the surface friction calculations within Cannon will equate to 0 and the spheres won't appear rotate when they collide. In Cannon, when bodies collide, If either body has mass 0 or the world gravity is 0,0,0, then any friction multiplications equate to 0.

Example Script

./src/client/client.ts

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import * as THREE from 'three'
import { OrbitControls } from 'three/examples/jsm/controls/OrbitControls'
import Stats from 'three/examples/jsm/libs/stats.module'
import { GUI } from 'dat.gui'
import * as CANNON from 'cannon-es'

const scene = new THREE.Scene()
scene.add(new THREE.AxesHelper(5))

const light1 = new THREE.SpotLight()
light1.position.set(2.5, 5, 5)
light1.angle = Math.PI / 4
light1.penumbra = 0.5
light1.castShadow = true
light1.shadow.mapSize.width = 1024
light1.shadow.mapSize.height = 1024
light1.shadow.camera.near = 0.5
light1.shadow.camera.far = 20
scene.add(light1)

const light2 = new THREE.SpotLight()
light2.position.set(-2.5, 5, 5)
light2.angle = Math.PI / 4
light2.penumbra = 0.5
light2.castShadow = true
light2.shadow.mapSize.width = 1024
light2.shadow.mapSize.height = 1024
light2.shadow.camera.near = 0.5
light2.shadow.camera.far = 20
scene.add(light2)

scene.background = new THREE.CubeTextureLoader().load([
    'img/px_eso0932a.jpg',
    'img/nx_eso0932a.jpg',
    'img/py_eso0932a.jpg',
    'img/ny_eso0932a.jpg',
    'img/pz_eso0932a.jpg',
    'img/nz_eso0932a.jpg',
])

const camera = new THREE.PerspectiveCamera(
    75,
    window.innerWidth / window.innerHeight,
    0.1,
    1000
)
camera.position.set(0.5, 0.5, 6)

const renderer = new THREE.WebGLRenderer()
renderer.setSize(window.innerWidth, window.innerHeight)
renderer.shadowMap.enabled = true
document.body.appendChild(renderer.domElement)

const controls = new OrbitControls(camera, renderer.domElement)
controls.enableDamping = true
controls.target.y = 0.5

const world = new CANNON.World()
world.gravity.set(0, -1, 0) // setting minimal gravity otherwise you lose friction calculations

const moonMaterial = new THREE.MeshStandardMaterial()
let texture = new THREE.TextureLoader().load('img/moon_540x270.jpg')
moonMaterial.map = texture

const sphereMeshes: THREE.Mesh[] = []
const sphereBodies: CANNON.Body[] = []

for (let x = 0; x < 100; x++) {
    const sphereGeometry = new THREE.SphereGeometry(0.5)
    sphereMeshes.push(new THREE.Mesh(sphereGeometry, moonMaterial))
    sphereMeshes[x].position.x = Math.random() * 100 - 50
    sphereMeshes[x].position.y = Math.random() * 100 - 50
    sphereMeshes[x].position.z = Math.random() * 100 - 50
    sphereMeshes[x].castShadow = true
    sphereMeshes[x].receiveShadow = true
    scene.add(sphereMeshes[x])

    const sphereShape = new CANNON.Sphere(0.5)
    sphereBodies.push(new CANNON.Body({ mass: 1 }))
    sphereBodies[x].addShape(sphereShape)
    sphereBodies[x].position.x = sphereMeshes[x].position.x
    sphereBodies[x].position.y = sphereMeshes[x].position.y
    sphereBodies[x].position.z = sphereMeshes[x].position.z
    world.addBody(sphereBodies[x])
}

world.addEventListener('postStep', function () {
    // Gravity towards (0,0,0)
    sphereBodies.forEach((s) => {
        const v = new CANNON.Vec3()
        v.set(-s.position.x, -s.position.y, -s.position.z).normalize()
        v.scale(9.8, s.force)
        s.applyLocalForce(v)
        s.force.y += s.mass //cancel out world gravity
    })
})

const button = {
    explode: function () {
        sphereBodies.forEach((s) => {
            s.force.set(s.position.x, s.position.y, s.position.z).normalize()
            s.velocity = s.force.scale(Math.random() * 50)
        })
    },
}

window.addEventListener('resize', onWindowResize, false)
function onWindowResize() {
    camera.aspect = window.innerWidth / window.innerHeight
    camera.updateProjectionMatrix()
    renderer.setSize(window.innerWidth, window.innerHeight)
    render()
}

const stats = Stats()
document.body.appendChild(stats.dom)

const gui = new GUI()
gui.add(button, 'explode')

const clock = new THREE.Clock()
let delta

function animate() {
    requestAnimationFrame(animate)

    controls.update()

    delta = Math.min(clock.getDelta(), 0.1)
    world.step(delta)

    sphereBodies.forEach((s, i) => {
        sphereMeshes[i].position.set(s.position.x, s.position.y, s.position.z)
        sphereMeshes[i].quaternion.set(
            s.quaternion.x,
            s.quaternion.y,
            s.quaternion.z,
            s.quaternion.w
        )
    })

    render()

    stats.update()
}

function render() {
    renderer.render(scene, camera)
}

animate()