Filipe Costa - 1 year ago 75
Javascript Question

# rotate around pendulum 0.85 cylinder height

im stuck with a work that i need to do, basicly first i needed to create a pendulum that basicly is 2 spheres and 1 cylinder the 2 spheres are in the up and down a bigger 1 and a smaller 1, what i need to do is the pendulum should move not around his own center but about a center that is 0.85 the height of my cylinder, my idea was to create a pivot point, but since i dont know very good how the pivot point work i tryed a lot of things, i tryed first to add the cylinder to the scene and the spheres to the cylinder, then i created the pivot point and add the pivot point to the cylinder, in the animate function i just tried to rotate the pivot point in the x axis but nothing happens :/

Here is my code guys hope someone can give me a hand

``````var scene, camera, renderer;
var caixaGrande, caixaPequena1, caixaPequena2,cylinder,Cylinder2,esferaGrande;
var pivotPoint1, pivotPoint2;

const RAIOCILINDRO = 2.5;
const ALTURACILINDRO = 100;
const RAIOESFERAGRANDE = 15;
const RAIOESFERAPEQUENA = 5;
var rotacao = Math.PI/180;

window.onload = function init() {

scene = new THREE.Scene();

camera = new THREE.PerspectiveCamera( 60, window.innerWidth / window.innerHeight, 1, 10000 );
camera.up=new THREE.Vector3(0,1,0);
camera.position.set(150,50,50);
camera.lookAt(new THREE.Vector3(0, 0, 0));

// Add a directional light to show off the object
var light = new THREE.DirectionalLight( 0xffffff, 1.5);
// Position the light out from the scene, pointing at the origin
light.position.set(1.5, 1.5, 1);
scene.add( light );

var textureCylindro = new THREE.TextureLoader().load("CylinderTexture.png");
var textureSphere = new THREE.TextureLoader().load("SphereTexture.png");

geometry = new THREE.CylinderGeometry(RAIOCILINDRO,RAIOCILINDRO,ALTURACILINDRO);
material = new THREE.MeshPhongMaterial( {color: 0xffffff , map:textureCylindro} );
cylinder = new THREE.Mesh(geometry,material);
scene.add(cylinder);

pivotPoint1 = new THREE.Object3D();
pivotPoint1.position.y=ALTURACILINDRO*0.15;
cylinder.add(pivotPoint1);

var geometry = new THREE.SphereGeometry(RAIOESFERAGRANDE);
var material = new THREE.MeshPhongMaterial( {color: 0xffffff,map:textureSphere} );
esferaGrande = new THREE.Mesh( geometry, material );
esferaGrande.position.y = -ALTURACILINDRO/2;
cylinder.add( esferaGrande );

var geometry = new THREE.SphereGeometry(RAIOESFERAPEQUENA);
var material = new THREE.MeshPhongMaterial( {color: 0xffffff,map:textureSphere} );
var esferaPequena = new THREE.Mesh( geometry, material );
esferaPequena.position.y = ALTURACILINDRO/2;
cylinder.add( esferaPequena );

Cylinder2 = cylinder.clone();
Cylinder2.position.z = 3 * RAIOESFERAGRANDE;
scene.add(Cylinder2);

renderer = new THREE.WebGLRenderer();
renderer.setSize( window.innerWidth, window.innerHeight );

document.body.appendChild( renderer.domElement );
animate();
}

function animate() {

pivotPoint1.rotation.x += 10;

requestAnimationFrame( animate );

renderer.render( scene, camera );

}
``````

Answer Source

What you should do is create an Object3D that you add the other 3 elements to, then you add that Object3D to the scene. Then when you want to rotate the whole thing, you rotate your Object3D.

So for example:

``````var axis = new Object3D();
sphere1.position.y = -.15 * pendulumLength;
sphere2.position.y = .85 * pendulumLength;
cylinder.position.y = .35 * pendulumLength;
//assuming the cylinder is pendulumLength long, this^ puts it right between the two balls
axis.add(sphere1);
axis.add(sphere2);
axis.add(cylinder);
scene.add(axis);
``````

then in your animate() function, simply rotate your Axis:

``````axis.rotation.z += .01;
``````

EDIT: So, here is my bad drawing of what happens. If you look here, the circle when positioned at 0,0 when rotated, rotates around the axis. Again when you move it up to 1,1 it rotates around its center point, because rotation is relative to the center point of the circle.

Again the same is true of the cylinder. At 0,0 it rotates around its center. At .5,.5 it also rotates around its center. It doesn't care where it is, it will rotate around its position point.

So if we want to rotate these two as a group relative to some other point, we need to make them the children of another object, because when we move the parent, the children maintain their relationship to the parent, even though their positions are still 1,1 and .5,.5 respectively.

The reason that they get rotated in the manner on the right is because their relationship to the parent object is 1,1 and .5,.5 relative to a parent object with a rotation of 0 radians. When we rotate the parent object by some number of radians, they need to move in order to maintain their original relationship.

Another way to think of it: You have a wooden board with a nail perfectly through the center and you have a shoe. You set the shoe in the upper right hand corner of the board with the toe pointing away from you. If you rotate the wooden board Math.PI/2 radians, the shoe will not stay in the upper right hand corner (relative to you), even though that's where you put it, because the shoe has been added to the board, and you have moved the board. Now, the shoe should be in the bottom right, and will be facing to your right. Object3D.add() is like the process of putting the shoe on the board.

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