问无法为matter.js设置动画

EN

    var Engine = Matter.Engine,
    World = Matter.World,
    Bodies = Matter.Bodies;

    var world;

    var engine;
    var particles=[];

function Particles(x,y,r){
  this.x=x;
  this.y=y;
  this.d=r*2;
  this.body=Bodies.circle(x,y,r);
  World.add(world,this.body);
  this.disp=function(){
    fill(255);
    ellipse(x,y,this.d, this.d);
  }

}



function setup() {
createCanvas(600,400);
engine = Engine.create();
world=engine.world;
Engine.run(engine);
rect1=Bodies.rectangle(width/2,height/2,10,150);
World.add(world,rect1);



}

function mousePressed(){
  particles.push(new Particles(mouseX,mouseY,5));
}

function draw() {
  background(51);

Engine.update(engine);
  push();
  rectMode(CENTER);
  rect(width/2,height/2,10,150);
  pop();

  for (var i = 0; i < particles.length; i++) {
    particles[i].disp();



}}