let P, arr = [], buttonStep;

​

function setup() {

  createCanvas(W, H);

  for(let i = 0; i < arrLength; i++){

    arr[i] = [floor(random(width)), floor(random(height))];

  }P = [width/2, height/2];

  //buttonStep = createButton("Step");

  //buttonStep.mousePressed(step);

  background(150);

  fill(255); stroke(255);

  arr.forEach(x => ellipse(x[0], x[1], S*width/600, S*height/600));

  fill(0); stroke(0); strokeWeight(0.1);

}

​

function draw() {

  ellipse(P[0], P[1], width/600, height/600);

  step(1,1,1,1);

}

​

function step(a,b,c,d){

  let r = arr[floor(random(arrLength))];

  P = [(a*P[0]+b*r[0])/(a+b),(c*P[1]+d*r[1])/(c+d)];

}