​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  //background(220);

  //noFill();

  rect(0, 0, p * width, p * height);

  rect(p * width, p * height, (1 - 2 * p) * width, (1 - 2 * p) * height);

  rect(width - (p * width), height - (p * height), p * width, p * height);

  //p = p * 1.01;

  p = p + 0.01;

  if (p > 1){

    p = 0;

  }

}