let g = 0;

let gspeed = 10;

​

// Size values

let s = 0;

let sspeed = 1;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  // Draw background at g

  background(g);

  // Change grayscale value

  g += gspeed;

  // Bounce grayscale value

  gspeed = bounce(g, 0, 255, gspeed);

​

  // Draw a rectangle of size s

  rect(width / 2, height / 2, s);

  // Change size value

  s += sspeed;

  // Bounce size value

  sspeed = bounce(s, 0, 200, sspeed);

}

​

// Bounce the direction of change of a value

function bounce(value, lower, upper, speed) {

  // Bounce the direction of change of a value

  if (value < lower || value > upper) {

    speed *= -1;

  }

​

  return speed;

}

​