Rectangular snow globes

int num = 2000;
int edge= 50;
Ball[] balls = new Ball[num];
color bg = #ffffff; //background + balls
color f = #2F3440; // fill rect

void setup() {
  size(500, 400);
  for (int i=0; i < num; i++) {
    balls[i]= new Ball();
  }
}

void draw() {
  background(bg);
  fill(f);
  rect(edge, 50, width-2*edge, 100, 15);
  rect(edge, 250, width-2*edge, 100, 15);
  for (int i=0; i < num; i++) {
    balls[i].run();
  }
}

class Ball {
  float x, y, sz;
  float speedx, speedy;
  float speed = 2;

  Ball() {
    x = random(width);
    y= random(height);
    speedx = random(-speed,speed);
    speedy = random(-speed,speed);
    sz = random(5,10);
  }

  void run() {
    move();
    bounce();
    display();
  }

  void move() {
    x += speedx;
    y += speedy;
  }

  void bounce() {
    if (x > width-sz/2 || x < sz/2) speedx *= -1;
    if (y > height+10 ||y < -10) speedy *= -1;
  }

  void display() {
    fill(bg);
    noStroke();
    ellipse(x, y, sz, sz);
  }
}