private float x1, y1, x2, y2;

void setup() 
{
  size(600, 800);
  PImage img = loadImage("CityNight.jpg");
  image(img, 0, 0);
  noStroke();
  colorMode(HSB);
  frameRate(30);
  rectMode(CENTER);
  for (int i=0; i<10; i++) {
    fill(255, 100);
    rect(random(120, width-120), random(120, height-120), 30, random(50, 100));
  }
}

void draw()
{
  if (frameCount<100) {
  fill(random(20, 80), random(50, 170), random(40, 170), 150);
  x1 = random(120, width - 120);
  y1 = random(120, height - 120);
  x2 = random(10, 30);
  y2 = random(20, 40);
  rect(x1, y1, x2, y2);
  }
}