float ang;

void setup() {
  size (200,200);
  background (255);
  framerate (50);
  rectMode(CENTER);
}
void draw () {
  background (255); 
  stroke(0,0,0,50);//color of red square
  translate (100,100);//move origin of red square

  line(0,-width, 0, width);// draw vertical line
  line(-height, 0, height, 0);// draw horizontal line
  rotate (ang);
  fill(255,0,0,50);//   red square
  rect (0,0,40,40);

  rotate (-ang);
  translate (40,40);
  rotate (ang);
  fill(0,255,0,50);//   green square
  rect (0,0,20,20);

  rotate (-ang);
  translate (25,25);
  rotate (ang);
  fill(0,0,255,50);//    blue square
  rect (0,0,10,10);

  ang+= TWO_PI/360;
}