void setup (){
  size(200,200);
  ellipseMode(CENTER_DIAMETER);
}
void loop(){
  background (0);
  if (mouseX< 100 || mouseY<100)
  for(int b=0;b<=200;b+=20){
    fill (0,255,0,b);
    rect (b,0,20,200);
  }else{
    background (0);
    for (int p=300;p>=0;p-=20){
      fill (0,p,0,150+p);
      ellipse (100,100,p,p);
    }
  }
}

    Source: geocities.com/ar/laventanitainteractiva

               ( geocities.com/ar)